Syntax structures semantics

General

This section specifies the meaning of the syntax elements read in the syntax structures.

Important variables and function calls are also described.

OBU semantics

General OBU semantics

An ordered series of OBUs is presented to the decoding process. Each OBU is given to the decoding process as a string of bytes along with a variable sz that identifies the total number of bytes in the OBU.

If the syntax element obu_has_size_field (in the OBU header) is equal to 1, then the variable sz will be unused and does not have to be provided.

obu_size contains the size in bytes of the OBU not including the bytes within obu_header or the obu_size syntax element.

Methods of framing the OBUs (i.e. of identifying the series of OBUs and their size and payload data) in a delivery or container format may be established in a manner outside the scope of this Specification. One simple method is described in Annex B.

OBU data starts on the first (most significant) bit and ends on the last bit of the given bytes. The payload of an OBU lies between the first bit of the given bytes and the last bit before the first trailing bit. Trailing bits are always present, unless the OBU consists of only the header. Trailing bits achieve byte alignment when the payload of an OBU is not byte aligned. The trailing bits may also used for additional byte padding, and if used are taken into account in the sz value. In all cases, the pattern used for the trailing bits guarantees that all OBUs (except header-only OBUs) end with the same pattern: one bit set to one, optionally followed by zeros.

Note: As a validity check for malformed encoded data and for operation in environments in which losses and errors can occur, decoders may detect an error if the end of the parsed data is not directly followed by the correct trailing bits pattern or if the parsing of the OBU header and payload leads to the consumption of bits within the trailing bits (except for tile group data which is allowed to read a small distance into the trailing bits as described in [section 8.2.4][]). {:.alert .alert-info }

drop_obu( ) is a function call that indicates when the decoding process should ignore an OBU because it is not contained in the selected operating point. When an OBU is not in the selected operating point the contents have no effect on the decoding process.

When this function is called, the bitstream position indicator should be advanced by obu_size * 8 bits.

OBU header semantics

OBUs are structured with a header and a payload. The header identifies the type of the payload using the obu_type header parameter.

obu_forbidden_bit must be set to 0.

Note: This ensures that MPEG2 transport is possible by preventing emulation of MPEG2 transport stream ids. {:.alert .alert-info }

obu_type specifies the type of data structure contained in the OBU payload:

obu_type	Name of obu_type
0	Reserved
1	OBU_SEQUENCE_HEADER
2	OBU_TEMPORAL_DELIMITER
3	OBU_FRAME_HEADER
4	OBU_TILE_GROUP
5	OBU_METADATA
6	OBU_FRAME
7	OBU_REDUNDANT_FRAME_HEADER
8	OBU_TILE_LIST
9-14	Reserved
15	OBU_PADDING
{:.table .table-sm .table-bordered }

Reserved units are for future use and shall be ignored by AV1 decoder.

obu_extension_flag indicates if the optional obu_extension_header is present.

obu_has_size_field equal to 1 indicates that the obu_size syntax element will be present. obu_has_size_field equal to 0 indicates that the obu_size syntax element will not be present.

obu_reserved_1bit must be set to 0. The value is ignored by a decoder.

OBU extension header semantics

temporal_id specifies the temporal level of the data contained in the OBU. When temporal_id is not present, temporal_id is inferred to be equal to 0.

spatial_id specifies the spatial level of the data contained in the OBU. When spatial_id is not present, spatial_id is inferred to be equal to 0.

Note: The term "spatial" refers to the fact that the enhancement here occurs in the spatial dimension: either as an increase in spatial resolution, or an increase in spatial fidelity (increased SNR). {:.alert .alert-info }

Tile group OBU data associated with spatial_id and temporal_id equal to 0 are referred to as the base layer, whereas tile group OBU data that are associated with spatial_id greater than 0 or temporal_id greater than 0 are referred to as enhancement layer(s).

Coded video data of a temporal level with temporal_id T and spatial level with spatial_id S are only allowed to reference previously coded video data of temporal_id T' and spatial_id S', where T' <= T and S' <= S.

extension_header_reserved_3bits must be set to 0. The value is ignored by a decoder.

Trailing bits semantics

Note: Tile group OBUs, tile List OBUs, and frame OBUs do end with trailing bits, but for these cases, the trailing bits are consumed by the exit_symbol process. {:.alert .alert-info }

trailing_one_bit shall be equal to 1.

When the syntax element trailing_one_bit is read, it is a requirement that nbBits is greater than zero.

trailing_zero_bit shall be equal to 0 and is inserted into the bitstream to align the bit position to a multiple of 8 bits and add optional zero padding bytes to the OBU.

Byte alignment semantics

zero_bit shall be equal to 0 and is inserted into the bitstream to align the bit position to a multiple of 8 bits.

Reserved OBU semantics

The reserved OBU allows the extension of this specification with additional OBU types in a way that allows older decoders to ignore them.

Sequence header OBU semantics

General sequence header OBU semantics

seq_profile specifies the features that can be used in the coded video sequence.

seq_profile	Bit depth	Monochrome support	Chroma subsampling
0	8 or 10	Yes	YUV 4:2:0
1	8 or 10	No	YUV 4:4:4
2	8 or 10	Yes	YUV 4:2:2
2	12	Yes	YUV 4:2:0, YUV 4:2:2, YUV 4:4:4
{:.table .table-sm .table-bordered }

It is a requirement of bitstream conformance that seq_profile is not greater than 2 (values 3 to 7 are reserved).

Monochrome can only be signaled when seq_profile is equal to 0 or 2.

AV1 profiles are defined in Annex A.

still_picture equal to 1 specifies that the coded video sequence contains only one coded frame. still_picture equal to 0 specifies that the coded video sequence contains one or more coded frames.

reduced_still_picture_header specifies that the syntax elements not needed by a still picture are omitted.

If reduced_still_picture_header is equal to 1, it is a requirement of bitstream conformance that still_picture is equal to 1.

Note: It is allowed to have still_picture equal to 1 and reduced_still_picture_header equal to 0. This allows a video frame to be converted to a still picture by changing a single bit. {:.alert .alert-info }

timing_info_present_flag specifies whether timing info is present in the coded video sequence.

decoder_model_info_present_flag specifies whether decoder model information is present in the coded video sequence.

initial_display_delay_present_flag specifies whether initial display delay information is present in the coded video sequence.

operating_points_cnt_minus_1 indicates the number of operating points minus 1 present in the coded video sequence.

An operating point specifies which spatial and temporal layers should be decoded.

operating_point_idc[ i ] contains a bitmask that indicates which spatial and temporal layers should be decoded for operating point i. Bit k is equal to 1 if temporal layer k should be decoded (for k between 0 and 7). Bit j+8 is equal to 1 if spatial layer j should be decoded (for j between 0 and 3).

However, if operating_point_idc[ i ] is equal to 0 then the coded video sequence has no scalability information in OBU extension headers and the operating point applies to the entire coded video sequence. This means that all OBUs must be decoded.

It is a requirement of bitstream conformance that operating_point_idc[ i ] is not equal to operating_point_idc[ j ] for j = 0..(i - 1).

Note: This constraint means it is not allowed for two operating points to have the same value of operating_point_idc. {:.alert .alert-info }

If operating_point_idc[ op ] is not equal to 0 for any value of op from 0 to operating_points_cnt_minus_1, it is a requirement of bitstream conformance that obu_extension_flag is equal to 1.

seq_level_idx[ i ] specifies the level that the coded video sequence conforms to when operating point i is selected.

Note: Encoders should select the lowest level that is satisfied by the operating point to maximize the number of decoders that can decode the stream, but this is not a requirement of bitstream conformance. {:.alert .alert-info }

seq_tier[ i ] specifies the tier that the coded video sequence conforms to when operating point i is selected.

decoder_model_present_for_this_op[ i ] equal to one indicates that there is a decoder model associated with operating point i. decoder_model_present_for_this_op[ i ] equal to zero indicates that there is not a decoder model associated with operating point i.

initial_display_delay_present_for_this_op[ i ] equal to 1 indicates that initial_display_delay_minus_1 is specified for operating point i. initial_display_delay_present_for_this_op[ i ] equal to 0 indicates that initial_display_delay_minus_1 is not specified for operating point i.

initial_display_delay_minus_1[ i ] plus 1 specifies, for operating point i, the number of decoded frames that should be present in the buffer pool before the first presentable frame is displayed. This will ensure that all presentable frames in the sequence can be decoded at or before the time that they are scheduled for display. If not signaled then initial_display_delay_minus_1[ i ] = BUFFER_POOL_MAX_SIZE - 1.

choose_operating_point( ) is a function call that indicates that the operating point should be selected.

The implementation of this function depends on the capabilities of the chosen implementation. The order of operating points indicates the preferred order for producing an output: a decoder should select the earliest operating point in the list that meets its decoding capabilities as expressed by the level associated with each operating point.

A decoder must return a value from choose_operating_point between 0 and operating_points_cnt_minus_1, or abandon the decoding process if no level within the decoder's capabilities can be found.

Note: To help with conformance testing, decoders may allow the operating point to be explicitly signaled by external means. {:.alert .alert-info }

Note: A decoder may need to change the operating point selection when a new coded video sequence begins. {:.alert .alert-info }

OperatingPointIdc specifies the value of operating_point_idc for the selected operating point.

It is a requirement of bitstream conformance that if OperatingPointIdc is equal to 0, then obu_extension_flag is equal to 0 for all OBUs that follow this sequence header until the next sequence header.

frame_width_bits_minus_1 specifies the number of bits minus 1 used for transmitting the frame width syntax elements.

frame_height_bits_minus_1 specifies the number of bits minus 1 used for transmitting the frame height syntax elements.

max_frame_width_minus_1 specifies the maximum frame width minus 1 for the frames represented by this sequence header.

max_frame_height_minus_1 specifies the maximum frame height minus 1 for the frames represented by this sequence header.

frame_id_numbers_present_flag specifies whether frame id numbers are present in the coded video sequence.

Note: The frame id numbers (represented in display_frame_id, current_frame_id, and RefFrameId[ i ]) are not needed by the decoding process, but allow decoders to spot when frames have been missed and take an appropriate action. {:.alert .alert-info }

additional_frame_id_length_minus_1 is used to calculate the number of bits used to encode the display_frame_id or current_frame_id syntax element.

delta_frame_id_length_minus_2 specifies the number of bits minus 2 used to encode delta_frame_id_minus_1 syntax elements.

use_128x128_superblock, when equal to 1, indicates that superblocks contain 128x128 luma samples. When equal to 0, it indicates that superblocks contain 64x64 luma samples. (The number of contained chroma samples depends on subsampling_x and subsampling_y.)

enable_filter_intra equal to 1 specifies that the use_filter_intra syntax element may be present. enable_filter_intra equal to 0 specifies that the use_filter_intra syntax element will not be present.

enable_intra_edge_filter specifies whether the intra edge filtering process should be enabled.

enable_interintra_compound equal to 1 specifies that the mode info for inter blocks may contain the syntax element interintra. enable_interintra_compound equal to 0 specifies that the syntax element interintra will not be present.

enable_masked_compound equal to 1 specifies that the mode info for inter blocks may contain the syntax element compound_type. enable_masked_compound equal to 0 specifies that the syntax element compound_type will not be present.

enable_warped_motion equal to 1 indicates that the allow_warped_motion syntax element may be present. enable_warped_motion equal to 0 indicates that the allow_warped_motion syntax element will not be present.

enable_order_hint equal to 1 indicates that tools based on the values of order hints may be used. enable_order_hint equal to 0 indicates that tools based on order hints are disabled.

enable_dual_filter equal to 1 indicates that the inter prediction filter type may be specified independently in the horizontal and vertical directions. If the flag is equal to 0, only one filter type may be specified, which is then used in both directions.

enable_jnt_comp equal to 1 indicates that the distance weights process may be used for inter prediction.

enable_ref_frame_mvs equal to 1 indicates that the use_ref_frame_mvs syntax element may be present. enable_ref_frame_mvs equal to 0 indicates that the use_ref_frame_mvs syntax element will not be present.

seq_choose_screen_content_tools equal to 0 indicates that the seq_force_screen_content_tools syntax element will be present. seq_choose_screen_content_tools equal to 1 indicates that seq_force_screen_content_tools should be set equal to SELECT_SCREEN_CONTENT_TOOLS.

seq_force_screen_content_tools equal to SELECT_SCREEN_CONTENT_TOOLS indicates that the allow_screen_content_tools syntax element will be present in the frame header. Otherwise, seq_force_screen_content_tools contains the value for allow_screen_content_tools.

seq_choose_integer_mv equal to 0 indicates that the seq_force_integer_mv syntax element will be present. seq_choose_integer_mv equal to 1 indicates that seq_force_integer_mv should be set equal to SELECT_INTEGER_MV.

seq_force_integer_mv equal to SELECT_INTEGER_MV indicates that the force_integer_mv syntax element will be present in the frame header (providing allow_screen_content_tools is equal to 1). Otherwise, seq_force_integer_mv contains the value for force_integer_mv.

order_hint_bits_minus_1 is used to compute OrderHintBits.

OrderHintBits specifies the number of bits used for the order_hint syntax element.

enable_superres equal to 1 specifies that the use_superres syntax element will be present in the uncompressed header. enable_superres equal to 0 specifies that the use_superres syntax element will not be present (instead use_superres will be set to 0 in the uncompressed header without being read).

Note: It is allowed to set enable_superres equal to 1 even when use_superres is not equal to 1 for any frame in the coded video sequence. {:.alert .alert-info }

enable_cdef equal to 1 specifies that cdef filtering may be enabled. enable_cdef equal to 0 specifies that cdef filtering is disabled.

Note: It is allowed to set enable_cdef equal to 1 even when cdef filtering is not used on any frame in the coded video sequence. {:.alert .alert-info }

enable_restoration equal to 1 specifies that loop restoration filtering may be enabled. enable_restoration equal to 0 specifies that loop restoration filtering is disabled.

Note: It is allowed to set enable_restoration equal to 1 even when loop restoration is not used on any frame in the coded video sequence. {:.alert .alert-info }

film_grain_params_present specifies whether film grain parameters are present in the coded video sequence.

Color config semantics

high_bitdepth and twelve_bit are syntax elements which, together with seq_profile, determine the bit depth.

mono_chrome equal to 1 indicates that the video does not contain U and V color planes. mono_chrome equal to 0 indicates that the video contains Y, U, and V color planes.

color_description_present_flag equal to 1 specifies that color_primaries, transfer_characteristics, and matrix_coefficients are present. color_description_present_flag equal to 0 specifies that color_primaries, transfer_characteristics and matrix_coefficients are not present.

color_primaries is an integer that is defined by the "Color primaries" section of ISO/IEC 23091-4/ITU-T H.273.

color_primaries	Name of color primaries	Description
1	CP_BT_709	BT.709
2	CP_UNSPECIFIED	Unspecified
4	CP_BT_470_M	BT.470 System M (historical)
5	CP_BT_470_B_G	BT.470 System B, G (historical)
6	CP_BT_601	BT.601
7	CP_SMPTE_240	SMPTE 240
8	CP_GENERIC_FILM	Generic film (color filters using illuminant C)
9	CP_BT_2020	BT.2020, BT.2100
10	CP_XYZ	SMPTE 428 (CIE 1921 XYZ)
11	CP_SMPTE_431	SMPTE RP 431-2
12	CP_SMPTE_432	SMPTE EG 432-1
22	CP_EBU_3213	EBU Tech. 3213-E
{:.table .table-sm .table-bordered }

transfer_characteristics is an integer that is defined by the "Transfer characteristics" section of ISO/IEC 23091-4/ITU-T H.273.

transfer_characteristics	Name of transfer characteristics	Description
0	TC_RESERVED_0	For future use
1	TC_BT_709	BT.709
2	TC_UNSPECIFIED	Unspecified
3	TC_RESERVED_3	For future use
4	TC_BT_470_M	BT.470 System M (historical)
5	TC_BT_470_B_G	BT.470 System B, G (historical)
6	TC_BT_601	BT.601
7	TC_SMPTE_240	SMPTE 240 M
8	TC_LINEAR	Linear
9	TC_LOG_100	Logarithmic (100 : 1 range)
10	TC_LOG_100_SQRT10	Logarithmic (100 * Sqrt(10) : 1 range)
11	TC_IEC_61966	IEC 61966-2-4
12	TC_BT_1361	BT.1361
13	TC_SRGB	sRGB or sYCC
14	TC_BT_2020_10_BIT	BT.2020 10-bit systems
15	TC_BT_2020_12_BIT	BT.2020 12-bit systems
16	TC_SMPTE_2084	SMPTE ST 2084, ITU BT.2100 PQ
17	TC_SMPTE_428	SMPTE ST 428
18	TC_HLG	BT.2100 HLG, ARIB STD-B67
{:.table .table-sm .table-bordered }

matrix_coefficients is an integer that is defined by the "Matrix coefficients" section of ISO/IEC 23091-4/ITU-T H.273.

matrix_coefficients	Name of matrix coefficients	Description
0	MC_IDENTITY	Identity matrix
1	MC_BT_709	BT.709
2	MC_UNSPECIFIED	Unspecified
3	MC_RESERVED_3	For future use
4	MC_FCC	US FCC 73.628
5	MC_BT_470_B_G	BT.470 System B, G (historical)
6	MC_BT_601	BT.601
7	MC_SMPTE_240	SMPTE 240 M
8	MC_SMPTE_YCGCO	YCgCo
9	MC_BT_2020_NCL	BT.2020 non-constant luminance, BT.2100 YCbCr
10	MC_BT_2020_CL	BT.2020 constant luminance
11	MC_SMPTE_2085	SMPTE ST 2085 YDzDx
12	MC_CHROMAT_NCL	Chromaticity-derived non-constant luminance
13	MC_CHROMAT_CL	Chromaticity-derived constant luminance
14	MC_ICTCP	BT.2100 ICtCp
{:.table .table-sm .table-bordered }

color_range is a binary value that is associated with the VideoFullRangeFlag variable specified in ISO/IEC 23091-4/ITU-T H.273. color range equal to 0 shall be referred to as the studio swing representation and color range equal to 1 shall be referred to as the full swing representation for all intents relating to this specification.

Note: Note that this specification does not enforce the range when signaled as Studio swing. Therefore the application should perform additional clamping and color conversion operations according to the specified range. {:.alert .alert-info }

subsampling_x, subsampling_y specify the chroma subsampling format:

subsampling_x	subsampling_y	mono_chrome	Description
0	0	0	YUV 4:4:4
1	0	0	YUV 4:2:2
1	1	0	YUV 4:2:0
1	1	1	Monochrome 4:0:0
{:.table .table-sm .table-bordered }

If matrix_coefficients is equal to MC_IDENTITY, it is a requirement of bitstream conformance that subsampling_x is equal to 0 and subsampling_y is equal to 0.

chroma_sample_position specifies the sample position for subsampled streams:

| chroma_sample_position | Name of chroma sample position| Description |:----------------------:|:------------------------------: ----------- | 0 | CSP_UNKNOWN | Unknown (in this case the source video transfer function must be signaled outside the AV1 bitstream) | 1 | CSP_VERTICAL | Horizontally co-located with (0, 0) luma sample, vertical position in the middle between two luma samples | 2 | CSP_COLOCATED | co-located with (0, 0) luma sample | 3 | CSP_RESERVED | {:.table .table-sm .table-bordered }

separate_uv_delta_q equal to 1 indicates that the U and V planes may have separate delta quantizer values. separate_uv_delta_q equal to 0 indicates that the U and V planes will share the same delta quantizer value.

Timing info semantics

num_units_in_display_tick is the number of time units of a clock operating at the frequency time_scale Hz that corresponds to one increment of a clock tick counter. A display clock tick, in seconds, is equal to num_units_in_display_tick divided by time_scale:

DispCT = num_units_in_display_tick ÷ time_scale

Note: The ÷ operator represents standard mathematical division (in contrast to the / operator which represents integer division). {:.alert .alert-info }

It is a requirement of bitstream conformance that num_units_in_display_tick is greater than 0.

time_scale is the number of time units that pass in one second.

It is a requirement of bitstream conformance that time_scale is greater than 0.

equal_picture_interval equal to 1 indicates that pictures should be displayed according to their output order with the number of ticks between two consecutive pictures (without dropping frames) specified by num_ticks_per_picture_minus_1 + 1. equal_picture_interval equal to 0 indicates that the interval between two consecutive pictures is not specified.

num_ticks_per_picture_minus_1 plus 1 specifies the number of clock ticks corresponding to output time between two consecutive pictures in the output order.

It is a requirement of bitstream conformance that the value of num_ticks_per_picture_minus_1 shall be in the range of 0 to (1 << 32) − 2, inclusive.

Note: The frame rate, when specified explicitly, applies to the top temporal layer of the bitstream. If bitstream is expected to be manipulated, e.g. by intermediate network elements, then the resulting frame rate may not match the specified one. In this case, an encoder is advised to use explicit time codes or some mechanisms that convey picture timing information outside the bitstream. {:.alert .alert-info }

Decoder model info semantics

buffer_delay_length_minus_1 plus 1 specifies the length of the decoder_buffer_delay and the encoder_buffer_delay syntax elements, in bits.

num_units_in_decoding_tick is the number of time units of a decoding clock operating at the frequency time_scale Hz that corresponds to one increment of a clock tick counter:

DecCT = num_units_in_decoding_tick ÷ time_scale

Note: The ÷ operator represents standard mathematical division (in contrast to the / operator which represents integer division). {:.alert .alert-info }

num_units_in_decoding_tick shall be greater than 0. DecCT represents the expected time to decode a single frame or a common divisor of the expected times to decode frames of different sizes and dimensions present in the coded video sequence.

buffer_removal_time_length_minus_1 plus 1 specifies the length of the buffer_removal_time syntax element, in bits.

frame_presentation_time_length_minus_1 plus 1 specifies the length of the frame_presentation_time syntax element, in bits.

Operating parameters info semantics

decoder_buffer_delay[ op ] specifies the time interval between the arrival of the first bit in the smoothing buffer and the subsequent removal of the data that belongs to the first coded frame for operating point op, measured in units of 1/90000 seconds. The length of decoder_buffer_delay is specified by buffer_delay_length_minus_1 + 1, in bits.

encoder_buffer_delay[ op ] specifies, in combination with decoder_buffer_delay[ op ] syntax element, the first bit arrival time of frames to be decoded to the smoothing buffer. encoder_buffer_delay is measured in units of 1/90000 seconds.

For a video sequence that includes one or more random access points the sum of decoder_buffer_delay and encoder_buffer_delay shall be kept constant.

low_delay_mode_flag[ op ] equal to 1 indicates that the smoothing buffer operates in low-delay mode for operating point op. In low-delay mode late decode times and buffer underflow are both permitted. low_delay_mode_flag[ op ] equal to 0 indicates that the smoothing buffer operates in strict mode, where buffer underflow is not allowed.

Temporal delimiter OBU semantics

SeenFrameHeader is a variable used to mark whether the frame header for the current frame has been received. It is initialized to zero.

Padding OBU semantics

Multiple padding units can be present, each padding with an arbitrary number of bytes.

obu_padding_byte is a padding byte. Padding bytes may have arbitrary values and have no effect on the decoding process.

Metadata OBU semantics

General metadata OBU semantics

metadata_type indicates the type of metadata:

metadata_type	Name of metadata_type
0	Reserved for AOM use
1	METADATA_TYPE_HDR_CLL
2	METADATA_TYPE_HDR_MDCV
3	METADATA_TYPE_SCALABILITY
4	METADATA_TYPE_ITUT_T35
5	METADATA_TYPE_TIMECODE
6-31	Unregistered user private
32 and greater	Reserved for AOM use
{:.table .table-sm .table-bordered }

Metadata ITUT T35 semantics

itu_t_t35_country_code shall be a byte having a value specified as a country code by Annex A of Recommendation ITU-T T.35.

itu_t_t35_country_code_extension_byte shall be a byte having a value specified as a country code by Annex B of Recommendation ITU-T T.35.

itu_t_t35_payload_bytes shall be bytes containing data registered as specified in Recommendation ITU-T T.35.

The ITU-T T.35 terminal provider code and terminal provider oriented code shall be contained in the first one or more bytes of the itu_t_t35_payload_bytes, in the format specified by the Administration that issued the terminal provider code. Any remaining bytes in itu_t_t35_payload_bytes data shall be data having syntax and semantics as specified by the entity identified by the ITU-T T.35 country code and terminal provider code.

Metadata high dynamic range content light level semantics

max_cll specifies the maximum content light level as specified in CEA-861.3, Appendix A.

max_fall specifies the maximum frame-average light level as specified in CEA-861.3, Appendix A.

Metadata high dynamic range mastering display color volume semantics

primary_chromaticity_x[ i ] specifies a 0.16 fixed-point X chromaticity coordinate as defined by CIE 1931, where i = 0,1,2 specifies Red, Green, Blue respectively.

primary_chromaticity_y[ i ] specifies a 0.16 fixed-point Y chromaticity coordinate as defined by CIE 1931, where i = 0,1,2 specifies Red, Green, Blue respectively.

white_point_chromaticity_x specifies a 0.16 fixed-point white X chromaticity coordinate as defined by CIE 1931.

white_point_chromaticity_y specifies a 0.16 fixed-point white Y chromaticity coordinate as defined by CIE 1931.

luminance_max is a 24.8 fixed-point maximum luminance, represented in candelas per square meter.

luminance_min is a 18.14 fixed-point minimum luminance, represented in candelas per square meter.

Metadata scalability semantics

Note: The scalability metadata OBU is intended for use by intermediate processing entities that may perform selective layer elimination. Its presence allows these entities to know the structure of the original coded video sequence without having to decode individual frames. If the received bitstream has been modified by an intermediate processing entity, then some of the layers and/or individual frames may be absent from the bitstream. {:.alert .alert-info }

If scalability metadata is present it should be placed between the first sequence header and the first frame header of a coded video sequence. The information present in a scalability metadata OBU applies to the entire coded video sequence in which it is contained, and only that sequence. Redundant copies of a scalability metadata OBU may occur in any temporal unit of a coded video sequence.

scalability_mode_idc indicates the picture prediction structure of the coded video sequence.

scalability_mode_idc	Name of scalability_mode_idc
0	SCALABILITY_L1T2
1	SCALABILITY_L1T3
2	SCALABILITY_L2T1
3	SCALABILITY_L2T2
4	SCALABILITY_L2T3
5	SCALABILITY_S2T1
6	SCALABILITY_S2T2
7	SCALABILITY_S2T3
8	SCALABILITY_L2T1h
9	SCALABILITY_L2T2h
10	SCALABILITY_L2T3h
11	SCALABILITY_S2T1h
12	SCALABILITY_S2T2h
13	SCALABILITY_S2T3h
14	SCALABILITY_SS
15	SCALABILITY_L3T1
16	SCALABILITY_L3T2
17	SCALABILITY_L3T3
18	SCALABILITY_S3T1
19	SCALABILITY_S3T2
20	SCALABILITY_S3T3
21	SCALABILITY_L3T2_KEY
22	SCALABILITY_L3T3_KEY
23	SCALABILITY_L4T5_KEY
24	SCALABILITY_L4T7_KEY
25	SCALABILITY_L3T2_KEY_SHIFT
26	SCALABILITY_L3T3_KEY_SHIFT
27	SCALABILITY_L4T5_KEY_SHIFT
28	SCALABILITY_L4T7_KEY_SHIFT
29-255	reserved
{:.table .table-sm .table-bordered }

The scalability metadata provides two mechanisms for describing the underlying picture prediction structure of the bitstream:

Selection among a set of preconfigured structures, or modes, covering a number of cases that have found wide use in applications.
A facility for specifying picture prediction structures to accommodate a variety of special cases.

The preconfigured modes are described below. The mechanism for describing alternative structures is described in scalability_structure() below.

All predefined modes follow a dyadic, hierarchical picture prediction structure. They support up to seven temporal layers, in combinations with up to four spatial layers. The second spatial layer may have twice or one and a half times the resolution of the base layer in each dimension, depending on the mode. There is also support for a spatial layer that uses no inter-layer prediction (i.e., the second spatial layer does not use its corresponding base layer as a reference) and a spatial layer that uses inter-layer prediction only at key frames. The following table lists the predefined scalability structures.

Name of scalability_mode_idc	Spatial Layers	Resolution Ratio	Temporal Layers	Inter-layer dependency
SCALABILITY_L1T2	1		2
SCALABILITY_L1T3	1		3
SCALABILITY_L2T1	2	2:1	1	Yes
SCALABILITY_L2T2	2	2:1	2	Yes
SCALABILITY_L2T3	2	2:1	3	Yes
SCALABILITY_S2T1	2	2:1	1	No
SCALABILITY_S2T2	2	2:1	2	No
SCALABILITY_S2T3	2	2:1	3	No
SCALABILITY_L2T1h	2	1.5:1	1	Yes
SCALABILITY_L2T2h	2	1.5:1	2	Yes
SCALABILITY_L2T3h	2	1.5:1	3	Yes
SCALABILITY_S2T1h	2	1.5:1	1	No
SCALABILITY_S2T2h	2	1.5:1	2	No
SCALABILITY_S2T3h	2	1.5:1	3	No
SCALABILITY_L3T1	3	2:1	1	Yes
SCALABILITY_L3T2	3	2:1	2	Yes
SCALABILITY_L3T3	3	2:1	3	Yes
SCALABILITY_S3T1	3	2:1	1	No
SCALABILITY_S3T2	3	2:1	2	No
SCALABILITY_S3T3	3	2:1	3	No
SCALABILITY_L3T2_KEY	3	2:1	2	Yes
SCALABILITY_L3T3_KEY	3	2:1	3	Yes
SCALABILITY_L4T5_KEY	4	2:1	5	Yes
SCALABILITY_L4T7_KEY	4	2:1	7	Yes
SCALABILITY_L3T2_KEY_SHIFT	3	2:1	2	Yes
SCALABILITY_L3T3_KEY_SHIFT	3	2:1	3	Yes
SCALABILITY_L4T5_KEY_SHIFT	4	2:1	5	Yes
SCALABILITY_L4T7_KEY_SHIFT	4	2:1	7	Yes
{:.table .table-sm .table-bordered }

The following figures show the picture prediction structures for certain modes:

L1T2

L1T3

L2T1

L2T2

L2T3

S2T1

S2T2

S2T3

L3T2_KEY

L3T3_KEY

L4T5_KEY

L4T7_KEY

L3T2_KEY_SHIFT

L3T3_KEY_SHIFT

L4T5_KEY_SHIFT

L4T7_KEY_SHIFT

Scalability structure semantics

General

Note: The scalability_structure is intended for use by intermediate processing entities that may perform selective layer elimination. Its presence allows these entities to know the structure of the video bitstream without have to decode individual frames. Scalability structures should be placed immediately after the sequence header so that these entities are informed of the scalability structure of the video sequence as early as possible. {:.alert .alert-info }

spatial_layers_cnt_minus_1 indicates the number of spatial layers present in the coded video sequence minus one.

spatial_layer_description_present_flag indicates when set to 1 that the spatial_layer_ref_id is present for each of the (spatial_layers_cnt_minus_1 + 1) layers, or that it is not present when set to 0.

spatial_layer_dimensions_present_flag indicates when set to 1 that the spatial_layer_max_width and spatial_layer_max_height parameters are present for each of the (spatial_layers_cnt_minus_1 + 1) layers, or that it they are not present when set to 0.

temporal_group_description_present_flag indicates when set to 1 that the temporal dependency information is present, or that it is not when set to 0. When any temporal unit in a coded video sequence contains OBU extension headers that have temporal_id values that are not equal to each other, temporal_group_description_present_flag must be equal to 0.

scalability_structure_reserved_3bits must be set to zero and be ignored by decoders.

spatial_layer_max_width[ i ] specifies the maximum frame width for the frames with spatial_id equal to i. This number must not be larger than max_frame_width_minus_1 + 1.

spatial_layer_max_height[ i ] specifies the maximum frame height for the frames with spatial_id equal to i. This number must not be larger than max_frame_height_minus_1 + 1.

spatial_layer_ref_id[ i ] specifies the spatial_id value of the frame within the current temporal unit that the frame of layer i uses for reference. If no frame within the current temporal unit is used for reference the value must be equal to 255.

temporal_group_size indicates the number of pictures in a temporal picture group. If the temporal_group_size is greater than 0, then the scalability structure data allows the inter-picture temporal dependency structure of the coded video sequence to be specified. If the temporal_group_size is greater than 0, then for temporal_group_size pictures in the temporal group, each picture's temporal layer id (temporal_id), switch up points (temporal_group_temporal_switching_up_point_flag and temporal_group_spatial_switching_up_point_flag), and the reference picture indices (temporal_group_ref_pic_diff) are specified.

The first picture specified in a temporal group must have temporal_id equal to 0.

If the parameter temporal_group_size is not present or set to 0, then either there is only one temporal layer or there is no fixed inter-picture temporal dependency present in the coded video sequence.

Note that for a given picture, all frames follow the same inter-picture temporal dependency structure. However, the frame rate of each layer can be different from each other. The specified dependency structure in the scalability structure data must be for the highest frame rate layer.

temporal_group_temporal_id[ i ] specifies the temporal_id value for the i-th picture in the temporal group.

temporal_group_temporal_switching_up_point_flag[ i ] is set to 1 if subsequent (in decoding order) pictures with a temporal_id higher than temporal_group_temporal_id[ i ] do not depend on any picture preceding the current picture (in coding order) with temporal_id higher than temporal_group_temporal_id[ i ].

Note: This condition ensures that switching up to a higher frame rate is possible at the current picture. {:.alert .alert-info }

temporal_group_spatial_switching_up_point_flag[ i ]] is set to 1 if spatial layers of the current picture in the temporal group (i.e., pictures with a spatial_id higher than zero) do not depend on any picture preceding the current picture in the temporal group.

temporal_group_ref_cnt[ i ] indicates the number of reference pictures used by the i-th picture in the temporal group.

temporal_group_ref_pic_diff[ i ][ j ] indicates, for the i-th picture in the temporal group, the temporal distance between the i-th picture and the j-th reference picture used by the i-th picture. The temporal distance is measured in frames, counting only frames of identical spatial_id values.

Note: The scalability structure description does not allow different temporal prediction structures across non-temporal layers (i.e., layers with different spatial_id values). It also only allows for a single reference picture for inter-layer prediction. {:.alert .alert-info }

The following sections contain the value of these syntax elements for certain predefined modes. Prediction structures having scalability_mode_idc values in the range 21 to 28, inclusive, cannot be described using temporal group description syntax and are not described in the sections that follow.

L1T2 (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 0 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 2 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 2 | 1 | temporal_group_temporal_id[1] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[0][0] | 1 {:.table .table-sm .table-bordered }

L1T3 (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 0 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 4 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 4 | 1 | temporal_group_temporal_id[1] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[1][0] | 1 | 2 | temporal_group_temporal_id[2] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[2] | 1 | | temporal_group_ref_pic_diff[2][0] | 2 | 3 | temporal_group_temporal_id[3] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[3] | 1 | | temporal_group_ref_pic_diff[3][0] | 1 {:.table .table-sm .table-bordered }

L2T1 / L2T1h (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 0 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 1 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 1 {:.table .table-sm .table-bordered }

L2T2 / L2T2h (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 0 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 2 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 2 | 1 | temporal_group_temporal_id[1] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[1][0] | 1 {:.table .table-sm .table-bordered }

L2T3 / L2T3h (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 0 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 4 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 4 | 1 | temporal_group_temporal_id[1] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[1][0] | 1 | 2 | temporal_group_temporal_id[2] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[2] | 1 | | temporal_group_ref_pic_diff[2][0] | 2 | 3 | temporal_group_temporal_id[3] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[3] | 1 | | temporal_group_ref_pic_diff[3][0] | 1 {:.table .table-sm .table-bordered }

S2T1 / S2T1h (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 255 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 1 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 1 {:.table .table-sm .table-bordered }

S2T2 / S2T2h (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 255 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 2 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 2 | 1 | temporal_group_temporal_id[1] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[1][0] | 1 {:.table .table-sm .table-bordered }

S2T3 / S2T3h (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 255 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 4 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 4 | 1 | temporal_group_temporal_id[1] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[1][0] | 1 | 2 | temporal_group_temporal_id[2] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[2] | 1 | | temporal_group_ref_pic_diff[2][0] | 2 | 3 | temporal_group_temporal_id[3] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[3] | 1 | | temporal_group_ref_pic_diff[3][0] | 1 {:.table .table-sm .table-bordered }

L3T1 (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 0 | 2 | spatial_layer_ref_id[1] | 1

| | | | Picture | Temporal Group Description | Value | | temporal_group_size | 1 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 1 {:.table .table-sm .table-bordered }

L3T2 (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 0 | 2 | spatial_layer_ref_id[1] | 1 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 2 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 2 | 1 | temporal_group_temporal_id[1] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[1][0] | 1 {:.table .table-sm .table-bordered }

L3T3 (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 0 | 2 | spatial_layer_ref_id[1] | 1 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 4 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 4 | 1 | temporal_group_temporal_id[1] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[1][0] | 1 | 2 | temporal_group_temporal_id[2] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[2] | 1 | | temporal_group_ref_pic_diff[2][0] | 2 | 3 | temporal_group_temporal_id[3] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[3] | 1 | | temporal_group_ref_pic_diff[3][0] | 1 {:.table .table-sm .table-bordered }

S3T1 (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 255 | 2 | spatial_layer_ref_id[1] | 255

| | | | Picture | Temporal Group Description | Value | | temporal_group_size | 1 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 1 {:.table .table-sm .table-bordered }

S3T2 (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 255 | 2 | spatial_layer_ref_id[1] | 255 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 2 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 2 | 1 | temporal_group_temporal_id[1] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[1][0] | 1 {:.table .table-sm .table-bordered }

S3T3 (Informative)

| ----------- | ------------------------------------------------ | ---------------- | | Layer | Spatial Layers Description | Value | | spatial_layers_cnt_minus_1 | 1 | 0 | spatial_layer_ref_id[0] | 255 | 1 | spatial_layer_ref_id[1] | 255 | 2 | spatial_layer_ref_id[1] | 255 | | | | Picture | Temporal Group Description | Value | | temporal_group_size | 4 | 0 | temporal_group_temporal_id[0] | 0 | | temporal_group_temporal_switching_up_point_flag[0] | 1 | | temporal_group_spatial_switching_up_point_flag[0] | 0 | | temporal_group_ref_cnt[0] | 1 | | temporal_group_ref_pic_diff[0][0] | 4 | 1 | temporal_group_temporal_id[1] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[1] | 1 | | temporal_group_ref_pic_diff[1][0] | 1 | 2 | temporal_group_temporal_id[2] | 1 | | temporal_group_temporal_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[2] | 1 | | temporal_group_ref_pic_diff[2][0] | 2 | 3 | temporal_group_temporal_id[3] | 2 | | temporal_group_temporal_switching_up_point_flag[1] | 1 | | temporal_group_spatial_switching_up_point_flag[1] | 0 | | temporal_group_ref_cnt[3] | 1 | | temporal_group_ref_pic_diff[3][0] | 1 {:.table .table-sm .table-bordered }

Metadata timecode semantics

counting_type specifies the method of dropping values of the n_frames syntax element as specified in the table below. counting_type should be the same for all pictures in the coded video sequence.

counting_type	Meaning
0	no dropping of n_frames count values and no use of time_offset_value
1	no dropping of n_frames count values
2	dropping of individual zero values of n_frames count
3	dropping of individual values of n_frames count equal to maxFps − 1
4	dropping of the two lowest (value 0 and 1) n_frames counts when seconds_value is equal to 0 and minutes_value is not an integer multiple of 10
5	dropping of unspecified individual n_frames count values
6	dropping of unspecified numbers of unspecified n_frames count values
7..31	reserved
{:.table .table-sm .table-bordered }

full_timestamp_flag equal to 1 indicates that the the seconds_value, minutes_value, hours_value syntax elements will be present. full_timestamp_flag equal to 0 indicates that there are flags to control the presence of these syntax elements.

When timing_info_present_flag is equal to 1, the contents of the clock timestamp indicate a time of origin, capture, or ideal display. This indicated time is computed as follows:

if ( equal_picture_interval ) {
  ticksPerPicture = num_ticks_per_picture_minus_1 + 1
} else {
  ticksPerPicture = 1
}
ss = ( ( hours_value * 60 + minutes_value) * 60 + seconds_value )
clockTimestamp = ss * time_scale + n_frames * ticksPerPicture + time_offset_value

clockTimestamp is in units of clock ticks of a clock with clock frequency equal to time_scale Hz, relative to some unspecified point in time for which clockTimestamp would be equal to 0.

discontinuity_flag equal to 0 indicates that the difference between the current value of clockTimestamp and the value of clockTimestamp computed from the previous set of timestamp syntax elements in output order can be interpreted as the time difference between the times of origin or capture of the associated frames or fields. discontinuity_flag equal to 1 indicates that the difference between the current value of clockTimestamp and the value of clockTimestamp computed from the previous set of clock timestamp syntax elements in output order should not be interpreted as the time difference between the times of origin or capture of the associated frames or fields.

When timing_info_present_flag is equal to 1 and discontinuity_flag is equal to 0, the value of clockTimestamp shall be greater than or equal to the value of clockTimestamp for the previous set of clock timestamp syntax elements in output order.

cnt_dropped_flag specifies the skipping of one or more values of n_frames using the counting method specified by counting_type.

n_frames is used to compute clockTimestamp. When timing_info_present_flag is equal to 1, n_frames shall be less than maxFps, where maxFps is specified by maxFps = ceil( time_scale / ( 2 * num_units_in_display_tick ) ).

seconds_flag equal to 1 specifies that seconds_value and minutes_flag are present when full_timestamp_flag is equal to 0. seconds_flag equal to 0 specifies that seconds_value and minutes_flag are not present.

seconds_value is used to compute clockTimestamp and shall be in the range of 0 to 59. When seconds_value is not present, its value is inferred to be equal to the value of seconds_value for the previous set of clock timestamp syntax elements in decoding order, and it is required that such a previous seconds_value shall have been present.

minutes_flag equal to 1 specifies that minutes_value and hours_flag are present when full_timestamp_flag is equal to 0 and seconds_flag is equal to 1. minutes_flag equal to 0 specifies that minutes_value and hours_flag are not present.

minutes_value specifies the value of mm used to compute clockTimestamp and shall be in the range of 0 to 59, inclusive. When minutes_value is not present, its value is inferred to be equal to the value of minutes_value for the previous set of clock timestamp syntax elements in decoding order, and it is required that such a previous minutes_value shall have been present.

hours_flag equal to 1 specifies that hours_value is present when full_timestamp_flag is equal to 0 and seconds_flag is equal to 1 and minutes_flag is equal to 1.

hours_value is used to compute clockTimestamp and shall be in the range of 0 to 23, inclusive. When hours_value is not present, its value is inferred to be equal to the value of hours_value for the previous set of clock timestamp syntax elements in decoding order, and it is required that such a previous hours_value shall have been present.

time_offset_length greater than 0 specifies the length in bits of the time_offset_value syntax element. time_offset_length equal to 0 specifies that the time_offset_value syntax element is not present. time_offset_length should be the same for all pictures in the coded video sequence.

time_offset_value is used to compute clockTimestamp. The number of bits used to represent time_offset_value is equal to time_offset_length. When time_offset_value is not present, its value is inferred to be equal to 0.

Frame header OBU semantics

General frame header OBU semantics

It is a requirement of bitstream conformance that a sequence header OBU has been received before a frame header OBU.

frame_header_copy is a function call that indicates that a copy of the previous frame_header_obu should be inserted at this point.

Note: Bitstreams may contain several copies of the frame_header_obu interspersed with tile_group_obu to allow for greater error resilience. However, the copies must contain identical contents to the original frame_header_obu. {:.alert .alert-info }

If obu_type is equal to OBU_FRAME_HEADER or obu_type is equal to OBU_FRAME, it is a requirement of bitstream conformance that SeenFrameHeader is equal to 0.

If obu_type is equal to OBU_REDUNDANT_FRAME_HEADER, it is a requirement of bitstream conformance that SeenFrameHeader is equal to 1.

Note: These requirements ensure that the first frame header for a frame has obu_type equal to OBU_FRAME_HEADER, while later copies of this frame header (if present) have obu_type equal to OBU_REDUNDANT_FRAME_HEADER. {:.alert .alert-info }

TileNum is a variable giving the index (zero-based) of the current tile.

decode_frame_wrapup is a function call that indicates that the decode frame wrapup process specified in [section 7.4][] should be invoked.

Uncompressed header semantics

show_existing_frame equal to 1, indicates the frame indexed by frame_to_show_map_idx is to be output; show_existing_frame equal to 0 indicates that further processing is required.

If obu_type is equal to OBU_FRAME, it is a requirement of bitstream conformance that show_existing_frame is equal to 0.

frame_to_show_map_idx specifies the frame to be output. It is only available if show_existing_frame is 1.

display_frame_id provides the frame id number for the frame to output. It is a requirement of bitstream conformance that whenever display_frame_id is read, the value matches RefFrameId[ frame_to_show_map_idx ] (the value of current_frame_id at the time that the frame indexed by frame_to_show_map_idx was stored), and that RefValid[ frame_to_show_map_idx ] is equal to 1.

It is a requirement of bitstream conformance that the number of bits needed to read display_frame_id does not exceed 16. This is equivalent to the constraint that idLen <= 16.

frame_type specifies the type of the frame:

frame_type	Name of frame_type
0	KEY_FRAME
1	INTER_FRAME
2	INTRA_ONLY_FRAME
3	SWITCH_FRAME
{:.table .table-sm .table-bordered }

show_frame equal to 1 specifies that this frame should be immediately output once decoded. show_frame equal to 0 specifies that this frame should not be immediately output. (It may be output later if a later uncompressed header uses show_existing_frame equal to 1).

showable_frame equal to 1 specifies that the frame may be output using the show_existing_frame mechanism. showable_frame equal to 0 specifies that this frame will not be output using the show_existing_frame mechanism.

It is a requirement of bitstream conformance that when show_existing_frame is used to show a previous frame, that the value of showable_frame for the previous frame was equal to 1.

It is a requirement of bitstream conformance that when show_existing_frame is used to show a previous frame with RefFrameType[ frame_to_show_map_idx ] equal to KEY_FRAME, that the frame is output via the show_existing_frame mechanism at most once.

Note: This requirement also forbids storing a frame with frame_type equal to KEY_FRAME into multiple reference frames and then using show_existing_frame for each reference frame. {:.alert .alert-info }

error_resilient_mode equal to 1 indicates that error resilient mode is enabled; error_resilient_mode equal to 0 indicates that error resilient mode is disabled.

Note: Error resilient mode allows the syntax of a frame to be parsed independently of previously decoded frames. {:.alert .alert-info }

disable_cdf_update specifies whether the CDF update in the symbol decoding process should be disabled.

current_frame_id specifies the frame id number for the current frame. Frame id numbers are additional information that do not affect the decoding process, but provide decoders with a way of detecting missing reference frames so that appropriate action can be taken.

If frame_type is not equal to KEY_FRAME or show_frame is equal to 0, it is a requirement of bitstream conformance that all of the following conditions are true:

current_frame_id is not equal to PrevFrameID,
DiffFrameID is less than 1 << ( idLen - 1 )

where DiffFrameID is specified as follows:

If current_frame_id is greater than PrevFrameID, DiffFrameID is equal to current_frame_id - PrevFrameID.
Otherwise, DiffFrameID is equal to ( 1 << idLen ) + current_frame_id - PrevFrameID.

frame_size_override_flag equal to 0 specifies that the frame size is equal to the size in the sequence header. frame_size_override_flag equal to 1 specifies that the frame size will either be specified as the size of one of the reference frames, or computed from the frame_width_minus_1 and frame_height_minus_1 syntax elements.

order_hint is used to compute OrderHint.

OrderHint specifies OrderHintBits least significant bits of the expected output order for this frame.

Note: There is no requirement that OrderHint should reflect the true output order. As a guideline, the motion vector prediction is expected to be more accurate if the true output order is used for frames that will be shown later. If a frame is never to be shown (e.g. it has been constructed as an average of several frames for reference purposes), the encoder is free to choose whichever value of OrderHint will give the best compression. {:.alert .alert-info }

primary_ref_frame specifies which reference frame contains the CDF values and other state that should be loaded at the start of the frame.

Note: It is allowed for primary_ref_frame to be coded as PRIMARY_REF_NONE, this will cause default values to be used for the CDF values and other state. {:.alert .alert-info }

buffer_removal_time_present_flag equal to 1 specifies that buffer_removal_time is present. buffer_removal_time_present_flag equal to 0 specifies that buffer_removal_time is not present.

buffer_removal_time[ opNum ] specifies the frame removal time in units of DecCT clock ticks counted from the removal time of the last random access point for operating point opNum. buffer_removal_time is signaled as a fixed length unsigned integer with a length in bits given by buffer_removal_time_length_minus_1 + 1.

buffer_removal_time is the remainder of a modulo 1 << ( buffer_removal_time_length_minus_1 + 1 ) counter.

allow_screen_content_tools equal to 1 indicates that intra blocks may use palette encoding; allow_screen_content_tools equal to 0 indicates that palette encoding is never used.

allow_intrabc equal to 1 indicates that intra block copy may be used in this frame. allow_intrabc equal to 0 indicates that intra block copy is not allowed in this frame.

Note: intra block copy is only allowed in intra frames, and disables all loop filtering. force_integer_mv will be equal to 1 for intra frames, so only integer offsets are allowed in block copy mode. {:.alert .alert-info }

force_integer_mv equal to 1 specifies that motion vectors will always be integers. force_integer_mv equal to 0 specifies that motion vectors can contain fractional bits.

ref_order_hint[ i ] specifies the expected output order hint for each reference frame.

Note: The values in the ref_order_hint array are provided to allow implementations to gracefully handle cases when some frames have been lost. {:.alert .alert-info }

Note: When scalability is used, the values in RefOrderHint during the decode process may depend on the selected operating point. {:.alert .alert-info }

refresh_frame_flags contains a bitmask that specifies which reference frame slots will be updated with the current frame after it is decoded.

If frame_type is equal to INTRA_ONLY_FRAME, it is a requirement of bitstream conformance that refresh_frame_flags is not equal to 0xff.

Note: This restriction encourages encoders to correctly label random access points (by forcing frame_type to be equal to KEY_FRAME when an intra frame is used to reset the decoding process). {:.alert .alert-info }

See [section 7.20][] for details of the frame update process.

frame_refs_short_signaling equal to 1 indicates that only two reference frames are explicitly signaled. frame_refs_short_signaling equal to 0 indicates that all reference frames are explicitly signaled.

last_frame_idx specifies the reference frame to use for LAST_FRAME.

gold_frame_idx specifies the reference frame to use for GOLDEN_FRAME.

set_frame_refs is a function call that indicates the conceptual point where the ref_frame_idx values are computed (in the case when frame_refs_short_signaling is equal to 1, these syntax elements are computed instead of being explicitly signaled). When this function is called, the set frame refs process specified in [section 7.8][] is invoked.

ref_frame_idx[ i ] specifies which reference frames are used by inter frames. It is a requirement of bitstream conformance that RefValid[ ref_frame_idx[ i ] ] is equal to 1, and that the selected reference frames match the current frame in bit depth, profile, chroma subsampling, and color space.

Note: Syntax elements indicate a reference (such as LAST_FRAME, ALTREF_FRAME). These references are looked up in the ref_frame_idx array to find which reference frame should be used during inter prediction. There is no requirement that the values in ref_frame_idx should be distinct. {:.alert .alert-info }

RefFrameSignBias specifies the intended direction of the motion vector in time for each reference frame. A sign bias equal to 0 indicates that the reference frame is a forwards reference (i.e. the reference frame is expected to be output before the current frame); a sign bias equal to 1 indicates that the reference frame is a backwards reference.

Note: The sign bias is just an indication that can improve the accuracy of motion vector prediction and is not constrained to reflect the actual output order of pictures. {:.alert .alert-info }

delta_frame_id_minus_1 is used to calculate DeltaFrameId.

DeltaFrameId specifies the distance to the frame id for the reference frame.

RefFrameId[ i ] specifies the frame id for each reference frame.

expectedFrameId[ i ] specifies the frame id for each frame used for reference. It is a requirement of bitstream conformance that whenever expectedFrameId[ i ] is calculated, the value matches RefFrameId[ ref_frame_idx[ i ] ] (this contains the value of current_frame_id at the time that the frame indexed by ref_frame_idx[ i ] was stored).

allow_high_precision_mv equal to 0 specifies that motion vectors are specified to quarter pel precision; allow_high_precision_mv equal to 1 specifies that motion vectors are specified to eighth pel precision.

is_motion_mode_switchable equal to 0 specifies that only the SIMPLE motion mode will be used.

use_ref_frame_mvs equal to 1 specifies that motion vector information from a previous frame can be used when decoding the current frame. use_ref_frame_mvs equal to 0 specifies that this information will not be used.

disable_frame_end_update_cdf equal to 1 indicates that the end of frame CDF update is disabled; disable_frame_end_update_cdf equal to 0 indicates that the end of frame CDF update is enabled.

Note: It can be useful to disable the CDF update because it means the next frame can start to be decoded as soon as the frame headers of the current frame have been processed. {:.alert .alert-info }

motion_field_estimation is a function call which indicates that the motion field estimation process in [section 7.9][] should be invoked.

OrderHints specifies the expected output order for each reference frame.

CodedLossless is a variable that is equal to 1 when all segments use lossless encoding. This indicates that the frame is fully lossless at the coded resolution of FrameWidth by FrameHeight. In this case, the loop filter and CDEF filter are disabled.

It is a requirement of bitstream conformance that delta_q_present is equal to 0 when CodedLossless is equal to 1.

AllLossless is a variable that is equal to 1 when CodedLossless is equal to 1 and FrameWidth is equal to UpscaledWidth. This indicates that the frame is fully lossless at the upscaled resolution. In this case, the loop filter, CDEF filter, and loop restoration are disabled.

allow_warped_motion equal to 1 indicates that the syntax element motion_mode may be present. allow_warped_motion equal to 0 indicates that the syntax element motion_mode will not be present (this means that LOCALWARP cannot be signaled if allow_warped_motion is equal to 0).

reduced_tx_set equal to 1 specifies that the frame is restricted to a reduced subset of the full set of transform types.

setup_past_independence is a function call that indicates that this frame can be decoded without dependence on previous coded frames. When this function is invoked the following takes place:

FeatureData[ i ][ j ] and FeatureEnabled[ i ][ j ] are set equal to 0 for i = 0..MAX_SEGMENTS-1 and j = 0..SEG_LVL_MAX-1.
PrevSegmentIds[ row ][ col ] is set equal to 0 for row = 0..MiRows-1 and col = 0..MiCols-1.
PrevGmParams[ ref ][ i ] is set equal to ( ( i % 3 == 2 ) ? 1 << WARPEDMODEL_PREC_BITS : 0 ) for ref = LAST_FRAME..ALTREF_FRAME, for i = 0..5.
loop_filter_delta_enabled is set equal to 1.
loop_filter_ref_deltas[ INTRA_FRAME ] is set equal to 1.
loop_filter_ref_deltas[ LAST_FRAME ] is set equal to 0.
loop_filter_ref_deltas[ LAST2_FRAME ] is set equal to 0.
loop_filter_ref_deltas[ LAST3_FRAME ] is set equal to 0.
loop_filter_ref_deltas[ BWDREF_FRAME ] is set equal to 0.
loop_filter_ref_deltas[ GOLDEN_FRAME ] is set equal to -1.
loop_filter_ref_deltas[ ALTREF_FRAME ] is set equal to -1.
loop_filter_ref_deltas[ ALTREF2_FRAME ] is set equal to -1.
loop_filter_mode_deltas[ i ] is set equal to 0 for i = 0..1.

init_non_coeff_cdfs is a function call that indicates that the CDF tables which are not used in the coeff( ) syntax structure should be initialised. When this function is invoked, the following steps apply:

YModeCdf is set to a copy of Default_Y_Mode_Cdf
UVModeCflNotAllowedCdf is set to a copy of Default_Uv_Mode_Cfl_Not_Allowed_Cdf
UVModeCflAllowedCdf is set to a copy of Default_Uv_Mode_Cfl_Allowed_Cdf
AngleDeltaCdf is set to a copy of Default_Angle_Delta_Cdf
IntrabcCdf is set to a copy of Default_Intrabc_Cdf
PartitionW8Cdf is set to a copy of Default_Partition_W8_Cdf
PartitionW16Cdf is set to a copy of Default_Partition_W16_Cdf
PartitionW32Cdf is set to a copy of Default_Partition_W32_Cdf
PartitionW64Cdf is set to a copy of Default_Partition_W64_Cdf
PartitionW128Cdf is set to a copy of Default_Partition_W128_Cdf
SegmentIdCdf is set to a copy of Default_Segment_Id_Cdf
SegmentIdPredictedCdf is set to a copy of Default_Segment_Id_Predicted_Cdf
Tx8x8Cdf is set to a copy of Default_Tx_8x8_Cdf
Tx16x16Cdf is set to a copy of Default_Tx_16x16_Cdf
Tx32x32Cdf is set to a copy of Default_Tx_32x32_Cdf
Tx64x64Cdf is set to a copy of Default_Tx_64x64_Cdf
TxfmSplitCdf is set to a copy of Default_Txfm_Split_Cdf
FilterIntraModeCdf is set to a copy of Default_Filter_Intra_Mode_Cdf
FilterIntraCdf is set to a copy of Default_Filter_Intra_Cdf
InterpFilterCdf is set to a copy of Default_Interp_Filter_Cdf
MotionModeCdf is set to a copy of Default_Motion_Mode_Cdf
NewMvCdf is set to a copy of Default_New_Mv_Cdf
ZeroMvCdf is set to a copy of Default_Zero_Mv_Cdf
RefMvCdf is set to a copy of Default_Ref_Mv_Cdf
CompoundModeCdf is set to a copy of Default_Compound_Mode_Cdf
DrlModeCdf is set to a copy of Default_Drl_Mode_Cdf
IsInterCdf is set to a copy of Default_Is_Inter_Cdf
CompModeCdf is set to a copy of Default_Comp_Mode_Cdf
SkipModeCdf is set to a copy of Default_Skip_Mode_Cdf
SkipCdf is set to a copy of Default_Skip_Cdf
CompRefCdf is set to a copy of Default_Comp_Ref_Cdf
CompBwdRefCdf is set to a copy of Default_Comp_Bwd_Ref_Cdf
SingleRefCdf is set to a copy of Default_Single_Ref_Cdf
MvJointCdf[ i ] is set to a copy of Default_Mv_Joint_Cdf for i = 0..MV_CONTEXTS-1
MvClassCdf[ i ] is set to a copy of Default_Mv_Class_Cdf for i = 0..MV_CONTEXTS-1
MvClass0BitCdf[ i ][ comp ] is set to a copy of Default_Mv_Class0_Bit_Cdf for i = 0..MV_CONTEXTS-1 and comp = 0..1
MvFrCdf[ i ] is set to a copy of Default_Mv_Fr_Cdf for i = 0..MV_CONTEXTS-1
MvClass0FrCdf[ i ] is set to a copy of Default_Mv_Class0_Fr_Cdf for i = 0..MV_CONTEXTS-1
MvClass0HpCdf[ i ][ comp ] is set to a copy of Default_Mv_Class0_Hp_Cdf for i = 0..MV_CONTEXTS-1 and comp = 0..1
MvSignCdf[ i ][ comp ] is set to a copy of Default_Mv_Sign_Cdf for i = 0..MV_CONTEXTS-1 and comp = 0..1
MvBitCdf[ i ][ comp ] is set to a copy of Default_Mv_Bit_Cdf for i = 0..MV_CONTEXTS-1 and comp = 0..1
MvHpCdf[ i ][ comp ] is set to a copy of Default_Mv_Hp_Cdf for i = 0..MV_CONTEXTS-1 and comp = 0..1
PaletteYModeCdf is set to a copy of Default_Palette_Y_Mode_Cdf
PaletteUVModeCdf is set to a copy of Default_Palette_Uv_Mode_Cdf
PaletteYSizeCdf is set to a copy of Default_Palette_Y_Size_Cdf
PaletteUVSizeCdf is set to a copy of Default_Palette_Uv_Size_Cdf
PaletteSize2YColorCdf is set to a copy of Default_Palette_Size_2_Y_Color_Cdf
PaletteSize2UVColorCdf is set to a copy of Default_Palette_Size_2_Uv_Color_Cdf
PaletteSize3YColorCdf is set to a copy of Default_Palette_Size_3_Y_Color_Cdf
PaletteSize3UVColorCdf is set to a copy of Default_Palette_Size_3_Uv_Color_Cdf
PaletteSize4YColorCdf is set to a copy of Default_Palette_Size_4_Y_Color_Cdf
PaletteSize4UVColorCdf is set to a copy of Default_Palette_Size_4_Uv_Color_Cdf
PaletteSize5YColorCdf is set to a copy of Default_Palette_Size_5_Y_Color_Cdf
PaletteSize5UVColorCdf is set to a copy of Default_Palette_Size_5_Uv_Color_Cdf
PaletteSize6YColorCdf is set to a copy of Default_Palette_Size_6_Y_Color_Cdf
PaletteSize6UVColorCdf is set to a copy of Default_Palette_Size_6_Uv_Color_Cdf
PaletteSize7YColorCdf is set to a copy of Default_Palette_Size_7_Y_Color_Cdf
PaletteSize7UVColorCdf is set to a copy of Default_Palette_Size_7_Uv_Color_Cdf
PaletteSize8YColorCdf is set to a copy of Default_Palette_Size_8_Y_Color_Cdf
PaletteSize8UVColorCdf is set to a copy of Default_Palette_Size_8_Uv_Color_Cdf
DeltaQCdf is set to a copy of Default_Delta_Q_Cdf
DeltaLFCdf is set to a copy of Default_Delta_Lf_Cdf
DeltaLFMultiCdf[ i ] is set to a copy of Default_Delta_Lf_Cdf for i = 0..FRAME_LF_COUNT-1
IntraTxTypeSet1Cdf is set to a copy of Default_Intra_Tx_Type_Set1_Cdf
IntraTxTypeSet2Cdf is set to a copy of Default_Intra_Tx_Type_Set2_Cdf
InterTxTypeSet1Cdf is set to a copy of Default_Inter_Tx_Type_Set1_Cdf
InterTxTypeSet2Cdf is set to a copy of Default_Inter_Tx_Type_Set2_Cdf
InterTxTypeSet3Cdf is set to a copy of Default_Inter_Tx_Type_Set3_Cdf
UseObmcCdf is set to a copy of Default_Use_Obmc_Cdf
InterIntraCdf is set to a copy of Default_Inter_Intra_Cdf
CompRefTypeCdf is set to a copy of Default_Comp_Ref_Type_Cdf
CflSignCdf is set to a copy of Default_Cfl_Sign_Cdf
UniCompRefCdf is set to a copy of Default_Uni_Comp_Ref_Cdf
WedgeInterIntraCdf is set to a copy of Default_Wedge_Inter_Intra_Cdf
CompGroupIdxCdf is set to a copy of Default_Comp_Group_Idx_Cdf
CompoundIdxCdf is set to a copy of Default_Compound_Idx_Cdf
CompoundTypeCdf is set to a copy of Default_Compound_Type_Cdf
InterIntraModeCdf is set to a copy of Default_Inter_Intra_Mode_Cdf
WedgeIndexCdf is set to a copy of Default_Wedge_Index_Cdf
CflAlphaCdf is set to a copy of Default_Cfl_Alpha_Cdf
UseWienerCdf is set to a copy of Default_Use_Wiener_Cdf
UseSgrprojCdf is set to a copy of Default_Use_Sgrproj_Cdf
RestorationTypeCdf is set to a copy of Default_Restoration_Type_Cdf

init_coeff_cdfs( ) is a function call that indicates that the CDF tables used in the coeff( ) syntax structure should be initialised. When this function is invoked, the following steps apply:

The variable idx is derived as follows:
- If base_q_idx is less than or equal to 20, idx is set equal to 0.
- Otherwise, if base_q_idx is less than or equal to 60, idx is set equal to 1.
- Otherwise, if base_q_idx is less than or equal to 120, idx is set equal to 2.
- Otherwise, idx is set equal to 3.
The cumulative distribution function arrays are reset to default values as follows:
- TxbSkipCdf is set to a copy of Default_Txb_Skip_Cdf[ idx ].
- EobPt16Cdf is set to a copy of Default_Eob_Pt_16_Cdf[ idx ].
- EobPt32Cdf is set to a copy of Default_Eob_Pt_32_Cdf[ idx ].
- EobPt64Cdf is set to a copy of Default_Eob_Pt_64_Cdf[ idx ].
- EobPt128Cdf is set to a copy of Default_Eob_Pt_128_Cdf[ idx ].
- EobPt256Cdf is set to a copy of Default_Eob_Pt_256_Cdf[ idx ].
- EobPt512Cdf is set to a copy of Default_Eob_Pt_512_Cdf[ idx ].
- EobPt1024Cdf is set to a copy of Default_Eob_Pt_1024_Cdf[ idx ].
- EobExtraCdf is set to a copy of Default_Eob_Extra_Cdf[ idx ].
- DcSignCdf is set to a copy of Default_Dc_Sign_Cdf[ idx ].
- CoeffBaseEobCdf is set to a copy of Default_Coeff_Base_Eob_Cdf[ idx ].
- CoeffBaseCdf is set to a copy of Default_Coeff_Base_Cdf[ idx ].
- CoeffBrCdf is set to a copy of Default_Coeff_Br_Cdf[ idx ].

load_cdfs( ctx ) is a function call that indicates that the CDF tables are loaded from frame context number ctx in the range 0 to (NUM_REF_FRAMES - 1). When this function is invoked, a copy of each CDF array mentioned in the semantics for init_coeff_cdfs and init_non_coeff_cdfs is loaded from an area of memory indexed by ctx. (The memory contents of these frame contexts have been initialized by previous calls to save_cdfs). Once the CDF arrays have been loaded, the last entry in each array, representing the symbol count for that context, is set to 0.

load_previous( ) is a function call that indicates that information from a previous frame may be loaded for use in decoding the current frame. When this function is invoked the following ordered steps apply:

The variable prevFrame is set equal to ref_frame_idx[ primary_ref_frame ].
PrevGmParams is set equal to SavedGmParams[ prevFrame ].
The function load_loop_filter_params( prevFrame ) specified in [section 7.21][] is invoked.
The function load_segmentation_params( prevFrame ) specified in [section 7.21][] is invoked.

load_previous_segment_ids( ) is a function call that indicates that a segmentation map from a previous frame may be loaded for use in decoding the current frame. When this function is invoked the segmentation map contained in PrevSegmentIds is set as follows:

The variable prevFrame is set equal to ref_frame_idx[ primary_ref_frame ].
If segmentation_enabled is equal to 1, RefMiCols[ prevFrame ] is equal to MiCols, and RefMiRows[ prevFrame ] is equal to MiRows, PrevSegmentIds[ row ][ col ] is set equal to SavedSegmentIds[ prevFrame ][ row ][ col ] for row = 0..MiRows-1, for col = 0..MiCols-1.

Otherwise, PrevSegmentIds[ row ][ col ] is set equal to 0 for row = 0..MiRows-1, for col = 0..MiCols-1.

Reference frame marking semantics

RefValid is an array which is indexed by a reference picture slot number. A value of 1 in the array signifies that the corresponding reference picture slot is valid for use as a reference picture, while a value of 0 signifies that the corresponding reference picture slot is not valid for use as a reference picture.

Note: RefValid is used for bitstream conformance checking. Frames are marked as invalid when they are too far in the past to be referenced by the frame id mechanism or when the order hints do not match. {:.alert .alert-info }

Frame size semantics

frame_width_minus_1 plus one is the width of the frame in luma samples.

frame_height_minus_1 plus one is the height of the frame in luma samples.

It is a requirement of bitstream conformance that frame_width_minus_1 is less than or equal to max_frame_width_minus_1.

It is a requirement of bitstream conformance that frame_height_minus_1 is less than or equal to max_frame_height_minus_1.

If FrameIsIntra is equal to 0 (indicating that this frame may use inter prediction), the requirements described in the frame size with refs semantics of [section 6.8.6] must also be satisfied.

Render size semantics

The render size is provided as a hint to the application about the desired display size. It has no effect on the decoding process.

render_and_frame_size_different equal to 0 means that the render width and height are inferred from the frame width and height. render_and_frame_size_different equal to 1 means that the render width and height are explicitly coded.

Note: It is allowed for the bitstream to explicitly code the render dimensions in the bitstream even if they are an exact match for the frame dimensions. {:.alert .alert-info }

render_width_minus_1 plus one is the render width of the frame in luma samples.

render_height_minus_1 plus one is the render height of the frame in luma samples.

Frame size with refs semantics

For inter frames, the frame size is either set equal to the size of a reference frame, or can be sent explicitly.

found_ref equal to 1 indicates that the frame dimensions can be inferred from reference frame i where i is the loop counter in the syntax parsing process for frame_size_with_refs. found_ref equal to 0 indicates that the frame dimensions are not inferred from reference frame i.

Once the FrameWidth and FrameHeight have been computed for an inter frame, it is a requirement of bitstream conformance that for all values of i in the range 0..(REFS_PER_FRAME - 1), all the following conditions are true:

2 * FrameWidth >= RefUpscaledWidth[ ref_frame_idx[ i ] ]
2 * FrameHeight >= RefFrameHeight[ ref_frame_idx[ i ] ]
FrameWidth <= 16 * RefUpscaledWidth[ ref_frame_idx[ i ] ]
FrameHeight <= 16 * RefFrameHeight[ ref_frame_idx[ i ] ]

Note: This is a requirement even if all the blocks in an inter frame are coded using intra prediction. {:.alert .alert-info }

Superres params semantics

use_superres equal to 0 indicates that no upscaling is needed. use_superres equal to 1 indicates that upscaling is needed.

coded_denom is used to compute the amount of upscaling.

SuperresDenom is the denominator of a fraction that specifies the ratio between the superblock width before and after upscaling. The numerator of this fraction is equal to the constant SUPERRES_NUM.

Compute image size semantics

MiCols is the number of 4x4 block columns in the frame.

MiRows is the number of 4x4 block rows in the frame.

Interpolation filter semantics

is_filter_switchable equal to 1 indicates that the filter selection is signaled at the block level; is_filter_switchable equal to 0 indicates that the filter selection is signaled at the frame level.

interpolation_filter specifies the filter selection used for performing inter prediction:

interpolation_filter	Name of interpolation_filter
0	EIGHTTAP
1	EIGHTTAP_SMOOTH
2	EIGHTTAP_SHARP
3	BILINEAR
4	SWITCHABLE
{:.table .table-sm .table-bordered }

Loop filter semantics

loop_filter_level is an array containing loop filter strength values. Different loop filter strength values from the array are used depending on the image plane being filtered, and the edge direction (vertical or horizontal) being filtered.

loop_filter_sharpness indicates the sharpness level. The loop_filter_level and loop_filter_sharpness together determine when a block edge is filtered, and by how much the filtering can change the sample values.

The loop filter process is described in [section 7.14][].

loop_filter_delta_enabled equal to 1 means that the filter level depends on the mode and reference frame used to predict a block. loop_filter_delta_enabled equal to 0 means that the filter level does not depend on the mode and reference frame.

loop_filter_delta_update equal to 1 means that additional syntax elements are present that specify which mode and reference frame deltas are to be updated. loop_filter_delta_update equal to 0 means that these syntax elements are not present.

update_ref_delta equal to 1 means that the syntax element loop_filter_ref_deltas is present; update_ref_delta equal to 0 means that this syntax element is not present.

loop_filter_ref_deltas contains the adjustment needed for the filter level based on the chosen reference frame. If this syntax element is not present, it maintains its previous value.

update_mode_delta equal to 1 means that the syntax element loop_filter_mode_deltas is present; update_mode_delta equal to 0 means that this syntax element is not present.

loop_filter_mode_deltas contains the adjustment needed for the filter level based on the chosen mode. If this syntax element is not present, it maintains its previous value.

Note: The previous values for loop_filter_mode_deltas and loop_filter_ref_deltas are intially set by the setup_past_independence function and can be subsequently modified by these syntax elements being coded in a previous frame. {:.alert .alert-info }

Quantization params semantics

The residual is specified via decoded coefficients which are adjusted by one of four quantization parameters before the inverse transform is applied. The choice depends on the plane (Y or UV) and coefficient position (DC/AC coefficient). The Dequantization process is specified in [section 7.12][].

base_q_idx indicates the base frame qindex. This is used for Y AC coefficients and as the base value for the other quantizers.

DeltaQYDc indicates the Y DC quantizer relative to base_q_idx.

diff_uv_delta equal to 1 indicates that the U and V delta quantizer values are coded separately. diff_uv_delta equal to 0 indicates that the U and V delta quantizer values share a common value.

DeltaQUDc indicates the U DC quantizer relative to base_q_idx.

DeltaQUAc indicates the U AC quantizer relative to base_q_idx.

DeltaQVDc indicates the V DC quantizer relative to base_q_idx.

DeltaQVAc indicates the V AC quantizer relative to base_q_idx.

using_qmatrix specifies that the quantizer matrix will be used to compute quantizers.

qm_y specifies the level in the quantizer matrix that should be used for luma plane decoding.

qm_u specifies the level in the quantizer matrix that should be used for chroma U plane decoding.

qm_v specifies the level in the quantizer matrix that should be used for chroma V plane decoding.

Delta quantizer semantics

delta_coded specifies that the delta_q syntax element is present.

delta_q specifies an offset (relative to base_q_idx) for a particular quantization parameter.

Segmentation params semantics

AV1 provides a means of segmenting the image and then applying various adjustments at the segment level.

Up to 8 segments may be specified for any given frame. For each of these segments it is possible to specify:

A quantizer (absolute value or delta).
A loop filter strength (absolute value or delta).
A prediction reference frame.
A block skip mode that implies both the use of a (0,0) motion vector and that no residual will be coded.

Each of these data values for each segment may be individually updated at the frame level. Where a value is not updated in a given frame, the value from the previous frame persists. The exceptions to this are key frames, intra only frames or other frames where independence from past frame values is required (for example to enable error resilience). In such cases all values are reset as described in the semantics for setup_past_independence.

The segment affiliation (the segmentation map) is stored at the resolution of 4x4 blocks. If no explicit update is coded for a block's segment affiliation, then it persists from frame to frame (until reset by a call to setup_past_independence).

SegIdPreSkip equal to 1 indicates that the segment id will be read before the skip syntax element. SegIdPreSkip equal to 0 indicates that the skip syntax element will be read first.

LastActiveSegId indicates the highest numbered segment id that has some enabled feature. This is used when decoding the segment id to only decode choices corresponding to used segments.

segmentation_enabled equal to 1 indicates that this frame makes use of the segmentation tool; segmentation_enabled equal to 0 indicates that the frame does not use segmentation.

segmentation_update_map equal to 1 indicates that the segmentation map is updated during the decoding of this frame. segmentation_update_map equal to 0 means that the segmentation map from the previous frame is used.

segmentation_temporal_update equal to 1 indicates that the updates to the segmentation map are coded relative to the existing segmentation map. segmentation_temporal_update equal to 0 indicates that the new segmentation map is coded without reference to the existing segmentation map.

segmentation_update_data equal to 1 indicates that new parameters are about to be specified for each segment. segmentation_update_data equal to 0 indicates that the segmentation parameters should keep their existing values.

feature_enabled equal to 0 indicates that the corresponding feature is unused and has value equal to 0. feature_enabled equal to 1 indicates that the feature value is coded.

feature_value specifies the feature data for a segment feature.

Tile info semantics

uniform_tile_spacing_flag equal to 1 means that the tiles are uniformly spaced across the frame. (In other words, all tiles are the same size except for the ones at the right and bottom edge which can be smaller.) uniform_tile_spacing_flag equal to 0 means that the tile sizes are coded.

increment_tile_cols_log2 is used to compute TileColsLog2.

TileColsLog2 specifies the base 2 logarithm of the desired number of tiles across the frame.

TileCols specifies the number of tiles across the frame. It is a requirement of bitstream conformance that TileCols is less than or equal to MAX_TILE_COLS.

increment_tile_rows_log2 is used to compute TileRowsLog2.

TileRowsLog2 specifies the base 2 logarithm of the desired number of tiles down the frame.

Note: For small frame sizes the actual number of tiles in the frame may be smaller than the desired number because the tile size is rounded up to a multiple of the maximum superblock size. {:.alert .alert-info }

TileRows specifies the number of tiles down the frame. It is a requirement of bitstream conformance that TileRows is less than or equal to MAX_TILE_ROWS.

tileWidthSb is used to specify the width of each tile in units of superblocks. It is a requirement of bitstream conformance that tileWidthSb is less than or equal to maxTileWidthSb.

tileHeightSb is used to specify the height of each tile in units of superblocks. It is a requirement of bitstream conformance that tileWidthSb * tileHeightSb is less than or equal to maxTileAreaSb.

If uniform_tile_spacing_flag is equal to 0, it is a requirement of bitstream conformance that startSb is equal to sbCols when the loop writing MiColStarts exits.

If uniform_tile_spacing_flag is equal to 0, it is a requirement of bitstream conformance that startSb is equal to sbRows when the loop writing MiRowStarts exits.

Note: The requirements on startSb ensure that the sizes of each tile add up to the full size of the frame when measured in superblocks. {:.alert .alert-info }

MiColStarts is an array specifying the start column (in units of 4x4 luma samples) for each tile across the image.

MiRowStarts is an array specifying the start row (in units of 4x4 luma samples) for each tile down the image.

width_in_sbs_minus_1 specifies the width of a tile minus 1 in units of superblocks.

height_in_sbs_minus_1 specifies the height of a tile minus 1 in units of superblocks.

maxTileHeightSb specifies the maximum height (in units of superblocks) that can be used for a tile (to avoid making tiles with too much area).

context_update_tile_id specifies which tile to use for the CDF update. It is a requirement of bitstream conformance that context_update_tile_id is less than TileCols * TileRows.

tile_size_bytes_minus_1 is used to compute TileSizeBytes.

TileSizeBytes specifies the number of bytes needed to code each tile size.

Quantizer index delta parameters semantics

delta_q_present specifies whether quantizer index delta values are present.

delta_q_res specifies the left shift which should be applied to decoded quantizer index delta values.

Loop filter delta parameters semantics

delta_lf_present specifies whether loop filter delta values are present.

delta_lf_res specifies the left shift which should be applied to decoded loop filter delta values.

delta_lf_multi equal to 1 specifies that separate loop filter deltas are sent for horizontal luma edges, vertical luma edges, the U edges, and the V edges. delta_lf_multi equal to 0 specifies that the same loop filter delta is used for all edges.

Global motion params semantics

is_global specifies whether global motion parameters are present for a particular reference frame.

is_rot_zoom specifies whether a particular reference frame uses rotation and zoom global motion.

is_translation specifies whether a particular reference frame uses translation global motion.

Global param semantics

absBits is used to compute the range of values that can be used for gm_params[ref][idx]. The values allowed are in the range -(1 << absBits) to (1 << absBits).

precBits specifies the number of fractional bits used for representing gm_params[ref][idx]. All global motion parameters are stored in the model with WARPEDMODEL_PREC_BITS fractional bits, but the parameters are encoded with less precision.

Decode subexp semantics

subexp_final_bits provide the final bits that are read once the appropriate range has been determined.

subexp_more_bits equal to 0 specifies that the parameter is in the range mk to mk+a-1. subexp_more_bits equal to 1 specifies that the parameter is greater than mk+a-1.

subexp_bits specifies the value of the parameter minus mk.

Film grain params semantics

apply_grain equal to 1 specifies that film grain should be added to this frame. apply_grain equal to 0 specifies that film grain should not be added.

reset_grain_params() is a function call that indicates that all the syntax elements read in film_grain_params should be set equal to 0.

grain_seed specifies the starting value for the pseudo-random numbers used during film grain synthesis.

update_grain equal to 1 means that a new set of parameters should be sent. update_grain equal to 0 means that the previous set of parameters should be used.

film_grain_params_ref_idx indicates which reference frame contains the film grain parameters to be used for this frame.

It is a requirement of bitstream conformance that film_grain_params_ref_idx is equal to ref_frame_idx[ j ] for some value of j in the range 0 to REFS_PER_FRAME - 1.

Note: This requirement means that film grain can only be predicted from the frames that the current frame is using as reference frames. {:.alert .alert-info }

load_grain_params(idx) is a function call that indicates that all the syntax elements read in film_grain_params should be set equal to the values stored in an area of memory indexed by idx.

tempGrainSeed is a temporary variable that is used to avoid losing the value of grain_seed when load_grain_params is called. When update_grain is equal to 0, a previous set of parameters should be used for everything except grain_seed.

num_y_points specifies the number of points for the piece-wise linear scaling function of the luma component.

It is a requirement of bitstream conformance that num_y_points is less than or equal to 14.

point_y_value[ i ] represents the x (luma value) coordinate for the i-th point of the piecewise linear scaling function for luma component. The values are signaled on the scale of 0..255. (In case of 10 bit video, these values correspond to luma values divided by 4. In case of 12 bit video, these values correspond to luma values divided by 16.)

If i is greater than 0, it is a requirement of bitstream conformance that point_y_value[ i ] is greater than point_y_value[ i - 1 ] (this ensures the x coordinates are specified in increasing order).

point_y_scaling[ i ] represents the scaling (output) value for the i-th point of the piecewise linear scaling function for luma component.

chroma_scaling_from_luma specifies that the chroma scaling is inferred from the luma scaling.

num_cb_points specifies the number of points for the piece-wise linear scaling function of the cb component.

It is a requirement of bitstream conformance that num_cb_points is less than or equal to 10.

Note: When chroma_scaling_from_luma is equal to 1, it is still allowed for num_y_points to take values up to 14. This means that the chroma scaling also needs to support up to 14 points. {:.alert .alert-info }

point_cb_value[ i ] represents the x coordinate for the i-th point of the piece-wise linear scaling function for cb component. The values are signaled on the scale of 0..255.

If i is greater than 0, it is a requirement of bitstream conformance that point_cb_value[ i ] is greater than point_cb_value[ i - 1 ].

point_cb_scaling[ i ] represents the scaling (output) value for the i-th point of the piecewise linear scaling function for cb component.

num_cr_points specifies represents the number of points for the piece-wise linear scaling function of the cr component.

It is a requirement of bitstream conformance that num_cr_points is less than or equal to 10.

If subsampling_x is equal to 1 and subsampling_y is equal to 1 and num_cb_points is equal to 0, it is a requirement of bitstream conformance that num_cr_points is equal to 0.

If subsampling_x is equal to 1 and subsampling_y is equal to 1 and num_cb_points is not equal to 0, it is a requirement of bitstream conformance that num_cr_points is not equal to 0.

Note: These requirements ensure that for 4:2:0 chroma subsampling, film grain noise will be applied to both chroma components, or to neither. There is no restriction for 4:2:2 or 4:4:4 chroma subsampling. {:.alert .alert-info }

point_cr_value[ i ] represents the x coordinate for the i-th point of the piece-wise linear scaling function for cr component. The values are signaled on the scale of 0..255.

If i is greater than 0, it is a requirement of bitstream conformance that point_cr_value[ i ] is greater than point_cr_value[ i - 1 ].

point_cr_scaling[ i ] represents the scaling (output) value for the i-th point of the piecewise linear scaling function for cr component.

grain_scaling_minus_8 represents the shift – 8 applied to the values of the chroma component. The grain_scaling_minus_8 can take values of 0..3 and determines the range and quantization step of the standard deviation of film grain.

ar_coeff_lag specifies the number of auto-regressive coefficients for luma and chroma.

ar_coeffs_y_plus_128[ i ] specifies auto-regressive coefficients used for the Y plane.

ar_coeffs_cb_plus_128[ i ] specifies auto-regressive coefficients used for the U plane.

ar_coeffs_cr_plus_128[ i ] specifies auto-regressive coefficients used for the V plane.

ar_coeff_shift_minus_6 specifies the range of the auto-regressive coefficients. Values of 0, 1, 2, and 3 correspond to the ranges for auto-regressive coefficients of [-2, 2), [-1, 1), [-0.5, 0.5) and [-0.25, 0.25) respectively.

grain_scale_shift specifies how much the Gaussian random numbers should be scaled down during the grain synthesis process.

cb_mult represents a multiplier for the cb component used in derivation of the input index to the cb component scaling function.

cb_luma_mult represents a multiplier for the average luma component used in derivation of the input index to the cb component scaling function.

cb_offset represents an offset used in derivation of the input index to the cb component scaling function.

cr_mult represents a multiplier for the cr component used in derivation of the input index to the cr component scaling function.

cr_luma_mult represents a multiplier for the average luma component used in derivation of the input index to the cr component scaling function.

cr_offset represents an offset used in derivation of the input index to the cr component scaling function.

overlap_flag equal to 1 indicates that the overlap between film grain blocks shall be applied. overlap_flag equal to 0 indicates that the overlap between film grain blocks shall not be applied.

clip_to_restricted_range equal to 1 indicates that clipping to the restricted (studio) range shall be applied to the sample values after adding the film grain (see the semantics for color_range for an explanation of studio swing). clip_to_restricted_range equal to 0 indicates that clipping to the full range shall be applied to the sample values after adding the film grain.

TX mode semantics

tx_mode_select is used to compute TxMode.

TxMode specifies how the transform size is determined:

TxMode	Name of TxMode
0	ONLY_4X4
1	TX_MODE_LARGEST
2	TX_MODE_SELECT
{:.table .table-sm .table-bordered }

For tx_mode equal to TX_MODE_LARGEST, the inverse transform will use the largest transform size that fits inside the block.

For tx_mode equal to ONLY_4X4, the inverse transform will use only 4x4 transforms.

For tx_mode equal to TX_MODE_SELECT, the choice of transform size is specified explicitly for each block.

Skip mode params semantics

SkipModeFrame[ list ] specifies the frames to use for compound prediction when skip_mode is equal to 1.

skip_mode_present equal to 1 specifies that the syntax element skip_mode will be present. skip_mode_present equal to 0 specifies that skip_mode will not be used for this frame.

Note: Skip mode tries to use the closest forward and backward references (as measured by values in the RefOrderHint array). If no backward reference is found, then the second closest forward reference is used. If no forward reference is found, then skip mode is disabled. (Forward prediction is when a frame is used for reference that is considered to be output before the current frame, backward prediction is when a frame is used that has not yet been output.) {:.alert .alert-info }

Frame reference mode semantics

reference_select equal to 1 specifies that the mode info for inter blocks contains the syntax element comp_mode that indicates whether to use single or compound reference prediction. reference_select equal to 0 specifies that all inter blocks will use single prediction.

Temporal point info semantics

frame_presentation_time specifies the presentation time of the frame in clock ticks DispCT counted from the removal time of the last random access point for the operating point that is being decoded. The syntax element is signaled as a fixed length unsigned integer with a length in bits given by frame_presentation_time_length_minus_1 + 1. The frame_presentation_time is the remainder of a modulo 1 << (frame_presentation_time_length_minus_1 + 1) counter.

Frame OBU semantics

A frame OBU consists of a frame header OBU and a tile group OBU packed into a single OBU.

Note: The intention is to provide a more compact way of coding the common use case where the frame header is immediately followed by tile group data. {:.alert .alert-info }

Tile group OBU semantics

General tile group OBU semantics

NumTiles specifies the total number of tiles in the frame.

tile_start_and_end_present_flag specifies whether tg_start and tg_end are present. If tg_start and tg_end are not present, this tile group covers the entire frame.

If obu_type is equal to OBU_FRAME, it is a requirement of bitstream conformance that the value of tile_start_and_end_present_flag is equal to 0.

tg_start specifies the zero-based index of the first tile in the current tile group.

It is a requirement of bitstream conformance that the value of tg_start is equal to the value of TileNum at the point that tile_group_obu is invoked.

tg_end specifies the zero-based index of the last tile in the current tile group.

It is a requirement of bitstream conformance that the value of tg_end is greater than or equal to tg_start.

It is a requirement of bitstream conformance that the value of tg_end for the last tile group in each frame is equal to NumTiles - 1.

Note: These requirements ensure that conceptually all tile groups are present and received in order for the purposes of specifying the decode process. {:.alert .alert-info }

frame_end_update_cdf is a function call that indicates that the frame CDF arrays are set equal to the saved CDFs. This process is described in [section 7.7][].

tile_size_minus_1 is used to compute tileSize.

tileSize specifies the size in bytes of the next coded tile.

Note: This size includes any padding bytes if added by the exit process for the Symbol decoder. The size does not include the bytes used for tile_size_minus_1 or syntax elements sent before tile_size_minus_1. For the last tile in the tile group, tileSize is computed instead of being read and includes the OBU trailing bits. {:.alert .alert-info }

decode_frame_wrapup is a function call that indicates that the decode frame wrapup process specified in [section 7.4][] should be invoked.

Decode tile semantics

clear_left_context is a function call that indicates that some arrays used to determine the probabilities are zeroed. When this function is invoked the arrays LeftLevelContext, LeftDcContext, and LeftSegPredContext are set equal to 0.

Note: LeftLevelContext[ plane ][ i ], LeftDcContext[ plane ][ i ], and LeftSegPredContext[ i ] need to be set to 0 for i = 0..MiRows-1, for plane = 0..2. {:.alert .alert-info }

clear_above_context is a function call that indicates that some arrays used to determine the probabilities are zeroed. When this function is invoked the arrays AboveLevelContext, AboveDcContext, and AboveSegPredContext are set equal to 0.

Note: AboveLevelContext[ plane ][ i ], AboveDcContext[ plane ][ i ], and AboveSegPredContext[ i ] need to be set to 0 for i = 0..MiCols-1, for plane = 0..2. {:.alert .alert-info }

ReadDeltas specifies whether the current block may read delta values for the quantizer index and loop filter. If the entire superblock is skipped the delta values are not read, otherwise delta values for the quantizer index and loop filter are read on the first block of a superblock. If delta_q_present is equal to 0, no delta values are read for the quantizer index. If delta_lf_present is equal to 0, no delta values are read for the loop filter.

Clear block decoded flags semantics

BlockDecoded is an array which stores one boolean value per 4x4 sample block per plane in the current superblock, plus a border of one 4x4 sample block on all sides of the superblock. Except for the borders, a value of 1 in BlockDecoded indicates that the corresponding 4x4 sample block has been decoded. The borders are used when computing above-right and below-left availability along the top and left edges of the superblock.

Decode partition semantics

partition specifies how a block is partitioned:

partition	Name of partition
0	PARTITION_NONE
1	PARTITION_HORZ
2	PARTITION_VERT
3	PARTITION_SPLIT
4	PARTITION_HORZ_A
5	PARTITION_HORZ_B
6	PARTITION_VERT_A
7	PARTITION_VERT_B
8	PARTITION_HORZ_4
9	PARTITION_VERT_4
{:.table .table-sm .table-bordered }

The variable subSize is computed from partition and indicates the size of the component blocks within this block:

subSize	Name of subSize
0	BLOCK_4X4
1	BLOCK_4X8
2	BLOCK_8X4
3	BLOCK_8X8
4	BLOCK_8X16
5	BLOCK_16X8
6	BLOCK_16X16
7	BLOCK_16X32
8	BLOCK_32X16
9	BLOCK_32X32
10	BLOCK_32X64
11	BLOCK_64X32
12	BLOCK_64X64
13	BLOCK_64X128
14	BLOCK_128X64
15	BLOCK_128X128
16	BLOCK_4X16
17	BLOCK_16X4
18	BLOCK_8X32
19	BLOCK_32X8
20	BLOCK_16X64
21	BLOCK_64X16
{:.table .table-sm .table-bordered }

The dimensions of these blocks are given in width, height order (e.g. BLOCK_8X16 corresponds to a block that is 8 samples wide, and 16 samples high).

It is a requirement of bitstream conformance that get_plane_residual_size( subSize, 1 ) is not equal to BLOCK_INVALID every time subSize is computed.

Note: This requirement prevents the UV blocks from being too tall or too wide (i.e. having aspect ratios outside the range 1:4 to 4:1). For example, when 4:2:2 chroma subsampling is used a luma partition of size 8x32 is invalid, as it implies a chroma partition of size 4x32, which results in an aspect ratio of 1:8. {:.alert .alert-info }

split_or_vert is used to compute partition for blocks when only split or vert partitions are allowed because of overlap with the right hand edge of the frame.

split_or_horz is used to compute partition for blocks when only split or horz partitions are allowed because of overlap with the bottom edge of the frame.

Decode block semantics

MiRow is a variable holding the vertical location of the block in units of 4x4 luma samples.

MiCol is a variable holding the horizontal location of the block in units of 4x4 luma samples.

MiSize is a variable holding the size of the block with values having the same interpretation for the variable subSize.

HasChroma is a variable that specifies whether chroma information is coded for this block.

Variable AvailU is equal to 0 if the information from the block above cannot be used on the luma plane; AvailU is equal to 1 if the information from the block above can be used on the luma plane.

Variable AvailL is equal to 0 if the information from the block to the left can not be used on the luma plane; AvailL is equal to 1 if the information from the block to the left can be used on the luma plane.

Variables AvailUChroma and AvailLChroma have the same significance as AvailU and AvailL, but on the chroma planes.

Intra frame mode info semantics

This syntax is used when coding an intra block within an intra frame.

use_intrabc equal to 1 specifies that intra block copy should be used for this block. use_intrabc equal to 0 specifies that intra block copy should not be used.

intra_frame_y_mode specifies the direction of intra prediction filtering:

intra_frame_y_mode	Name of intra_frame_y_mode
0	DC_PRED
1	V_PRED
2	H_PRED
3	D45_PRED
4	D135_PRED
5	D113_PRED
6	D157_PRED
7	D203_PRED
8	D67_PRED
9	SMOOTH_PRED
10	SMOOTH_V_PRED
11	SMOOTH_H_PRED
12	PAETH_PRED
{:.table .table-sm .table-bordered }

uv_mode specifies the chrominance intra prediction mode using values with the same interpretation as in the semantics for intra_frame_y_mode, with an additional mode UV_CFL_PRED.

uv_mode	Name of uv_mode
0	DC_PRED
1	V_PRED
2	H_PRED
3	D45_PRED
4	D135_PRED
5	D113_PRED
6	D157_PRED
7	D203_PRED
8	D67_PRED
9	SMOOTH_PRED
10	SMOOTH_V_PRED
11	SMOOTH_H_PRED
12	PAETH_PRED
13	UV_CFL_PRED
{:.table .table-sm .table-bordered }

Note: Due to the way the uv_mode syntax element is read, uv_mode can only be read as UV_CFL_PRED when Max( Block_Width[ MiSize ], Block_Height[ MiSize ] ) <= 32. {:.alert .alert-info }

Intra segment ID semantics

Lossless is a variable which, if equal to 1, indicates that the block is coded using a special 4x4 transform designed for encoding frames that are bit-identical with the original frames.

Read segment ID semantics

segment_id specifies which segment is associated with the current intra block being decoded. It is first read from the stream, and then postprocessed based on the predicted segment id.

It is a requirement of bitstream conformance that the postprocessed value of segment_id (i.e. the value returned by neg_deinterleave) is in the range 0 to LastActiveSegId (inclusive of endpoints).

Inter segment ID semantics

seg_id_predicted equal to 1 specifies that the segment_id is taken from the segmentation map. seg_id_predicted equal to 0 specifies that the syntax element segment_id is parsed.

Note: It is allowed for seg_id_predicted to be equal to 0 even if the value coded for the segment_id is equal to predictedSegmentId. {:.alert .alert-info }

Skip mode semantics

skip_mode equal to 1 indicates that this block will use some default settings (that correspond to compound prediction) and so most of the mode info is skipped. skip_mode equal to 0 indicates that the mode info is not skipped.

Skip semantics

skip equal to 0 indicates that there may be some transform coefficients to read for this block; skip equal to 1 indicates that there are no transform coefficients.

Quantizer index delta semantics

delta_q_abs specifies the absolute value of the quantizer index delta value being decoded. If delta_q_abs is equal to DELTA_Q_SMALL, the value is encoded using delta_q_rem_bits and delta_q_abs_bits.

delta_q_rem_bits and delta_q_abs_bits encode the absolute value of the quantizer index delta value being decoded, where the absolute value of the quantizer index delta value is of the form:

(1 << delta_q_rem_bits) + delta_q_abs_bits + 1

delta_q_sign_bit equal to 0 indicates that the quantizer index delta value is positive; delta_q_sign_bit equal to 1 indicates that the quantizer index delta value is negative.

Loop filter delta semantics

delta_lf_abs specifies the absolute value of the loop filter delta value being decoded. If delta_lf_abs is equal to DELTA_LF_SMALL, the value is encoded using delta_lf_rem_bits and delta_lf_abs_bits.

delta_lf_rem_bits and delta_lf_abs_bits encode the absolute value of the loop filter delta value being decoded, where the absolute value of the loop filter delta value is of the form:

( 1 << ( delta_lf_rem_bits + 1 ) ) + delta_lf_abs_bits + 1

delta_lf_sign_bit equal to 0 indicates that the loop filter delta value is positive; delta_lf_sign_bit equal to 1 indicates that the loop filter delta value is negative.

CDEF params semantics

cdef_damping_minus_3 controls the amount of damping in the deringing filter.

cdef_bits specifies the number of bits needed to specify which CDEF filter to apply.

cdef_y_pri_strength and cdef_uv_pri_strength specify the strength of the primary filter.

cdef_y_sec_strength and cdef_uv_sec_strength specify the strength of the secondary filter.

Loop restoration params semantics

lr_type is used to compute FrameRestorationType.

FrameRestorationType specifies the type of restoration used for each plane as follows:

lr_type	FrameRestorationType	Name of FrameRestorationType
0	0	RESTORE_NONE
1	3	RESTORE_SWITCHABLE
2	1	RESTORE_WIENER
3	2	RESTORE_SGRPROJ
{:.table .table-sm .table-bordered }

UsesLr indicates if any plane uses loop restoration.

lr_unit_shift specifies if the luma restoration size should be halved.

lr_unit_extra_shift specifies if the luma restoration size should be halved again.

lr_uv_shift is only present for 4:2:0 formats and specifies if the chroma size should be half the luma size.

LoopRestorationSize[plane] specifies the size of loop restoration units in units of samples in the current plane.

TX size semantics

tx_depth is used to compute TxSize. tx_depth is inverted with respect to TxSize, i.e. it specifies how much smaller the transform size should be made than the largest possible transform size for the block.

TxSize specifies the transform size to be used for this block:

TxSize	Name of TxSize
0	TX_4X4
1	TX_8X8
2	TX_16X16
3	TX_32X32
4	TX_64X64
5	TX_4X8
6	TX_8X4
7	TX_8X16
8	TX_16X8
9	TX_16X32
10	TX_32X16
11	TX_32X64
12	TX_64X32
13	TX_4X16
14	TX_16X4
15	TX_8X32
16	TX_32X8
17	TX_16X64
18	TX_64X16
{:.table .table-sm .table-bordered }

Note: TxSize is determined for skipped intra blocks because TxSize controls the granularity of the intra prediction. {:.alert .alert-info }

Block TX size semantics

InterTxSizes is an array that holds the transform sizes within inter frames.

Note: TxSizes and InterTxSizes contain different values. All the values in TxSizes across a residual block will share the same value, while InterTxSizes can represent several different transform sizes within a residual block. {:.alert .alert-info }

Var TX size semantics

txfm_split equal to 1 specifies that the block should be split into smaller transform sizes. txfm_split equal to 0 specifies that the block should not be split any more.

Transform type semantics

set specifies the transform set.

is_inter	set	Name of transform set
Don't care	0	TX_SET_DCTONLY
0	1	TX_SET_INTRA_1
0	2	TX_SET_INTRA_2
1	1	TX_SET_INTER_1
1	2	TX_SET_INTER_2
1	3	TX_SET_INTER_3
{:.table .table-sm .table-bordered }

The transform sets determine what subset of transform types can be used, according to the following table.

Transform type	TX_SET_ DCTONLY	TX_SET_ INTRA_1	TX_SET_ INTRA_2	TX_SET_ INTER_1	TX_SET_ INTER_2	TX_SET_ INTER_3
DCT_DCT	X	X	X	X	X	X
ADST_DCT		X	X	X	X
DCT_ADST		X	X	X	X
ADST_ADST		X	X	X	X
FLIPADST_DCT				X	X
DCT_FLIPADST				X	X
FLIPADST_FLIPADST				X	X
ADST_FLIPADST				X	X
FLIPADST_ADST				X	X
IDTX		X	X	X	X	X
V_DCT		X		X	X
H_DCT		X		X	X
V_ADST				X
H_ADST				X
V_FLIPADST				X
H_FLIPADST				X
{:.table .table-sm .table-bordered }

inter_tx_type specifies the transform type for inter blocks.

intra_tx_type specifies the transform type for intra blocks.

Is inter semantics

is_inter equal to 0 specifies that the block is an intra block; is_inter equal to 1 specifies that the block is an inter block.

Intra block mode info semantics

This syntax is used when coding an intra block within an inter frame.

y_mode specifies the direction of luminance intra prediction using values with the same interpretation as for intra_frame_y_mode.

uv_mode specifies the chrominance intra prediction mode using values with the same interpretation as in the semantics for intra_frame_y_mode, with an additional mode UV_CFL_PRED.

Note: Due to the way the uv_mode syntax element is read, uv_mode can only be read as UV_CFL_PRED when Max( Block_Width[ MiSize ], Block_Height[ MiSize ] ) <= 32. {:.alert .alert-info }

Inter block mode info semantics

This syntax is used when coding an inter block.

compound_mode specifies how the motion vector used by inter prediction is obtained when using compound prediction. An offset is added to compound_mode to compute YMode as follows:

YMode	Name of YMode
14	NEARESTMV
15	NEARMV
16	GLOBALMV
17	NEWMV
18	NEAREST_NEARESTMV
19	NEAR_NEARMV
20	NEAREST_NEWMV
21	NEW_NEARESTMV
22	NEAR_NEWMV
23	NEW_NEARMV
24	GLOBAL_GLOBALMV
25	NEW_NEWMV
{:.table .table-sm .table-bordered }

Note: The intra modes take values 0..13 so these YMode values start at 14. {:.alert .alert-info }

new_mv equal to 0 means that a motion vector difference should be read.

zero_mv equal to 0 means that the motion vector should be set equal to default motion for the frame.

ref_mv equal to 0 means that the most likely motion vector should be used (called NEAREST), ref_mv equal to 1 means that the second most likely motion vector should be used (called NEAR).

interp_filter specifies the type of filter used in inter prediction. Values 0..3 are allowed with the same interpretation as for interpolation_filter. One filter type is specified for the vertical filter direction and one for the horizontal filter direction.

Note: The syntax element interpolation_filter from the uncompressed header can specify the type of filter to be used for the whole frame. If it is set to SWITCHABLE then the interp_filter syntax element is read from the bitstream for every inter block. {:.alert .alert-info }

RefMvIdx specifies which candidate in the RefStackMv should be used.

drl_mode is a bit sent for candidates in the motion vector stack to indicate if they should be used. drl_mode equal to 0 means to use the current value of idx. drl_mode equal to 1 says to continue searching. DRL stands for "Dynamic Reference List".

Filter intra mode info semantics

use_filter_intra is a bit specifying whether or not intra filtering can be used.

filter_intra_mode specifies the type of intra filtering, and can take on any of the following values:

filter_intra_mode	Name of filter_intra_mode
0	FILTER_DC_PRED
1	FILTER_V_PRED
2	FILTER_H_PRED
3	FILTER_D157_PRED
4	FILTER_PAETH_PRED
{:.table .table-sm .table-bordered }

Ref frames semantics

comp_mode specifies whether single or compound prediction is used:

comp_mode	Name of comp_mode
0	SINGLE_REFERENCE
1	COMPOUND_REFERENCE
{:.table .table-sm .table-bordered }

SINGLE_REFERENCE indicates that the inter block uses only a single reference frame to generate motion compensated prediction.

COMPOUND_REFERENCE indicates that the inter block uses compound mode.

There are two reference frame groups:

Group 1: LAST_FRAME, LAST2_FRAME, LAST3_FRAME, and GOLDEN_FRAME.
Group 2: BWDREF_FRAME, ALTREF2_FRAME, and ALTREF_FRAME.

Note: Encoders are free to assign these references to any of the reference frames (via the ref_frame_idx array). For example, there is no requirement of bitstream conformance that LAST_FRAME should indicate a frame that appears before the current frame in output order. Similarly, encoders can assign multiple references to the same reference frame. {:.alert .alert-info }

comp_ref_type is used for compound prediction to specify whether both reference frames come from the same group or not:

| comp_ref_type | Name of comp_ref_type| Description |:-------------:|:---------------------: ----------- | 0 | UNIDIR_COMP_REFERENCE| Both reference frames from the same group | 1 | BIDIR_COMP_REFERENCE | One from Group 1 and one from Group 2 {:.table .table-sm .table-bordered }

uni_comp_ref, uni_comp_ref_p1, and uni_comp_ref_p2 specify which reference frames are in use when both come from the same group.

comp_ref, comp_ref_p1, and comp_ref_p2 specify the first reference frame when the two reference frames come from different groups.

comp_bwdref and comp_bwdref_p1 specify the second reference frame when the two reference frames come from different groups.

single_ref_p1, single_ref_p2, single_ref_p3, single_ref_p4, single_ref_p5, and single_ref_p6 specify the reference frame when only a single reference frame is in use.

RefFrame[ 0 ] specifies which frame is used to compute the predicted samples for this block:

RefFrame[ 0 ]	Name of ref_frame
0	INTRA_FRAME
1	LAST_FRAME
2	LAST2_FRAME
3	LAST3_FRAME
4	GOLDEN_FRAME
5	BWDREF_FRAME
6	ALTREF2_FRAME
7	ALTREF_FRAME
{:.table .table-sm .table-bordered }

RefFrame[ 1 ] specifies which additional frame is used in compound prediction:

RefFrame[ 1 ]	Name of ref_frame
-1	NONE (this block uses single prediction)
0	INTRA_FRAME (this block uses interintra prediction)
1	LAST_FRAME
2	LAST2_FRAME
3	LAST3_FRAME
4	GOLDEN_FRAME
5	BWDREF_FRAME
6	ALTREF2_FRAME
7	ALTREF_FRAME
{:.table .table-sm .table-bordered }

Note: Not all combinations of RefFrame[0] and RefFrame[1] can be coded. {:.alert .alert-info }

Assign mv semantics

It is a requirement of bitstream conformance that whenever assign_mv returns, the function is_mv_valid(isCompound) would return 1, where is_mv_valid is defined as:

is_mv_valid( isCompound ) {
    for ( i = 0; i < 1 + isCompound; i++ ) {
        for ( comp = 0; comp < 2; comp++ ) {
            if ( Abs( Mv[ i ][ comp ] ) >= ( 1 << 14 ) )
                return 0
        }
    }
    if ( !use_intrabc ) {
        return 1
    }
    bw = Block_Width[ MiSize ]
    bh = Block_Height[ MiSize ]
    if ( (Mv[ 0 ][ 0 ] & 7) || (Mv[ 0 ][ 1 ] & 7) ) {
        return 0
    }
    deltaRow = Mv[ 0 ][ 0 ] >> 3
    deltaCol = Mv[ 0 ][ 1 ] >> 3
    srcTopEdge = MiRow * MI_SIZE + deltaRow
    srcLeftEdge = MiCol * MI_SIZE + deltaCol
    srcBottomEdge = srcTopEdge + bh
    srcRightEdge = srcLeftEdge + bw
    if ( HasChroma ) {
        if ( bw < 8 && subsampling_x )
            srcLeftEdge -= 4
        if ( bh < 8 && subsampling_y )
            srcTopEdge -= 4
    }
    if ( srcTopEdge < MiRowStart * MI_SIZE ||
         srcLeftEdge < MiColStart * MI_SIZE ||
         srcBottomEdge > MiRowEnd * MI_SIZE ||
         srcRightEdge > MiColEnd * MI_SIZE ) {
        return 0
    }
    sbSize = use_128x128_superblock ? BLOCK_128X128 : BLOCK_64X64
    sbH = Block_Height[ sbSize ]
    activeSbRow = (MiRow * MI_SIZE) / sbH
    activeSb64Col = (MiCol * MI_SIZE) >> 6
    srcSbRow = (srcBottomEdge - 1) / sbH
    srcSb64Col = (srcRightEdge - 1) >> 6
    totalSb64PerRow = ((MiColEnd - MiColStart - 1) >> 4) + 1
    activeSb64 = activeSbRow * totalSb64PerRow + activeSb64Col
    srcSb64 = srcSbRow * totalSb64PerRow + srcSb64Col
    if ( srcSb64 >= activeSb64 - INTRABC_DELAY_SB64) {
        return 0
    }
    gradient = 1 + INTRABC_DELAY_SB64 + use_128x128_superblock
    wfOffset = gradient * (activeSbRow - srcSbRow)
    if ( srcSbRow > activeSbRow ||
         srcSb64Col >= activeSb64Col - INTRABC_DELAY_SB64 + wfOffset ) {
        return 0
    }
    return 1
}

Note: The purpose of this function is to limit the maximum size of motion vectors and also, if use_intrabc is equal to 1, to additionally constrain the motion vector in order that the data is fetched from parts of the tile that have already been decoded, and that are not too close to the current block (in order to make a pipelined decoder implementation feasible). {:.alert .alert-info }

Read motion mode semantics

use_obmc equal to 1 means that OBMC should be used. use_obmc equal to 0 means that simple translation should be used.

motion_mode specifies the type of motion compensation to perform:

motion_mode	Name of motion_mode
0	SIMPLE
1	OBMC
2	LOCALWARP
{:.table }

Note: A motion_mode equal to SIMPLE is used for blocks requiring global motion. {:.alert .alert-info }

Read inter intra semantics

interintra equal to 1 specifies that an inter prediction should be blended with an intra prediction.

interintra_mode specifies the type of intra prediction to be used:

interintra_mode	Name of interintra_mode
0	II_DC_PRED
1	II_V_PRED
2	II_H_PRED
3	II_SMOOTH_PRED
{:.table .table-sm .table-bordered }

wedge_interintra equal to 1 specifies that wedge blending should be used. wedge_interintra equal to 0 specifies that intra blending should be used.

wedge_index is used to derive the direction and offset of the wedge mask used during blending.

Read compound type semantics

comp_group_idx equal to 0 indicates that the compound_idx syntax element should be read. comp_group_idx equal to 1 indicates that the compound_idx syntax element is not present.

compound_idx equal to 0 indicates that a distance based weighted scheme should be used for blending. compound_idx equal to 1 indicates that the averaging scheme should be used for blending.

compound_type specifies how the two predictions should be blended together:

compound_type	Name of compound_type
0	COMPOUND_WEDGE
1	COMPOUND_DIFFWTD
2	COMPOUND_AVERAGE
3	COMPOUND_INTRA
4	COMPOUND_DISTANCE
{:.table .table-sm .table-bordered }

Note: COMPOUND_AVERAGE, COMPOUND_INTRA, and COMPOUND_DISTANCE cannot be directly signaled with the compound_type syntax element but are inferred from other syntax elements. {:.alert .alert-info }

wedge_index is used to derive the direction and offset of the wedge mask used during blending.

wedge_sign specifies the sign of the wedge blend.

mask_type specifies the type of mask to be used during blending:

mask_type	Name of mask_type
0	UNIFORM_45
1	UNIFORM_45_INV
{:.table .table-sm .table-bordered }

MV semantics

MvCtx is used to determine which CDFs to use for the motion vector syntax elements.

mv_joint specifies which components of the motion vector difference are non-zero:

mv_joint	Name of mv_joint	Changes row	Changes col
0	MV_JOINT_ZERO	No	No
1	MV_JOINT_HNZVZ	No	Yes
2	MV_JOINT_HZVNZ	Yes	No
3	MV_JOINT_HNZVNZ	Yes	Yes
{:.table .table-sm .table-bordered }

The motion vector difference is added to the PredMv to compute the final motion vector in Mv.

MV component semantics

mv_sign equal to 0 means that the motion vector difference is positive; mv_sign equal to 1 means that the motion vector difference is negative.

mv_class specifies the class of the motion vector difference. A higher class means that the motion vector difference represents a larger update:

mv_class	Name of mv_class
0	MV_CLASS_0
1	MV_CLASS_1
2	MV_CLASS_2
3	MV_CLASS_3
4	MV_CLASS_4
5	MV_CLASS_5
6	MV_CLASS_6
7	MV_CLASS_7
8	MV_CLASS_8
9	MV_CLASS_9
10	MV_CLASS_10
{:.table .table-sm .table-bordered }

mv_class0_bit specifies the integer part of the motion vector difference. This is only present for class 0 motion vector differences.

mv_class0_fr specifies the first 2 fractional bits of the motion vector difference. This is only present for class 0 motion vector differences.

mv_class0_hp specifies the third fraction bit of the motion vector difference. This is only present for class 0 motion vector differences.

mv_bit specifies bit i of the integer part of the motion vector difference.

mv_fr specifies the first 2 fractional bits of the motion vector difference.

mv_hp specifies the third fractional bit of the motion vector difference.

Compute prediction semantics

The prediction for inter and interintra blocks is triggered within compute_prediction. However, intra prediction is done at the transform block granularity so predict_intra is also called from transform_block.

predW and predH are variables containing the smallest size that can be used for inter prediction. (This size may be increased for chroma blocks if not all blocks use inter prediction.)

predict_inter is a function call that indicates the conceptual point where inter prediction happens. When this function is called, the inter prediction process specified in [section 7.11.3][] is invoked.

predict_intra is a function call that indicates the conceptual point where intra prediction happens. When this function is called, the intra prediction process specified in [section 7.11.2][] is invoked.

Note: The predict_inter and predict_intra functions do not affect the syntax decode process. {:.alert .alert-info }

someUseIntra is a variable that indicates if some of the blocks corresponding to this residual require intra prediction.

Note: The chroma residual block size is always at least 4 in width and height. This means that no transform width or height smaller than 4 is required. As such, a chroma residual may actually cover several luma blocks. If any of these blocks are intra, a single prediction is performed for the entire chroma residual block based on the mode info of the bottom right luma block. However, if all the constituent blocks are inter blocks, a special case is triggered and inter prediction is done using the smaller chroma block size that corresponds to each of the luma blocks. {:.alert .alert-info }

Residual semantics

The residual consists of a number of transform blocks.

If the block is wider or higher than 64 luma samples, then the residual is split into 64 by 64 chunks.

Within each chunk, the transform blocks are either sent in raster order (if use_inter is equal to 0 or LossLess is equal to 1), or within a recursive transform tree.

Transform block semantics

reconstruct is a function call that indicates the conceptual point where inverse transform and reconstruction happens. When this function is called, the reconstruction process specified in [section 7.12.3][] is invoked.

predict_palette is a function call that indicates the conceptual point where palette prediction happens. When this function is called, the palette prediction process specified in [section 7.11.4][] is invoked.

predict_chroma_from_luma is a function call that indicates the conceptual point where predicting chroma from luma happens. When this function is called, the predict chroma from luma process specified in [section 7.11.5][] is invoked.

MaxLumaW and MaxLumaH are needed for chroma from luma prediction and store the extent of luma samples that can be used for prediction.

LoopfilterTxSizes is an array that stores the transform size for each plane and position for use in loop filtering. LoopfilterTxSizes[ plane ][ row ][ col ] stores the transform size where row and col are in units of 4x4 samples.

Note: The transform size is always equal for planes 1 and 2. {:.alert .alert-info }

Coefficients semantics

TxTypes is an array which stores at a 4x4 luma sample granularity the transform type to be used.

Note: The transform type is only read for luma transform blocks, the chroma uses the transform type for a corresponding luma block. Chroma blocks will only use transform types that have been written for the current residual block. {:.alert .alert-info }

Quant is an array storing the quantised coefficients for the current transform block.

all_zero equal to 1 specifies that all coefficients are zero.

Note: The transform type is only present when this is a luminance block and all_zero is equal to 0. If all_zero is equal to 1 for a luminance block, the transform type is set to DCT_DCT. {:.alert .alert-info }

eob_extra and eob_extra_bit specify the position of the last non-zero coefficient by being used to compute the variable eob.

eob_pt_16, eob_pt_32, eob_pt_64, eob_pt_128, eob_pt_256, eob_pt_512, eob_pt_1024: syntax elements used to compute eob.

eob is a variable that indicates the index of the end of block. This index is equal to one plus the index of the last non-zero coefficient.

coeff_base_eob is a syntax element used to compute the base level of the last non-zero coefficient.

Note: The base level is set to coeff_base_eob plus 1. Since this coefficient is known to be non-zero, only base levels of 1, 2, or 3 can be coded via coeff_base_eob. {:.alert .alert-info }

coeff_base specifies the base level of a coefficient (this syntax element is used for all coefficients except the last non-zero coefficient).

Note: The base level can take values of 0, 1, 2, or 3. If the base level is less than 3, then it contains the actual level of the coefficient. Otherwise, the syntax element coeff_br is used to optionally increase the level. {:.alert .alert-info }

dc_sign specifies the sign of the DC coefficient.

sign_bit specifies the sign of a non-zero AC coefficient.

coeff_br specifies an increment to the coefficient.

Note: Each quantized coefficient can use coeff_br to provide up to 4 increments. If an increment less than 3 is coded, it signifies that this was the final increment. {:.alert .alert-info }

golomb_length_bit is used to compute the number of extra bits required to code the coefficient.

If length is equal to 20, it is a requirement of bitstream conformance that golomb_length_bit is equal to 1.

golomb_data_bit specifies the value of one of the extra bits.

AboveLevelContext and LeftLevelContext are arrays that store at a 4 sample granularity the cumulative sum of coefficient levels.

AboveDcContext and LeftDcContext are arrays that store at a 4 sample granularity 2 bits signaling the sign of the DC coefficient (zero being counted as a separate sign).

Intra angle info semantics

angle_delta_y specifies the offset to be applied to the intra prediction angle specified by the prediction mode in the luma plane, biased by MAX_ANGLE_DELTA so as to encode a positive value.

angle_delta_uv specifies the offset to be applied to the intra prediction angle specified by the prediction mode in the chroma plane biased by MAX_ANGLE_DELTA so as to encode a positive value.

AngleDeltaY is computed from angle_delta_y by removing the MAX_ANGLE_DELTA offset to produce the final luma angle offset value, which may be positive or negative.

AngleDeltaUV is computed from angle_delta_uv by removing the MAX_ANGLE_DELTA offset to produce the final chroma angle offset value, which may be positive or negative.

Read CFL alphas semantics

cfl_alpha_signs contains the sign of the alpha values for U and V packed together into a single syntax element with 8 possible values. (The combination of two zero signs is prohibited as it is redundant with DC Intra prediction.)

cfl_alpha_signs	Name of signU	Name of signV
0	CFL_SIGN_ZERO	CFL_SIGN_NEG
1	CFL_SIGN_ZERO	CFL_SIGN_POS
2	CFL_SIGN_NEG	CFL_SIGN_ZERO
3	CFL_SIGN_NEG	CFL_SIGN_NEG
4	CFL_SIGN_NEG	CFL_SIGN_POS
5	CFL_SIGN_POS	CFL_SIGN_ZERO
6	CFL_SIGN_POS	CFL_SIGN_NEG
7	CFL_SIGN_POS	CFL_SIGN_POS
{:.table .table-sm .table-bordered }

signU contains the sign of the alpha value for the U component:

signU	Name of signU
0	CFL_SIGN_ZERO
1	CFL_SIGN_NEG
2	CFL_SIGN_POS
{:.table .table-sm .table-bordered }

signV contains the sign of the alpha value for the V component with the same interpretation as for signU.

cfl_alpha_u contains the absolute value of alpha minus one for the U component.

cfl_alpha_v contains the absolute value of alpha minus one for the V component.

CflAlphaU contains the signed value of the alpha component for the U component.

CflAlphaV contains the signed value of the alpha component for the V component.

Palette mode info semantics

has_palette_y is a boolean value specifying whether a palette is encoded for the Y plane.

has_palette_uv is a boolean value specifying whether a palette is encoded for the UV plane.

palette_size_y_minus_2 is used to compute PaletteSizeY.

PaletteSizeY is a variable holding the Y plane palette size.

palette_size_uv_minus_2 is used to compute PaletteSizeUV.

PaletteSizeUV is a variable holding the UV plane palette size.

use_palette_color_cache_y, if equal to 1, indicates that for a particular palette entry in the luma palette, the cached entry should be used.

use_palette_color_cache_u, if equal to 1, indicates that for a particular palette entry in the U chroma palette, the cached entry should be used.

palette_colors_y is an array holding the Y plane palette colors.

palette_colors_u is an array holding the U plane palette colors.

palette_colors_v is an array holding the V plane palette colors.

delta_encode_palette_colors_v, if equal to 1, indicates that the V chroma palette is encoded using delta encoding.

palette_num_extra_bits_y is used to calculate the number of bits used to store each palette delta value for the luma palette.

palette_num_extra_bits_u is used to calculate the number of bits used to store each palette delta value for the U chroma palette.

palette_num_extra_bits_v is used to calculate the number of bits used to store each palette delta value for the V chroma palette.

palette_delta_y is a delta value for the luma palette.

palette_delta_u is a delta value for the U chroma palette.

palette_delta_v is a delta value for the V chroma palette.

Note: Luma and U delta values give a positive offset relative to the previous palette entry in the same plane. V delta values give a signed offset relative to the U palette entries. {:.alert .alert-info }

palette_delta_sign_bit_v, if equal to 1, indicates that the decoded V chroma palette delta value should be negated.

Palette tokens semantics

color_index_map_y holds the index in palette_colors_y for the block's Y plane top left sample.

color_index_map_uv holds the index in palette_colors_u and palette_colors_v for the block's UV plane top left sample.

palette_color_idx_y holds the index in ColorOrder for a sample in the block's Y plane.

palette_color_idx_uv holds the index in ColorOrder for a sample in the block's UV plane.

Palette color context semantics

ColorOrder is an array holding the mapping from an encoded index to the palette. ColorOrder is ranked in order of frequency of occurrence of each color in the neighborhood of the current block, weighted by closeness to the current block.

ColorContextHash is a variable derived from the distribution of colors in the neighborhood of the current block, which is used to determine the probability context used to decode palette_color_idx_y and palette_color_idx_uv.

Read CDEF semantics

cdef_idx specifies which CDEF filtering parameters should be used for a particular 64 by 64 block. A value of -1 means that CDEF is disabled for that block.

Read loop restoration unit semantics

use_wiener specifies if the Wiener filter should be used.

use_sgrproj specifies if the self guided filter should be used.

restoration_type specifies the restoration filter that should be used with the same interpretation as FrameRestorationType.

lr_sgr_set specifies which set of parameters to use for the self guided filter.

subexp_more_bools equal to 0 specifies that the parameter is in the range mk to mk+a-1. subexp_more_bools equal to 1 specifies that the parameter is greater than mk+a-1.

subexp_unif_bools specifies the value of the parameter minus mk.

subexp_bools specifies the value of the parameter minus mk.

Tile list OBU semantics

General tile list OBU semantics

output_frame_width_in_tiles_minus_1 plus one is the width of the output frame, in tile units.

output_frame_height_in_tiles_minus_1 plus one is the height of the output frame, in tile units.

tile_count_minus_1 plus one is the number of tile_list_entry in the list.

It is a requirement of bitstream conformance that tile_count_minus_1 is less than or equal to 511.

Tile list entry semantics

anchor_frame_idx is the index into an array AnchorFrames of the frames that the tile uses for prediction. The AnchorFrames array is provided by external means and may change for each tile list OBU. The process for creating the AnchorFrames array is outside of the scope of this specification.

It is a requirement of bitstream conformance that anchor_frame_idx is less than or equal to 127.

anchor_tile_row is the row coordinate of the tile in the frame that it belongs, in tile units.

It is a requirement of bitstream conformance that anchor_tile_row is less than TileRows.

anchor_tile_col is the column coordinate of the tile in the frame that it belongs, in tile units.

It is a requirement of bitstream conformance that anchor_tile_col is less than TileCols.

tile_data_size_minus_1 plus one is the size of the coded tile data, coded_tile_data, in bytes.

coded_tile_data are the tile_data_size_minus_1 + 1 bytes of the coded tile.

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07.bitstream.semantics.md

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Syntax structures semantics

General

OBU semantics

General OBU semantics

OBU header semantics

OBU extension header semantics

Trailing bits semantics

Byte alignment semantics

Reserved OBU semantics

Sequence header OBU semantics

General sequence header OBU semantics

Color config semantics

Timing info semantics

Decoder model info semantics

Operating parameters info semantics

Temporal delimiter OBU semantics

Padding OBU semantics

Metadata OBU semantics

General metadata OBU semantics

Metadata ITUT T35 semantics

Metadata high dynamic range content light level semantics

Metadata high dynamic range mastering display color volume semantics

Metadata scalability semantics

Scalability structure semantics

General

L1T2 (Informative)

L1T3 (Informative)

L2T1 / L2T1h (Informative)

L2T2 / L2T2h (Informative)

L2T3 / L2T3h (Informative)

S2T1 / S2T1h (Informative)

S2T2 / S2T2h (Informative)

S2T3 / S2T3h (Informative)

L3T1 (Informative)

L3T2 (Informative)

L3T3 (Informative)

S3T1 (Informative)

S3T2 (Informative)

S3T3 (Informative)

Metadata timecode semantics

Frame header OBU semantics

General frame header OBU semantics

Uncompressed header semantics

Reference frame marking semantics

Frame size semantics

Render size semantics

Frame size with refs semantics

Superres params semantics

Compute image size semantics

Interpolation filter semantics

Loop filter semantics

Quantization params semantics

Delta quantizer semantics

Segmentation params semantics

Tile info semantics

Quantizer index delta parameters semantics

Loop filter delta parameters semantics

Global motion params semantics

Global param semantics

Decode subexp semantics

Film grain params semantics

TX mode semantics

Skip mode params semantics

Frame reference mode semantics

Temporal point info semantics

Frame OBU semantics

Tile group OBU semantics

General tile group OBU semantics

Decode tile semantics

Clear block decoded flags semantics

Decode partition semantics

Decode block semantics

Intra frame mode info semantics

Intra segment ID semantics