Setting time_base when adding a stream to an output container

[daveisfera]

Overview

Accessing time_base now raises an error with 13.0 when specifying time_base when calling add_stream on an output container

Expected behavior

Able to specify the time_base on a stream like previous versions

Actual behavior

RuntimeError raised when it's called

Traceback:

Traceback (most recent call last):
  File \"/usr/src/app/test.py\", line 129, in test
    main()
  File \"/usr/src/app/test.py\", line 81, in _test
    stream = container.add_stream(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File \"av/container/output.pyx\", line 132, in av.container.output.OutputContainer.add_stream
  File \"av/stream.pyx\", line 111, in av.stream.Stream.__setattr__
  File \"av/codec/context.pyx\", line 511, in av.codec.context.CodecContext.time_base.__set__
RuntimeError: Cannot access 'time_base' as a decoder"

Investigation

Caused by this commit

Reproduction

Call add_stream with time_base, like this:

container.add_stream("h264", time_base=Fraction(1, 1000))

Versions

• OS: Debian Bookworm
• PyAV runtime: 13.0

PyAV v13.0.0
library configuration: --prefix=/usr --enable-shared --disable-static --disable-debug --disable-doc --disable-ffplay --enable-gpl --enable-libfdk_aac --enable-libx264 --enable-libx265 --enable-librsvg --enable-openssl --enable-nonfree --enable-libaom --extra-libs=-lpthread
library license: nonfree and unredistributable
libavcodec     60. 31.102
libavdevice    60.  3.100
libavfilter     9. 12.100
libavformat    60. 16.100
libavutil      58. 29.100
libswresample   4. 12.100
libswscale      7.  5.100

• PyAV build:

pip3 install av==13.0.0 --no-binary av

Research

I have done the following:

• Checked the PyAV documentation
• Searched on Google
• Searched on Stack Overflow
• Looked through old GitHub issues

Additional context

If there's another way to specify this, then I'm definitely up for switching how I specify this, but I believe this is a valid use case since it's an encoder rather than a decoder and maybe the issue is that something is wrong with the is_decoder check?

[WyattBlue]

Can't repo. "done" is printed successfully.

import av
from fractions import Fraction

container = av.open("test.mp4", "w")

container.add_stream("h264", time_base=Fraction(1, 1000))

container.close()
print("done")

[WyattBlue]

Your issue might be caused by compiling with Debian's old system libraries.

[daveisfera]

As I dug in more, it's a little more complicated than I realized because I had put in a workaround that I forgot about. I'm using av.Codec("h264", "r") to force it to be a decoder because the encoder version doesn't work. The first issue is that it doesn't allow you to set extradata (which I submitted a PR to allow) but then it still doesn't result in a valid output even though the workaround I put in does. I'm guessing that I'm doing something wrong or there's a sneaky bug with some value not being set since I'm not just copying a codec from a template.

So running this with no second arg so av.Codec("h264", "r") will be used, results in a working file but adding h264 as the second arg so it uses an encoder codec makes it not work.

import sys

import av

input_ = av.open(sys.argv[1])
output = av.open("output.mp4", "w")

# Make an output stream using the input as a template. This copies the stream
# setup from one to the other.
in_stream = input_.streams.video[0]
try:
    codec = sys.argv[2]
except IndexError:
    codec = av.Codec("h264", "r")
out_stream = output.add_stream(
    codec, width=in_stream.width, height=in_stream.height, rate=in_stream.average_rate, time_base=in_stream.time_base
)
out_stream.extradata = in_stream.extradata

for packet in input_.demux(in_stream):
    # print(packet)

    # We need to skip the "flushing" packets that `demux` generates.
    if packet.dts is None:
        continue

    # We need to assign the packet to the new stream.
    packet.stream = out_stream

    output.mux(packet)

input_.close()
output.close()

[daveisfera]

I noticed that the pixel format is missing when it uses the encoder, so is it doing something to set that when the decoder is used? Is there a way for me to ensure all of the codec values are set properly?

[daveisfera]

To check that theory, I did print(name, getattr(out_stream.codec_context, name)) for each of the out_stream values with the different codec inputs and they were basically the same (the only difference was the GOP, name and the encoder/decoder flags), so dug some more and what's happening is the extradata on out_stream is changing the first time output.mux() is being called and that's why nothing ends up being written

[daveisfera]

The issue is that template=in_stream sets the codec to h264 as a decoder, so the init of that codec is called and that allows everything to happen as expected. When h264 is used directly, the PyAV interface sets mode to w, so it's an encoder (as it should be), but that means it's actually libx264 and the init for that is called, which overwrites extradata and causes the problems I'm seeing here.
So long story short, the PR that I submitted allows this to all work correctly and it should be good now.