Stream quality problems with simple h264 encoder

[martin19]

I've written a simple h264 encoder using libva for a screen capture tool. The encoder is set up to create a single IDR frame with only P frames following. I've copied most of the code from the h264 example in libva-utils. I'm facing a problem with image quality of the encoded stream. Errors in the picture seem to accumulate over time and the visual quality deteriorates as more frames are encoded/decoded.

If i understand correctly, the quality should not get worse over time (as long as there are no transmission errors) - the encoder should know about the "reconstruction error" of individual frames and respect them when encoding another. The video I've attached shows a kind of "ghost picture" of past frames which gets more visible over time. I think I might be doing something wrong in my code, however I cannot find out.

The code can be found here:
https://github.com/martin19/snpserver/blob/main/src/stream/video/SnpEncoderVaH264.cpp

A dump of an encoded stream with QP set to constant value of 30 (can be viewed with VLC for example):
https://drive.google.com/file/d/1IdjtmxVxz6KTW8twdLvYodD7vrFhLU-a/view?usp=sharing

(..rgb components are swapped in the stream, so colors are not correct)

Can anyone please help here?

[martin19]

Did this experiment:

I'm using h264encode (from the libutils package) to encode 1000 frames of 1080p video. If i set the encoder to produce no I-frames the decoder "chokes" in the middle of the stream while and produces erroneous frames. If i keep the idr/intra period on standard everything works ok.

This works:

./h264encode -w 1920 -h 1080 -framecount 1000 --minqp 30 --initialqp 30 --entropy 0 --rcmode CQP --profile HP --srcyuv input.yuv

This creates erroneous frames

./h264encode -w 1920 -h 1080 -framecount 1000 --minqp 30 --initialqp 30 --idr_period 1000 -intra_period 1000 --entropy 0 --rcmode CQP --profile HP --srcyuv input.yuv

I suppose keyframes are required for the encoder to create a proper stream without errors.