PSA: OpenH264 update coming in M129 (with a bitstream change)

[Philipp Hancke]

tl;dr: if you depend on OpenH264 please read this carefully and test in Canary and Beta.

I just landed a commit that updates the OpenH264 dependency Chromium uses for H264 software encoding (on the constrained baseline profile)

  https://chromiumdash.appspot.com/commit/9d7bae44ba5835e40c248a818673b7c94901c67e

The full changelog is linked in the commit. If everything goes as planned this will land in Chromium 129 stable on September 17th.

The primary reason for updating this was a change in the H264 pic_order_cnt in the SPS generated by OpenH264 from 0 to 2 from

  https://github.com/cisco/openh264/pull/3752

which can be found in the SPS in Wireshark like this:

The value 2 means that the stream can be decoded instantly as the display order is the same as the decode order and there is no

need to wait for the next frame to determine if the current frame can be played out. See

  https://www.vcodex.com/h264avc-picture-management/

for a great explanation (or the excellent book on H264 by the same author)

What originally spurred the change was this behavior observed in a webrtc-internals dump:

which basically shows the decoder waiting for 30ms or one frame for no reason.

This behavior is a known issue on some Android hardware decoders like the one used in Qualcomms Snapdragon chips and quite undesirable for RTC scenarios in general.

For WebRTC on those devices you should see a drop in the per-frame totalDecodeTime returned from getStats to more reasonable values of 4 milliseconds.

As a bonus this also removes two bytes from each frame encoded with OpenH264. This may not sound much but adds up!

If you depend on H264 please test in Canary and Beta

cheers

Philipp

[R simons]

Hi Philipp, I'm facing a h264 hw decocing issue recently in Qualcomms Snapdragon 680, cannot decoding two video stream with 720p and 30fps same time. I posted a discuss here: https://groups.google.com/g/discuss-webrtc/c/0T7uBRv-ACg

I wonder to know is there a way to force sw h264 decoder on Android Chrome?

I had tried disable hw decoding in chrome://flags, did not work, seems the sw decoder(ffmpeg) is still not supported on Android Chrome yet.