[Ffmpeg-devel] H264 playback problems related to num_reorder_frames check

[Frank Otto]

Hi,

recent ffmpeg versions have trouble playing back H264 video,
at least when it was created with x264 (I have no other H264
encoder). The playback is all jerky as if the frames are in
the wrong order.

Mplayer gives the following warning at the start of playback:
  [h264 @ 0xb6cbfba8]illegal num_reorder_frames -1

It seems this warning and the playback problems are caused
by this piece of code inside "decode_vui_parameters" from "h264.c"
(added in rev. 7663 by "michael"):

        if(num_reorder_frames > 16 /*max_dec_frame_buffering || max_dec_frame_buffering > 16*/){
            av_log(h->s.avctx, AV_LOG_ERROR, "illegal num_reorder_frames %d\n", num_reorder_frames);
            return -1;
        }
        sps->num_reorder_frames= num_reorder_frames;

And locally "num_reorder_frames" is declared *unsigned*, while in
the SPS structure it is declared as signed, so the check is hit
and the function returns before setting sps->num_reorder_frames .

Now the question is, who is at fault:
- x264 for creating files which result in negative num_reorder_frames
- ffmpeg for not allowing negative num_reorder_frames

Versions of ffmpeg/mplayer from before this check was added play
the video just fine. The old behaviour can easily be restored by
declaring num_reorder_frames as signed.

Regards and thanks,
Frank

P.S. please CC as I'm not on the list.