New submission from Anssi Hannula <anssi.hann...@iki.fi>: 23.976 H.264 Matroska files are commonly detected as 24fps (r_frame_rate).
The most visible issue caused by this I've seen is probably frame-based subtitle misalignment on MPlayer and XBMC (though the latter has workarounded it). Background and the issue specifics: - normally lavf assumes that the (1 / codec->time_base / codec->ticks_per_frame) is the fps (r_frame_rate), unless it would be too high to be represented by the st->time_base, in which case (1 / st->time_base) is taken instead (utils.c 2431-2440) - if ((1/5) > codec->time_base >= (1/101)) is false, the codec->time_base is considered unreliable and instead a custom fps probing code is used (tb_unreliable()) which reads the timestamps of the first 20 packets - some H.264 (and I guess MPEG2) streams have a specific interlacing mode that causes there to be 2x more packets (as output from the demuxer) than codec->time_base and codec->ticks_per_frame would indicate (well, I guess technically they are correct, if the packets actually contain half-frames due to interlacing), e.g. http://samples.ffmpeg.org/V-codecs/h264/hdtv-interlaced/ sky_720p_test_why-cant-i-overwrite.ts - due to the above (AFAIK), H264 and MPEG2 are always assumed to have unreliable timebase and the fps probing code is always used - mkv tracks have generally millisecond precision for timestamps - 23.976fps therefore requires a pattern of 41ms and 42ms frames, that add up to 1.001s in 24 frames - the fps probing code doesn't detect the difference between 24fps and 23.976fps from just 20 frames, it would need more than that (25-30) => 23.976fps files are wrongly detected as 24fps - 23.976fps files are progressive (unless insane), so the codec timebase as got from the decoder would actually be reliable and show an exact rate of 24/1.001. - mkv tracks also contain a default_duration field that contains the length of frames in nanosecond precision. In the file I checked it was accurate to within 1.5ns (some rounding issue I guess), which corresponds to an error of about 0.000001 fps. The issue was discussed at http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/2011- January/104856.html ---------- messages: 13770 nosy: anssi priority: normal status: open substatus: open title: 23.976 H.264 matroska files detected as 24fps topic: avformat type: bug ________________________________________________ FFmpeg issue tracker <iss...@roundup.ffmpeg.org> <https://roundup.ffmpeg.org/issue2629> ________________________________________________
