[FFmpeg-devel] [PATCH] lavf/matroskadec: fix is_keyframe for early Blocks

wm4 nfxjfg at googlemail.com
Wed Feb 1 08:18:51 EET 2017

On Tue, 31 Jan 2017 12:02:01 -0800
Chris Cunningham <chcunningham at chromium.org> wrote:

> Thanks for taking a look.
> Definitely missing a "break;" - will fix in subsequent patch.
> Agree timestamps should be relative (didn't realize this). Vignesh points
> out that "0" in the test file is due to a bug in ffmpeg (and probably other
> muxers) where this value is not written as a relative timestamp, but
> instead as the timestamp of the previous frame. https://github.com/FFmp
> eg/FFmpeg/blob/master/libavformat/matroskaenc.c#L2053
> <https://www.google.com/url?q=https%3A%2F%2Fgithub.com%2FFFmpeg%2FFFmpeg%2Fblob%2Fmaster%2Flibavformat%2Fmatroskaenc.c%23L2053&sa=D&sntz=1&usg=AFQjCNGs8m6GsWbhTvCZl0Q_juGAldQblA>

Just a few lines below this reads

   mkv->last_track_timestamp[track_number - 1] = ts - mkv->cluster_pts;

which looks like it intends to write a relative value. Though "ts" can
be a DTS, while the other value is always a PTS.

> Anyway, I'm all for following Haali. Its not obvious how best to do this. I
> don't think there's any great default value to indicate "not-set" and the
> generic embl parsing code that reads the reference timestamp doesn't really
> lend itself to setting an additional field like
> MatroskaBlock.has_reference. I can sort this out, but I'll pause in case
> you have a recommendation in-mind.

I don't know a nice way either. Here are 3 potential ways I came up

1. Use a better dummy value, like INT64_MIN, which is almost 100%
   unlikely to appear in a valid file. (This is probably equivalent to
   your intention in original patch.)
2. Add a new "presence_flag_offset" field to EbmlSyntax - the EBML
   reader will write a 1 to it if the element was found. (This is a bit
   annoying, because 0 is a valid offset, and you surely wouldn't want
   to change all the other element definitions.)
3. Add a new type like EBML_PRESENCE, which writes whether the element
   was present or not to data_offset, instead of the element's content
   or its default value. (There are some other special "types" like
   EBML_STOP, so maybe this idea isn't all too hacky.)

More information about the ffmpeg-devel mailing list