a note with this change:
If we set the timecodescale to microsecond, and we encode a 30 fps video,
the duration of each frame are then 33333 us.
In this case,
(int16_t)cluster_time != cluster_time
Will almost every time faile and we will need to create a new block per
frame (it seems to me at least)
Because in the block header, there is a timestamp relative to Cluster
timestamp (signed int16) which cannot represent 33333
On Wed, Jan 13, 2021 at 12:02 PM James Almer <jamr...@gmail.com> wrote:
> On 1/13/2021 2:46 PM, Thierry Foucu wrote:
> > By default the time code scale in a MKV file in millisecond. With this
> > option we can set the time code scale to microsecond or nanoseconds for
> > very high frame rate.
> > ---
> > libavformat/matroskaenc.c | 11 +++++++----
> > 1 file changed, 7 insertions(+), 4 deletions(-)
> >
> > diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
> > index 233c472b8f..cfad6a4693 100644
> > --- a/libavformat/matroskaenc.c
> > +++ b/libavformat/matroskaenc.c
> > @@ -158,6 +158,8 @@ typedef struct MatroskaMuxContext {
> > int default_mode;
> >
> > uint32_t segment_uid[4];
> > +
> > + int64_t timecodescale;
> > } MatroskaMuxContext;
> >
> > /** 2 bytes * 7 for EBML IDs, 7 1-byte EBML lengths, 6 1-byte uint,
> > @@ -1827,7 +1829,7 @@ static int mkv_write_header(AVFormatContext *s)
> > return ret;
> > pb = mkv->info.bc;
> >
> > - put_ebml_uint(pb, MATROSKA_ID_TIMECODESCALE, 1000000);
> > + put_ebml_uint(pb, MATROSKA_ID_TIMECODESCALE, mkv->timecodescale);
> > if ((tag = av_dict_get(s->metadata, "title", NULL, 0)))
> > put_ebml_string(pb, MATROSKA_ID_TITLE, tag->value);
> > if (!(s->flags & AVFMT_FLAG_BITEXACT)) {
> > @@ -1927,12 +1929,12 @@ static int mkv_write_header(AVFormatContext *s)
> > // after 4k and on a keyframe
> > if (IS_SEEKABLE(pb, mkv)) {
> > if (mkv->cluster_time_limit < 0)
> > - mkv->cluster_time_limit = 5000;
> > + mkv->cluster_time_limit = 5*(1000000000/mkv->timecodescale);
>
> Either 1000000000LL, or use av_rescale(). Same for the two below
>
> > if (mkv->cluster_size_limit < 0)
> > mkv->cluster_size_limit = 5 * 1024 * 1024;
> > } else {
> > if (mkv->cluster_time_limit < 0)
> > - mkv->cluster_time_limit = 1000;
> > + mkv->cluster_time_limit = 1*(1000000000/mkv->timecodescale);
> > if (mkv->cluster_size_limit < 0)
> > mkv->cluster_size_limit = 32 * 1024;
> > }
> > @@ -2708,7 +2710,7 @@ static int mkv_init(struct AVFormatContext *s)
> > }
> >
> > // ms precision is the de-facto standard timescale for mkv
> files
> > - avpriv_set_pts_info(st, 64, 1, 1000);
> > + avpriv_set_pts_info(st, 64, 1, 1000000000/mkv->timecodescale);
> >
> > if (st->codecpar->codec_type == AVMEDIA_TYPE_ATTACHMENT) {
> > if (mkv->mode == MODE_WEBM) {
> > @@ -2795,6 +2797,7 @@ static const AVOption options[] = {
> > { "infer", "For each track type, mark the first track of
> disposition default as default; if none exists, mark the first track as
> default.", 0, AV_OPT_TYPE_CONST, { .i64 = DEFAULT_MODE_INFER }, 0, 0,
> FLAGS, "default_mode" },
> > { "infer_no_subs", "For each track type, mark the first track of
> disposition default as default; for audio and video: if none exists, mark
> the first track as default.", 0, AV_OPT_TYPE_CONST, { .i64 =
> DEFAULT_MODE_INFER_NO_SUBS }, 0, 0, FLAGS, "default_mode" },
> > { "passthrough", "Use the disposition flag as-is", 0,
> AV_OPT_TYPE_CONST, { .i64 = DEFAULT_MODE_PASSTHROUGH }, 0, 0, FLAGS,
> "default_mode" },
> > + { "timecodescale", "Time code scale for all tracks in nanoseconds",
> OFFSET(timecodescale), AV_OPT_TYPE_INT64, { .i64 = 1000000 }, 1,
> 1000000000, FLAGS },
> > { NULL },
> > };
>
> Does this cover all cases? A timecodescale of 1000000 was implied until
> now, so many parts of the code could be hardcoding it in a non obvious way.
>
> I see for example MATROSKA_ID_DURATION is set as av_rescale(s->duration,
> 1000, AV_TIME_BASE). Should that AV_TIME_BASE be changed to
> mkv->timecodescale? The spec says "Duration of the Segment in
> nanoseconds based on TimestampScale."
>
Interesting. I did not have to change this because if I check the Metadata
DURATION coming from the demuxer with a default ms and set to microsecond,
I do see in both:
DURATION : 00:00:01.001000000
or is it another duration I need to look at, like this one:
https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/matroskaenc.c#L1863
See output:
# default behavoir
./ffmpeg -f lavfi -i "testsrc=r=30000/1001" -debug_ts -t 1 -y -f matroska
/tmp/test.mkv
./ffmpeg -i /tmp/test.mkv
ffmpeg version N-100413-g3799e77f93 Copyright (c) 2000-2020 the FFmpeg
developers
built with gcc 10 (Debian 10.2.0-19)
configuration: --disable-optimizations
libavutil 56. 62.100 / 56. 62.100
libavcodec 58.115.102 / 58.115.102
libavformat 58. 65.100 / 58. 65.100
libavdevice 58. 11.103 / 58. 11.103
libavfilter 7. 93.100 / 7. 93.100
libswscale 5. 8.100 / 5. 8.100
libswresample 3. 8.100 / 3. 8.100
Input #0, matroska,webm, from '/tmp/test.mkv':
Metadata:
ENCODER : Lavf58.65.100
Duration: 00:00:01.00, start: 0.000000, bitrate: 412 kb/s
Stream #0:0: Video: mpeg4 (Simple Profile), yuv420p(tv, progressive),
320x240 [SAR 1:1 DAR 4:3], 29.97 fps, 29.97 tbr, 1k tbn, 30k tbc (default)
Metadata:
ENCODER : Lavc58.115.102 mpeg4
DURATION : 00:00:01.001000000
# set the timecodescale to microsecond
./ffmpeg -f lavfi -i "testsrc=r=30000/1001" -debug_ts -t 1 -timecodescale
1000 -y -f matroska /tmp/test_2.mkv
./ffmpeg -i /tmp/test_2.mkv
ffmpeg version N-100413-g3799e77f93 Copyright (c) 2000-2020 the FFmpeg
developers
built with gcc 10 (Debian 10.2.0-19)
configuration: --disable-optimizations
libavutil 56. 62.100 / 56. 62.100
libavcodec 58.115.102 / 58.115.102
libavformat 58. 65.100 / 58. 65.100
libavdevice 58. 11.103 / 58. 11.103
libavfilter 7. 93.100 / 7. 93.100
libswscale 5. 8.100 / 5. 8.100
libswresample 3. 8.100 / 3. 8.100
Input #0, matroska,webm, from '/tmp/test_2.mkv':
Metadata:
ENCODER : Lavf58.65.100
Duration: 00:00:01.00, start: 0.000000, bitrate: 416 kb/s
Stream #0:0: Video: mpeg4 (Simple Profile), yuv420p(tv, progressive),
320x240 [SAR 1:1 DAR 4:3], 29.97 fps, 29.97 tbr, 1000k tbn, 30k tbc
(default)
Metadata:
ENCODER : Lavc58.115.102 mpeg4
DURATION : 00:00:01.001000000
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Thierry Foucu
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