[FFmpeg-devel] [PATCH] libavformat/matroskaenc.c: Add option to set timecodescale
James Almer
jamrial at gmail.com
Wed Jan 13 23:21:06 EET 2021
On 1/13/2021 5:50 PM, Thierry Foucu wrote:
> 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 <jamrial at 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
That and the one immediately after it are the ones i was talking about,
yes. In any case, it seems to be a temporary value written to the output
that will afterwards be overwritten in
https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/matroskaenc.c#L2559
when writing the trailer.
The commit that added the code in mkv_write_header() is 70c1647a350. I
guess you could ensure the temp value written there is correct by
forcing the muxing process to stop without calling mkv_write_trailer(),
or commenting out the code writing the final duration value.
>
> 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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