[FFmpeg-devel] [PATCH] avformat/matroskaenc: Allow customizing the time stamp precision via option

James Almer jamrial at gmail.com
Wed May 19 18:23:01 EEST 2021


On 5/19/2021 12:14 PM, Lynne wrote:
> May 19, 2021, 17:12 by dev at lynne.ee:
> 
>> May 19, 2021, 03:44 by michael.dirks at xaymar.com:
>>
>>> From: Michael Fabian 'Xaymar' Dirks <michael.dirks at xaymar.com>
>>>
>>> Adds the "timestamp_precision" option to matroskaenc, which allows users
>>> to increase the time stamp precision up to 1 nanosecond. This aids with
>>> certain demuxers being unable to detect constant rate. It is a work
>>> around fix for the problems reported in 259, 6406, 7927, 8909 and 9124.
>>>
>>> Signed-off-by: Michael Fabian 'Xaymar' Dirks <michael.dirks at xaymar.com>
>>> ---
>>>   libavformat/matroskaenc.c | 25 ++++++++++++++++++-------
>>>   1 file changed, 18 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
>>> index b4284a8778..8e2813c36e 100644
>>> --- a/libavformat/matroskaenc.c
>>> +++ b/libavformat/matroskaenc.c
>>> @@ -158,6 +158,8 @@ typedef struct MatroskaMuxContext {
>>>   int                 default_mode;
>>>   
>>>   uint32_t            segment_uid[4];
>>> +
>>> +    AVRational          time_base;
>>>   } MatroskaMuxContext;
>>>   
>>>   /** 2 bytes * 7 for EBML IDs, 7 1-byte EBML lengths, 6 1-byte uint,
>>> @@ -1816,6 +1818,7 @@ static int mkv_write_header(AVFormatContext *s)
>>>   const AVDictionaryEntry *tag;
>>>   int ret, i, version = 2;
>>>   int64_t creation_time;
>>> +    int64_t time_base = 1;
>>>   
>>>   if (mkv->mode != MODE_WEBM ||
>>>   av_dict_get(s->metadata, "stereo_mode", NULL, 0) ||
>>> @@ -1852,7 +1855,10 @@ static int mkv_write_header(AVFormatContext *s)
>>>   return ret;
>>>   pb = mkv->info.bc;
>>>   
>>> -    put_ebml_uint(pb, MATROSKA_ID_TIMECODESCALE, 1000000);
>>> +    time_base = av_rescale_q(time_base, mkv->time_base, (AVRational){1, 1000000000});
>>> +    av_log(s, AV_LOG_DEBUG, "TimestampScale is: %" PRId64 " ns\n", time_base);
>>> +    put_ebml_uint(pb, MATROSKA_ID_TIMECODESCALE, time_base);
>>> +
>>>   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)) {
>>> @@ -1885,11 +1891,11 @@ static int mkv_write_header(AVFormatContext *s)
>>>   int64_t metadata_duration = get_metadata_duration(s);
>>>   
>>>   if (s->duration > 0) {
>>> -            int64_t scaledDuration = av_rescale(s->duration, 1000, AV_TIME_BASE);
>>> +            int64_t scaledDuration = av_rescale_q(s->duration, AV_TIME_BASE_Q, mkv->time_base);
>>>   put_ebml_float(pb, MATROSKA_ID_DURATION, scaledDuration);
>>>   av_log(s, AV_LOG_DEBUG, "Write early duration from recording time = %" PRIu64 "\n", scaledDuration);
>>>   } else if (metadata_duration > 0) {
>>> -            int64_t scaledDuration = av_rescale(metadata_duration, 1000, AV_TIME_BASE);
>>> +            int64_t scaledDuration = av_rescale_q(metadata_duration, AV_TIME_BASE_Q, mkv->time_base);
>>>   put_ebml_float(pb, MATROSKA_ID_DURATION, scaledDuration);
>>>   av_log(s, AV_LOG_DEBUG, "Write early duration from metadata = %" PRIu64 "\n", scaledDuration);
>>>   } else if (s->pb->seekable & AVIO_SEEKABLE_NORMAL) {
>>> @@ -1950,12 +1956,12 @@ 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 = av_rescale_q(5000, (AVRational){1, 1000}, mkv->time_base);
>>>   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 = av_rescale_q(1000, (AVRational){1, 1000}, mkv->time_base);
>>>   if (mkv->cluster_size_limit < 0)
>>>   mkv->cluster_size_limit = 32 * 1024;
>>>   }
>>> @@ -2736,8 +2742,8 @@ static int mkv_init(struct AVFormatContext *s)
>>>   track->uid = mkv_get_uid(mkv->tracks, i, &c);
>>>   }
>>>   
>>> -        // ms precision is the de-facto standard timescale for mkv files
>>> -        avpriv_set_pts_info(st, 64, 1, 1000);
>>> +        // Use user-defined timescale.
>>> +        avpriv_set_pts_info(st, 64, mkv->time_base.num, mkv->time_base.den);
>>>   
>>>   if (st->codecpar->codec_type == AVMEDIA_TYPE_ATTACHMENT) {
>>>   if (mkv->mode == MODE_WEBM) {
>>> @@ -2757,6 +2763,10 @@ static int mkv_init(struct AVFormatContext *s)
>>>   track->track_num_size = ebml_num_size(track->track_num);
>>>   }
>>>   
>>> +    // Scale the configured cluster_time_limit.
>>> +    if (mkv->cluster_time_limit >= 0)
>>> +        mkv->cluster_time_limit = av_rescale_q(mkv->cluster_time_limit, (AVRational){1, 1000}, mkv->time_base);
>>> +
>>>   if (mkv->is_dash && nb_tracks != 1)
>>>   return AVERROR(EINVAL);
>>>   
>>> @@ -2824,6 +2834,7 @@ 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" },
>>> +    { "timestamp_precision", "Timestamp precision to use for the entire container", OFFSET(time_base), AV_OPT_TYPE_RATIONAL, { .dbl = 0.001 }, 0.000000001, 1.0, FLAGS},
>>>   { NULL },
>>>   };
>>>
>>
>> Make the default 1 ns.
>> This is 2021. I'm not accepting any compromises. Matroska has
>> dragged its heels for long enough, video players have resorted to hacks
>> to fix time stamps, and high-framerate video is still unacceptably jittery.
>>
>> We're bumping to 5.0 next release anyway, and if any users have issues,
>> they can manually set this to 1ms.
>>
> 
> Or better yet, use the timebase to decide the value.
> That way, remuxing Matroska won't break anything, and the overhead
> will be lower.

Unlike mp4, there's no per-track timebase in Matroska. You can't 
feasibly derive a timebase for the whole thing from one of them.
And no, we absolutely must not change the default from the 
recommended/defacto value defined by the spec. I worry about the amount 
of software out there that may not expect anything else. It's going to 
give us more headaches than benefits.
Those who have issues with timestamps and know what they are doing can 
set timebase to whatever they need. But people doing "ffmpeg -i input 
output" because they read a tutorial online should not start finding 
their encodes potentially not playing on their TVs or set-top boxes.

Also, this patch needs to either fix or remove the chunk in 
https://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavformat/matroskaenc.c;h=186a25d9205c4c7950563ab749febbab10f82f0d;hb=HEAD#l2321
When you request a higher time scale value like 1/1000000 it will 
constantly trigger, spam stderr, and create files with as low as one 
block per cluster.


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