[FFmpeg-devel] [PATCH] libavformat/matroskaenc.c: Add option to set timecodescale

[James Almer]

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."