Thierry Foucu:
> On Fri, Jun 4, 2021 at 10:18 PM Andreas Rheinhardt <
> andreas.rheinha...@outlook.com> wrote:
>
>> Michael Fabian 'Xaymar' Dirks:
>>> On 2021-05-24 22:15, Andreas Rheinhardt wrote:
>>>> michael.di...@xaymar.com:
>>>>> From: Michael Fabian 'Xaymar' Dirks <michael.di...@xaymar.com>
>>>>>
>>>>> Adds "timestamp_precision" to the available options for Matroska
>> muxing.
>>>>> The option enables users and developers to change the precision of the
>>>>> time stamps in the Matroska container up to 1 nanosecond, which can aid
>>>>> with the proper detection of constant and variable rate content.
>>>>>
>>>>> Work-around fix for: 259, 6406, 7927, 8909 and 9124.
>>>>>
>>>>> Signed-off-by: Michael Fabian 'Xaymar' Dirks <michael.di...@xaymar.com
>>>
>>>>> ---
>>>>> doc/muxers.texi | 8 ++++++++
>>>>> libavformat/matroskaenc.c | 33 ++++++++++++++++++++++++++-------
>>>>> 2 files changed, 34 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/doc/muxers.texi b/doc/muxers.texi
>>>>> index e1c6ad0829..8655be94ff 100644
>>>>> --- a/doc/muxers.texi
>>>>> +++ b/doc/muxers.texi
>>>>> @@ -1583,6 +1583,14 @@ bitmap is stored bottom-up. Note that this
>>>>> option does not flip the bitmap
>>>>> which has to be done manually beforehand, e.g. by using the vflip
>>>>> filter.
>>>>> Default is @var{false} and indicates bitmap is stored top down.
>>>>> +@item timestamp_precision
>>>>> +Sets the timestamp precision up to 1 nanosecond for Matroska/WebM,
>>>>> which can
>>>>> +improve detection of constant rate content in demuxers. Note that
>>>>> some poorly
>>>>> +implemented demuxers may require a timestamp precision of 1
>>>>> millisecond, so
>>>>> +increasing it past that point may result in playback issues. Higher
>>>>> precision
>>>>> +also reduces the maximum possible timestamp significantly.
>>>> This should mention that a too high precision will increase container
>>>> overhead.
>>> Good point.
>>>>
>>>>> +Default is @var{1/1000} (1 millisecond).
>>>>> +
>>>>> @end table
>>>>> @anchor{md5}
>>>>> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
>>>>> index 186a25d920..1b911a648c 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,
>>>>> @@ -1814,6 +1816,7 @@ 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) ||
>>>>> @@ -1850,7 +1853,10 @@ 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);
>>>> There is a serious problem here: mkv->time_base is the time base that
>>>> the muxer uses; yet if mkv->time_base is not a multiple of 1/1000000000,
>>>> then av_rescale_q will have to round a bit and then the demuxer will
>>>> read a different time base, leading to a drift of all timestamps. This
>>>> is not acceptable.
>>> This issue is already present in the current version of FFmpeg.
>>
>> Matroska's timestamp imprecision currently lead to jitter, but not to
>> drift (this of course presumes that one actually uses the timestamps
>> as-is (instead of summing the durations)). But your patch as-is can lead
>> to drift (when using a wrong timebase), because it adds a whole new type
>> of error: One in which the demuxer and the muxer disagree about the
>> timebase that is in use. Consider a user using 1/750000000 as timebase.
>> 48kHz audio (with a timebase of 1/48000) can be converted accurately to
>> it, so that the muxer receives precise timestamps. Yet the muxer
>> actually writes that the timebase is 1/1000000000 and that is what a
>> demuxer sees. This means that all timestamps (except those for which
>> Matroska uses "absolute timestamps" (Matroska-speak for a time/duration
>> that is always in 1/1000000000 like default duration)) are skewed as-if
>> multiplied by 3/4.
>>
>>> Unfortunately even if you tell me this, this is not something I can
>>> change: Matroska uses nanosecond precision, and nobody has agreed on
>>> what the way forward for timestamps is. You will have to bring up such
>>> nanosecond-precision problems with the Matroska specification
>>> maintainers itself, which are currently seeking IETF standardization:
>>> https://github.com/ietf-wg-cellar/matroska-specification/issues/422
>>>>
>>
>> Already did so (I am mkver).
>>
>>>>> +
>>>>> 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)) {
>>>>> @@ -1883,11 +1889,11 @@ 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) {
>>>>> @@ -1948,12 +1954,12 @@ 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);
>>>> There are three places where you rescale this; it would be better if you
>>>> did it unconditionally in one place (namely after this block here).
>>>>
>>> I did not want to cut into existing code too much, merely adjust it to
>>> the change without changing the main parts.
>>>>> if (mkv->cluster_size_limit < 0)
>>>>> mkv->cluster_size_limit = 32 * 1024;
>>>>> }
>>>>> @@ -2713,6 +2719,14 @@ static int mkv_init(struct AVFormatContext *s)
>>>>> } else
>>>>> mkv->mode = MODE_MATROSKAv2;
>>>>> + // WebM requires a timestamp precision of 1ms.
>>>>> + if (mkv->mode == MODE_WEBM) {
>>>>> + if (av_cmp_q(mkv->time_base, (AVRational){1, 1000}) != 0) {
>>>>> + av_log(s, AV_LOG_ERROR, "WebM requires 1ms timestamp
>>>>> precision\n");
>>>>> + return AVERROR(EINVAL);
>>>>> + }
>>>> (This could be put into the same block as the one just above that sets
>>>> the mode.)
>>> Fair point.
>>>>> + }
>>>>> +
>>>>> mkv->cur_audio_pkt = av_packet_alloc();
>>>>> if (!mkv->cur_audio_pkt)
>>>>> return AVERROR(ENOMEM);
>>>>> @@ -2738,8 +2752,8 @@ 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) {
>>>>> @@ -2759,6 +2773,10 @@ 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);
>>>>> @@ -2826,6 +2844,7 @@ 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, INT_MAX, FLAGS},
>>>> Using a default value that is a valid value makes it impossible to
>>>> distinguish whether the user set a value at all; but I'd like to
>>>> preserve that possibility.
>>> Sorry, could you explain why you want to do this further? I see no code
>>> path that would even need to check if the user/caller has set precision,
>>> if it's left at default we get the default of 1/1000 - just as previous
>>> FFmpeg versions.
>>>>
>>
>> The default value of 1/1000 is good for most use-cases, but there might
>> be scenarios where it is not (e.g. TrueHD packets have a duration < 1ms,
>> so different packets will end up with the same timestamp when using
>> 1/1000). If the user has set a timestamp precision, then that should be
>> honoured; but if not, then we might use a different value than 1/1000
>> (in the future; I know that currently no code path that makes use of
>> this exists).
>>
>>
> Is there any problem to submit this flag while keeping the default value to
> millisecond. So users of matroska muxer who want to increase the precision
> can still do it.
> And the discussion of which default value should be could be done in
> another patch?
>
There is no such problem in general; yet there is a problem with this
patch, namely that it allows to choose timebases which are not a
multiple of the basic Matroska timebase of 1/1000000000. See above.
- Andreas
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