[FFmpeg-devel] [PATCH] avformat/mov: parse the last moof box when mp4 segment format
wangyaqiang
1035567130 at qq.com
Fri Sep 30 12:50:48 EEST 2022
> 2022年9月29日 16:29，zhilizhao(赵志立) <quinkblack at foxmail.com <mailto:quinkblack at foxmail.com>> 写道：
>
>
>
>> On Sep 1, 2022, at 18:22, 1035567130 at qq.com <mailto:1035567130 at qq.com> wrote:
>>
>> From: Wang Yaqiang <wangyaqiang03 at kuaishou.com <mailto:wangyaqiang03 at kuaishou.com>>
>>
>> In the format of mp4 segment, the bitrate calculation of
>> stream depends on the sample_size in moof->traf->trun box.
>> In the original logic, when the last sidx box is read,
>> it is not parsed backwards, and the total sample_size calculation is smaller.
>> As a result, the bitrate displayed by ffprobe is also smaller than the actual.
>> Increasing the moof_count variable ensures that the last moof is parsed.
>>
>> Test method: You can use -c copy remux a fmp4 file as mp4
>> and ffprobe the two files will find that the bitrate is inconsistent
>> Befor patch:
>> Stream #0:1[0x2](und): Audio: aac (HE-AAC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 59 kb/s
>> After patch:
>> Stream #0:1[0x2](und): Audio: aac (HE-AAC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 96 kb/s (default)
>>
>> Signed-off-by: Wang Yaqiang <wangyaqiang03 at kuaishou.com <mailto:wangyaqiang03 at kuaishou.com>>
>> ---
>> libavformat/isom.h | 1 +
>> libavformat/mov.c | 8 +++++++-
>> 2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/libavformat/isom.h b/libavformat/isom.h
>> index f05c2d9c28..183a3c486b 100644
>> --- a/libavformat/isom.h
>> +++ b/libavformat/isom.h
>> @@ -296,6 +296,7 @@ typedef struct MOVContext {
>> int has_looked_for_mfra;
>> int use_tfdt;
>> MOVFragmentIndex frag_index;
>> + int moof_count; //ensure last fragment parse moof box
>> int atom_depth;
>> unsigned int aax_mode; ///< 'aax' file has been detected
>> uint8_t file_key[20];
>> diff --git a/libavformat/mov.c b/libavformat/mov.c
>> index 14550e6456..396658e342 100644
>> --- a/libavformat/mov.c
>> +++ b/libavformat/mov.c
>> @@ -7784,12 +7784,15 @@ static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom)
>> int64_t start_pos = avio_tell(pb);
>> int64_t left;
>> int err = parse(c, pb, a);
>> + if (a.type == MKTAG('m','o','o','f')) {
>> + c->moof_count ++;
>> + }
>> if (err < 0) {
>> c->atom_depth --;
>> return err;
>> }
>> if (c->found_moov && c->found_mdat && a.size <= INT64_MAX - start_pos &&
>> - ((!(pb->seekable & AVIO_SEEKABLE_NORMAL) || c->fc->flags & AVFMT_FLAG_IGNIDX || c->frag_index.complete) ||
>> + ((!(pb->seekable & AVIO_SEEKABLE_NORMAL) || c->fc->flags & AVFMT_FLAG_IGNIDX || (c->frag_index.complete && c->moof_count >= c->frag_index.nb_items)) ||
>> start_pos + a.size == avio_size(pb))) {
>> if (!(pb->seekable & AVIO_SEEKABLE_NORMAL) || c->fc->flags & AVFMT_FLAG_IGNIDX || c->frag_index.complete)
>> c->next_root_atom = start_pos + a.size;
>
> No, it breaks the use case with global sidx. We can achieve fast
> startup with global sidx.
This patch indeed have an effect on startup, will add some option.
>
> The patch describes an issue with multiple sidx cases, actually
> it’s more serious with global sidx. In the first case, the bitrate
> is a little inaccurate. Bitrate is near zero for the second case.
>
> I have two ideas:
>
> 1. Just skip bitrate calculation from sc->data_size if sidx exist,
> e.g.,
>
> @@ -8500,7 +8500,7 @@ static int mov_read_header(AVFormatContext *s)
> for (i = 0; i < s->nb_streams; i++) {
> AVStream *st = s->streams[i];
> MOVStreamContext *sc = st->priv_data;
> - if (st->duration > 0) {
> + if (st->duration > 0 && !sc->has_sidx) {
> /* Akin to sc->data_size * 8 * sc->time_scale / st->duration but accounting for overflows. */
> st->codecpar->bit_rate = av_rescale(sc->data_size, ((int64_t) sc->time_scale) * 8, st->duration);
> if (st->codecpar->bit_rate == INT64_MIN) {
>
> It’s simple, and bitrate information has multiple sources.
>
In this way, will be no bitrate info in the stream, will affect the users？
> 2. Add a option to prevent mark complete when reading sidx
>
> @@ -5446,7 +5446,7 @@ static int mov_read_sidx(MOVContext *c, AVIOContext *pb, MOVAtom atom)
> // See if the remaining bytes are just an mfra which we can ignore.
> is_complete = offset == stream_size;
> - if (!is_complete && (pb->seekable & AVIO_SEEKABLE_NORMAL) && stream_size > 0 ) {
> + if (!is_complete && (pb->seekable & AVIO_SEEKABLE_NORMAL) && stream_size > 0 && !parse_full) {
>
> Then
Mfra box is not required in fmp4. for example, adding skip_trailer parameter when generate fm4, bitrate will still be incorrect.
>
> @@ -8500,7 +8500,7 @@ static int mov_read_header(AVFormatContext *s)
> for (i = 0; i < s->nb_streams; i++) {
> AVStream *st = s->streams[i];
> MOVStreamContext *sc = st->priv_data;
> - if (st->duration > 0) {
> + if (st->duration > 0 && (!sc->has_sidx || parse_full)) {
>
> It’s such a corner case and I can’t find a suitable name for this option.
>
> Any ideas?
>
>> @@ -8361,6 +8364,9 @@ static int mov_read_header(AVFormatContext *s)
>> av_log(s, AV_LOG_ERROR, "moov atom not found\n");
>> return AVERROR_INVALIDDATA;
>> }
>> + if (mov->frag_index.nb_items > mov->moof_count) {
>> + av_log(s, AV_LOG_WARNING, "the number of moof is less then fragment count\n");
>> + }
>> av_log(mov->fc, AV_LOG_TRACE, "on_parse_exit_offset=%"PRId64"\n", avio_tell(pb));
>>
>> if (pb->seekable & AVIO_SEEKABLE_NORMAL) {
>> --
>> 2.33.0
>>
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