[FFmpeg-devel] [PATCH] avformat/mov: parse the last moof box when mp4 segment format
"zhilizhao(赵志立)"
quinkblack at foxmail.com
Fri Sep 30 14:47:38 EEST 2022
> On Sep 30, 2022, at 18:22, zhilizhao(赵志立) <quinkblack at foxmail.com> wrote:
>
>
>
>> On Sep 30, 2022, at 17:50, wangyaqiang <1035567130 at qq.com> wrote:
>>
>>
>>
>>> 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？
>
> It depends. We have multiple sources to get bitrate.
>
>>
>>> 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.
>
> You missed the point. If mfra doesn’t exist in the file,
> the issue doesn’t exist neither.
Sorry you are right. offset is not current file position but accumulated
value from sidx boxes.
c->frag_index.complete should be marked conditionally, if we can’t find
a better solution.
>
>>
>>>
>>> @@ -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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