Re: [FFmpeg-devel] [PATCH] avformat/hlsenc: fix CODECS attribute of H.264

["\"zhilizhao(赵志立)\"" <quinkblack@foxmail.com>]

> On May 30, 2023, at 12:44, Lance Wang <lance.lmwang@gmail.com> wrote:
> 
> On Tue, May 30, 2023 at 11:09 AM "zhilizhao(赵志立)" <quinkblack@foxmail.com>
> wrote:
> 
>> 
>> 
>>> On May 30, 2023, at 09:03, Lance Wang <lance.lmwang@gmail.com> wrote:
>>> 
>>> On Mon, May 29, 2023 at 10:16 PM Zhao Zhili <quinkblack@foxmail.com>
>> wrote:
>>> 
>>>> From: Zhao Zhili <zhilizhao@tencent.com>
>>>> 
>>>> Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
>>>> ---
>>>> libavformat/hlsenc.c | 14 ++++++++++++--
>>>> 1 file changed, 12 insertions(+), 2 deletions(-)
>>>> 
>>>> diff --git a/libavformat/hlsenc.c b/libavformat/hlsenc.c
>>>> index 871afb571b..1e0848ce3d 100644
>>>> --- a/libavformat/hlsenc.c
>>>> +++ b/libavformat/hlsenc.c
>>>> @@ -355,9 +355,19 @@ static void write_codec_attr(AVStream *st,
>>>> VariantStream *vs)
>>>> 
>>>>    if (st->codecpar->codec_id == AV_CODEC_ID_H264) {
>>>>        uint8_t *data = st->codecpar->extradata;
>>>> -        if (data && (data[0] | data[1] | data[2]) == 0 && data[3] == 1
>> &&
>>>> (data[4] & 0x1F) == 7) {
>>>> +        if (data) {
>>>> +            const uint8_t *p;
>>>> +
>>>> +            if (AV_RB32(data) == 0x01 && (data[4] & 0x1F) == 7)
>>>> +                p = &data[5];
>>>> +            else if (AV_RB24(data) == 0x01 && (data[3] & 0x1F) == 7)
>>>> +                p = &data[4];
>>>> +            else if (data[0] == 0x01)  /* avcC */
>>>> +                p = &data[1];
>>>> +            else
>>>> +                goto fail;
>>>> 
>>> 
>>> how to reproduce the issue? I recall mpegts is annex b format and sps/pps
>>> start code is 4 byte always.
>> 
>> For example:
>> 
>> ffmpeg -i foo.mp4 -c copy -hls_playlist_type vod -master_pl_name bar.m3u8
>> test.m3u8
>> 
>> 1. The input of hls muxer can be avcc
>> 
>> 2. The output of hls muxer can be avcc too, with fmp4 segments.
>> 
>> 
> Yes, this is the case. I had to add mp4toannexb bsf after the copy for avcc
> input.  If we
> don't add the format conversion, hevc may  have the same issue I guess.

Use mp4toannexb for TS segments is a good practice, but not for
fmp4.

No comments on hevc.

> Also we should
> check the size of extradata.

extradata has a AV_INPUT_BUFFER_PADDING_SIZE, so not necessary.

> 
> 
> 
>> 3. Start code of SPS/PPS should be 0 0 0 1, however, 0 0 1 exist in wild.
>> ff_isom_write_avcc() does the same:
>> 
>>    /* check for H.264 start code */
>>    if (AV_RB32(data) != 0x00000001 &&
>>        AV_RB24(data) != 0x000001) {
>>        avio_write(pb, data, len);
>>        return 0;
>>    }
>> 
>> ff_isom_write_avcc() can be used here, and get/create CODECS attribute can
>> be
>> shared by multiple muxers. I won't go that further by now.
>> 
>> 
> The next NALU checking is for SPS only, so one more byte is zero_byte. It's
> required by specs:
> 
> zero_byte is a single byte equal to 0x00.
> 
> When any of the following conditions are true, the zero_byte syntax element
> shall be present:
> 
>   -
> 
>   –  the nal_unit_type within the nal_unit( ) is equal to 7 (sequence
>   parameter set) or 8 (picture parameter set),
>   -
> 
>   –  the byte stream NAL unit syntax structure contains the first NAL unit
>   of an access unit in decoding order, as specified in clause 7.4.1.2.3.

Yes, but it’s not the first time people don’t follow spec.


> 
> 
> 
>> 
>>> 
>>> 
>>>>            snprintf(attr, sizeof(attr),
>>>> -                     "avc1.%02x%02x%02x", data[5], data[6], data[7]);
>>>> +                     "avc1.%02x%02x%02x", p[0], p[1], p[2]);
>>>>        } else {
>>>>            goto fail;
>>>>        }
>>>> --
>>>> 2.25.1
>>>> 
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