Re: [FFmpeg-devel] [PATCH 5/7] avformat/matroskaenc: write a MaxBlockAdditionID element

[James Almer <jamrial@gmail.com>]

On 3/24/2023 8:37 AM, Anton Khirnov wrote:
> Quoting James Almer (2023-03-21 18:06:35)
>> A non zero value is mandatory for Matroska if the track has blocks with BlockAdditions.
>>
>> Signed-off-by: James Almer <jamrial@gmail.com>
>> ---
>>   libavformat/matroskaenc.c | 36 ++++++++++++++++++++++++++++++++----
>>   1 file changed, 32 insertions(+), 4 deletions(-)
>>
>> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
>> index 46f4331a18..0687d9c32e 100644
>> --- a/libavformat/matroskaenc.c
>> +++ b/libavformat/matroskaenc.c
>> @@ -188,6 +188,8 @@ typedef struct mkv_track {
>>       int64_t         last_timestamp;
>>       int64_t         duration;
>>       int64_t         duration_offset;
>> +    uint64_t        max_blockaddid;
>> +    int64_t         blockadditionmapping_offset;
>>       int             codecpriv_offset;
>>       unsigned        codecpriv_size;     ///< size reserved for CodecPrivate excluding header+length field
>>       int64_t         ts_offset;
>> @@ -1597,12 +1599,21 @@ static int mkv_write_stereo_mode(AVFormatContext *s, EbmlWriter *writer,
>>       return 0;
>>   }
>>   
>> -static void mkv_write_dovi(AVFormatContext *s, AVIOContext *pb, AVStream *st)
>> +static void mkv_write_blockadditionmapping(AVFormatContext *s, MatroskaMuxContext *mkv,
>> +                                           AVIOContext *pb, mkv_track *track, AVStream *st)
>>   {
>>   #if CONFIG_MATROSKA_MUXER
>>       AVDOVIDecoderConfigurationRecord *dovi = (AVDOVIDecoderConfigurationRecord *)
>>                                                av_stream_get_side_data(st, AV_PKT_DATA_DOVI_CONF, NULL);
>>   
>> +    if (IS_SEEKABLE(s->pb, mkv)) {
>> +        track->blockadditionmapping_offset = avio_tell(pb);
>> +        // We can't know at this point if there will be a block with BlockAdditions, so
>> +        // we either write the default value here, or a void element. Either of them will
>> +        // be overwritten when finishing the track.
> 
> IIRC matroska uints are variable-size, so the value you write later may
> have a longer representation than the dummy placeholder you're writing
> here.
> Or am I wrong?

It might if we were to write any arbitrary value passed to the muxer in 
AV_PKT_DATA_MATROSKA_BLOCKADDITIONAL side data, but we will not. We're 
only going to write 1 (opaque, codec specific, currently used for alpha 
channel) at this point in the patchset, or 4 (itu-t t35) after patch 
6/7, for now.
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