[FFmpeg-devel] [PATCH 04/12] avformat/mxfdec: compute both essence_offset and essence_length in mxf_compute_essence_containers

[Marton Balint]

On Wed, 13 Jun 2018, Tomas Härdin wrote:

> sön 2018-06-10 klockan 12:36 +0200 skrev Marton Balint:
>> Also compute the correct essence_offset and essence_length for all clip wrapped
>> essences.
>> 
>> > Signed-off-by: Marton Balint <cus at passwd.hu>
>> ---
>>  libavformat/mxfdec.c | 108 +++++++++++++++++++++++++++------------------------
>>  1 file changed, 57 insertions(+), 51 deletions(-)
>> 
>> diff --git a/libavformat/mxfdec.c b/libavformat/mxfdec.c
>> index 1ab34f4873..67b0028e88 100644
>> --- a/libavformat/mxfdec.c
>> +++ b/libavformat/mxfdec.c
>> @@ -98,6 +98,7 @@ typedef struct MXFPartition {
>>      int pack_length;
>>      int64_t pack_ofs;               ///< absolute offset of pack in file, including run-in
>>      int64_t body_offset;
>> +    KLVPacket first_essence_klv;
>>  } MXFPartition;
>>  
>>  typedef struct MXFCryptoContext {
>> @@ -2782,47 +2783,76 @@ static int mxf_parse_handle_partition_or_eof(MXFContext *mxf)
>>      return mxf->parsing_backward ? mxf_seek_to_previous_partition(mxf) : 1;
>>  }
>>  
>> +static int64_t round_to_kag(int64_t position, int kag_size)
>> +{
>> +    /* TODO: account for run-in? the spec isn't clear whether KAG should account for it */
>> +    /* NOTE: kag_size may be any integer between 1 - 2^10 */
>> +    int64_t ret = (position / kag_size) * kag_size;
>> +    return ret == position ? ret : ret + kag_size;
>
> This should probably just be
>
> return ((position + kag_size - 1) / kag_size) * kag_size;
>
> but maybe there's some fine point I'm overlooking. What happens if
> kag_size is a smallish value like 16?
>
> A check for position > INT64_MAX - kag_size might also be appropriate.

Well, this is your code as well :), I just moved it to avoid a forward 
declaration.

>
>> +}
>> +
>> +static MXFWrappingScheme mxf_get_wrapping_by_body_sid(AVFormatContext *s, int body_sid)
>> +{
>> +    for (int i = 0; i < s->nb_streams; i++) {
>> +        MXFTrack *track = s->streams[i]->priv_data;
>> +        if (track && track->body_sid == body_sid && track->wrapping != UnknownWrapped)
>> +            return track->wrapping;
>> +    }
>> +    return UnknownWrapped;
>> +}
>> +
>>  /**
>>   * Figures out the proper offset and length of the essence container in each partition
>>   */
>> -static void mxf_compute_essence_containers(MXFContext *mxf)
>> +static void mxf_compute_essence_containers(AVFormatContext *s)
>>  {
>> +    MXFContext *mxf = s->priv_data;
>>      int x;
>>  
>> -    /* everything is already correct */
>> -    if (mxf->op == OPAtom)
>> -        return;
>> -
>>      for (x = 0; x < mxf->partitions_count; x++) {
>>          MXFPartition *p = &mxf->partitions[x];
>> +        MXFWrappingScheme wrapping;
>>  
>>          if (!p->body_sid)
>>              continue;       /* BodySID == 0 -> no essence */
>>  
>> -        if (x >= mxf->partitions_count - 1)
>> -            break;          /* FooterPartition - can't compute length (and we don't need to) */
>> +        /* for non clip-wrapped essences we compute essence_offset
>> +         * for clip wrapped essences we point essence_offset after the KL (usually klv.offset + 20 or 25)
>> +         */
>>  
>> -        /* essence container spans to the next partition */
>> -        p->essence_length = mxf->partitions[x+1].this_partition - p->essence_offset;
>> +        wrapping = (mxf->op == OPAtom) ? ClipWrapped : mxf_get_wrapping_by_body_sid(s, p->body_sid);
>>  
>> -        if (p->essence_length < 0) {
>> -            /* next ThisPartition < essence_offset */
>> -            p->essence_length = 0;
>> -            av_log(mxf->fc, AV_LOG_ERROR,
>> -                   "partition %i: bad ThisPartition = %"PRIX64"\n",
>> -                   x+1, mxf->partitions[x+1].this_partition);
>> +        if (wrapping == ClipWrapped) {
>> +            p->essence_offset = p->first_essence_klv.next_klv - p->first_essence_klv.length;
>> +            p->essence_length = p->first_essence_klv.length;
>
> Much nicer
>
>> +        } else {
>> +            int64_t op1a_essence_offset =
>> +                p->this_partition +
>> +                round_to_kag(p->pack_length,       p->kag_size) +
>> +                round_to_kag(p->header_byte_count, p->kag_size) +
>> +                round_to_kag(p->index_byte_count,  p->kag_size);
>
> Is this really always the case? I guess with OP1a it isn't a huge
> concern since the demuxer will find the next essence packet anyway. But
> still..
>
> I'm also fairly sure this is my code originally so.. :)

I think this tends to work well, if kag_size is not guessed. However, if 
it _is_ guessed, then there might be problems, and having a 0 kagsize is 
valid (only means an unknown KAG). So I am inclined to simply 
use p->first_essence_klv.offset unconditionally, as you suggested in

http://ffmpeg.org/pipermail/ffmpeg-devel/2018-February/225629.html

See the attached patch.

Regards,
Marton
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