[FFmpeg-devel] [PATCH 09/12] avformat/mxfdec: add support for clip wrapped essences

[Marton Balint]

On Thu, 14 Jun 2018, Tomas Härdin wrote:

> ons 2018-06-13 klockan 22:11 +0200 skrev Marton Balint:
>> 
>> On Wed, 13 Jun 2018, Tomas Härdin wrote:
>> 
>> > sön 2018-06-10 klockan 12:36 +0200 skrev Marton Balint:
>> > > Also use common code with opAtom.
>> > > 
>> > > Fixes ticket #2776.
>> > > Partially fixes ticket #5671.
>> > > Fixes ticket #5866.
>> > > 
>> > > > Signed-off-by: Marton Balint <cus at passwd.hu>
>> > > 
>> > > ---
>> > >  libavformat/mxfdec.c | 281 ++++++++++++++++++++++++---------------------------
>> > >  1 file changed, 130 insertions(+), 151 deletions(-)
>> > > 
>> > > diff --git a/libavformat/mxfdec.c b/libavformat/mxfdec.c
>> > >  
>> > > -            next_ofs = mxf_set_current_edit_unit(mxf, klv.offset);
>> > > -
>> > > -            if (next_ofs >= 0 && klv.next_klv > next_ofs) {
>> > > -                /* if this check is hit then it's possible OPAtom was treated as OP1a
>> > > -                 * truncate the packet since it's probably very large (>2 GiB is common) */
>> > > -                avpriv_request_sample(s,
>> > > -                                      "OPAtom misinterpreted as OP1a? "
>> > > -                                      "KLV for edit unit %"PRId64" extending into "
>> > > -                                      "next edit unit",
>> > > -                                      mxf->current_edit_unit);
>> > > -                klv.length = next_ofs - avio_tell(s->pb);
>> > > +            next_ofs = mxf_set_current_edit_unit(mxf, st, pos, 1);
>> > > +
>> > > +            if (track->wrapping != FrameWrapped) {
>> > > +                int64_t size;
>> > > +
>> > > +                if (next_ofs <= 0) {
>> > > +                    // If we have no way to packetize the data, then return it in chunks...
>> > > +                    int64_t max_packet_size = 33554432;
>> > 
>> > Any reason for this particular number?
>> 
>> Not really, I chose 32 MB because it is big enough to not fragment typical 
>> exotic files (e.g.: GOP wrapped, etc), but small enough so allocating 
>> this much RAM in one packet does not cause any issues if we encounter a 
>> huge KLV with some unknown (clip?) wrapping.
>
> Perhaps add a #define MXF_MAX_CHUNK_SIZE (32 << 20) or so on the top of
> the file

Sure.

>
>> To be frank, I am still not sure if this should actually work or not, or 
>> if the parser system of libavformat can make something useful out of a 
>> chunked source like this or not, I don't seem to have an MXF file which 
>> triggers this.
>
> There's no way to split it into edit units based on duration? Surely
> outputing packets of size klv.length / duration should be fine? If
> duration == 1 then surely the entire chunk should be read? It's easy
> enough imagine DPX being wrapped inside MXF, and DPX frames can easily
> be 50+ MiB. Take 8k raw 8-bit 4:4:4 video for example. 8192 x 4608 x 3
> = 108 MiB. An error could be output if klv.size % duration != 0.

That is a good idea in general, but I think it is better if we implement 
it in mxf_handle_missing_index_table_segment and generate a fake index 
table segment with a constant edit unit byte count for clip wrapped 
essences without a proper index.

Regards,
Marton