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

[Tomas Härdin]

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

> 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.

/Tomas