[FFmpeg-devel] [PATCH] avcodec/jpeg2000dec: support of 2 fields in 1 AVPacket

[Tomas Härdin]

sön 2024-02-04 klockan 23:25 -0300 skrev James Almer:
> On 2/3/2024 7:00 AM, Tomas Härdin wrote:
> > fre 2024-02-02 klockan 16:55 +0100 skrev Jerome Martinez:
> > > Before this patch, the FFmpeg MXF parser correctly detects
> > > content
> > > with
> > > 2 fields in 1 AVPacket as e.g. interlaced 720x486 but the FFmpeg
> > > JPEG
> > > 2000 decoder reads the JPEG 2000 SIZ header without understanding
> > > that
> > > the indicated height is the height of 1 field only so overwrites
> > > the
> > > frame size info with e.g. 720x243, and also completely discards
> > > the
> > > second frame, which lead to the decoding of only half of the
> > > stored
> > > content as "progressive" 720x243 flagged interlaced.
> > 
> > Is the decoder really the right place to do this? Surely this
> > happens
> > with more codecs than just j2k. Isnt it also a parser's job to do
> > this
> > kind of stuff?
> 
> An AVParser must not split (or assemble) a packet in a form that is
> not 
> meant to be encapsulated in a container.

This is two independent packets that are concatenated. A bsf changes
the bits, right? Whereas in this case no changes in the actual data is
necessary. Parsers are typically used to split raw essence into
packets, right? This is a similar case.

A use-case for this kind of splitting could be upping EditRate to
60000/1001 (SampleRate still being 60000/1001), allowing for field-
level editing. The resulting MXF would still be SEPARATE_FIELDS. This
is mode F1 in the table that pal posted.

This ties into the work Anton is doing on interlacing I suspect.

/Tomas