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

[Jerome Martinez <jerome@mediaarea.net>]

On 03/02/2024 11:00, 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?

Both solutions have pro and con:
- Doing it in the decoder fixes the issue for all containers and only 
one codec
- Doing it in the demuxer fixes the issue for one container and all codecs
And currently I know only one container and only one codec having this 
issue.

My choice to implement in the decoder comes from the idea that it seems 
more hacky to decode 2 AVPackets (crafted from a single MXF packet), 
then do a RAM copy of the decoded (so huge) content for interleaving 
fields into 1 frame (pressure on RAM bandwidth) in the MXF demuxer + 
adapt frame metadata (height is overwritten by the decoder then need to 
overwrite again the value), doing it in the decoder seems less intrusive.

If moving to the demuxer is the only acceptable solution, I will do it 
but I would like to be sure that there is a consensus on that by FFmpeg 
developers before doing it, in order to not have this extra work 
rejected due to a future disagreement about where it should go.

> The logic that scans the packet for two SOI markers shouldn't be
> necessary if the relevant information is carried over from the MXF
> demuxer.

As far as I know there is nothing in the MXF file saying where is the 
second field in the packet, at which MXF metadata do you think?


>   It also makes the j2k decoder harder to ||ize. You might also
> need the StoredF2Offset

I don't change the field order detection by current FFmpeg code, I rely 
on FFmpeg code directly.
In my opinion if MXF field order detection needs to be changed, it would 
be in a separate patch dedicated to that (the field order detection in 
MXF) and it does not impact this patch as it is independent from which 
container is used, using AVCodecContext field order information. Jérôme
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