Re: [FFmpeg-devel] [PATCH] MXF - Add jpeg2000 subdescriptor - Sponsored by INA

["Cédric Le Barz" <clebarz@ektacom.com>]

Le 04/06/2023 à 20:24, Tomas Härdin a écrit :
> tor 2023-06-01 klockan 17:19 +0200 skrev Cédric Le Barz:
>> Attach to this mail, is my new patch for adding jpeg2000 sub-
>> descriptor
>> in MXF file  taking into account remarks from FFmpeg community
>> (remarks
>> from Pierre-Anthony above as well as this from Tomas), i.e. :
>>
>> 1 - When there is a mismatch in components number, this is now
>> process
>> as an error.
>>
>> 2 - When defining and using the J2K descriptor items, I now refer to
>> the
>> symbol name from the SMPTE registers (X0siz, XT0siz...)
>>
>> 3 - As suggested, I make use of  jpeg2000_read_main_headers() in
>> libavcodec. But, I let the parsing function in mxfenc.c file as all
>> other parsing functions are in this file (mxf_parse_mpeg2_frame,
>> mxf_parse_h264_frame...). The use of jpeg2000_read_main_headers() in
>> mxfenc implies some minor modifications in jpeg2000 files :
>>
>> * rename of Jpeg2000Tile structure to J2kTile in j2kenc.c (to avoid
>> redefinition)
>>
>> * move some structure declarations from jpeg2000dec.c to jpeg2000.h
>>
>> * make jpeg2000_read_main_headers() function "public"
> For this you need to prefix the name with ff_ and bump libavcodec's
> minor version number
>
>> +static int mxf_parse_jpeg2000_frame(AVFormatContext *s, AVStream
>> *st, AVPacket *pkt)
>> +{
>> +    MXFContext *mxf = s->priv_data;
>> +    MXFStreamContext *sc = st->priv_data;
>> +    int component_count = av_pix_fmt_count_planes(st->codecpar-
>>> format);
>> +    Jpeg2000DecoderContext jpeg2000ctx;
> This makes sizeof(Jpeg2000DecoderContext) part of the public API which
> is a big no-no. Consider making the fields part of a different smaller
> struct instead that is also included in Jpeg2000DecoderContext. The
> safest option is to have a function that allocates that struct. The
> reason for this is because we might want to read other things from the
> headers at a later date. Another possibility is to pass a bunch of
> pointers to ints that the parsed values get written to. If at a later
> date we need to parse more fields then we can introduce
> ff_jpeg2000_read_main_headers_2() and so on. The latter seems easier
> API stability wise, if a bit tedious with the number of function
> arguments.
>
> /Tomas
>
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Hi,

I'm facing a problem to get the jpeg2000 information I need to build the 
mxf file.

All the functions to extract jpeg2000 information exist : they are 
located in libavcodec side and used Jpeg2000DecoderContext, which is not 
obviously public and therefore unknown from libavformat side.

Is there a way to get a pointer on the Jpeg2000DecoderContext (at least 
a void*) from libavcodec side ?

Thanks for your help.

Cédric



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