[FFmpeg-devel] How to satisfy two codecs with one decoder?

[Carl Eugen Hoyos]

Am Sa., 18. Apr. 2020 um 00:46 Uhr schrieb Roger Pack <rogerdpack2 at gmail.com>:
>
> I want to add to the closed caption libavcodec/ccaption_dec.c decoder
> to be able to handle both 608 "over CEA 708" (which it already does)
> and also "raw EIA 608 byte pairs" which it doesn't.
>
> My idea was to introduce a new codec id for the raw 608 pairs.
> AV_CODEC_ID_EIA_RAW_608 or the like.
>
> The decoding shares a lot of functionality.
>
> Currently the decoder code is set up like this:
>
> AVCodec ff_ccaption_decoder = {
> .name = "cc_dec",
> .long_name = NULL_IF_CONFIG_SMALL("Closed Caption (EIA-608 / CEA-708)"),
> .type = AVMEDIA_TYPE_SUBTITLE,
> .id = AV_CODEC_ID_EIA_608,
> ...
>
> I was hoping to "add" another codec like AV_CODEC_ID_EIA_RAW_608 and
> having this decoder be able to decode both types.
>
> I was anticipating being able to define some kind of "query" method so
> it would check if the requested codec id was "either
> AV_CODEC_ID_EIA_608 or AV_CODEC_ID_EIA_RAW_608" and accept decoding
> either codec.
>
> Is there a way for a codec to decode two id's like this?
> If not any recommendations on how to best handle this?

I don't know how relevant this is but the mjpeg decoder also
decodes ljpeg.

Carl Eugen