Re: [FFmpeg-devel] SWS cleanup / SPI Funding Suggestion

[Niklas Haas <ffmpeg@haasn.xyz>]

On Fri, 13 Oct 2023 19:42:44 -0300 James Almer <jamrial@gmail.com> wrote:
> Anton wrote and pushed an AVFrame based API. It can surely be 
> improved/extended to use AVFrame metadata.

Yes, this is actually a good idea. This API endpoint already has the
"correct" signature, so we could definitely re-use it (and SwsContext)
instead of introducing a new header file.

But to be clear, even with this sws_scale_frame API, you currently still
need to configure the SwsContext up-front - and that is the source of
problems IMO. I think the entire family of
sws_getContext/sws_init_context/sws_setColorspaceDetails are buggy,
unmaintainable nightmares.

Starting from scratch, this context would not exist at all. All required
metadata is available in the AVFrame itself, and it's trivial to
invalidate the internal state when something changes. The function
itself should be effectively stateless, with the SwsContext serving as a
mere cache.

So maybe a good path forwards is:

1. Make sws_scale_frame explicitly ignore the configured colorspace
   details in favor of AVFrame metadata.
2. Allow using sws_scale_frame with an SwsContext that has not been
   initialized, but merely allocated.
3. Deprecate sws_scale and the old configuration API

Then SwsContext would be serving double-duty between being the
configuration struct for the legacy API, while also being a cache for
the new API, until eventually being just the latter.
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