Re: [FFmpeg-devel] [PATCH] avfilter: add alphamerge_cuda filter

[Jorge Estrada <jestrada.list@gmail.com>]

Thanks for the pointer. I'll submit a patch for vf_alphamerge to use
refcounting. Tested it locally and it seems like the refounting approach
works with CUDA inputs as well so there's no need for the CUDA kernel here.
Would it be preferable to add the CUDA format to the supported input
formats in alphamerge or keep anything CUDA related separated into
alphamerge_cuda which would basically be doing the same thing?

On Thu, Aug 7, 2025 at 3:15 AM Nicolas George <george@nsup.org> wrote:

> Jorge Estrada (HE12025-08-06):
> > This patch adds the alphamerge_cuda video filter
>
> Thanks, but sorry, no: this is wrong, although not by your fault.
>
> Your filter supports only planar formats. Changing a whole plane in a
> planar format should involve a little reference counting gymnastics and
> updating a pointer, no data copy.
>
> See <https://code.ffmpeg.org/FFmpeg/FFmpeg/issues/20153>:
>
> # `vf_alphamerge` was added in July 2012 (82ecae8a70), it used memcpy() to
> # replace the alpha plane of the frame, that was later (d6b9f2b7da)
> # updated to use av_image_copy_plane().
> #
> # But in March 2013 (c4e8821732), we gained the ability to share plane
> # data between multiple frames, thanks to refcounted buffers.
> # `vf_alphamerge` could have used that ability, but it was never done,
> # probably because `vf_alphamerge` came from the FFmpeg side while the
> # refcounting came from the libav side.
> #
> # `vf_alphamerge` needs to be updated to only create a new reference to
> # the alpha plane of the input frame and use it in the output frame. No
> # data copy involved at all.
>
> The same probably applies to vf_extractplanes.c, although the excuse of
> having been added before refcounting is not valid for this one.
>
> These are rather low-hanging fruits.
>
> Regards,
>
> --
>   Nicolas George
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