Re: [FFmpeg-devel] [PATCH v2] avfilter: propagate colorspace and color_range from buffer filter and between AVFilterLink.

[Niklas Haas <ffmpeg@haasn.xyz>]

On Mon, 25 Mar 2024 16:02:23 +0100 Niklas Haas <ffmpeg@haasn.xyz> wrote:
> On Mon, 25 Mar 2024 15:36:26 +0100 Damiano Galassi <damiog@gmail.com> wrote:
> > On Mon, Mar 25, 2024 at 2:40 PM Niklas Haas <ffmpeg@haasn.xyz> wrote:
> > 
> > > > ffmpeg -i in.mp4 -vf
> > > > "scale='width=1920:height=1080',zscale='width=1920:height=1080'" out.mp4
> > > > ffmpeg -i in.jpg -vf "zscale='width=1920:height=1080'" out.jpg
> > > >
> > > > it doesn't need a specific mp4 or jpg file.
> > >
> > > I cannot reproduce any error with your first command line.
> > 
> > 
> >  here's a sample file, and the output I get:
> > 
> > https://subler.org/downloads/test.mp4
> > https://gist.github.com/galad87/a6ddc3318cf20cd2f1ac7d053e1a0786
> 
> Okay, I see the problem. Basically, swap_color_spaces() doesn't re-run
> after pick_formats() settles a filter's format. So swap_color_spaces()
> never actually sets zscale's preferred output format, because at that
> point there are still multiple options for swscale's output format.
> 
> This patch appears to help, but I'm honestly not sure what exactly the
> difference is between this REDUCE_FORMATS() and swap_color_spaces(), or
> actually, for what case the latter even exists.

I *think* that the former is intended for picking an exact match,
whereas the latter is intended for picking the *best* remaining match
(where bestness can be meaningfully measured by some score)

So based on this, given that csp/range is only concerned with exact
matches, after the suggest patch we can just delete swap_color_spaces()
and swap_color_ranges(). Good riddance, too.

> 
> diff --git a/libavfilter/avfiltergraph.c b/libavfilter/avfiltergraph.c
> index bb5399c55e8..3b8f7464889 100644
> --- a/libavfilter/avfiltergraph.c
> +++ b/libavfilter/avfiltergraph.c
> @@ -794,6 +794,10 @@ static int reduce_formats_on_filter(AVFilterContext *filter)
>                     nb_formats, ff_add_format);
>      REDUCE_FORMATS(int,      AVFilterFormats,        samplerates,     formats,
>                     nb_formats, ff_add_format);
> +    REDUCE_FORMATS(int,      AVFilterFormats,        color_spaces,    formats,
> +                   nb_formats, ff_add_format);
> +    REDUCE_FORMATS(int,      AVFilterFormats,        color_ranges,    formats,
> +                   nb_formats, ff_add_format);
>  
>      /* reduce channel layouts */
>      for (i = 0; i < filter->nb_inputs; i++) {
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