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

[Damiano Galassi]

On Sun, Mar 24, 2024 at 6:53 PM Niklas Haas <ffmpeg at haasn.xyz> wrote:

> But wait - aren't all filter's lists set to the same reference? Isn't that
> the
> point of the design? If they share the same format list, they will all
> inherit the correct setting (via pick_format). Except for filters like
> vf_scale which deliberately use separate format lists, since they can
> perform conversion. But even in this case,
> swap_color_spaces_on_filters() should ensure that both lists are set to
> the same value IFF they support the same color spaces.
>
> Upon further inspection, it actually appears like the propagation you
> propose in this patch is only needed for things which *aren't* subject
> to format negotiation, which is why width, height and sample aspect
> ratio are propagated here, but things like format or sample rates are not.
>
> So this patch as written should not be applied, I think.
>
> > What I meant was that your patch didn't make any difference from the
> > existing behavior
> > because even if it sets the default AVFilterLink values to AVCOL_SPC_NONE
> > and AVCOL_SPC_NONE,
> > when it's the time to call ff_filter_config_links(), those two values
> have
> > already been reset
> > to a default unspecified value, so you still get a wrongly configure
> graph
> > with something like:
> >
> > Buffer
> >     Link:  AVCOL_SPC_BT709 AVCOL_RANGE_MPEG
> > Scale
> >     Link: AVCOL_SPC_UNSPECIFIED AVCOL_RANGE_UNSPECIFIED
> > Whateverfilter
> >     Link: AVCOL_SPC_UNSPECIFIED AVCOL_RANGE_UNSPECIFIED
> > Buffersink
> >
> > instead of
> >
> > Buffer
> >     Link: AVCOL_SPC_BT709 AVCOL_RANGE_MPEG
> > Scale
> >     Link: AVCOL_SPC_BT709 AVCOL_RANGE_MPEG
> > Whateverfilter
> >     Link: AVCOL_SPC_BT709 AVCOL_RANGE_MPEG
> > Buffersink
>
> Perhaps you can describe this scenario more thoroughly? What filters are
> you using downstream of 'scale' here, and does it advertise BT709 as
> supported?
>

It's entirely possible that I am completely wrong, I took a look at
avfilter code
for the first time just a few days ago.

Here's two way to reproduce with the cli something similar to what I am
seeing
when using the API directly:

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.