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

[Niklas Haas]

On Sun, 24 Mar 2024 20:03:03 +0100 Damiano Galassi <damiog at gmail.com> wrote:
> 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.

I cannot reproduce any error with your first command line.