Re: [FFmpeg-devel] [PATCH v2 16/17] fftools/ffmpeg_filter: propagate codec yuv metadata to filters

[Niklas Haas <ffmpeg@haasn.xyz>]

On Wed, 10 Apr 2024 03:25:45 +0200 Michael Niedermayer <michael@niedermayer.cc> wrote:
> On Mon, Apr 08, 2024 at 02:57:20PM +0200, Niklas Haas wrote:
> > From: Niklas Haas <git@haasn.dev>
> > 
> > To convert between color spaces/ranges, if needed by the codec
> > properties.
> > ---
> >  fftools/ffmpeg_filter.c | 34 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 34 insertions(+)
> 
> I presume this is intended to change some cases
> iam asking because it does
> for example, this one
> -i aletrek.mkv -t 1 -bitexact  /tmp/file-aletrek-palette.mkv
> has the output file change slightly
> 
> https://trac.ffmpeg.org/attachment/ticket/5071/aletrek.mkv
> 
> also given fate does not change, it would make sense to add a testcase
> to fate that does cover this

Two notes:

1. The only difference between the `master` behavior and the new
   behavior is that the file is marked as limited range instead of as
   unspecified. However, this is the correct tagging, as the actual
   output *is* limited range.

2. While not *broken* per se, this is actually still a bug - in this
   case, since the input is basically full range, we should actually try
   and output full range by default.

The reason it doesn't output full range here is because a consequence of
the fact that format reduction happens *before* the logic in
pick_format() fixes all non-YUV links to be tagged as PC/RGB-only. So
the format reduction logic just sees that vf_scale can output any range
in this scenario (true) and picks TV range output as the default,
resulting in a conversion from the PC range input to a TV range output.

The easiest solution would be to not blindly pick the first here, but to
instead try and pick a colorspace and range matching the input (if one
exists). But this may still break in more complicated scenarios where
the dependence on the forced format spans several filters.

The more correct solution would probably be to explicitly reduce only
the format first (going through all the steps) and then negotiate
everything that depends on the format in an entirely separate step.

I'll see if I can do something about this situation, though it's
ultimately not a high priority as it's not a regression compared to the
status quo - just something that we could definitely improve.

> 
> thx
> 
> [...]
> -- 
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> 
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