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

From: Michael Niedermayer <michael@niedermayer.cc>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH v2 16/17] fftools/ffmpeg_filter: propagate codec yuv metadata to filters
Date: Wed, 10 Apr 2024 14:59:15 +0200
Message-ID: <20240410125915.GN6420@pb2> (raw)
In-Reply-To: <20240410122906.GB9919@haasn.xyz>

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On Wed, Apr 10, 2024 at 12:29:06PM +0200, Niklas Haas wrote:
> 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.

I have the feeling the colorspace negotiation has become a bit messy
IIRC The way it was intended to work originally was to have a
arbitrary filtergraph and then randomly simplify/merge bits of the
filtergraph before picking "colorspaces" for each part.
Then repeat this whole process a few times starting from the unsimplified
graph. Then pick the best and that would be within a constant factor
of the optimal solution for any arbitrary complex graph.

Somehow this was never implemented as things worked okesich with just
doing a single pass instead of multiple and then picking the best.

I dont know how much this applies here now but i thought bringing
up the original intended design was a good idea. Because it seems
every problem in the negotiation is solved by adjusting the single pass
steps or its orders while really it should have been multiple randomized
passes.
Maybe we never have to deal with complex enough filtergraphs where a single
pass cant be made to work well and surely this here is not a complex one
at all.
But wanted to bring this up anyway because i think many people forgot
or didnt even know the original idea was to do multiple passes so as to
handle any arbitrary complex graph well.

thx

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

If you think the mosad wants you dead since a long time then you are either
wrong or dead since a long time.

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