Re: [FFmpeg-devel] [PATCH] lavfi: protection against premultiplied alpha (was: The patch series about premultiplied alpha)

From: Nicolas George <george@nsup.org>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH] lavfi: protection against premultiplied alpha (was: The patch series about premultiplied alpha)
Date: Thu, 14 Aug 2025 16:00:12 +0200
Message-ID: <aJ3r7KXGa46jJNqx@phare.normalesup.org> (raw)
In-Reply-To: <20250813162538.GC676537@haasn.xyz>

Niklas Haas (HE12025-08-13):
> Updated https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20031 with full negotiation
> for the alpha mode. I also went ahead and fixed the drawutils filters to support
> premultiplied alpha, since it was low-hanging fruit.

Nice. I must confess, I was not paying enough attention when the new
fields were added to the negotiation. I will need a little time to
re-familiarize myself with the code.

The new patch series is not arrived on the mailing-list, apparently the
link is random. I tried reviewing on the web thing, it is way too
inconvenient, especially for jumping between related parts of the code.
Please send it to the mailing-list.

> Now all the filters you were concerned about should be protected from receiving
> premultiplied alpha, at filter graph configuration time.

Uh… no, absolutely not. Filters using drawutils were only an obvious
example, with drawbox a very easy test case, but the list I gave was
never supposed to be exhaustive, it was only meant to explain the issue
to you. There are a few other obvious examples and certainly several
less obvious cases.

Furthermore, it would not be reasonable to demand that the authors of
future filters implement it or even think about it.

Which is why the default, i.e. the alpha format list set by the famework
if the filter did not set it, must not include premultiplied.

Note that it is not a sudden random requirement from me. We did it the
same way when we added support for unknown channel layouts while keeping
API compatibility with the forks: filters that had not been explicitly
vetted for support were assumed to not support them. Now like before, it
is the only reasonable way to do things.

-- 
  Nicolas George
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