Re: [FFmpeg-devel] Again pre-multiplied alpha

[Nicolas George <george@nsup.org>]

Niklas Haas (HE12025-07-29):
> I think this would involve more invasive changes (i.e. adding negotiation to
> the avfilter code for this and ideally other properties) that you and I both
> agreed is out of scope of this series.

No it does not. You just need to add a flag “this filter supports
premultiplied alpha” and have the framework throw an error if the flag
is not set.

We can later use the flag for negotiation or replace it with something
else, as it is purely internal API.

> If it makes you feel better, I could add an error message to the stacking
> filters specifically, for now?

Do you really think the stacking filters are the only ones where the
problem can happen?

> You argue that premultiplied frames were an "anecdotal experimental feature"
> only up until this series, but this is flat out untrue; because this series
> does not change how files are decoded. In every scenario that you worry about
> now getting the wrong result, you would have gotten the wrong result even
> before my changes.

That is the point: you are making a change to make it work. You are
officializing support, do it properly.

The other time that would have been the right time would have been when
Paul added the premultiply filter. But unless you have a TARDIS, that is
not an option.

You want the glory? Do the work. As I pointed in the first paragraph, it
is quite easy. It certainly would have taken you less time than arguing
with me.

I do not understand why you want to use past sloppiness to justify
present sloppiness when you could treat it as low-hanging fruits.

By the way, there is another flaw that I just noticed in this series: it
does not have any transverse user documentation. Sure you added the
documentation for the options, but it also need a few paragraphs to
explain to the user what premultiplied alpha is and what to do with it.

-- 
  Nicolas George
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