[FFmpeg-devel] [PATCH] swscale: set sane alpha blending defaults

wm4 nfxjfg at googlemail.com
Sun Aug 30 20:22:50 CEST 2015

On Sun, 30 Aug 2015 18:09:05 +0000 (UTC)
Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:

> wm4 <nfxjfg <at> googlemail.com> writes:
> > Change the default to blend with black, which 
> > gives generally expected results.
> Given that this introduces a speed regression, is 
> rarely needed and it is immediately visible that 
> the function is needed, I don't think this is a 
> sensible change.

Performance is no argument in this case. Alpha video is rare in the
first place, so it won't affect most ffmpeg uses. Also, you can assume
the alpha channel exists for a reason.

As for speed, the user can always disable alpha blending. I'd rather
prefer sane, correct default behavior, over fast and broken.

> Note that contrary to visual issue, the performance 
> regression is not immediately visible.

It's the other way around. Did you even look at the test video in the
issue tracker? Judging whether the result is correct requires manual
checking of the whole video. On the other hand, libswscale is an
obvious thing to check when the conversion seems to be going too
slowly. libswscale is known not to be the fastest, especially with
"obscure" pixel formats.

> And "black" may be sensible for prores, in all 
> other cases, it is either suboptimal or bad.

Such as? Name all the counter examples you know. I'm curious.

But side from that, blending with _any_ color is still more reasonable
than discarding the alpha.

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