I'm not sure if we want to make the transitions more complicated. Pretty much
all of MLT expects straight alpha. It might be easier to modify the producer to
convert premultiplied alpha to straight alpha before passing the image along to
the rest of MLT.
________________________________
From: Jan Starzak <jan.star...@gmail.com>
To: mlt-devel@lists.sourceforge.net
Sent: Tuesday, April 15, 2014 5:22 AM
Subject: [Mlt-devel] Alpha support in Composite
Hello everyone,
I was doing a little experimenting with MLT and ran into the following problem:
both the affine and composite transitions do not seem to follow proper alpha
handling. Industry standard in video is to use premultiplied alpha, however
these two transitions seem to expect straight alpha. To use an example: I have
a few clips (QTRLE with alpha) that I want to combine using the composite
transition with a "background" clip that has all-solid alpha. What i get is
this:
http://s30.postimg.org/60jrz8qpt/alpha_black_outline.jpg
However, I should not get these black outlines around the edges. I think this
is because the mixing done by the affine and composite transitions seems to
assume, that the source is "straight alpha".
Am I doing something wrong, or is this just a weak spot in MLT? This would seem
like something that needs fixing, as this affects everything that doesn't use
fully opaque or fully transparent alpha.
I would really like to fix this, however I would like some clarification: the
composite transition works line by line and the SSE2 acceleration is only used
for "Over" (default) line compositing function, correct? So if I wanted to fix
it, what I would need to do is create a new line compositing function, right?
I hope this is not an issue that has already been covered, however I found no
such mention in the list archives.
Kind regards,
Jan Starzak
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