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Alpha blending using affine transition

jstarzak
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Hello everyone,

I have the following problem: I am trying to use Kdenlive to blend a couple of videos together (QT-RLE mov-files with alpha). I have a single "no-transparency" clip on my bottom track and then I overlay additional clips onto it using the affine transition connected to the bottom track. This works fairly well, however I am seeing some alpha-blending issues, that seem like Kdenlive is interpreting my source clips wrong. Here's an example:

Image

What shouldn't appear in my composite (in my opinion) are the black outlines around the clouds. My clips are using premultiplied alpha (against black), which is an industry-standard procedure. Should I use some different technique than an affine transition? I don't see any option to set "Alpha interpretation" when it comes to the Affine transition. What do you recommend?

I have made a couple of searches against google, this forum and also mlt-devel mailing list, however no such issue was ever raised.

Thank you for your help and time.
User avatar TheDiveO
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You've hit a weak spot in MLT. I've run into a similar situation with a much simpler setup: just a title screen that also contains alpha values other than 0.0 and 1.0 at the antialiased edges of text. To my understanding the problem is that the Affine transition does not offer the usual alpha operations.

There is OVER, OR, XOR, and AND, but these are bitwise operation on the alpha values ... which really does not make any sense except for alpha mattes using only alpha 0 and 255. I don't remember if the alpha op is for the composite transition only.

At least I now know that I'm not the only person missing proper alpha op support for affine and composite transitions.


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