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maxlover
02-22-2005, 10:01 AM
May be this question is very noob but i dont know answer:), what is premultiply color, found in any footage property in combustion. color+alpha and premultiply.. I know what color and alpha is :) but what is that..premultiply color......

lightwolf
02-22-2005, 11:29 AM
Basically, the colour can be stored in two different ways in an image with an alpha channel.
Straight, or pre-multiplied.
Straight is easy. Basically, the RGB is left as it is, and the transparency is stored in the alpha channel.
Pre-multiplied means that the RGB is actually (mathematically) multiplied by the value of the alpha channel.
This basically makes it easier to layer during compositing, less calculations.
Mathematically:
straight alpha:
final_rgb = (background_rgb * (1.0 - alpha)) + (foreground_rgb * alpha)
pre-multiplied:
final_rgb = (background_rgb * (1.0 - alpha)) + foreground_rgb

There are a couple more issues, you can pre-multiply to different background colours, in the example above I assum a pre-multiply to black (which is imho the most common).

Cheers,
Mike

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