"Alexander Enzmann" wrote:
> When using alpha/opacity, you have four channels in the image.
> Three carry color, one carries alpha/opacity.

I know.

> Once you have collected together background color and object
> color into a pixel (the three color channels), you can't
> (easily) separate them back out.

Yes you can. It can be done internally just as easily as externally.

> If you wanted to remove the background color from the pixel,
> you would also need to know the color of the background that
> got added in (three more channels in the image).

Not if POV-Ray itself remove the background from the colour channels. Then
you don't need to do it yourself later, and then you don't need any extra
channels.

> If you are going to do compositing, you should probably use
> a black background.  That way there's no extraneous color
> contributions.

Yes there is. If I make a red sphere with 50% transparency on a black
background then the colour of the sphere will be darkened.

> > (C) Around my white torus there's a red halo - a leftover
> >     from the red background I originally used. I don't like that.
>
> Then you shouldn't have used a red background.

There is leftovers from the background no matter which background colour I
use.

> However, since you know the color you originally used for a
> background, you can remove it from the image:

Why should I do it myself if POV-Ray can do it?

> That would be different from almost every image processing
> package in existence

So with other programs I have to remove the background manually while
POV-Ray could do it automatically? I wouldn't mind that.

> (besides not being possible without also storing
> the entire background along with every image).

Again, you don't need to store the background in some extra channels,
because it is already removed from the colour channels by POV-Ray and then
you don't need the information anymore.

Rune
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