On Fri, 29 Sep 2000 08:28:05 -0400, Alexander Enzmann wrote:
>> 
>> (A) I render a white torus on a red background with AA on
>>     and alpha channel on too.
>
>If you are going to do compositing, you should probably use a black
>background.  That way there's no extraneous color contributions.

That's not true.  You'll get dark fringes as the black darkens whatever it's
averaged with.

>Then you shouldn't have used a red background.  However, since you know
>the color you originally used for a background, you can remove it from
>the image:
>   - multiply the original background color by the alpha value
>   - subtract from the pixel
>   - hope there weren't any out of range values anywhere along the way.

It seems that if this works manually, it should work for POV when +UA is
specified.  I agree with Rune; I can't think of a single instance where it
would be a good thing to average in the background color when the user 
requests an alpha channel.

>I disagree.  That would be different from almost every image processing
>package in existence (besides not being possible without also storing
>the entire background along with every image).

I'm not sure I follow this statement.

-- 
Ron Parker   http://www2.fwi.com/~parkerr/traces.html
My opinions.  Mine.  Not anyone else's.
Proudly not helping RIAA and SDMI steal my rights -- 
  http://www.eff.org/Misc/EFF/Newsletters/EFFector/HTML/effect13.08.html

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