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From: Jeric@cup.portal.com
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To: lightwave-l@netcom.com
Subject: Re: alpha's
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Date: Sun, 29 Jan 95 14:29:53 PST
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Mark Thompson explains:>
>Lens flares are an additive effect, ie. adding them to some image should
>not in any way diminish the intensity contribution of any pixel. The
>standard formula for combining two images using alpha is: PixNew = [PixFG *
>alpha] + [PixBG * (1 - alpha)] where alpha is scaled to a value from 0 to
>1. As you can see, with a translucent object (say alpha=0.5), the
>background will be decreased in value by 50% to arrive at the new pixel
>value. Therefore, what you really want to do is render your lens flares
>over a black background, and then when you composite them, simply do an
>absolute add of flare to the background (or using the format of the
>equation above: PixNew = PixFG + PixBG). Once your flares are rendered over
>a black background, you simply load the image(s) into the forground in
>LW and composite them over your other graphics using a completely black
>image for alpha (or using the alpha that was generated by LW when the
>flares were rendered, which will be all black unless there were objects
>other than lights in the scene). The other option is to composite the
>flares externally in something like ImageFX. In this case, you would
>use the "absolute add" compositing mode for the flares.
This is good for compositing within the computer, but what if
you are trying to composite an animation WITH lens flares off a
tape in a post-production suite?
Your reply seems to imply that this would not work, directly using
the alpha-channel animation generted by Lightwave thru ALPHA-SAVE.