EnigmA Amiga Run 1997 July/EnigmA AMIGA RUN 20 (1997)(G.R. Edizioni)(IT)[!][issue 1997-07 & 08][EAR-CD IV].iso/lightwave/tips/lightwave-jan95.txt/000337_owner-lightwave-l _Fri Jan 27 21:35:17 1995.msg

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         (from Brett Feeney <[e-mail]>)
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From: [e-mail] (Mark Thompson)
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Subject: Re: alpha's
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Brett Feeney <[e-mail]> writes:
> is there a way to produce an alpha channel for lensflare,
> could sumone explain the theory behind it to me .
>     thanx in advance to any help recieved

You should really check out my article on the alpha channel in the
July issue of LWPro. I covered this as well as many other issues.

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.

For more details, see the LWPro article.
     *~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*
     *           Mark Thompson               [phone]       *
     *         Fusion Films Inc.           [e-mail]     *
     *     Radiant Image Productions                              *
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~