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Internet Message Format | 1995-11-07 | 2KB

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From: [e-mail] (Mark Thompson)
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Subject: Re: Additive?
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> >> Can some one explain additive?
> Additive does just what it sounds like it does.. it adds the color of the
> surface to the background color.

To elaborate...Normally in LW, when a transparent color is rendered, it
is blended with the background according to the standard blending
algorithm: PixelResult = PixelNew * Trans + PixelOld * (1 - Trans),
where Trans is the transparency of the pixel. In other words, as the
new pixel gets more opaque, the pixel behind it is faded out. This is
the behaviour you would want for standard compositing, but not for
many lighting effects. In contrast, with Additive selected, the new pixel
is still multiplied by the transparency, but the old pixel behind it is
added at its full value. PixelResult = PixelNew * Trans + PixelOld.
This is how light effects should behave, and its how lens flares are
applied.
     *~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*
     *           Mark Thompson                  [phone]    *
     *         Fusion Films, Inc.             [e-mail]  *
     * Director of Animation and Special FX                       *
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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