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Date: Tue, 31 Oct 1995 23:53:49 -0500 (EST)
From: Ernie Wright <[e-mail]>
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Subject: Re: Additive?
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Denny Armenta Ramade asked:

> Can some one explain additive?

This is another curious holdover from Videoscape, which had "bright-
ening" and "darkening" surface color codes.  Polygons with an Additive
surface don't show on their own; their color is "added" to that of the
polygons behind them, a brightening effect (LW emulates Videoscape
darkening with dark semi-transparent polygons).

I used Additive recently in a solar eclipse simulation showing the
moon's shadow falling on the earth.  I needed to simulate umbra and
penumbra shadowing, and since LW doesn't have extended light sources,
I did this by raytracing two concentric spheres, a small one for the
umbra and a larger semi-transparent one with edge transparency for the
penumbra.

I needed these to cast shadows without ever being visible themselves,
and it turns out this is possible by setting the Luminosity, Diffuse
and Specular levels to 0 and turning Additive on.

- Ernie



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