VolumeRendering
Open GL Transparency
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Geometric primitives can be semi-transparent by defining a fourth alpha channel of the color:
glColor4f(r,g,b, alpha);
A red color with 10% transparency:
glColor4f(1,0,0, 0.9f);
$alpha = Opacity$
$Opacity = 1 - Transparency$
Composition of multiple primitives with $alpha\ne1$ needs to be done with blending enabled in back to front fashion:
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
glEnable(GL_BLEND);
If blending is not done back to front, colors will be awkward:
Left: Ok - from back to front,
Right: Wrong - from front to back
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