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Alpha testing vs alpha blending

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retrieved from: http://www.gamedev.net/topic/460604-alphatest-vs-alphablend/

Nit: Alpha testing is a rejection test for source pixels. Alpha blending is a combination of source and destination pixels.

Alpha testing is a way to eliminate pixels whose alpha fails some test. Usually you set it to discard pixels with a very low alpha, since they’ll have little to no visual effect on the scene and aren’t worth blending into the framebuffer.

Alpha testing also allows you to do some other neat effects. Since pixels that are alpha tested away behave as if they never existed (as far as the framebuffer is concerned), you can create geometric cutouts of polygons by using alpha from a texture. They don’t write to the Z-buffer either, meaning that if you disable alpha blending you don’t even have to do a depth sort. This is great for rendering solid billboards (i.e. foliage, impostors, etc.).

Written by gomiski

2012/03/06 at 2:40 pm

Posted in GPU / 3D, Lecture