OpenGL 4 Shading Language Cookbook - Third Edition

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This technique is fairly simple and straightforward, and is a nice alternative to traditional blending techniques. It is a great way to make holes in objects or to present the appearance of decay. If your alpha map has a gradual change in the alpha throughout the map (for example, an alpha map where the alpha values make a smoothly varying height field), then it can be used to animate the decay of an object. We could vary the alpha threshold (0.15, in the preceding example) from 0.0 to 1.0 to create an animated effect of the object gradually decaying away to nothing.