In this recipe, we'll use the latter technique, and sample a constant number of texels around
the fragment's position in the shadow map. We'll calculate an average of the resulting
comparisons and use that result to scale the diffuse and specular components.
We'll make use of OpenGL's built in support for PCF, by using linear filtering on the depth
texture. When linear filtering is used with this kind of texture, the hardware can automatically
sample four nearby texels (execute four depth......
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