The first step enables linear filtering on the shadow map texture. When this is enabled, the OpenGL driver can repeat the depth comparison on the four nearby texels within the texture. The results of the four comparisons will be averaged and returned.
Within the fragment shader, we use the textureProjOffset function to sample the four texels (diagonally) surrounding the texel nearest to ShadowCoord. The third argument is the offset. It is added to the texel's coordinates (not the texture coordinates) before the lookup takes place.
As linear filtering is enabled, each lookup will sample an additional four texels, for a total of 16 texels. The results are then averaged together and stored within the variable shadow.
As before, ...
