GPU Pro 6

80 II Rendering

Light source

Caster

Shadow map

Search region

Figure 1.11. Blockers in a 8 × 8 search grid.

Shader Model 5.0’s new intrinsic GatherRed() accelerates this step by sampling

four values at once. In Thief,wedecidedtousea8× 8 kernel size, which

actually performs 16 samples instead of 64 for a Shader Model 4.0 implementation

(see Listing 1.4). Increasing the size of the kernel will allow a larger penumbra,

since points that are farther from the shaded one can be tested, but it obviously

increases the cost as the number of texture fetches grows.

Because the penumbra width (or blurriness) is tightly related to the size of

the kernel, which depends ...