Dynamic Tessellation - Practical Rendering and Computation with Direct3D 11 - page 431

Figure 9.15. A "best fit" plane.

normalizing these 4 raw normal vectors results in a single unit-length normal vector for the
patch, one that is generally representative of the underlying surface. 5 By taking any of the four
corner positions, it is possible to derive a standard plane equation, as shown in Equation (9.1):
Ax + By + Cz + D = 0,

A =N ,
B=N y , (9.1)

C = N z ,
D=-(N*P).

With this plane equation known, the compute shader can evaluate each height map
sample for the distance between it and the plane.

Implementing with a Compute Shader

Notation and indexes in the compute shader are not immediately obvious; Figure 9.16 in-
troduces two of the key variables in the context of a terrain rendering pre-pass. 6 The core
HLSL shader has an entry point, shown in Listing 9.10, with the [numthreads(x, y,z)] at-
tribute attached to it.

5 The process of calculating a normal vector is demonstrated in the vertex shader and rasterizer sections
of Chapter 3.
6 Compute shaders are discussed in detail in Chapter 5.