ShaderX Shader Programming Tips & Tricks With DirectX 9

Making It Fast Using a Linear Basis
Evaluating N-Patches via a vertex shader can be quite expensive. If you are also
using a displacement map, the inherent surface curve usually isn’t very important
anyway. Usually when using displacement compression, we would like a basis that
has a smooth surface normal but relies on the displacement map to handle the
position. A linear basis has all these properties: The surface normal is smooth
between patches (assuming the vertex normals are smooth), but the position
before the displacement is planar. The surface normal is generated from the linear
interpolation of the vertex normals (in a similar manner to how Phong shading
interpolates the lighting normal).
A linear basis only requires the mesh vertex data, and as these can be shared
between patches, it’s usually better to store vertex indices rather than a triangle
index at every interior point. This usually increases the number of vertices that
can be stored in constant memory, which increases performance as more patches
can be evaluated per call at the expense of slightly larger per-vertex data.
//HLSL for a displaced linear basis surface with indexed vertices
float MAX DISPLACEMENT HEIGHT = 100; // this is just an example value
float3 VertexPos[NUM BASE VERTICES];
float3 VertexNormal[NUM BASE VERTICES];
float2 VertexUV[NUM BASE VERTICES];
struct VS IN
{
float2 barycentric;
float3 indices;
float displacement;
};
void main( VS IN input )
{
float i = input.barycentric.x;
float j = input.barycentric.y
float k=1.0–i–j;
float i0 = input.indices.x * 256;
float i1 = input.indices.y * 256;
float i2 = input.indices.z * 256;
}
Section I — Geometry Manipulation Tricks
Using Vertex Shaders for Geometry Compression
float3 pos = i*VertexPos[i0] + j*VertexPos[i1] + k*VertexPos[i2];
float3 normal = i* VertexNormal[i0] + j* VertexNormal[i1] + k* VertexNormal[i2];
float2 uv = i* VertexUV[i0] + j* VertexUV[i1] + k* VertexUV[i2];
normal = normalized( normal );
pos = pos + input.displacement * normal * MAX DISPLACEMENT HEIGHT;
Barycentric coordinates are in the range [0,1] and are the same for each triangle
at a particular subdivision. Indices only require a maximum of 256 values (there
are currently only 256 constants). So a byte per index is enough. For the triangle
indexed version, this is 1 byte + 1 byte displacement and a shared 8 bytes (two
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