ShaderX Shader Programming Tips & Tricks With DirectX 9

Points Inside a Triangle
N-Patches
A unique point inside a triangle can be computed via the three vertices defining
the triangle and the barycentric coordinates of this interior point. The three vertices
for each triangle are placed into constant memory, and we store two of the
barycentric coordinates in the vertex stream (k can be computed from i and j). A
vertex stream triangle index is used to select which set of three vertices in constant
memory makes up the triangle with which we are currently working.
Here we hit a small issue: Some vertices belong to more than one triangle.
We have to duplicate each vertex attached to more than one triangle and give each
one a separate index.
//HLSL code for calculating interior points of a number of triangles.
float3 VertexPos[3 * NUM BASE TRIANGLE];
void main(float3 vertexStream : POSITION0)
{
float i = vertexStream.x;
float j = vertexStream.y
float k=1.0–i–j;
float baseIndex = vertexStream.z * 256; // un-normalize index
float3 pos = i*VertexPos[ (baseIndex*3) +0]+
j*VertexPos[ (baseIndex*3) +1]+
k*VertexPos[(baseIndex*3) +2];
}
N-Patches (Curved PN Patches [3]) are a type of bicubic patch where the control
points are determined from a triangle’s vertex positions and normals. N-Patches
come in two variations, both with cubic interpolated position, but they differ in
whether the normal is interpolated linearly or quadratically. The algorithm calculates
the control points for the patch and then evaluates at each point on the base
triangle.
Effectively, there are two frequencies at which this vertex shader needs executing;
the control points need calculating only once per patch, whereas the evaluation
needs running at every vertex. Some consoles can execute this pattern on
the GPU, but on current PC architectures you can either generate the control
points on the CPU and upload them to vertex constant memory or recalculate the
control points at every vertex. The first uses CPU power per patch, and each
patch uses more constant memory (for linear normal N-Patches, 39 floats versus
18 for vertices), whereas recalculating at every vertex uses a lot of vertex shader
power but allows better batching and has lower CPU overhead.
float3 VertexPos[3 * NUM BASE TRIANGLE];
float3 VertexNormals[3 * NUM BASE TRIANGLE];
Section I — Geometry Manipulation Tricks
Using Vertex Shaders for Geometry Compression
// bicubic control points
float3 b300,b030,b003, b210,b120,b021, b201,b102,b012, b111;
9