#Tessellation in games Patch#

After all, we did already transform the control points in the vertex shader. You might be wondering what the point of the hull program is. The hull program is run per control point, but unlike the vertex shader, it can see all of the control points for the entire patch.

#Tessellation in games Patch#

The patch constant function is responsible for computing data that remains constant over the entire patch. The hull shader consists of two parts: a patch constant function and the hull program. Once all 16 control points have been transformed, the hull shader executes. The same principle holds when using tessellation: the next stage isn't executed until all 16 control points have been transformed by 16 executions of the vertex shader.

For example, when using the triangle primitive type, the next stage is run once per every three executions of the vertex shader. When not using tessellation, the next shader stage is executed once the vertex shader has operated on all of the vertices of a single primitive. Similar to the non-patch primitive types, the vertex shader can only see one control point at a time it cannot see all 16 of the control points on the patch. The vertex shader is executed once per control point, and each execution is given one control point worth of data from the input assembler. This buffer is bound to the input assembler as usual. Since we're rendering one patch, we'll need a buffer with 16 points in it - in this context, these points are control points. That is, it's possible to describe a patch with anywhere from 1 to 32 control points.ДЇor the purpose of this example, say we've configured the input assembler to handle patches with 16 control points, and also that we're only rendering one patch. D3D11 adds 32 new primitive types: each represents a patch with a different number of control points. It turns out that it is quite elegant to add new primitive types for patches. Normally, the input assembler can be configured to handle points, lines, line strips, triangles and triangle strips. Input assembler -> Vertex shader -> Hull shader -> Tessellator -> Domain shader -> Pixel shader

To solve this problem, D3D11 adds two new programmable stages: the hull shader and the domain shader. The vertex shader by itself isn't particularly well-suited for handling the manipulation of patches we could store the control points in a buffer and index with SV_VertexID, but this is not very efficient especially when dealing with 16+ control points per patch. Say, however, that we wish to process control points and patches instead. The vertex shader is executed once per vertex, and each execution is given one vertex worth of data from the input assembler. Input assembler -> Vertex shader -> Pixel shaderД«uffers containing per-vertex data are bound to the input assembler. Consider the typical flow of data through the programmable pipeline: