Domain shaders are a crucial component of the tessellation pipeline in DirectX. They work in conjunction with hull shaders to generate new vertices from a patch, allowing for dynamic level of detail and intricate geometric detail on the fly.
Tessellation is a hardware-accelerated technique that subdivides a mesh's primitives (like triangles or quads) into smaller primitives. This process is typically controlled by two programmable shader stages:
A domain shader executes for each vertex that needs to be generated within a tessellated patch. It receives:
Using these inputs, the domain shader calculates the position of a new vertex in object space. This output vertex is then passed to the rasterizer for further processing.
This is a simplified example of a domain shader that might be used to tessellate a quad.
// Define the output structure for the domain shader
struct DS_OUTPUT
{
float4 Position : SV_POSITION; // Clip-space position
float2 Tex : TEXCOORD0; // Texture coordinates
};
// Define the input structure for the domain shader
// This structure receives data from the hull shader
struct DS_INPUT
{
float3 DomainCoordinates : SV_DOMAIN; // Barycentric coordinates (u, v, w)
float4 ControlPoint : POSITION; // Control point data
float2 Tex : TEXCOORD0; // Texture coordinates from control point
};
// Assume PatchConstantHS is defined in Hull Shader output
// Domain Shader function
DS_OUTPUT DomainShader(float4 patchDistance : SV_DOMAIN,
const OutputPatch tri, // Assuming a quad (4 control points)
float4 patchConstants[1] : PATCHCONSTANT) // Patch constants from Hull Shader
{
DS_OUTPUT result;
// Simplified example: Interpolate position and texture coordinates
// In a real scenario, you'd use patchDistance and patchConstants to perform complex calculations
// Linear interpolation of position
result.Position = tri[0].ControlPoint * (1.0f - patchDistance.x - patchDistance.y) +
tri[1].ControlPoint * patchDistance.x +
tri[2].ControlPoint * patchDistance.y;
// For quads, you'd need more complex interpolation based on patchDistance.x and patch.y
// Linear interpolation of texture coordinates
result.Tex = tri[0].Tex * (1.0f - patchDistance.x - patchDistance.y) +
tri[1].Tex * patchDistance.x +
tri[2].Tex * patchDistance.y;
// Transform to clip space (typically done by vertex shader, but shown here for completeness if it's the end)
// In a full pipeline, this would be handled by the VS or subsequent stages.
// For a tessellated pipeline, the domain shader output is often in object or world space,
// and then transformed by a subsequent vertex shader stage.
// Let's assume it's outputting world space for simplicity here.
// The SV_POSITION semantic typically expects clip-space coordinates.
// This highlights that the exact stage and its output space can vary.
// For demonstration, let's assume the domain shader outputs world space and a final VS handles the projection.
// If this were the final output stage before rasterization, transformation to clip-space would be required.
// Example transformation to clip space (if this were the final stage)
// This requires projection matrix from constants or uniforms
// result.Position = mul(WorldViewProjectionMatrix, result.Position);
// result.Position.w = 1.0f; // Ensure w is 1 for perspective division
return result;
}
The domain shader is executed after the hull shader and before the geometry shader (if present) or rasterizer. The tessellation pipeline typically looks like this: