DirectX Computational Graphics

Tutorials on Advanced Graphics Techniques

Tessellation in DirectX

Tessellation is a powerful technique in modern graphics pipelines that allows for the dynamic subdivision of geometry at runtime. This enables the creation of highly detailed models from simpler base meshes, improving visual fidelity and reducing the need for extremely complex static models. DirectX 11 and later versions provide hardware-accelerated tessellation stages.

Understanding Tessellation

The tessellation process involves three primary hardware stages:

Tessellation Stages Explained

Hull Shader (HS)

The hull shader is where you define the tessellation level for each patch of geometry. It typically involves two types of functions:

The tessellation factors control the level of detail. Higher factors result in more triangles and thus a more detailed mesh.

Tessellator Stage

This fixed-function stage takes the tessellation factors from the hull shader and subdivides the input primitives (e.g., a single quad) into smaller primitives based on the desired tessellation level. It generates new vertices that lie on the edges and within the surface of the original primitive.

Domain Shader (DS)

The domain shader is responsible for calculating the final attributes of the tessellated vertices. For each vertex generated by the tessellator, the domain shader receives:

Using these inputs, the domain shader can compute new vertex positions, normals, texture coordinates, and other attributes, effectively defining the detailed geometry.

Common Use Cases

Example HLSL Code Snippets

Hull Shader - Patch Constant Function (Example for Quads)

This function sets the tessellation factors. You might use distance from camera or other heuristics.


// HS_PATCH_CONSTANT_FUNC.hlsl
struct HS_CONTROL_POINT_OUTPUT
{
    float3 pos : POSITION;
};

struct HS_PATCH_CONSTANT_DATA
{
    float EdgeTessFactor[4] : SV_TessFactor; // For a quad: front, back, left, right
    float InsideTessFactor  : SV_InsideTessFactor; // For a quad: inside
};

HS_PATCH_CONSTANT_DATA HS_PatchConstantFunc(InputPatch<HS_CONTROL_POINT_OUTPUT, 4> patch, uint patchID : SV_PrimitiveID)
{
    HS_PATCH_CONSTANT_DATA patchConstantData;

    // Example: Simple tessellation factors (can be dynamic)
    float tessFactor = 8.0f; // Higher value means more tessellation

    patchConstantData.EdgeTessFactor[0] = tessFactor; // Front edge
    patchConstantData.EdgeTessFactor[1] = tessFactor; // Back edge
    patchConstantData.EdgeTessFactor[2] = tessFactor; // Left edge
    patchConstantData.EdgeTessFactor[3] = tessFactor; // Right edge
    patchConstantData.InsideTessFactor = tessFactor;

    return patchConstantData;
}

Domain Shader (Example)

This function interpolates and displaces vertices based on barycentric coordinates.


// DS_MAIN.hlsl
struct DS_INPUT
{
    float3 pos : POSITION;
    float2 uv  : TEXCOORD;
    float3 normal : NORMAL;
    // ... other attributes
};

struct DS_OUTPUT
{
    float4 pos : SV_POSITION;
    float2 uv  : TEXCOORD;
    float3 normal : NORMAL;
    // ... other attributes
};

// Assume displacementMap is a texture sampler
Texture2D displacementMap;
SamplerState displacementSampler;

DS_OUTPUT DS_Main(HS_PATCH_CONSTANT_DATA input, const OutputPatch<DS_INPUT, 4> patch, float3 barycentricCoords : SV_DomainLocation)
{
    DS_OUTPUT output;

    // Interpolate vertex attributes using barycentric coordinates
    // For quads, barycentricCoords are typically (u, v) representing position on the patch.
    float u = barycentricCoords.x;
    float v = barycentricCoords.y;

    // Interpolate position (example assumes patch is a quad)
    // This is a simplified linear interpolation. For true Bezier/NURBS, more complex math is needed.
    float3 interpolatedPos = patch[0].pos * (1.0f - u - v) + patch[1].pos * u + patch[2].pos * v; // Incorrect for general quad
    // Correct interpolation for a quad (linear interpolation along u and v directions)
    // Let's assume patch[0] is bottom-left, patch[1] is top-left, patch[2] is bottom-right, patch[3] is top-right
    float3 P0 = lerp(patch[0].pos, patch[1].pos, v); // Interpolate along left edge
    float3 P1 = lerp(patch[2].pos, patch[3].pos, v); // Interpolate along right edge
    interpolatedPos = lerp(P0, P1, u); // Interpolate between left and right edges

    // Interpolate UVs
    float2 interpolatedUV = patch[0].uv * (1.0f - u - v) + patch[1].uv * u + patch[2].uv * v; // Simplified
    // Correct for quad
    float2 UV0 = lerp(patch[0].uv, patch[1].uv, v);
    float2 UV1 = lerp(patch[2].uv, patch[3].uv, v);
    interpolatedUV = lerp(UV0, UV1, u);


    // Sample displacement map and apply displacement
    float displacement = displacementMap.Sample(displacementSampler, interpolatedUV).r;
    interpolatedPos += float3(0, 0, displacement); // Assuming displacement is along Z axis for simplicity. Adjust as needed.

    output.pos = mul(float4(interpolatedPos, 1.0f), WorldViewProjectionMatrix); // Needs ViewProjection matrix
    output.uv = interpolatedUV;
    output.normal = interpolatedPos; // Placeholder for normal calculation

    return output;
}

Performance Considerations

While tessellation offers great visual benefits, it can be computationally expensive. Carefully consider:

By understanding and effectively utilizing the hull shader, tessellator, and domain shader, you can unlock new levels of geometric detail and visual realism in your DirectX applications.