Tessellation in DirectX

Introduction to Tessellation

Tessellation is a powerful technique in DirectX that allows for the dynamic subdivision of geometry at runtime. This enables the creation of highly detailed surfaces from simpler base meshes, significantly enhancing visual fidelity without requiring complex pre-modeled geometry. It's particularly effective for procedural content generation, adaptive level-of-detail (LOD), and complex material effects.

DirectX 11 introduced dedicated tessellation stages to the graphics pipeline, providing explicit control over this process. These stages work in conjunction with existing pipeline stages like the vertex shader and geometry shader, but offer a more specialized and efficient approach to mesh subdivision.

Tessellation Stages

The tessellation pipeline consists of three primary stages:

  1. Hull Shader: This shader stage is responsible for orchestrating the tessellation process. It determines which parts of the geometry should be tessellated, how densely, and it outputs control points that guide the tessellation factors and tessellated vertices.
  2. Tessellator: This is a fixed-function stage that takes the tessellation factors calculated by the hull shader and generates a set of tessellation indices. These indices define the subdivision pattern.
  3. Domain Shader: This shader stage takes the tessellated indices from the tessellator and the hull shader's output control points. It then calculates the final vertex positions, normals, texture coordinates, and other attributes for the newly generated vertices.
DirectX Tessellation Pipeline Diagram

Conceptual diagram of the DirectX Tessellation Pipeline.

Hull Shader Explained

The hull shader acts as the manager of tessellation. It typically consists of two parts:

Tessellation factors are typically represented by a 3-component vector, controlling tessellation along the edges and face of a patch. Common partitioning schemes include:

Hull Shader Example (Conceptual HLSL)


struct HS_CONTROL_POINT_OUTPUT
{
    float3 position : POSITION;
    float2 texCoord : TEXCOORD;
};

struct HS_OUTPUT
{
    float4 SV_Position : SV_POSITION;
    float2 texCoord : TEXCOORD;
};

[outputtopology(Patches< PatchConstantData, 3 >)]
[patchconstantfunc](input patch const HS_CONTROL_POINT_OUTPUT inputCP[3]) : SV_ConstantBuffer
{
    PatchConstantData outputConsts;
    // Calculate tessellation factors based on inputCP
    // e.g., outputConsts.TessFactor[0] = calculateEdgeTessellation(inputCP);
    // e.g., outputConsts.TessFactor[1] = calculateInsideTessellation(inputCP);
    outputConsts.TessFactor[0] = 4.0; // Example: 4 divisions along edges
    outputConsts.TessFactor[1] = 4.0; // Example: 4 divisions inside
    return outputConsts;
}

[domain(tri)]
HS_OUTPUT MainHS(InputPatch<HS_CONTROL_POINT_OUTPUT, 3> ip,
                       uint id : SV_DomainLocation)
{
    HS_OUTPUT output;
    // Output control point from the input patch
    output.texCoord = ip[id].texCoord;
    // Pass through original vertex data to the domain shader
    return output;
}

Domain Shader Explained

The domain shader is responsible for calculating the final geometric attributes of the tessellated vertices. It receives:

Using this information, the domain shader interpolates the attributes of the control points to produce the final vertex data for the rendered triangle/quad.

Domain Shader Example (Conceptual HLSL)


struct HS_CONTROL_POINT_OUTPUT
{
    float3 position : POSITION;
    float2 texCoord : TEXCOORD;
};

struct HS_OUTPUT
{
    float4 SV_Position : SV_POSITION;
    float2 texCoord : TEXCOORD;
};

[domain(tri)]
float4 MainDS(PatchConstantData patchConsts,
                const float3 DomainLocation : SV_DomainLocation,
                const float3 cp[3] : CONTROLPOINTID) : SV_Position
{
    // Interpolate position based on DomainLocation (barycentric coords)
    float3 interpolatedPos = DomainLocation.x * cp[0] + DomainLocation.y * cp[1] + DomainLocation.z * cp[2];

    // A simple pass-through and positioning for demonstration
    float4 finalPos = float4(interpolatedPos, 1.0);

    // In a real scenario, you'd use DomainLocation to interpolate
    // texture coordinates, normals, etc., from the hull shader's output
    // based on the corresponding control points.

    return finalPos;
}

Benefits and Use Cases

Considerations

While powerful, tessellation introduces overhead. Careful profiling and optimization are necessary. Understanding the interplay between the hull shader, tessellator, and domain shader is crucial for achieving desired results efficiently. Choose appropriate partitioning schemes and tessellation factors based on the content and performance targets.

Tessellation requires support from the graphics hardware and is typically enabled through DirectX 11 or later APIs.

Back to Computational Graphics

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