Domain Shading in DirectX Computational Graphics

Domain Shading is an advanced technique in DirectX computational graphics that allows for more sophisticated and dynamic control over the appearance of surfaces. It extends the traditional shading pipeline by introducing a "domain" where shading calculations can be performed with greater flexibility, enabling effects such as procedural texturing, adaptive tessellation, and complex surface deformations directly within the shader stages.

Introduction

Traditionally, surface appearance is determined by texture mapping, vertex colors, and fixed-function lighting models. While powerful, these methods can be limited when dealing with highly complex or procedurally generated geometry. Domain Shading, often implemented using Hull and Domain Shaders in the DirectX 11 (and later) tessellation pipeline, provides a programmable solution to generate or modify surface geometry and its associated shading properties on the fly.

The Tessellation Pipeline

Domain Shading is intrinsically linked to the DirectX tessellation pipeline, which consists of the following stages:

• Input Assembler: Takes vertex data and prepares it for rendering.
• Vertex Shader: Processes each vertex.
• Hull Shader: Prepares tessellation factors and patch data for the next stage.
• Tessellator: Generates new vertices based on tessellation factors.
• Domain Shader: Processes the newly generated vertices, interpolating data from the patch and performing shading calculations.
• Geometry Shader (Optional): Can further process primitives.
• Pixel Shader: Shades the final pixels.

The Hull Shader and Domain Shader are the core components that enable Domain Shading.

Hull Shader

The Hull Shader's primary role is to determine how a patch of geometry (e.g., a quad or triangle) should be tessellated. It calculates tessellation factors, which control the level of detail, and outputs patch constants that are accessible by the Domain Shader.

Key functions:

• Calculating tessellation factors (e.g., edge factors, inside factor).
• Outputting patch constants for shared data across tessellated vertices.

Domain Shader

The Domain Shader receives tessellation factors and patch constants from the Hull Shader, along with the output of the Hull Shader for each tessellated vertex. It's responsible for generating the final position and other attributes (like texture coordinates, normals) for each tessellated vertex. This is where the "domain shading" truly takes place, as complex calculations can be performed to define the surface's detailed appearance.

Key functions:

• Generating new vertex positions by interpolating within the original patch.
• Calculating surface normals, tangents, and other attributes.
• Applying procedural texturing or displacement mapping.
• Performing adaptive tessellation based on screen space or object properties.

Applications of Domain Shading

Procedural Texturing

Instead of relying on pre-baked textures, Domain Shading can procedurally generate intricate patterns, noise, or material properties directly on the surface. This is particularly useful for generating organic materials like wood grain, marble, or complex terrain features without the memory overhead of high-resolution textures.

Adaptive Tessellation and Displacement Mapping

Domain Shading allows for dynamic control over the level of detail. Surfaces can be tessellated more finely in areas closer to the camera or where more detail is required, and less finely elsewhere. This is often combined with displacement mapping, where the Domain Shader displaces vertices along their normals based on a height map or procedural function, creating realistic geometric detail.

Example HLSL Code Snippets

Hull Shader Example (Simplified)

This snippet shows how to output tessellation factors and patch constants.

struct HS_CONTROL_POINT_OUTPUT {
    float4 pos : POSITION;
    float2 tex : TEXCOORD0;
};

struct HS_PATCH_CONSTANT_OUTPUT {
    float EdgeTessFactor[3] : SV_TessFactor;
    float InsideTessFactor : SV_InsideTessFactor;
};

[domain("tri")]
[partitioning("fractional_even")]
[outputtopology("triangle_cw")]
[patchconstantfunc("PatchConstantFunc")]
[domainkernel("DomainKernel")]
void MainHullShader(
    in InputPatch patch,
    out HS_PATCH_CONSTANT_OUTPUT patchConstant,
    in uniform int TessellationLevel
) {
    // Calculate tessellation factors based on TessellationLevel
    patchConstant.EdgeTessFactor[0] = TessellationLevel;
    patchConstant.EdgeTessFactor[1] = TessellationLevel;
    patchConstant.EdgeTessFactor[2] = TessellationLevel;
    patchConstant.InsideTessFactor = TessellationLevel;
}

HS_PATCH_CONSTANT_OUTPUT PatchConstantFunc(
    in InputPatch patch,
    in uniform int TessellationLevel
) {
    HS_PATCH_CONSTANT_OUTPUT output;
    output.EdgeTessFactor[0] = TessellationLevel;
    output.EdgeTessFactor[1] = TessellationLevel;
    output.EdgeTessFactor[2] = TessellationLevel;
    output.InsideTessFactor = TessellationLevel;
    return output;
}
                

Domain Shader Example (Simplified)

This snippet demonstrates interpolating vertex positions and calculating normals.

struct DS_INPUT {
    float4 pos : SV_POSITION;
    float2 tex : TEXCOORD0;
};

struct DS_OUTPUT {
    float4 pos : SV_POSITION;
    float2 tex : TEXCOORD0;
    float3 normal : NORMAL;
};

[domain("tri")]
void MainDomainShader(
    in DS_OUTPUT domainPos : SV_DOMAINPOSITION,
    in InputPatch patch,
    out DS_OUTPUT output
) {
    // Interpolate vertex positions based on barycentric coordinates (domainPos.pos)
    float3 v0 = patch[0].pos.xyz;
    float3 v1 = patch[1].pos.xyz;
    float3 v2 = patch[2].pos.xyz;

    output.pos.xyz = v0 * domainPos.pos.x + v1 * domainPos.pos.y + v2 * domainPos.pos.z;

    // Interpolate texture coordinates
    output.tex.x = patch[0].tex.x * domainPos.pos.x + patch[1].tex.x * domainPos.pos.y + patch[2].tex.x * domainPos.pos.z;
    output.tex.y = patch[0].tex.y * domainPos.pos.x + patch[1].tex.y * domainPos.pos.y + patch[2].tex.y * domainPos.pos.z;

    // In a real scenario, you'd calculate normals here, potentially based on displacement
    // For simplicity, we'll just pass a placeholder or a calculated normal.
    // Example: Calculate normal from original vertices
    float3 edge0 = v1 - v0;
    float3 edge1 = v2 - v0;
    output.normal = normalize(cross(edge0, edge1));

    // Apply World transform if needed
    // output.pos = mul(worldMatrix, output.pos);
    // output.normal = mul((float3x3)worldMatrix, output.normal);

    // Clip space position
    output.pos.w = 1.0f;
    output.pos = mul(projectionMatrix, mul(viewMatrix, output.pos));
}
                

Conclusion

Domain Shading, powered by the tessellation pipeline in DirectX, offers developers unprecedented control over surface generation and appearance. By enabling complex, adaptive, and procedural shading directly on the GPU, it unlocks a new level of visual fidelity and artistic expression in real-time graphics.