MSDN Documentation

Pixel Shading

Pixel shaders, also known as fragment shaders in OpenGL, are programs that run on the GPU to determine the final color of each pixel (or fragment) that is rendered. They are a crucial part of modern graphics pipelines, enabling sophisticated lighting, texturing, and visual effects.

The Role of Pixel Shaders

After a vertex shader has transformed and processed vertices, and the rasterizer has determined which pixels are covered by the primitives (triangles, lines, points), the pixel shader takes over. For each pixel that an object covers, a pixel shader instance is executed. Its primary job is to calculate the color of that pixel, considering factors such as:

• Texture Sampling: Reading color or other data from textures.
• Lighting Calculations: Applying complex lighting models (e.g., Phong, Blinn-Phong, physically-based rendering).
• Material Properties: Incorporating the surface properties of the object.
• Interpolated Data: Using values interpolated from vertex attributes (like color, texture coordinates, normals).
• Post-processing Effects: Modifying the output color for effects like bloom, depth of field, or color grading.

Shader Languages (HLSL)

In DirectX, High-Level Shading Language (HLSL) is used to write pixel shaders. HLSL is a C-like language that allows developers to express complex algorithms in a way that can be compiled and executed efficiently on the GPU. A typical pixel shader function in HLSL might look like this:

struct PixelShaderInput
{
    float4 Position : SV_POSITION;
    float2 Tex : TEXCOORD0;
    float3 Normal : NORMAL;
    float3 WorldPos : POSITION;
};

// Samplers and textures
Texture2D myTexture;
SamplerState samLinear;

float4 main(PixelShaderInput input) : SV_TARGET
{
    // Sample the texture
    float4 texColor = myTexture.Sample(samLinear, input.Tex);

    // Simple diffuse lighting (example)
    float3 lightDir = normalize(float3(0.5, 0.8, -0.2));
    float diffuseFactor = saturate(dot(input.Normal, -lightDir));

    // Combine texture color and lighting
    float4 finalColor = texColor * diffuseFactor;

    return finalColor;
}
                

Interpolation

Data from vertex shaders is interpolated across the surface of the primitive. For example, texture coordinates or vertex colors defined at the vertices of a triangle will have their values smoothly blended for each pixel inside that triangle. This interpolation is crucial for applying textures and other per-pixel effects seamlessly.

The diagram above illustrates the general flow. After vertex processing, primitives are rasterized. For each pixel covered, the pixel shader is invoked. It uses interpolated data and potentially texture lookups and calculations to output a final color to the render target.

Advanced Techniques

Modern pixel shaders are capable of implementing a wide array of advanced graphical effects:

• Normal Mapping: Using a texture to provide per-pixel surface normals, creating the illusion of intricate detail without extra geometry.
• Specular Mapping: Controlling the shininess and specular highlights of surfaces.
• Environment Mapping (Cubemaps): Simulating reflections based on the surrounding environment.
• Shader-Based Anti-Aliasing: Techniques like FXAA (Fast Approximate Anti-Aliasing) are often implemented in post-processing pixel shaders.
• Post-Processing Effects: Bloom, depth of field, motion blur, color correction, and tone mapping are all commonly achieved using pixel shaders after the main scene is rendered.

Understanding and mastering pixel shaders is fundamental to creating visually rich and compelling graphics with DirectX.