Windows API Reference

Graphics - Shaders - Pixel Shaders

Pixel Shaders

Pixel shaders, also known as fragment shaders, are programmable shaders that run on the graphics processing unit (GPU). They are responsible for determining the final color of each pixel on the screen. This process typically involves reading from textures, performing calculations based on lighting and material properties, and outputting a color value.

Core Concepts

  • Per-Pixel Operations: Pixel shaders operate on individual pixels, allowing for fine-grained control over visual effects.
  • Texture Sampling: They read color, normal, and other data from textures to define surface appearance.
  • Lighting and Shading Models: Pixel shaders implement complex lighting calculations, such as Phong, Blinn-Phong, or custom physically based rendering (PBR) models.
  • Color Blending and Output: The final computed color is blended with existing framebuffer content according to blending modes.
  • Shader Languages: Typically written in high-level shader languages like High-Level Shading Language (HLSL) for DirectX or OpenGL Shading Language (GLSL) for OpenGL.

HLSL Pixel Shader Example (Simplified)


// Input structure from vertex shader
struct VS_OUTPUT {
    float4 Position : SV_POSITION;
    float2 Tex : TEXCOORD0;
    float3 Normal : NORMAL;
    float3 WorldPos : POSITION1;
};

// Sampler and texture declaration
Texture2D myTexture;
SamplerState mySampler;

// Constant buffer for lighting parameters
cbuffer LightInfo {
    float3 lightDirection;
    float3 ambientColor;
    float3 diffuseColor;
};

// Main pixel shader function
float4 PSMain(VS_OUTPUT input) : SV_TARGET {
    // Sample color from texture
    float4 texColor = myTexture.Sample(mySampler, input.Tex);

    // Normalize the normal vector
    float3 normal = normalize(input.Normal);

    // Calculate diffuse lighting
    float diffuseFactor = max(0, dot(normal, -lightDirection));
    float3 diffuseLight = diffuseFactor * diffuseColor;

    // Combine ambient and diffuse lighting
    float3 finalColor = ambientColor + diffuseLight;

    // Apply lighting and texture color
    finalColor *= texColor.rgb;

    return float4(finalColor, texColor.a);
}

Key Functions and Structures

Pixel shaders interact with various GPU hardware features and use specific functions for tasks like texture sampling, mathematical operations, and color manipulation. The exact functions and structures depend on the graphics API and shader model version.

Common Tasks and Corresponding Functions (HLSL):

Task HLSL Functions/Concepts Description
Texture Sampling Texture2D.Sample(), SamplerState Retrieves color values from textures at specified coordinates.
Vector/Matrix Operations dot(), normalize(), lerp(), mul() Performs common mathematical operations for lighting and transformations.
Color Manipulation float4(), (color.rgb), (color.a) Accessing and modifying color components (RGBA).
Output SV_TARGET semantic Specifies the render target where the pixel color is written.

Performance Considerations

  • Minimize texture fetches.
  • Use simpler lighting models if possible.
  • Avoid complex branching and loops.
  • Optimize mathematical calculations.
  • Leverage hardware features for common operations.

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