DirectX Computational Graphics

Pixel Processing: The Heart of Rendering

After rasterization, the graphics pipeline has determined which pixels on the screen are covered by a primitive. The next crucial stage is pixel processing, also known as fragment processing in some contexts. This stage determines the final color of each pixel, taking into account various factors like textures, lighting, and material properties.

The Role of the Pixel Shader

The primary component responsible for pixel processing is the Pixel Shader. This programmable stage executes a small program for each pixel (or fragment) that needs to be rendered. Its goal is to compute the final color value that will be written to the render target.

A pixel shader takes interpolated vertex data and outputs a final color.

Inputs to the Pixel Shader

The pixel shader receives data that has been interpolated from the vertex shader output across the primitive. Common inputs include:

• Interpolated vertex attributes (e.g., texture coordinates, normals, colors).
• Constant values defined by the application.
• Texture samples fetched from texture units.

Key Operations within the Pixel Shader

Pixel shaders can perform a wide range of operations to determine the final pixel color:

• Texture Sampling: Fetching color information from textures using interpolated texture coordinates. This is fundamental for applying surface details, patterns, and images.
• Lighting Calculations: Applying lighting models (e.g., Phong, Blinn-Phong) using interpolated normals and light source information to simulate how light interacts with the surface.
• Color Blending and Modulations: Combining fetched texture colors, lighting contributions, and base material colors.
• Fog and Atmospheric Effects: Applying fog or other environmental effects based on depth or other factors.
• Alpha Blending: Determining the transparency of the pixel, crucial for rendering translucent objects.

Shader Language (HLSL) Example

High-Level Shading Language (HLSL) is commonly used for writing DirectX shaders. Here's a simplified example of a pixel shader that samples a texture and applies a basic lighting factor:

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

// Texture sampler and sampler state
Texture2D      g_Texture;
SamplerState   g_Sampler;

// Light direction and color
float3         g_LightDirection;
float4         g_LightColor;

float4 PSMain(PixelShaderInput input) : SV_TARGET {
    // Sample the texture
    float4 texColor = g_Texture.Sample(g_Sampler, input.Tex);

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

    // Calculate diffuse lighting
    float diffuseFactor = max(0.0, dot(normal, -g_LightDirection));
    float4 lighting = g_LightColor * diffuseFactor;

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

    return finalColor;
}
            

Output and Beyond

The output of the pixel shader is typically a color value (often in RGBA format). This color is then subject to further stages in the pipeline, such as:

• Output Merging: This stage handles depth testing, stencil testing, and alpha blending. The pixel shader's output color is only written to the render target if it passes these tests.

Mastering pixel processing is essential for achieving realistic and visually stunning graphics in DirectX applications, from simple color determination to complex material rendering and post-processing effects.