Rendering Pipelines

A rendering pipeline is a series of stages that an application uses to render a 3D scene into a 2D image on the screen. Each stage performs specific operations on the geometry and associated data to transform it and prepare it for display.

Figure 1: A simplified conceptual rendering pipeline.

Core Stages of a Rendering Pipeline

Modern graphics APIs like DirectX and Vulkan define a programmable rendering pipeline, allowing developers fine-grained control over each stage. While specific implementations may vary, the fundamental stages remain consistent:

1. Input Assembler Stage

This stage is responsible for fetching vertex data from memory buffers and organizing it into primitives (points, lines, triangles) that the pipeline can process. It reads data from vertex buffers and index buffers.

Vertex Buffers: Contain vertex attributes like position, normals, texture coordinates, and color.
Index Buffers: Define the order in which vertices are processed to form primitives, allowing for efficient reuse of vertex data.

2. Vertex Shader Stage

The vertex shader is a programmable stage that processes each vertex individually. Its primary roles include:

Transforming vertex positions from model space to clip space.
Calculating per-vertex attributes like lighting information or texture coordinates.
Passing data to subsequent stages.

// Example Vertex Shader (HLSL)
struct VertexInput {
    float4 position : POSITION;
    float2 texCoord : TEXCOORD;
};

struct VertexOutput {
    float4 clipPos : SV_POSITION;
    float2 texCoord : TEXCOORD;
};

VertexOutput main(VertexInput input) {
    VertexOutput output;
    output.clipPos = mul(worldViewProjectionMatrix, input.position);
    output.texCoord = input.texCoord;
    return output;
}

3. Tessellation Stages (Optional)

These stages (Hull Shader, Tessellator, Domain Shader) allow for dynamic subdivision of primitives on the GPU, enabling higher detail in scenes by generating more geometry from less complex base models. They are crucial for advanced techniques like adaptive tessellation.

4. Geometry Shader Stage (Optional)

The geometry shader is a programmable stage that can process entire primitives (points, lines, triangles). It can:

Generate new primitives from existing ones.
Discard primitives.
Modify the vertex data of primitives.

While powerful, geometry shaders can be performance-intensive and are often replaced by compute shaders or tessellation for modern workflows.

5. Rasterization Stage

This fixed-function stage takes the transformed primitives (typically triangles) and determines which pixels on the screen are covered by each primitive. It performs:

Clipping: Removes geometry outside the view frustum.
Triangle Setup: Calculates edge functions and interpolates vertex attributes across the triangle.
Pixel Generation: Generates potential fragments (pixel candidates) for each covered pixel.

6. Pixel Shader (Fragment Shader) Stage

The pixel shader is the most computationally intensive programmable stage. It runs for each fragment generated by the rasterizer and determines the final color of the pixel. Its tasks include:

Sampling textures.
Calculating lighting and shading.
Applying materials and effects.
Outputting the final color to a render target.

// Example Pixel Shader (HLSL)
Texture2D diffuseTexture : DIFFUSE_TEXTURE;
SamplerState samplerState : SAMPLER_STATE;

struct PixelInput {
    float2 texCoord : TEXCOORD;
};

float4 main(PixelInput input) : SV_TARGET {
    return diffuseTexture.Sample(samplerState, input.texCoord);
}

7. Output Merger Stage

This final stage blends the output of the pixel shader with the existing contents of the render targets. It performs operations such as:

Depth Testing: Determines if a fragment is visible or occluded by previously rendered geometry.
Stencil Testing: Allows for advanced rendering effects using a stencil buffer.
Blending: Combines the new fragment color with the existing pixel color (e.g., for transparency).
Render Target Output: Writes the final pixel color to the frame buffer.

Key Concepts

Understanding the flow of data and transformations through these stages is crucial for efficient and high-quality graphics rendering. Each programmable stage offers opportunities for customization and optimization.

Modern Rendering Techniques

While the core pipeline remains, many modern rendering techniques leverage and extend these stages:

Deferred Rendering: Separates geometry processing from lighting calculations, improving performance for scenes with many lights.
Physically Based Rendering (PBR): Aims to simulate the physical behavior of light interactions with surfaces for more realistic visuals.
Compute Shaders: General-purpose computation on the GPU, often used for post-processing effects, physics simulations, or even parts of the rendering pipeline itself.

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