Core Stages of the Graphics Pipeline

The modern graphics pipeline, particularly in DirectX, can be conceptually divided into several key stages. While the exact implementation details vary between hardware and API versions, the fundamental flow remains consistent:

1. Input Assembler (IA) Stage

This stage is responsible for fetching vertex data from the application and organizing it into primitives like points, lines, or triangles. The data is typically read from vertex buffers and index buffers.

• Vertex Buffers: Contain per-vertex data such as position, color, texture coordinates, and normals.
• Index Buffers: Provide indices into the vertex buffer to define which vertices form each primitive, allowing for data reuse and efficiency.

2. Vertex Shader (VS) Stage

The vertex shader operates on each vertex individually. Its primary tasks include transforming vertex positions from model space to clip space and performing per-vertex calculations like lighting and color blending. The output of the vertex shader is a set of transformed vertices.

// Example of a simplified vertex shader input and output structure
struct VS_INPUT {
    float4 Position : POSITION;
    float2 TexCoord : TEXCOORD;
};

struct VS_OUTPUT {
    float4 Position : SV_POSITION; // Clip-space position
    float2 TexCoord : TEXCOORD;
};

VS_OUTPUT main(VS_INPUT input) {
    VS_OUTPUT output;
    // Transform position
    output.Position = mul(input.Position, WorldViewProjectionMatrix);
    // Pass through texture coordinates
    output.TexCoord = input.TexCoord;
    return output;
}

3. Hull Shader (HS) / Domain Shader (DS) Stages (Tessellation)

These optional stages enable hardware tessellation, allowing for dynamic subdivision of primitives to create complex geometry on the fly. The Hull Shader determines the tessellation factors, and the Domain Shader generates new vertices within the tessellated patches.

4. Geometry Shader (GS) Stage

The geometry shader can operate on entire primitives (points, lines, triangles) rather than individual vertices. It can create new primitives, discard existing ones, or modify vertex attributes. This stage is less commonly used in high-performance scenarios compared to tessellation.

5. Rasterizer (RS) Stage

The rasterizer converts the geometric primitives (output from previous stages) into a set of pixel fragments that cover the screen. It performs tasks like clipping primitives to the view frustum and determining which screen pixels are covered by each primitive.

6. Pixel Shader (PS) Stage

The pixel shader, also known as the fragment shader, operates on each pixel fragment generated by the rasterizer. Its main role is to determine the final color of each pixel, often involving texture sampling, lighting calculations, and other per-pixel effects. The output is a color value that is written to the render target.

// Example of a simplified pixel shader
struct PS_INPUT {
    float4 Position : SV_POSITION;
    float2 TexCoord : TEXCOORD;
};

Texture2D DiffuseTexture;
SamplerState Sampler;

float4 main(PS_INPUT input) : SV_TARGET {
    // Sample texture color
    float4 texColor = DiffuseTexture.Sample(Sampler, input.TexCoord);
    // Return final pixel color
    return texColor;
}

7. Output Merger (OM) Stage

This final stage combines the pixel shader output with the contents of the render target and depth/stencil buffers. It handles blending, depth testing, stencil testing, and write operations to produce the final image on the screen.

• Render Target: The buffer where the final pixel colors are written.
• Depth Buffer: Used for depth testing (Z-buffering) to determine which objects are in front of others.
• Stencil Buffer: Used for more complex masking and effect operations.

DirectX 12 and the Modern Pipeline

DirectX 12 introduces significant changes to the graphics pipeline, emphasizing explicit control and reduced CPU overhead. While the core stages remain conceptually similar, DirectX 12 allows for more fine-grained management of resources and pipeline states:

• Programmable Pipeline: Most stages are programmable by shaders written in High-Level Shading Language (HLSL).
• Pipeline State Objects (PSOs): In DirectX 12, the graphics pipeline state is encapsulated in a PSO, which is created upfront and binds together various states like shaders, render target formats, and blending modes. This reduces runtime state changes.
• Command Lists: Commands are recorded into command lists and then submitted for execution by the GPU, enabling parallel command list generation.

Key Concepts and APIs

• HLSL (High-Level Shading Language): The shader language used for programming the programmable stages of the graphics pipeline.
• DirectX Device: The primary interface to the graphics hardware.
• Swap Chain: Manages the presentation of rendered frames to the display.
• Render Target View (RTV): A view into a render target resource.
• Depth Stencil View (DSV): A view into a depth-stencil resource.
• Shader Resource View (SRV): A view into a resource that can be read by shaders (e.g., textures).
• Unordered Access View (UAV): A view into a resource that can be read and written by shaders.