DirectX Computational Graphics

Physically Based Rendering (PBR)

Physically Based Rendering aims to simulate the behavior of light in the real world more accurately. Instead of using artistic guesses for specular highlights and diffuse color, PBR relies on material properties that are physically measurable, such as:

• Albedo: The base color of the surface.
• Metallic: Indicates whether the surface is a metal or a dielectric.
• Roughness: Controls the micro-facet distribution, affecting the sharpness of reflections.
• Specular/IOR (Index of Refraction): For non-metals, defines how light reflects.

DirectX supports PBR workflows through its shader model capabilities. A typical PBR shader might look like this (simplified HLSL conceptual example):

// Simplified PBR lighting function
float3 CalculatePBR(float3 N, float3 V, float3 L,
                    float3 albedo, float metallic, float roughness) {
    float NdotL = max(dot(N, L), 0.0);
    float NdotV = max(dot(N, V), 0.0);

    // Calculate Fresnel terms, Specular, Diffuse, etc.
    // ... (complex calculations based on PBR models like Cook-Torrance)

    float3 specular = CalculateSpecular(N, V, L, metallic, roughness);
    float3 diffuse = CalculateDiffuse(albedo, metallic, roughness);

    return (diffuse + specular) * NdotL;
}
                

Compute Shaders

Compute shaders are a versatile addition to the graphics pipeline, allowing the GPU to be used for general-purpose computation (GPGPU). They are not tied to the traditional rendering pipeline stages and can be invoked independently.

Applications include:

• Complex physics simulations.
• Advanced image processing and post-effects.
• AI and machine learning inference.
• Particle systems.

Key concepts include thread groups, UAVs (Unordered Access Views), and read-only textures. A basic compute shader structure in HLSL:

// Example of a simple compute shader for data manipulation
#define THREAD_GROUP_SIZE_X 8
#define THREAD_GROUP_SIZE_Y 8

RWStructuredBuffer<float4> outputBuffer : register(u0);
StructuredBuffer<float4> inputBuffer : register(t0);

[numthreads(THREAD_GROUP_SIZE_X, THREAD_GROUP_SIZE_Y, 1)]
void CSMain(uint3 dispatchThreadID : SV_DispatchThreadID) {
    uint index = dispatchThreadID.x + dispatchThreadID.y * 1024; // Assuming 1024 width
    outputBuffer[index] = inputBuffer[index] * 2.0;
}
                

Tessellation

Tessellation allows for dynamic subdivision of geometry at runtime, enabling the creation of highly detailed surfaces from low-polygon models. This is particularly useful for adding detail to terrains, characters, or complex organic shapes.

DirectX features three tessellation stages:

• Hull Shader: Controls the tessellation factors and outputs control points for the patches.
• Tessellator Stage: Performs the geometric subdivision based on tessellation factors.
• Domain Shader: Computes the final vertex positions in the new tessellated geometry.

This technique can significantly reduce the need for high-poly models and allows for adaptive detail based on camera distance or screen space coverage.

GPU Profiling and Analysis

Tools like the DirectX PIX are essential for understanding GPU performance. PIX allows developers to capture frame captures and analyze GPU performance at a granular level.

Key metrics to monitor include:

• GPU Occupancy: How effectively the GPU's processing units are utilized.
• Draw Call Overhead: The cost of submitting drawing commands.
• Shader Execution Time: The time taken for shaders to complete.
• Memory Bandwidth: Data transfer rates between CPU and GPU, and between GPU memory and caches.
• Vertex/Pixel Throughput: The number of vertices or pixels processed per unit of time.

Optimizations often involve reducing draw calls, improving shader efficiency, and managing memory effectively.

Culling Techniques

Culling is the process of discarding geometry that will not be visible to the camera, thereby reducing the workload on the GPU.

• Frustum Culling: Objects outside the camera's viewing frustum are not rendered.
• Occlusion Culling: Objects hidden behind other opaque objects are not rendered. Techniques include hardware occlusion queries and software occlusion culling (e.g., Hierarchical Z-Buffer).
• Back-face Culling: Polygons whose normals face away from the camera are discarded.

Level of Detail (LOD)

LOD is a technique where simpler representations of an object are used when it is further away from the camera. This reduces the number of vertices and triangles that need to be processed, significantly improving performance without a noticeable degradation in visual quality.

DirectX can implement LOD through:

• Pre-authored mesh variations.
• Procedural LOD generation.
• Tessellation dynamically adjusting detail.

Ray Tracing and Path Tracing

Ray tracing and its more advanced form, path tracing, simulate the path of light rays through a scene. Unlike rasterization, which projects 3D geometry onto a 2D screen, ray tracing traces rays from the camera into the scene, simulating reflections, refractions, and shadows with unparalleled realism.

DirectX Raytracing (DXR) provides a framework for leveraging GPU hardware acceleration for ray tracing. This enables:

• Soft shadows
• Accurate reflections and refractions
• Global illumination effects

DXR introduces new shader stages like ray-generation, closest-hit, any-hit, and miss shaders to manage the ray tracing process.

Global Illumination (GI)

Global illumination refers to techniques that simulate the indirect lighting in a scene – how light bounces off surfaces and illuminates other surfaces. This creates more natural and believable lighting compared to techniques that only consider direct light sources.

Methods include:

• Baked Lighting: Pre-calculating lightmaps for static scenes.
• Screen-Space Global Illumination (SSGI): Approximations using screen-space information.
• Real-time Ray-Traced GI: Leveraging DXR for dynamic indirect lighting.

GI is critical for achieving photorealism in games and architectural visualizations.

Post-Processing Effects

Post-processing effects are applied to the final rendered image after the main scene geometry has been drawn. These effects are typically implemented using pixel shaders or compute shaders.

Common examples include:

• Bloom: Simulates the effect of bright light sources spreading.
• Depth of Field (DoF): Mimics the focus effects of a camera lens.
• Motion Blur: Simulates the blurring of fast-moving objects.
• Color Grading: Adjusting the color balance and tone of the image.
• Anti-aliasing (e.g., MSAA, FXAA, TAA): Smoothing out jagged edges.

Modern engines often combine multiple post-processing passes to achieve complex visual styles.