MSDN Documentation

Physically Based Rendering (PBR)

Physically Based Rendering aims to simulate the way light interacts with surfaces in the real world. This approach leads to more realistic and consistent visuals across different lighting conditions.

Key PBR concepts include:

• Microfacet Theory: Modeling surface roughness at a microscopic level.
• Energy Conservation: Ensuring that reflected light does not exceed incoming light.
• Material Properties: Using parameters like Albedo, Metallic, Roughness, and Specular.

In DirectX, PBR is typically implemented using shaders. Here's a simplified example of a PBR fragment shader calculation:

// Simplified PBR lighting equation fragment shader

float3 NdotV = max(dot(normal, viewDir), 0.0);
float3 F = fresnelSchlick(max(dot(halfDir, viewDir), 0.0), metallic, roughness);

float NdotL = max(dot(normal, lightDir), 0.0);
float LoH = max(dot(lightDir, halfDir), 0.0);
float NdotH = max(dot(normal, halfDir), 0.0);

float G = geometricOcclusion(NdotV, NdotL, roughness);
float D = distributionGGX(NdotH, roughness);

float3 numerator = D * G * F;
float denominator = 4.0 * NdotV * NdotL + 0.001; // Add epsilon to avoid division by zero
float3 specular = numerator / denominator;

// Add diffuse component (e.g., Cook-Torrance specular model)
float3 k_s = F;
float3 k_d = (1.0 - k_s) * (1.0 - metallic); // Diffuse color for non-metals

// Simplified diffuse term
float3 diffuse = k_d * albedo / PI;

float3 color = diffuse + specular;

// Apply lighting and tone mapping...
            

Refer to the PBR Shaders documentation for detailed implementations.

Global Illumination Techniques

Global Illumination (GI) accounts for indirect lighting, where light bounces off surfaces and illuminates other parts of the scene. This adds significant realism.

Screen Space Global Illumination (SSGI)

SSGI is a real-time approximation of GI that uses information available in screen-space buffers (depth, normals, color).

• Ray Marching: Casting short rays from the current pixel into screen space to find nearby scene geometry.
• Probe Tracing: Simulating light probes that capture indirect illumination.

Baked Lighting

For static scenes, lightmaps can be pre-computed to store indirect lighting information. This is computationally inexpensive at runtime.

Explore the Global Illumination Overview for more details.

Real-time Ray Tracing

DirectX Raytracing (DXR) enables real-time ray tracing on compatible hardware, offering superior accuracy for effects like:

• Ray-Traced Reflections: Realistic reflections that account for multiple bounces.
• Ray-Traced Shadows: Soft and accurate shadows.
• Ray-Traced Ambient Occlusion (RTAO): Natural shadowing in crevices and corners.

DXR utilizes shader stages like Ray Generation, Intersection, Any Hit, Closest Hit, Miss, and Callable shaders.

// Example DXR Ray Generation Shader snippet

struct RayPayload {
    float3 Color;
    uint   RaysMissed;
};

[shader("raygeneration")]
void RGShader() {
    uint2 launchID = DispatchRaysIndex();
    uint2 dimension = DispatchRaysDimensions();

    float u = (float(launchID.x) + 0.5) / float(dimension.x);
    float v = (float(launchID.y) + 0.5) / float(dimension.y);

    float3 origin;
    float3 direction;
    // Calculate ray origin and direction based on camera
    
    RayDesc ray;
    ray.Origin = origin;
    ray.Direction = direction;
    ray.TMin = 0.001; // Small epsilon to avoid self-intersection
    ray.HitGroupMask = 1;
    ray.MaxPayloadSizeInBytes = sizeof(RayPayload);
    ray.RecursionDepth = MAX_RECURSION_DEPTH;

    RayPayload payload;
    payload.Color = float3(0.0, 0.0, 0.0);
    payload.RaysMissed = 0;

    TraceRay(
        SceneObject(), // Acceleration structure
        RAY_FLAG_FORCE_OPAQUE | RAY_FLAG_SKIP_PROCEDURAL_PRIMITIVES, // Ray flags
        0xFF, // Instance index mask
        0, // Ray type index
        0, // Hit group index
        0, // Closest hit shader index
        1, // Any hit shader index
        2, // Miss shader index
        1, // Payload index
        &payload // Payload pointer
    );

    OutputImage[launchID] = payload.Color;
}
            

Learn more in the DXR Introduction.

Compute Shaders for Simulation

Compute shaders are powerful for general-purpose computation on the GPU, beyond graphics rendering. They are ideal for simulations:

• Particle Systems: Simulating smoke, fire, fluids, or crowds.
• Physics Engines: Collision detection, cloth simulation, rigid body dynamics.
• Data Processing: Complex algorithms that benefit from massive parallelism.

Key compute shader features include shared memory, thread groups, and UAVs (Unordered Access Views) for data read/write.

// Example Compute Shader for Particle Velocity Update

struct Particle {
    float3 Position;
    float3 Velocity;
    float Life;
};

RWStructuredBuffer<Particle> Particles : register(u0);

// Other parameters like deltaTime, forces, etc.

[numthreads(64, 1, 1)]
void CSMain(uint3 id : SV_DispatchThreadID) {
    Particle p = Particles[id.x];

    if (p.Life <= 0.0) {
        return; // Particle is dead
    }

    // Apply forces (e.g., gravity, wind)
    float3 forces = float3(0.0, -9.8, 0.0);
    float3 acceleration = forces / p.Mass; // Assuming Mass is a uniform

    p.Velocity += acceleration * deltaTime;

    // Integrate position
    p.Position += p.Velocity * deltaTime;

    // Update life
    p.Life -= deltaTime;

    Particles[id.x] = p;
}
            

See Compute Shaders Overview for more.

Tessellation and Geometry Shaders

These programmable pipeline stages allow for dynamic mesh generation and modification:

Tessellation Shaders

• Hull Shader: Controls tessellation factors.
• Tessellator: Generates new vertices.
• Domain Shader: Computes the position and attributes of new vertices.

Used for creating detailed surfaces from low-polygon models, procedural displacement, and LOD.

Geometry Shaders

Can create or destroy primitives (points, lines, triangles) based on input primitives. Useful for:

• Generating billboards.
• Creating particle trails.
• Implementing GS-based physics.

Check out Tessellation and Geometry Shaders.

VR and AR Rendering

Rendering for Virtual and Augmented Reality presents unique challenges:

• Stereoscopic Rendering: Rendering two slightly different viewpoints for each eye.
• High Frame Rates: Typically 90Hz or higher is required to prevent motion sickness.
• Low Latency: Minimizing the delay between user movement and visual feedback.
• Distortion Correction: Applying lens distortion correction per eye.

DirectX provides APIs and best practices for integrating with VR/AR SDKs (e.g., OpenXR, SteamVR).

For more on this topic, visit VR/AR Development with DirectX.