09 Motion Blur - Tutorial¶

09 Motion blur result

This tutorial demonstrates how to implement realistic motion blur in ray tracing using the VK_NV_ray_tracing_motion_blur extension. Motion blur simulates the temporal effects of fast-moving objects by interpolating between different states over time and accumulating multiple temporal samples per pixel.

Key Changes from 02_basic.cpp¶

1. Extension Setup¶

Modified: Application initialization - Added VK_NV_ray_tracing_motion_blur extension support - Enabled motion blur features in Vulkan context initialization - This extension is required for all motion blur functionality

2. Scene Data Changes¶

Modified: createScene() - Added a modified cube mesh with vertex position changes for vertex motion demonstration - Created three colored cubes to demonstrate different motion types: - Green cube: Matrix motion (translation) - Red cube: SRT motion (rotation) - Blue cube: Vertex motion (morphing)

3. Acceleration Structure Changes¶

Modified: createBottomLevelAS() and createTopLevelAS() - Motion-enabled BLAS creation using VK_BUILD_ACCELERATION_STRUCTURE_MOTION_BIT_NV - Vertex motion BLAS with dual vertex data (T0 and T1 positions) - Motion instances with 160-byte stride alignment requirement - Three motion instance types: Matrix, SRT, and Vertex motion

4. Shader Changes¶

Modified: Ray generation shader - Replaced TraceRay() with TraceMotionRay() for temporal ray tracing - Added temporal sampling loop with random time generation - Sample accumulation and averaging for smooth motion blur

[shader("raygeneration")]
void rgenMain()
{
    // Accumulate multiple temporal samples
    float3 result = float3(0, 0, 0);
    for(int i = 0; i < pushConst.numSamples; i++)
    {
        float time = rand(seed);  // Random time between 0.0-1.0
        TraceMotionRay(topLevelAS, rayFlags, 0xff, 0, 0, 0, ray, time, payload);
        result += payload.color;
    }
    outImage[int2(launchID)] = float4(result / float(pushConst.numSamples), 1.0);
}

5. Pipeline Changes¶

Modified: createRayTracingPipeline() - Added VK_PIPELINE_CREATE_RAY_TRACING_ALLOW_MOTION_BIT_NV flag - Enables motion blur support in the ray tracing pipeline

6. UI Changes¶

Modified: onUIRender() - Added sample count slider for motion blur quality control - Explanatory text about the three motion types - Performance guidance for sample count selection

How It Works¶

Motion blur in ray tracing works by creating acceleration structures that can interpolate between two states (T0 and T1) based on a time parameter. When tracing rays, each ray gets a random time value between 0.0 and 1.0, causing the acceleration structure to interpolate the object's position, rotation, or vertex positions at that specific moment in time.

The final image is created by accumulating multiple temporal samples per pixel and averaging the results, creating the characteristic motion blur effect.

Motion Blur Types¶

Matrix Motion¶

Purpose: General object transformation (translation, scaling, rotation)
Data: Two 4x4 transformation matrices
Interpolation: Linear matrix interpolation
Use Case: Moving objects, camera motion

SRT Motion¶

Purpose: Independent scale, rotation, and translation
Data: Separate scale, rotation (quaternion), and translation components
Interpolation: Component-wise (slerp for rotation)
Use Case: Complex animations with independent motion components

Vertex Motion¶

Purpose: Object deformation and morphing
Data: Two complete sets of vertex positions
Interpolation: Per-vertex linear interpolation
Use Case: Character animation, fluid simulation, morphing effects

Benefits¶

Realistic Motion: Simulates real-world camera and object motion blur
Temporal Anti-aliasing: Reduces temporal flickering and aliasing
Performance: Hardware-accelerated motion blur without post-processing
Flexibility: Three motion types cover most animation scenarios

Technical Details¶

Performance Considerations¶

Sample Count: 10-20 samples recommended for good quality/performance balance
Memory Overhead: Motion instances are 2.5x larger than standard instances (160 vs 64 bytes)
Build Time: Motion-enabled acceleration structures take longer to build
Shader Overhead: TraceMotionRay() has additional computational cost

Memory Requirements¶

Motion instances require 160-byte alignment
Vertex motion BLAS needs storage for both T0 and T1 vertex data
Increased scratch buffer requirements for motion-enabled builds

Quality vs Performance¶

1-5 samples: Fast but noisy motion blur
10-20 samples: Good quality (recommended)
50-100 samples: Very smooth but significantly slower

Usage Instructions¶

Adjust Sample Count: Use the UI slider to control motion blur quality
Observe Motion Types: Each cube demonstrates a different motion technique
Performance Tuning: Start with low sample counts and increase as needed
Debug Mode: Set samples to 1 to verify motion setup without accumulation

The motion blur effect becomes more pronounced with higher sample counts, but performance decreases proportionally. The three motion types can be combined in the same scene for complex animations.