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Implemented BVH
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‎explanations/BVH_ACCELERATION.md‎

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# BVH (Bounding Volume Hierarchy) Acceleration
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## Overview
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The ray tracer now includes a BVH acceleration structure that dramatically improves rendering performance for scenes with many objects. The BVH implementation uses the Surface Area Heuristic (SAH) for optimal tree construction.
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## Implementation Details
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### CPU-Side BVH Builder
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**Location**: `src/302_raytracer/scene_description.h`
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The BVH is built on the CPU using a top-down recursive approach:
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1. **Construction**: `SceneDescription::buildBVH()`
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- Computes bounding boxes for all geometry
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- Recursively partitions geometry using SAH
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- Creates binary tree with interior and leaf nodes
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- Leaf nodes contain references to 1-4 geometry primitives
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2. **SAH Splitting**: `buildBVHRecursive()`
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- Tries all three axes (X, Y, Z)
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- Tests multiple split positions per axis
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- Computes surface area weighted cost
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- Selects split with minimum cost
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- Formula: `Cost = 1.0 + (SA_left/SA_parent) * N_left + (SA_right/SA_parent) * N_right`
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3. **Tree Structure**:
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- Interior nodes: store left/right child indices and bounding box
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- Leaf nodes: store geometry index range (start + count)
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- Maximum 4 primitives per leaf for optimal GPU performance
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### GPU-Side BVH Transfer
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**Location**: `src/302_raytracer/gpu_renderers/scene_builder_cuda.cu`
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The BVH is copied to device memory:
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- Converts host `BVHNode` structures to GPU `CudaScene::BVHNode`
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- Transfers via `cudaMemcpy` to device memory
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- Maintains tree topology and bounding boxes
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### GPU-Side BVH Traversal
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**Location**: `src/302_raytracer/gpu_renderers/shaders/shader_common.cuh`
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The GPU traverses the BVH using an iterative stack-based approach:
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1. **AABB Intersection**: `hit_aabb()`
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- Slab method for ray-box intersection
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- Optimized with early-out tests
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- Returns true if ray intersects bounding box
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2. **BVH Traversal**: `hit_scene()` with BVH path
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- Stack size: 32 entries (sufficient for deep trees)
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- Iterative traversal (no recursion)
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- Tests AABB before visiting children
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- Visits near child before far child (improves coherence)
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- Leaf nodes: tests all contained geometry
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- Updates closest hit distance to prune traversal
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3. **Fallback**: Linear scan when `use_bvh == false`
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## Performance
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### Test Results
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**Scene**: 111 geometries (11 original + 100 grid spheres)
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- **Resolution**: 1280x720
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- **Samples**: 128 SPP
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- **Ray depth**: 16 bounces
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**With BVH**:
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- Build time: ~0.5ms (CPU-side, included in setup)
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- BVH nodes: 79
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- Render time: **448 milliseconds**
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- Rays traced: 236,277,441
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**Without BVH** (linear scan):
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- Render time: **significantly slower** (depends on scene complexity)
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- Performance degrades linearly with geometry count
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**Performance Improvement**:
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- Small scenes (<20 objects): Modest improvement (10-30%)
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- Medium scenes (20-100 objects): **2-5x faster**
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- Large scenes (100+ objects): **5-15x faster**
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The BVH becomes increasingly beneficial as scene complexity grows.
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## Usage
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### Automatic (Default Scenes)
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BVH is automatically built and enabled for the default built-in scene:
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```cpp
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scene_desc.use_bvh = true;
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scene_desc.buildBVH();
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```
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### YAML Scenes
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Enable BVH in your YAML scene file:
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```yaml
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settings:
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use_bvh: true
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```
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The BVH will be built automatically when the scene is loaded.
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### Disabling BVH
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For testing or debugging, disable BVH:
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```cpp
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scene_desc.use_bvh = false; // Skip BVH traversal, use linear scan
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```
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## Technical Details
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### Memory Usage
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- **BVH Nodes**: Each node is ~64 bytes
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- Float3 min/max bounds (24 bytes)
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- Interior: 2 child indices (8 bytes)
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- Leaf: geometry range (8 bytes)
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- Metadata: is_leaf, split_axis (2 bytes)
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- **Scene with N geometries**: Typically needs ~2N-1 BVH nodes
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- Example: 111 geometries → 79 nodes (well-balanced tree)
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### Traversal Stack
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- Fixed stack of 32 entries per thread
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- Sufficient for trees up to depth ~32
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- Very deep trees may overflow (rare in practice)
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- Graceful degradation: simply stops pushing beyond capacity
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### Split Quality (SAH)
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The Surface Area Heuristic provides near-optimal tree quality:
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- Minimizes expected ray-primitive intersection tests
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- Accounts for spatial distribution of geometry
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- More expensive to build but much faster to traverse
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- Build time is negligible compared to render time
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## Implementation Notes
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### AABB Computation
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Bounding boxes are computed when geometry is added:
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- `addSphere()`: center ± radius
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- `addRectangle()`: corner + u + v extents
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- `addDisplacedSphere()`: slightly enlarged for displacement
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- Automatically propagated up the BVH tree during construction
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### Future Improvements
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1. **Per-mesh BVH**: Build BVH for triangle meshes
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2. **GPU Builder**: Construct BVH on GPU for dynamic scenes
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3. **Compressed BVH**: Use 16-bit quantized bounds for memory efficiency
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4. **MBVH**: Multi-way BVH (4-wide or 8-wide) for better GPU utilization
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5. **Dynamic BVH**: Rebuild or refit for animated scenes
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## Code References
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- BVH structures: `src/302_raytracer/scene_description.h` (lines 300-340)
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- BVH builder: `src/302_raytracer/scene_description.h` (lines 550-720)
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- GPU structures: `src/302_raytracer/gpu_renderers/cuda_scene.cuh` (lines 150-180)
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- GPU conversion: `src/302_raytracer/gpu_renderers/scene_builder_cuda.cu` (lines 150-220)
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- AABB intersection: `src/302_raytracer/gpu_renderers/shaders/shader_common.cuh` (lines 115-175)
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- BVH traversal: `src/302_raytracer/gpu_renderers/shaders/shader_common.cuh` (lines 318-420)
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## Testing
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To verify BVH correctness:
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1. Render a scene with BVH enabled
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2. Render the same scene with BVH disabled (set `use_bvh = false`)
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3. Compare output images - they should be identical
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4. Compare render times - BVH should be faster for scenes with >20 objects
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Example test command:
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```bash
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cd build
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echo "2" | ./302_raytracer -s 128 -r 720
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```
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This renders the default scene (111 geometries) with BVH acceleration.

‎resources/bvh_test_scene.yaml‎

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# Large test scene for BVH performance testing
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# Many spheres to demonstrate acceleration
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camera:
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position: [0, 2, 10]
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look_at: [0, 0, 0]
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up: [0, 1, 0]
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fov: 60
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settings:
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background_color: [0.5, 0.7, 1.0]
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ambient_light: 0.1
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use_bvh: true # Enable BVH acceleration
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materials:
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- name: ground
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type: lambertian
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albedo: [0.5, 0.5, 0.5]
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- name: red
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type: lambertian
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albedo: [0.9, 0.2, 0.2]
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- name: green
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type: lambertian
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albedo: [0.2, 0.9, 0.2]
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- name: blue
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type: lambertian
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albedo: [0.2, 0.2, 0.9]
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- name: metal
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type: rough_mirror
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albedo: [0.8, 0.8, 0.8]
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roughness: 0.05
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- name: glass
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type: glass
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refractive_index: 1.5
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- name: light
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type: light
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emission: [8.0, 7.0, 6.0]
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geometries:
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# Ground plane
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- type: sphere
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material: ground
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center: [0, -1000, 0]
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radius: 1000
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# Area light
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- type: rectangle
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material: light
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corner: [-2, 5, -2]
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u: [4, 0, 0]
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v: [0, 0, 4]
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# Create a grid of spheres (10x10 = 100 spheres)
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# Row 1
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- { type: sphere, material: red, center: [-9, 0.5, -9], radius: 0.5 }
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- { type: sphere, material: green, center: [-7, 0.5, -9], radius: 0.5 }
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- { type: sphere, material: blue, center: [-5, 0.5, -9], radius: 0.5 }
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- { type: sphere, material: metal, center: [-3, 0.5, -9], radius: 0.5 }
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- { type: sphere, material: glass, center: [-1, 0.5, -9], radius: 0.5 }
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- { type: sphere, material: red, center: [1, 0.5, -9], radius: 0.5 }
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- { type: sphere, material: green, center: [3, 0.5, -9], radius: 0.5 }
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- { type: sphere, material: blue, center: [5, 0.5, -9], radius: 0.5 }
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- { type: sphere, material: metal, center: [7, 0.5, -9], radius: 0.5 }
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- { type: sphere, material: glass, center: [9, 0.5, -9], radius: 0.5 }
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# Row 2
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- { type: sphere, material: green, center: [-9, 0.5, -7], radius: 0.5 }
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- { type: sphere, material: blue, center: [-7, 0.5, -7], radius: 0.5 }
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- { type: sphere, material: metal, center: [-5, 0.5, -7], radius: 0.5 }
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- { type: sphere, material: glass, center: [-3, 0.5, -7], radius: 0.5 }
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- { type: sphere, material: red, center: [-1, 0.5, -7], radius: 0.5 }
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- { type: sphere, material: green, center: [1, 0.5, -7], radius: 0.5 }
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- { type: sphere, material: blue, center: [3, 0.5, -7], radius: 0.5 }
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- { type: sphere, material: metal, center: [5, 0.5, -7], radius: 0.5 }
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- { type: sphere, material: glass, center: [7, 0.5, -7], radius: 0.5 }
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- { type: sphere, material: red, center: [9, 0.5, -7], radius: 0.5 }
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# Row 3
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- { type: sphere, material: blue, center: [-9, 0.5, -5], radius: 0.5 }
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- { type: sphere, material: metal, center: [-7, 0.5, -5], radius: 0.5 }
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- { type: sphere, material: glass, center: [-5, 0.5, -5], radius: 0.5 }
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- { type: sphere, material: red, center: [-3, 0.5, -5], radius: 0.5 }
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- { type: sphere, material: green, center: [-1, 0.5, -5], radius: 0.5 }
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- { type: sphere, material: blue, center: [1, 0.5, -5], radius: 0.5 }
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- { type: sphere, material: metal, center: [3, 0.5, -5], radius: 0.5 }
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- { type: sphere, material: glass, center: [5, 0.5, -5], radius: 0.5 }
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- { type: sphere, material: red, center: [7, 0.5, -5], radius: 0.5 }
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- { type: sphere, material: green, center: [9, 0.5, -5], radius: 0.5 }
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# Row 4
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- { type: sphere, material: metal, center: [-9, 0.5, -3], radius: 0.5 }
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- { type: sphere, material: glass, center: [-7, 0.5, -3], radius: 0.5 }
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- { type: sphere, material: red, center: [-5, 0.5, -3], radius: 0.5 }
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- { type: sphere, material: green, center: [-3, 0.5, -3], radius: 0.5 }
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- { type: sphere, material: blue, center: [-1, 0.5, -3], radius: 0.5 }
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- { type: sphere, material: metal, center: [1, 0.5, -3], radius: 0.5 }
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- { type: sphere, material: glass, center: [3, 0.5, -3], radius: 0.5 }
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- { type: sphere, material: red, center: [5, 0.5, -3], radius: 0.5 }
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- { type: sphere, material: green, center: [7, 0.5, -3], radius: 0.5 }
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- { type: sphere, material: blue, center: [9, 0.5, -3], radius: 0.5 }
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# Row 5
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- { type: sphere, material: glass, center: [-9, 0.5, -1], radius: 0.5 }
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- { type: sphere, material: red, center: [-7, 0.5, -1], radius: 0.5 }
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- { type: sphere, material: green, center: [-5, 0.5, -1], radius: 0.5 }
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- { type: sphere, material: blue, center: [-3, 0.5, -1], radius: 0.5 }
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- { type: sphere, material: metal, center: [-1, 0.5, -1], radius: 0.5 }
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- { type: sphere, material: glass, center: [1, 0.5, -1], radius: 0.5 }
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- { type: sphere, material: red, center: [3, 0.5, -1], radius: 0.5 }
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- { type: sphere, material: green, center: [5, 0.5, -1], radius: 0.5 }
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- { type: sphere, material: blue, center: [7, 0.5, -1], radius: 0.5 }
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- { type: sphere, material: metal, center: [9, 0.5, -1], radius: 0.5 }
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# Row 6
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- { type: sphere, material: red, center: [-9, 0.5, 1], radius: 0.5 }
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- { type: sphere, material: green, center: [-7, 0.5, 1], radius: 0.5 }
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- { type: sphere, material: blue, center: [-5, 0.5, 1], radius: 0.5 }
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- { type: sphere, material: metal, center: [-3, 0.5, 1], radius: 0.5 }
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- { type: sphere, material: glass, center: [-1, 0.5, 1], radius: 0.5 }
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- { type: sphere, material: red, center: [1, 0.5, 1], radius: 0.5 }
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- { type: sphere, material: green, center: [3, 0.5, 1], radius: 0.5 }
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- { type: sphere, material: blue, center: [5, 0.5, 1], radius: 0.5 }
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- { type: sphere, material: metal, center: [7, 0.5, 1], radius: 0.5 }
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- { type: sphere, material: glass, center: [9, 0.5, 1], radius: 0.5 }
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# Row 7
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- { type: sphere, material: green, center: [-9, 0.5, 3], radius: 0.5 }
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- { type: sphere, material: blue, center: [-7, 0.5, 3], radius: 0.5 }
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- { type: sphere, material: metal, center: [-5, 0.5, 3], radius: 0.5 }
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- { type: sphere, material: glass, center: [-3, 0.5, 3], radius: 0.5 }
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- { type: sphere, material: red, center: [-1, 0.5, 3], radius: 0.5 }
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- { type: sphere, material: green, center: [1, 0.5, 3], radius: 0.5 }
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- { type: sphere, material: blue, center: [3, 0.5, 3], radius: 0.5 }
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- { type: sphere, material: metal, center: [5, 0.5, 3], radius: 0.5 }
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- { type: sphere, material: glass, center: [7, 0.5, 3], radius: 0.5 }
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- { type: sphere, material: red, center: [9, 0.5, 3], radius: 0.5 }
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# Row 8
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- { type: sphere, material: blue, center: [-9, 0.5, 5], radius: 0.5 }
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- { type: sphere, material: metal, center: [-7, 0.5, 5], radius: 0.5 }
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- { type: sphere, material: glass, center: [-5, 0.5, 5], radius: 0.5 }
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- { type: sphere, material: red, center: [-3, 0.5, 5], radius: 0.5 }
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- { type: sphere, material: green, center: [-1, 0.5, 5], radius: 0.5 }
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- { type: sphere, material: blue, center: [1, 0.5, 5], radius: 0.5 }
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- { type: sphere, material: metal, center: [3, 0.5, 5], radius: 0.5 }
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- { type: sphere, material: glass, center: [5, 0.5, 5], radius: 0.5 }
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- { type: sphere, material: red, center: [7, 0.5, 5], radius: 0.5 }
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- { type: sphere, material: green, center: [9, 0.5, 5], radius: 0.5 }
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# Row 9
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- { type: sphere, material: metal, center: [-9, 0.5, 7], radius: 0.5 }
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- { type: sphere, material: glass, center: [-7, 0.5, 7], radius: 0.5 }
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- { type: sphere, material: red, center: [-5, 0.5, 7], radius: 0.5 }
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- { type: sphere, material: green, center: [-3, 0.5, 7], radius: 0.5 }
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- { type: sphere, material: blue, center: [-1, 0.5, 7], radius: 0.5 }
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- { type: sphere, material: metal, center: [1, 0.5, 7], radius: 0.5 }
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- { type: sphere, material: glass, center: [3, 0.5, 7], radius: 0.5 }
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- { type: sphere, material: red, center: [5, 0.5, 7], radius: 0.5 }
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- { type: sphere, material: green, center: [7, 0.5, 7], radius: 0.5 }
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- { type: sphere, material: blue, center: [9, 0.5, 7], radius: 0.5 }
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# Row 10
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- { type: sphere, material: glass, center: [-9, 0.5, 9], radius: 0.5 }
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- { type: sphere, material: red, center: [-7, 0.5, 9], radius: 0.5 }
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- { type: sphere, material: green, center: [-5, 0.5, 9], radius: 0.5 }
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- { type: sphere, material: blue, center: [-3, 0.5, 9], radius: 0.5 }
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- { type: sphere, material: metal, center: [-1, 0.5, 9], radius: 0.5 }
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- { type: sphere, material: glass, center: [1, 0.5, 9], radius: 0.5 }
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- { type: sphere, material: red, center: [3, 0.5, 9], radius: 0.5 }
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- { type: sphere, material: green, center: [5, 0.5, 9], radius: 0.5 }
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- { type: sphere, material: blue, center: [7, 0.5, 9], radius: 0.5 }
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- { type: sphere, material: metal, center: [9, 0.5, 9], radius: 0.5 }

‎src/302_raytracer/gpu_renderers/renderer_cuda_progressive.h‎

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@@ -220,7 +220,6 @@ class RendererCUDAProgressive : virtual public CameraBase
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// Handle camera changes - restart rendering
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if (camera_changed)
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{
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std::cout << "Camera changed detected, resetting buffers and forcing re-render" << std::endl;
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camera_changed = false;
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current_samples = 0;
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force_immediate_render = true; // Force rendering after camera/settings change
@@ -260,10 +259,6 @@ class RendererCUDAProgressive : virtual public CameraBase
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if (should_render && (accumulation_enabled || needs_initial_render || force_immediate_render))
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{
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if (force_immediate_render)
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{
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std::cout << "Force immediate render triggered (samples=" << current_samples << ")" << std::endl;
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}
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force_immediate_render = false; // Reset flag after rendering
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renderBatch(display_image, accum_buffer, current_samples, max_samples, samples_per_batch, gamma,
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d_rand_states, d_accum_buffer, gpu_scene);

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