Phase 3 — GPU and Vulkan Ray Tracing

• Moving from CPU to GPU. Vulkan’s ray tracing extension (VK_KHR_ray_tracing_pipeline) exposes hardware RT units (NVIDIA RTX, AMD RDNA2+).
• Parent: PathTracer Learning

3.1 Vulkan Ray Tracing Overview

• PathTracer Learning - Vulkan RT Pipeline
  • Full pipeline breakdown: ray gen, intersection, any-hit, closest-hit, miss shaders
  • Shader Binding Table (SBT) construction and indexing
• PathTracer Learning - BLAS and TLAS
  • How geometry is organized for hardware traversal
• Key extensions required
  • VK_KHR_ray_tracing_pipeline — the RT pipeline itself
  • VK_KHR_acceleration_structure — BLAS/TLAS management
  • VK_KHR_ray_query — inline ray queries from any shader stage
  • VK_KHR_buffer_device_address — GPU pointers (required for SBT)
  • VK_KHR_deferred_host_operations — async BLAS builds
  • VK_EXT_descriptor_indexing — bindless textures/buffers

3.2 Acceleration Structures in Vulkan

• PathTracer Learning - BLAS and TLAS
• BLAS (Bottom-Level Acceleration Structure)
  • Contains actual geometry: triangles or AABBs
  • Built once per mesh (or rebuilt for skinned meshes)
  • VkAccelerationStructureGeometryKHR — describes the geometry
  • VkAccelerationStructureBuildGeometryInfoKHR — build parameters
  • Build flags: PREFER_FAST_TRACE vs PREFER_FAST_BUILD vs ALLOW_UPDATE
  • Compaction: reduces memory by 50-70% after build
• TLAS (Top-Level Acceleration Structure)
  • Contains instances of BLASes with transform matrices
  • Rebuilt every frame (instances move, appear, disappear)
  • VkAccelerationStructureInstanceKHR:

    struct VkAccelerationStructureInstanceKHR {
        VkTransformMatrixKHR transform;        // 3x4 row-major matrix
        uint32_t instanceCustomIndex : 24;     // gl_InstanceCustomIndexEXT in shader
        uint32_t mask : 8;                     // visibility mask
        uint32_t instanceShaderBindingTableRecordOffset : 24;
        uint32_t flags : 8;
        uint64_t accelerationStructureReference; // BLAS device address
    };
    

• PathTracer Learning - Concept - Device Address Bit
  • VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT — required for SBT and AS input buffers
  • vkGetBufferDeviceAddress() returns a VkDeviceAddress (uint64)
  • This is a raw GPU pointer — enables pointer arithmetic in shaders

3.3 Shader Binding Table (SBT)

• See PathTracer Learning - Vulkan RT Pipeline for full details
• The SBT maps ray types to shader programs
• Layout: [RayGen | Miss shaders | Hit groups | Callable shaders]
• Each entry is a shader handle (32 bytes) + optional inline data
• Alignment requirements
  • shaderGroupHandleSize — typically 32 bytes
  • shaderGroupBaseAlignment — typically 64 bytes
  • Each region must be aligned to shaderGroupBaseAlignment
• Indexing formula
  • hitGroupIndex = instanceSBTOffset + geometryIndex * sbtStride + rayContributionToHitGroupIndex

3.4 Ray Tracing Shaders (GLSL)

• See PathTracer Learning - Vulkan RT Pipeline for complete shader examples
• Ray generation shader (.rgen)
  • Entry point for each pixel
  • Calls traceRayEXT() to launch rays
  • For path tracing: implement the full bounce loop here (not recursively)
• Closest-hit shader (.rchit)
  • Called for the nearest confirmed hit
  • Fetch vertex data via buffer device address (GL_EXT_buffer_reference)
  • Evaluate BRDF, trace shadow rays
• Miss shader (.rmiss)
  • Called when ray hits nothing
  • Sample environment map
• Any-hit shader (.rahit)
  • Called for every potential hit — use for alpha testing
  • ignoreIntersectionEXT() to reject transparent pixels

3.5 Bindless Resources

• Path tracers need access to many textures and buffers (one per mesh/material)
• Bindless: bind a large array of descriptors, index at runtime
• VK_EXT_descriptor_indexing — enables unbounded descriptor arrays

#extension GL_EXT_nonuniform_qualifier : require
layout(set=1, binding=0) uniform sampler2D textures[];
 
// In closest-hit shader:
vec4 albedo = texture(textures[nonuniformEXT(materialIndex)], uv);

• nonuniformEXT — required when index varies per invocation (non-uniform)
• Alternative: push constants or inline data in SBT entries

3.6 Iterative Path Tracing on GPU

• Recursive approach: hit shader calls traceRayEXT → limited by maxPipelineRayRecursionDepth
• Iterative approach (preferred): loop in ray generation shader

  // In rgen shader — full path tracing loop
  vec3 throughput = vec3(1.0);
  vec3 radiance   = vec3(0.0);
  Ray  ray        = generateCameraRay(pixel);
   
  for (int bounce = 0; bounce < MAX_BOUNCES; bounce++) {
      traceRayEXT(tlas, ..., ray.origin, 0.001, ray.direction, 1e4, 0);
      // payload contains: hit info, new ray direction, BRDF weight, direct light
      
      radiance   += throughput * payload.emission;
      radiance   += throughput * payload.directLight;
      throughput *= payload.brdfWeight;
      
      if (payload.missed) { radiance += throughput * payload.envLight; break; }
      if (all(lessThan(throughput, vec3(0.001)))) break;  // early termination
      
      ray = Ray(payload.nextOrigin, payload.nextDirection);
  }

• Payload carries all data needed between bounces

3.7 Async Compute

• PathTracer Learning Concept Async Compute
  • Run compute work in parallel with graphics on separate queue
  • BLAS builds can overlap with rendering
  • Denoising passes can overlap with next frame’s ray tracing

3.8 Ray Query (Inline RT)

• Alternative to RT pipeline: query rays from any shader stage
• VK_KHR_ray_query — no SBT, no separate shader stages

#extension GL_EXT_ray_query : require
 
// In a compute shader:
rayQueryEXT rq;
rayQueryInitializeEXT(rq, tlas, gl_RayFlagsOpaqueEXT, 0xFF,
                      origin, 0.001, direction, 1e4);
while (rayQueryProceedEXT(rq)) {}  // traverse
 
if (rayQueryGetIntersectionTypeEXT(rq, true) == gl_RayQueryCommittedIntersectionTriangleEXT) {
    float t = rayQueryGetIntersectionTEXT(rq, true);
    // hit!
}

• Useful for: shadow rays in compute shaders, AO, simple occlusion queries
• Less flexible than RT pipeline but simpler to integrate

Phase 3 Checklist

• Create BLAS from triangle mesh
• Create TLAS with instances
• Build and upload SBT
• Write ray generation shader
• Write closest-hit shader with BRDF evaluation
• Write miss shader for sky/environment
• Trace a primary ray and display result
• Add shadow rays for direct lighting (NEE)
• Implement iterative path tracing loop in rgen shader
• Set up bindless texture array for materials
• Implement BLAS compaction

Project

• PathTracer Learning - Project - Vulkan RT

Navigation

• Previous: PathTracer Learning - Phase 2 - CPU Ray Tracing
• Next: PathTracer Learning - Phase 4 - Godot Internals