QuaternionEngine/docs/RayTracing.md

Ray Tracing Manager: BLAS Cache and Per-Frame TLAS

Optional subsystem that enables hybrid or full ray traced shadows via Ray Query. It builds and caches BLAS per mesh and rebuilds a TLAS from the current DrawContext when enabled.

• Files: src/core/vk_raytracing.h/.cpp

Device Feature & Extension Enablement

• Feature detection happens in DeviceManager::init_vulkan() and sets:
  • VK_KHR_acceleration_structure, VK_KHR_ray_query, and VK_KHR_deferred_host_operations (if supported).
  • Device features are appended via DeviceBuilder.add_pNext(...).
• DescriptorManager::gpuSceneDataLayout() adds a TLAS binding at (set=0, binding=1) when AS is supported.

BLAS Build & Cache

• AccelStructureHandle getOrBuildBLAS(const std::shared_ptr<MeshAsset>& mesh):
  • One GAS per MeshAsset, keyed by vertex buffer VkBuffer.
  • Populated with one triangle geometry per GeoSurface.
  • Built with VK_BUILD_ACCELERATION_STRUCTURE_PREFER_FAST_TRACE_BIT_KHR and device-local storage + scratch.
  • Cached in _blasByVB for reuse across frames.
  • Called from AssetManager::createMesh(...) and from GLTF loader after mesh upload.

TLAS Rebuild Per Frame

• VkAccelerationStructureKHR buildTLASFromDrawContext(const DrawContext& dc):
  • Iterates dc.OpaqueSurfaces and creates one instance per render object.
  • Looks up BLAS by RenderObject::vertexBuffer; if missing, instance is skipped.
  • Uploads instances to a CPU→GPU buffer with device address.
  • Builds TLAS with immediate_submit and stores device address for Ray Query.

Renderer Integration

• In VulkanEngine::draw() before building passes:
  • If RT mode is enabled (shadowSettings.mode != 0) and manager exists, TLAS is rebuilt from the latest draw context.
• Lighting pass binds the TLAS at set=0,binding=1 when available.

Modes & UI

• Mode 0: Shadow maps only (CSM).
• Mode 1: Hybrid — selected cascades assisted by Ray Query (configurable bitmask).
• Mode 2: Ray Query only (no shadow maps).

Notes & Caveats

• BLAS cache key is the vertex buffer handle; if you rebuild meshes in-place, BLAS must be invalidated.
• CPU→GPU memory is used for the TLAS instance buffer to simplify updates. On some platforms, you may prefer staging + device-local.
• The RT path requires Vulkan 1.2+ with Ray Query and Acceleration Structure features available.