Crafter.Graphics/examples/Sponza/README.md

Sponza example

Loads the Sponza atrium as a .cmesh + one albedo .ctex and renders it via ray tracing on both Vulkan (native) and WebGPU (wasm). Same main.cpp, #ifdef CRAFTER_GRAPHICS_WINDOW_DOM selects the backend.

What this example proves

• .cmesh and .ctex decompression round-trip on both backends (GPU via VK_EXT_memory_decompression on Vulkan, CPU via Compression::DecompressCPU on WebGPU).
• A single texture binding flowing from Image2D<RGBA8> through the RT pipeline's closest-hit on both backends. The closest-hit samples at the barycentric attribs as UVs — proof-of-binding, not visually accurate. Per-vertex UV interpolation is the next step.

The closest-hit reads its texture through a spec constant (albedo[albedoSlot]), not a runtime index. That is deliberate — see below.

Native RT limitation: dynamic descriptor_heap indexing in hit shaders

On NVIDIA driver 610.43.02 (Vulkan 1.4), indexing a layout(descriptor_heap) array with a runtime (non-constant) index inside a ray-tracing hit shader aborts the device with VK_ERROR_DEVICE_LOST (an instruction-pointer / READ_INVALID device-fault) with validation off. GPU-Assisted Validation masks it — the scene runs fine under GPU-AV — which is why a validated run doesn't catch it. It is a driver-side fault, the same family as the descriptor-heap AS-read fault (#7 / #15) and the RT recursion / compute TLAS-push issues (#21 / #22), but here for plain SSBO and sampled-image descriptors read with a non-constant heap index (issue #23).

What was isolated (NVIDIA RTX 4090, driver 610.43.02)

Driving a native bindless RT scene headlessly and bisecting the closest-hit:

• A closest-hit that reads only lightHeap[lightSlot] where lightSlot is a spec constant survives indefinitely. ✅ (This example's albedo[albedoSlot] is exactly this case.)
• Reading indexHeap[assetIndexStart + gl_InstanceCustomIndexEXT] / vertexHeap[...] — a heap index offset by a runtime value — device-losts on the first geometry hit. ❌
• Reading a texture dynamically, textureHeap[assetColorStart + gl_InstanceCustomIndexEXT], also device-losts. ❌ So it is SSBO and sampled-image descriptors.
• nonuniformEXT() on the dynamic index does not help.
• The identical dynamic-heap-index pattern works fine in fragment shaders (the UI renderer indexes uiTextures[] / ui*Heap[] by per-item runtime slots), so this is RT-stage-specific, not a general descriptor_heap problem.
• Reading a spec-constant-indexed SSBO in raygen works; only the dynamic index in the hit stage faults.

Why there is no transparent engine workaround

The AS-read fault (#15) is worked around transparently because an acceleration structure can be reached two ways: through a descriptor, or through its device address via OpConvertUToAccelerationStructureKHR (which reads no descriptor). There is exactly one TLAS, so the engine rewrites the heap AS read into an address load and feeds the address in as push data.

Neither half of that applies here:

• Sampled images have no device-address path. A texture must be reached through a descriptor; there is no OpConvertUToImage. A dynamic heap texture index cannot be rewritten into anything that avoids dynamic descriptor selection.
• There are many buffers, dynamically selected. SSBOs can be reached by address (buffer_reference / OpConvertUToPtr), but a per-mesh array selected by gl_InstanceCustomIndexEXT would need the engine to maintain and bind an address-table buffer and a SPIR-V rewrite far larger than the single-TLAS AS case — and it would still leave the texture half broken.

So the engine cannot paper over this the way it does the AS read. The fix is on the consumer side: avoid dynamically selecting a descriptor in a hit shader.

Recommended pattern

The fault is dynamic selection of a descriptor. Indexing within a single bound resource — an element offset into one SSBO, a layer into one array texture — is ordinary memory / layer addressing and is unaffected. So bind one resource and index inside it, rather than indexing the heap:

• Geometry — pack all meshes' vertices/indices into a single SSBO bound at a spec-constant slot and index it by a runtime element offset, or reach each mesh's buffer via buffer_reference (a device address loaded from one bound table). Either way the descriptor is constant; only the offset/address is dynamic.

  // ❌ faults in a hit shader on NVIDIA: dynamic descriptor selection
  layout(descriptor_heap) buffer Verts { Vertex v[]; } vertexHeap[];
  Vertex vtx = vertexHeap[assetVertexStart + gl_InstanceCustomIndexEXT].v[i];
  
  // ✅ one descriptor (spec-constant slot), dynamic element offset
  layout(constant_id = 0) const uint16_t vertexSlot = 0us;
  layout(descriptor_heap) buffer Verts { Vertex v[]; } vertexHeap[];
  uint base = assetVertexStart[gl_InstanceCustomIndexEXT]; // from a bound SSBO
  Vertex vtx = vertexHeap[vertexSlot].v[base + i];
  

• Materials / textures — put them in one texture2DArray (or a small number of arrays bucketed by format/size) bound at a spec-constant slot and index by layer:

  // ✅ one array texture (spec-constant slot), dynamic layer index
  layout(constant_id = 1) const uint16_t materialArraySlot = 0us;
  layout(descriptor_heap) uniform sampler2DArray materials[];
  uint layer = materialLayer[gl_InstanceCustomIndexEXT]; // from a bound SSBO
  vec3 albedo = texture(materials[materialArraySlot], vec3(uv, layer)).rgb;
  

This is precisely what the WebGPU path already does — bucketed texture arrays plus a single geometry buffer — so it is a proven, cross-backend pattern, and it sidesteps the NVIDIA RT fault on the native path.

Remove this section once a fixed NVIDIA driver ships and dynamic descriptor_heap indexing in hit shaders stops faulting.

Asset fetch

project.cpp calls Crafter::GitFetch(...) on https://github.com/jimmiebergmann/Sponza (pinned to commit 222338979d32f4f4818466291bdbc29f192b86ba). The clone lands in the per-user crafter-build cache; first build pulls ~280 MB once, subsequent builds reuse it.

cfg.assets then picks two files out of that clone:

Source	Compressed output
sponza.obj	sponza.cmesh
textures/sponza_arch_diff.tga	sponza_arch_diff.ctex

Both land flat in the example's bin directory.

Building

crafter build                          # native Vulkan
crafter build --target=wasm32-wasip1   # WebGPU / wasm

License & attribution

Sponza geometry, materials, and textures are licensed under CC BY 3.0.

• Original model: Frank Meinl, Crytek (2010).
• OBJ packaging / cleanup: Morgan McGuire, McGuire Computer Graphics Archive — https://casual-effects.com/data.
• GitHub mirror used here: Jimmie Bergmann's roof-material fixup — https://github.com/jimmiebergmann/Sponza.

When redistributing builds of this example that bundle the compressed Sponza outputs (*.cmesh, *.ctex), the CC BY 3.0 attribution requirement applies. Quoting the original credit somewhere visible to end users (about-screen, credits page, etc.) is enough.

The Crafter.Graphics library code itself is LGPL-3.0; the two licenses are compatible for data + code distribution.