Module rt_topology 

Backend-agnostic planner for the incremental RT acceleration-structure topology refresh. When the participating draw set changes at runtime (a cloned prop, a streamed chunk added/removed), the BLAS head must be brought back in line with the current set without rebuilding every BLAS: reuse every BLAS whose geometry slice is unchanged, build only the new ones, and retire the orphans. This module owns only the pure decision (which slot reuses which old BLAS, which are orphaned); the actual GPU allocation / build / retire is per-backend (directx/raytrace.rs, vulkan/raytrace.rs). Split out so the plan is unit-testable without a GPU.

Consumed by the DirectX + Vulkan backends. The Metal backend predates this module and keeps its own equivalent copy (metal/raytrace.rs); a future cleanup could converge it here once Metal can be rebuilt alongside.

Structs

GeomSig

Identifies the geometry a draw-object BLAS traces, on the shared vertex/index buffers. Two draw objects with the same signature trace identical geometry, so a topology refresh can reuse the existing BLAS instead of building a new one. A streamed mesh is placed wherever the sub-allocator has room, so a slot that streams out and back in generally returns on a different slice; the signature moves with it and the BLAS is rebuilt rather than wrongly reused. base_vertex + index_offset + index_count are exactly the inputs the per-backend geometry descriptor uses; vertex_offset is carried too so a static draw (whose base_vertex is 0) still distinguishes distinct vertex regions.

TopologyPlan

Per-new-slot decision for a topology refresh of the draw-object BLAS head.

Functions

plan_topology_refresh

Decide, for the draw-object BLAS head only, which BLAS to reuse, which to build, and which to retire when the participating draw set changes. Matches old and new slots by draw_objects index AND geometry signature: a slot whose geometry moved (a chunk slot recycled for a different chunk) does not match, so it rebuilds. Pure so it is unit-testable without a GPU.