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Phase 3B: per-instance LOD via meshoptimizer simplifySloppy

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Decimate each unique mesh once at sidecar-build time and swap to the
reduced index slice per-instance per-frame when projected sphere radius
drops below IFC_LOD1_PX (default 30).  Same VBO, same SSBO, just a
different firstIndex/count in the indirect command.

Extends MeshInfo (48→56 B) with lod1_ebo_byte_offset + lod1_index_count
and bumps the sidecar to v5.  buildLods() runs inside
onStreamingFinished, appends decimated indices to sd.indices,
applyLodExtension pushes the EBO suffix to the live GPU state, and the
sidecar is written with LOD1 baked in.

simplifySloppy (voxel clustering) is used instead of the default
edge-collapse meshopt_simplify because BIM brep output is per-triangle-
unwelded and non-manifold after welding — simplify returned the input
unchanged for every mesh tested.  Sloppy ignores topology.  Knobs
(IFC_LOD_SLOPPY, IFC_LOD_ERROR, IFC_LOD_RATIO, IFC_LOD_MIN_SAVINGS,
IFC_LOD_LOCK_BORDER, IFC_LOD_DEBUG) are available for A/B tuning.

Result on the 128M-tri 10-model test scene (GTX 1650, 2px contribution
cull): 20.2 → 43.2 fps, 40M → 14M visible triangles, no change in
object count.  LOD build adds 100–600 ms per model on first open,
cached thereafter.

README Phase 3B section is now a full writeup of pipeline, selection,
decimator-choice rationale, env vars, and measured numbers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Dion Moult
2026-04-13 18:31:43 +10:00
parent 68fea7bd45
commit c78e16eafb
10 changed files with 532 additions and 40 deletions
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src/ifcviewer/SidecarCache.cpp
+5 -1
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@@ -17,7 +17,11 @@
* *
********************************************************************************/
// v4 layout (all multi-byte fields native-endian; endianness marker in header):
// v5 layout (all multi-byte fields native-endian; endianness marker in header).
// Same sequence as v4; the only change is that MeshInfo grew two uint32_ts
// (lod1_ebo_byte_offset + lod1_index_count) and `indices` may contain extra
// appended LOD1 slices pointed at by those offsets.
//
//
// SidecarHeader (16 bytes)
// uint64_t source_file_size