Phase 3B: per-instance LOD via meshoptimizer simplifySloppy

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>
2026-09-22 18:42:33 +00:00 · 2026-04-13 18:31:43 +10:00
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+/********************************************************************************
+ *                                                                              *
+ * This file is part of IfcOpenShell.                                           *
+ *                                                                              *
+ * IfcOpenShell is free software: you can redistribute it and/or modify         *
+ * it under the terms of the Lesser GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3.0 of the License, or          *
+ * (at your option) any later version.                                          *
+ *                                                                              *
+ * IfcOpenShell is distributed in the hope that it will be useful,              *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
+ * Lesser GNU General Public License for more details.                          *
+ *                                                                              *
+ * You should have received a copy of the Lesser GNU General Public License     *
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+#ifndef LODBUILDER_H
+#define LODBUILDER_H
+
+#include "SidecarCache.h"
+
+// Build a LOD1 index slice for every mesh in `sd` whose triangle count is
+// above `min_triangles`, using meshoptimizer's edge-collapse decimator.  The
+// LOD1 indices are appended to `sd.indices`; each MeshInfo's
+// lod1_ebo_byte_offset + lod1_index_count are populated to point at the
+// appended range.  Meshes that don't qualify (too small) or where the
+// decimator couldn't meet the target within the error budget have
+// lod1_index_count left at 0 (renderer falls back to LOD0).
+//
+// Defaults match the Phase 3B first-iteration design:
+//   min_triangles = 500     — below this the overhead dominates
+//   target_ratio  = 0.25    — aim for 25% of original tris
+//   target_error  = 0.05    — stop if relative error exceeds 5%
+//
+// `sd.vertices` is read (position is the first 3 floats of each
+// INSTANCED_VERTEX_STRIDE_FLOATS-wide vertex) but not modified — LOD1
+// reuses the same vertex buffer, just with a different index list.
+void buildLods(SidecarData& sd,
+               int min_triangles = 500,
+               float target_ratio = 0.25f,
+               float target_error = 0.05f);
+
+// Cheap summary for logging.  Safe to call before or after buildLods.
+struct LodStats {
+    uint32_t meshes_total       = 0;
+    uint32_t meshes_with_lod1   = 0;
+    uint32_t tris_lod0          = 0;   // sum across all meshes
+    uint32_t tris_lod1          = 0;   // only for meshes that got LOD1
+    uint32_t tris_lod0_for_lod1 = 0;   // LOD0 tris of the meshes that got LOD1
+};
+LodStats summariseLods(const SidecarData& sd);
+
+#endif // LODBUILDER_H