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ifcviewer: remove meshopt_simplify path, keep only simplifySloppy
Edge-collapse decimation (meshopt_simplify) returns BIM meshes unchanged due to per-triangle vertex duplication and non-manifold topology. The sloppy voxel-clustering decimator is faster, needs no shadow index welding, and produces good results at the sub-30px LOD1 threshold. Remove the non-sloppy branch, shadow buffer, IFC_LOD_SLOPPY and IFC_LOD_LOCK_BORDER env vars. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Dion Moult
@@ -37,28 +37,17 @@ void buildLods(SidecarData& sd,
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const size_t total_vertex_count = sd.vertices.size() / vtx_stride_bytes;
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// Env var knobs so we can tune without rebuilding.
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// IFC_LOD_LOCK_BORDER=1 re-enable LockBorder (off by default: BIM
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// geometry is often non-manifold so locking
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// borders prevents any collapse).
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// IFC_LOD_ERROR=<float> override target_error (default 0.05 → 0.2).
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// IFC_LOD_RATIO=<float> override target_ratio.
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// IFC_LOD_MIN_SAVINGS=<0..1> minimum fraction of tris saved to accept
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// (default 0.25).
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// IFC_LOD_DEBUG=1 print per-mesh diagnostics for the first
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// few meshes of each call.
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// IFC_LOD_SLOPPY=0 disable sloppy (clustering) decimator.
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// Default ON: BIM brep output is usually
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// non-manifold, so edge-collapse simplify
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// returns the input unchanged.
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const char* env_lock = std::getenv("IFC_LOD_LOCK_BORDER");
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const char* env_err = std::getenv("IFC_LOD_ERROR");
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const char* env_ratio = std::getenv("IFC_LOD_RATIO");
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const char* env_savings = std::getenv("IFC_LOD_MIN_SAVINGS");
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const char* env_debug = std::getenv("IFC_LOD_DEBUG");
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const char* env_sloppy = std::getenv("IFC_LOD_SLOPPY");
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const bool lock_border = env_lock && env_lock[0] == '1';
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const bool use_sloppy = !(env_sloppy && env_sloppy[0] == '0');
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if (env_err) target_error = static_cast<float>(std::atof(env_err));
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if (env_ratio) target_ratio = static_cast<float>(std::atof(env_ratio));
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float min_savings = 0.25f;
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@@ -71,10 +60,8 @@ void buildLods(SidecarData& sd,
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// Scratch buffers reused across meshes so we only allocate once.
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std::vector<uint32_t> simplified;
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std::vector<uint32_t> shadow;
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std::vector<float> dequant_pos; // 3 floats/vertex, dequantized
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simplified.reserve(1024);
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shadow.reserve(1024);
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dequant_pos.reserve(1024 * 3);
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int dbg_printed = 0;
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@@ -128,46 +115,18 @@ void buildLods(SidecarData& sd,
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const size_t target_index_count = std::max<size_t>(
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3, static_cast<size_t>(mesh.index_count * target_ratio) / 3 * 3);
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// The instanced VBO stores each triangle's vertices separately, so the
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// mesh's index buffer is topologically disconnected — every edge is
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// boundary, every vertex is unique, and meshopt_simplify can't collapse
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// anything. Build a shadow index buffer that welds by position, so
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// shared-position vertices share an ID; then simplify on that. Output
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// indices are still valid mesh-local IDs (canonical representatives),
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// usable directly as LOD1 indices against the same VBO.
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shadow.resize(mesh.index_count);
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meshopt_generateShadowIndexBuffer(
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shadow.data(),
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indices, mesh.index_count,
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positions, mesh.vertex_count,
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sizeof(float) * 3, // compare only xyz
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local_pos_stride);
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// Cluster-based (sloppy) decimator. Ignores topology entirely;
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// ideal for BIM brep output which is usually non-manifold / has
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// T-junctions / per-triangle vertex duplication. Quantises
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// positions into voxel cells — no welding needed.
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simplified.resize(mesh.index_count);
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float result_error = 0.0f;
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size_t new_index_count = 0;
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if (use_sloppy) {
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// Cluster-based decimator. Ignores topology entirely; great for
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// BIM brep output which is usually non-manifold / has T-junctions.
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// Operates directly on the original indices — welding isn't
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// needed since it quantises positions into voxel cells.
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new_index_count = meshopt_simplifySloppy(
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simplified.data(),
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indices, mesh.index_count,
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positions, mesh.vertex_count, local_pos_stride,
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target_index_count, target_error,
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&result_error);
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} else {
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const unsigned int options =
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lock_border ? static_cast<unsigned int>(meshopt_SimplifyLockBorder) : 0u;
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new_index_count = meshopt_simplify(
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simplified.data(),
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shadow.data(), mesh.index_count,
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positions, mesh.vertex_count, local_pos_stride,
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target_index_count, target_error,
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options, &result_error);
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}
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size_t new_index_count = meshopt_simplifySloppy(
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simplified.data(),
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indices, mesh.index_count,
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positions, mesh.vertex_count, local_pos_stride,
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target_index_count, target_error,
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&result_error);
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if (debug && dbg_printed < 8) {
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std::fprintf(stderr,
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@@ -202,9 +161,9 @@ void buildLods(SidecarData& sd,
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if (debug) {
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std::fprintf(stderr,
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" [lod] summary: accepted=%d rejected_noreduce=%d rejected_savings=%d "
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"(lock_border=%d target_error=%.3f target_ratio=%.3f min_savings=%.3f)\n",
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"(target_error=%.3f target_ratio=%.3f min_savings=%.3f)\n",
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dbg_accepted, dbg_rejected_noreduce, dbg_rejected_savings,
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lock_border ? 1 : 0, target_error, target_ratio, min_savings);
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target_error, target_ratio, min_savings);
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}
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}
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@@ -23,8 +23,8 @@
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#include "SidecarCache.h"
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// Build a LOD1 index slice for every mesh in `sd` whose triangle count is
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// above `min_triangles`, using meshoptimizer's edge-collapse decimator. The
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// LOD1 indices are appended to `sd.indices`; each MeshInfo's
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// above `min_triangles`, using meshoptimizer's sloppy (voxel-clustering)
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// decimator. The LOD1 indices are appended to `sd.indices`; each MeshInfo's
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// lod1_ebo_byte_offset + lod1_index_count are populated to point at the
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// appended range. Meshes that don't qualify (too small) or where the
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// decimator couldn't meet the target within the error budget have
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+11
-27
@@ -540,44 +540,28 @@ shader) is unchanged.
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##### Decimator choice: `meshopt_simplifySloppy`
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The first attempt used `meshopt_simplify`, which is an edge-collapse
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decimator. It returned every input mesh unchanged (`err = 0.0`) for two
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reasons, both inherent to BIM brep output:
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1. **Per-triangle vertex duplication.** The instanced VBO stores each
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triangle's vertices separately so that hard-edge normals can differ
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across triangles. Topologically there are no shared vertices, so no
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edges exist for `meshopt_simplify` to collapse. A
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`meshopt_generateShadowIndexBuffer` welding pass (hash xyz only,
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ignore the interleaved normal/colour) fixes this half cheaply — the
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VBO isn't touched, only a per-call shadow index buffer is built.
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2. **Non-manifold topology even after welding.** BIM brep output has
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T-junctions, coplanar slivers, separate solids meeting at a plane,
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and multi-material cuts. `meshopt_simplify` needs valid 2-manifold
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edge pairs to score collapses; it refuses the non-manifold ones, the
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priority queue never fires, and it returns the input untouched.
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`meshopt_simplifySloppy` is a **voxel-clustering decimator** — it
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quantises positions into cells and merges everything in a cell to a
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single point. Topology is irrelevant, so it works directly on the
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original indices (welding isn't even needed). The trade-off is that it
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rounds off sharp corners and can produce slightly degenerate triangles,
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so it doesn't look great at mid-screen size. For a LOD1 that only
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activates below 30 px projected radius that's invisible in practice. If
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you ever want LOD1 to remain active at larger sizes, the only robust
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fix is to pre-process BIM meshes into manifold form (fuse coplanar
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faces, split at T-junctions) — a significant project unto itself.
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single point. This is the only meshoptimizer decimator that works on
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BIM brep output, which has per-triangle vertex duplication (hard-edge
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normals) and non-manifold topology (T-junctions, coplanar slivers,
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separate solids meeting at a plane). The edge-collapse decimator
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(`meshopt_simplify`) needs 2-manifold edge pairs to score collapses;
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on BIM geometry it returns the input unchanged.
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`simplifySloppy` rounds off sharp corners and can produce slightly
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degenerate triangles, so it doesn't look great at mid-screen size.
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For a LOD1 that only activates below 30 px projected radius that's
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invisible in practice.
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##### Tuning knobs (env vars)
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| Var | Default | Effect |
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|-----|---------|--------|
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| `IFC_LOD1_PX` | `30` | Projected sphere radius (px) below which LOD1 kicks in. `0` disables LOD1 entirely. |
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| `IFC_LOD_SLOPPY` | `1` | `0` falls back to edge-collapse (`meshopt_simplify`) on shadow-welded indices. Typically produces zero LOD1 output for BIM — useful only for A/B comparison. |
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| `IFC_LOD_ERROR` | `0.2` | Target relative error passed to meshopt. |
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| `IFC_LOD_RATIO` | `0.25` | Target triangle-count ratio (LOD1 aims for 25 % of LOD0 tris). |
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| `IFC_LOD_MIN_SAVINGS` | `0.25` | Reject the LOD1 result if it doesn't shave at least this fraction of triangles. |
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| `IFC_LOD_LOCK_BORDER` | `0` | `1` re-enables `meshopt_SimplifyLockBorder` (only meaningful with `IFC_LOD_SLOPPY=0`). |
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| `IFC_LOD_DEBUG` | `0` | `1` prints per-mesh `tris / target / got / err` for the first 8 candidate meshes plus an accept/reject summary per model. |
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##### Measured results
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