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ifcviewer: shrink vertex format from 16 to 12 bytes (oct i8x2 normals)

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Replace i16x2 octahedral normals with i8x2, filling the 2-byte padding
after position and saving 4 bytes per vertex. int8 gives ~1.4 deg
worst-case angular error — invisible for BIM geometry which is
overwhelmingly axis-aligned. 25% VBO reduction; sidecar files shrink
~15% overall (5.4 GB -> 4.6 GB on a 111-model test scene). Bumps
sidecar format to v7.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Dion Moult
2026-04-20 12:26:29 +10:00
parent dbe68b48f2
commit 3015f758ba
4 changed files with 85 additions and 155 deletions
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src/ifcviewer/InstancedGeometry.h
+9 -7
View File
@@ -25,16 +25,18 @@
#include <vector>
// Per-vertex layout for instanced meshes, stored in local coordinates,
// quantized against each mesh's local AABB. 16 bytes per vertex:
// quantized against each mesh's local AABB. 12 bytes per vertex:
// offset 0 pos 3 x uint16 normalized -> [0,1]; dequant to
// mix(mesh.aabb_min, mesh.aabb_max, t)
// offset 6 _pad 2 bytes
// offset 8 normal 2 x int16 normalized -> [-1,1]; octahedral-decoded
// offset 12 color 4 x uint8 normalized -> [0,1]
// offset 6 normal 2 x int8 normalized -> [-1,1]; octahedral-decoded
// offset 8 color 4 x uint8 normalized -> [0,1]
//
// int8 normals give ~1.4° worst-case angular error — invisible for BIM
// geometry which is overwhelmingly axis-aligned (walls, floors, slabs).
//
// Quantization basis is per mesh, stored in the MeshGpu SSBO bound at
// binding=2. The vertex shader looks up its basis via the instance's mesh_id.
static constexpr int INSTANCED_VERTEX_STRIDE_BYTES = 16;
static constexpr int INSTANCED_VERTEX_STRIDE_BYTES = 12;
// Streamer-side intermediate format: 7 floats per vertex (pos3 + normal3 +
// color-as-float). GeometryStreamer writes this into MeshChunk.vertices;
@@ -43,8 +45,8 @@ static constexpr int INSTANCED_VERTEX_STRIDE_BYTES = 16;
static constexpr int INSTANCED_VERTEX_STRIDE_FLOATS = 7;
static constexpr int INSTANCED_VERTEX_POS_OFFSET = 0;
static constexpr int INSTANCED_VERTEX_NORMAL_OFFSET = 8;
static constexpr int INSTANCED_VERTEX_COLOR_OFFSET = 12;
static constexpr int INSTANCED_VERTEX_NORMAL_OFFSET = 6;
static constexpr int INSTANCED_VERTEX_COLOR_OFFSET = 8;
// Per-mesh quantization basis, uploaded to a std430 SSBO. Two vec4s so
// std430 layout is trivial (no alignment surprises). w components unused.