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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/ViewportWindow.cpp
+10 -10
View File
@@ -46,9 +46,9 @@ static_assert(sizeof(DrawElementsIndirectCommand) == 20, "indirect cmd must be 2
// Shaders
// -----------------------------------------------------------------------------
//
// Vertex layout (GL side, 16 bytes — quantized; see InstancedGeometry.h):
// Vertex layout (GL side, 12 bytes — quantized; see InstancedGeometry.h):
// location 0: vec3 a_position_q (u16x3 normalized, per-mesh AABB basis)
// location 1: vec2 a_normal_oct (i16x2 normalized, octahedral)
// location 1: vec2 a_normal_oct (i8x2 normalized, octahedral)
// location 2: vec4 a_color (u8x4 normalized)
//
// Per-instance record in SSBO std430 (80 bytes):
@@ -66,7 +66,7 @@ static const char* MAIN_VERTEX_SHADER = R"(
#extension GL_ARB_shader_draw_parameters : require
// Quantized vertex inputs — see InstancedGeometry.h for layout.
layout(location = 0) in vec3 a_position_q; // u16x3 normalized -> [0,1]
layout(location = 1) in vec2 a_normal_oct; // i16x2 normalized -> [-1,1]
layout(location = 1) in vec2 a_normal_oct; // i8x2 normalized -> [-1,1]
layout(location = 2) in vec4 a_color;
struct InstanceRecord {
@@ -307,7 +307,7 @@ static void octEncode(const float n[3], float out[2]) {
}
// Quantize a streamer-format vertex (pos3 + normal3 + color-as-float) into
// the 16 B VBO record, given the mesh's tight local AABB. `extent_recip`
// the 12 B VBO record, given the mesh's tight local AABB. `extent_recip`
// is 1/(max-min) per axis, or 0 for degenerate axes (quantum becomes 0).
static void quantizeVertex(const float src[7],
const float aabb_min[3],
@@ -320,14 +320,14 @@ static void quantizeVertex(const float src[7],
if (t < 0.0f) t = 0.0f; else if (t > 1.0f) t = 1.0f;
p[a] = static_cast<uint16_t>(t * 65535.0f + 0.5f);
}
// Normal -> oct i16x2.
// Normal -> oct i8x2. int8 gives ~1.4° worst-case error — fine for BIM.
float oct[2];
octEncode(src + 3, oct);
int16_t* n = reinterpret_cast<int16_t*>(dst + INSTANCED_VERTEX_NORMAL_OFFSET);
int8_t* n = reinterpret_cast<int8_t*>(dst + INSTANCED_VERTEX_NORMAL_OFFSET);
for (int a = 0; a < 2; ++a) {
float v = oct[a];
if (v < -1.0f) v = -1.0f; else if (v > 1.0f) v = 1.0f;
n[a] = static_cast<int16_t>(std::lrintf(v * 32767.0f));
n[a] = static_cast<int8_t>(std::lrintf(v * 127.0f));
}
// Color passes through — streamer packs 4 bytes into the 7th float slot.
std::memcpy(dst + INSTANCED_VERTEX_COLOR_OFFSET, src + 6, 4);
@@ -498,13 +498,13 @@ void ViewportWindow::setupVaoLayout(GLuint vao, GLuint vbo, GLuint ebo) {
INSTANCED_VERTEX_POS_OFFSET);
gl_->glVertexArrayAttribBinding(vao, 0, 0);
// normal oct-encoded (2 x i16 normalized @ 8)
// normal oct-encoded (2 x i8 normalized @ 6)
gl_->glEnableVertexArrayAttrib(vao, 1);
gl_->glVertexArrayAttribFormat(vao, 1, 2, GL_SHORT, GL_TRUE,
gl_->glVertexArrayAttribFormat(vao, 1, 2, GL_BYTE, GL_TRUE,
INSTANCED_VERTEX_NORMAL_OFFSET);
gl_->glVertexArrayAttribBinding(vao, 1, 0);
// color (4 x u8 normalized @ 12)
// color (4 x u8 normalized @ 8)
gl_->glEnableVertexArrayAttrib(vao, 2);
gl_->glVertexArrayAttribFormat(vao, 2, 4, GL_UNSIGNED_BYTE, GL_TRUE,
INSTANCED_VERTEX_COLOR_OFFSET);