IfcOpenShell/src/ifcviewer/README.md

# IfcViewer

A high-performance native IFC viewer built on IfcOpenShell's C++ geometry
engine with a Qt6 interface and OpenGL 4.5 rendering.

## Architecture

```
+---------------------------------------------------+
|  Qt6 Application (MainWindow)                     |
|  +----------+ +----------------------------------+|
|  | Element  | | 3D Viewport                      ||
|  | Tree     | | (QWindow + OpenGL 4.5 Core)      ||
|  | (per-    | |                                  ||
|  |  model)  | | Per-model: VAO/VBO/EBO           ||
|  +----------+ |            instance SSBO         ||
|  | Property | |            visible SSBO          ||
|  | Table    | |            indirect buffer       ||
|  +----------+ | glMultiDrawElementsIndirect      ||
|  | Status / Progress / Stats                      |
+---------------------------------------------------+
        ^                        ^
        |                        |
  element metadata      MeshChunk / InstanceChunk / Sidecar
        |                        |
+---------------------------------------------------+
|  GeometryStreamer (one per loaded model)          |
|  IfcGeom::Iterator with N threads                 |
|  Dedups representations -> MeshChunk              |
|  Emits one InstanceChunk per placement            |
+---------------------------------------------------+
```

### Key design decisions

- **QWindow viewport** embedded via `QWidget::createWindowContainer()`. Gives
  us a raw native surface for OpenGL, bypassing `QOpenGLWidget`'s compositor
  overhead.
- **GPU instancing as the central pillar.** IFC models are dominated by
  repeated geometry — identical doors, windows, studs, pipes placed at
  different transforms. IfcOpenShell's iterator surfaces representation
  identity, so we upload each unique mesh exactly once and keep per-placement
  data (transform, object id, optional colour override) in a separate SSBO.
  For real projects this collapses tens of millions of triangles of duplicate
  vertex data into a few hundred MB of unique meshes.
- **Per-model GPU buffers**: each loaded model gets its own
  VAO/VBO/EBO/instance-SSBO/visible-SSBO/indirect-buffer. No cross-model
  growth copies. Removing a model frees its GPU memory immediately.
- **Quantized local-coordinate vertex format (12 B):** position as
  `u16x3` normalised against each mesh's local AABB, octahedral-encoded
  normal as `i8x2`, packed RGBA8 colour. The normal fills what was
  previously 2 bytes of padding, and shrinks from `i16x2` to `i8x2` —
  int8 gives ~1.4° worst-case angular error, invisible for BIM geometry
  which is overwhelmingly axis-aligned (walls, floors, slabs encode
  exactly). Dequantisation basis is per mesh, uploaded once in a
  `MeshGpu` SSBO at binding 2. The per-instance transform is applied in
  the vertex shader. No world-baked vertex data. 12/28 = 57 % smaller
  VBO than the original 28 B float layout (sidecar files shrink ~15 %
  overall since indices/instances/metadata are unchanged).
- **Multi-draw indirect:** every frame the CPU builds a flat list of visible
  instance indices and one `DrawElementsIndirectCommand` per non-empty mesh,
  then issues a single `glMultiDrawElementsIndirect` per model. 50k visible
  instances across 8k unique meshes collapse to one driver-side command
  submission per model.
- **BVH frustum culling over instances**: per-model BVH trees cull whole
  subtrees of placements with one frustum test. Falls back to a linear scan
  during progressive upload and for very small models (< 32 instances).
- **Parallel per-model cull:** each model's CPU cull (frustum + contribution
  + HiZ + bucketing + indirect-command emit) is independent, so `render()`
  fans them out via `std::async` and joins before the serial GL-upload
  pass. On an 18-model scene this took wall-clock cull from ~25 ms to
  ~5 ms. The cull scratch buffers live on `ModelGpuData` so each worker
  owns its output storage; phase-timer counters are atomic for the same
  reason. `IFC_CULL_THREADS=0` forces single-threaded fallback.
- **Reflection-aware two-pass draw:** IFC placements can have negative-
  determinant transforms (mirrored families). These flip the screen-space
  winding of their triangles, which would make them vanish under
  `GL_CULL_FACE`. The cull pass buckets visible instances into forward
  (det ≥ 0) and reverse (det < 0) slices and the renderer issues two MDI
  calls per model with `glFrontFace` toggled between them.
- **`reorient-shells` enabled in the iterator:** makes face winding
  consistent within a shell at geometry-gen time — the only place this can
  actually be fixed. Without it, files with inside-out faces produce dark
  patches and swiss-cheese under backface culling. Costs iterator time but
  is cached in the sidecar.
- **Progressive rendering during streaming:** the viewport is drawable
  before `finalizeModel()`. Instances are pushed to the SSBO one at a time
  via `glNamedBufferSubData` as they arrive, and the linear-scan cull path
  handles them until the BVH is built. Orbit and pan remain interactive
  through load.
- **Non-blocking sidecar loading**: sidecars are read on a background
  thread; only the final GPU upload touches the main thread.
- **Event-driven rendering:** no continuous render timer. Frames are
  scheduled via `QWindow::requestUpdate()` only when something changes
  (camera move, streaming chunk, hover, settings). When the camera and
  scene are idle the cull pass and HiZ readback are skipped entirely
  and the main thread blocks in the Qt event loop — the viewer costs
  zero CPU/GPU on a static scene. FPS is still reported accurately
  because frame cost is measured *inside* `render()`, not as wall-clock
  between frames.
- **GPU object picking**: a second render pass writes object IDs into an
  R32UI framebuffer. Click reads back one pixel. No CPU-side raycasting.
- **Multi-model support**: multiple IFCs can be loaded simultaneously.
  Each gets its own `GeometryStreamer` (which owns the `ifcopenshell::file`
  for property lookup). Models load sequentially. Per-model
  hide/show/remove.

### Files

| File | Purpose |
|------|---------|
| `main.cpp` | Application entry, GL 4.5 surface format, CLI argument parsing |
| `MainWindow.h/cpp` | Qt main window: multi-model project, element tree, properties, status |
| `ViewportWindow.h/cpp` | OpenGL 4.5 Core renderer: shaders, buffers, camera, culling, MDI draw, picking |
| `GeometryStreamer.h/cpp` | Background iterator runner; emits `MeshChunk` + `InstanceChunk` |
| `InstancedGeometry.h` | Shared structs: `MeshInfo`, `InstanceCpu`, `InstanceGpu`, chunk records |
| `BvhAccel.h/cpp` | Median-split BVH builder; operates on instance world-AABBs |
| `LodBuilder.h/cpp` | Post-stream decimation of unique meshes via meshoptimizer (`simplifySloppy`) |
| `SidecarCache.h/cpp` | Raw binary `.ifcview` (v8) sidecar read/write |
| `AppSettings.h/cpp` | Persisted preferences (geometry library, stats overlay, backface culling) |
| `SettingsWindow.h/cpp` | Settings dialog |
| `CMakeLists.txt` | Build configuration |

## Dependencies

- **Qt6** (Core, Gui, Widgets, OpenGL)
- **OpenGL 4.5** with `GL_ARB_direct_state_access` and
  `GL_ARB_shader_draw_parameters` — available on Windows and Linux. macOS
  will need a Vulkan/MoltenVK backend (not yet implemented; macOS caps out
  at GL 4.1).
- **IfcOpenShell C++ libraries** (IfcParse, IfcGeom, and their
  dependencies: Open CASCADE, Boost, Eigen3, optionally CGAL).
- **[meshoptimizer](https://github.com/zeux/meshoptimizer)** — linked via
  `find_package(meshoptimizer REQUIRED)`. Used at sidecar-build time for LOD
  decimation; not needed at runtime once a sidecar exists.

## Building

IfcViewer is part of the IfcOpenShell CMake project. From the repo root:

```sh
mkdir build && cd build

cmake ../cmake \
    -DCMAKE_BUILD_TYPE=Release \
    -DBUILD_IFCVIEWER=ON \
    -DBUILD_CONVERT=OFF \
    -DBUILD_IFCPYTHON=OFF \
    -DBUILD_GEOMSERVER=OFF \
    -DBUILD_DOCUMENTATION=OFF \
    -DBUILD_EXAMPLES=OFF \
    -DCOLLADA_SUPPORT=OFF \
    -DGLTF_SUPPORT=OFF \
    -DHDF5_SUPPORT=OFF

make -j$(nproc) IfcViewer
```

If Qt6 is not in a standard location, pass `-DQT_DIR=/path/to/qt6`.

## Usage

```sh
./IfcViewer arch.ifc struct.ifc mep.ifc
./IfcViewer                   # then File -> Add Files
```

### Controls

| Input | Action |
|-------|--------|
| Middle mouse drag | Orbit camera |
| Shift + middle mouse drag | Pan camera |
| Scroll wheel | Zoom |
| Left click | Select object |

### Keyboard

| Key | Action |
|-----|--------|
| Ctrl+O | Add files |
| Ctrl+Q | Quit |

### Settings

- **Geometry Library** — kernel string passed to IfcOpenShell (default
  `hybrid-cgal-simple-opencascade`).
- **Show Performance Stats** — overlay FPS / object / triangle / draw
  counts in the status bar.
- **Backface Culling** — `GL_CULL_FACE` on closed solids. Default on.
  Disable if a model uses open shells and you see missing faces.

## Performance Strategy

The viewer targets smooth orbiting at 60 fps on real-world multi-discipline
BIM projects (a "real job" being ~50 models, several million placements,
hundreds of millions of rasterised triangles when everything is in view).

Rendering performance has evolved in phases. Each builds on the previous,
and smaller models never pay for optimisations they don't need.

### Phase 1 — Per-object Frustum Culling

**Status:** implemented (and still the fallback for small models / during
streaming).

Six view-frustum planes are extracted from the view-projection matrix each
frame. Each instance's world AABB is tested with the p-vertex / n-vertex
method (one dot product + one compare per plane, 6 planes).

Surviving instance indices are written into a per-mesh bucket, then
flattened into a single `uint[]` (the "visible SSBO", binding = 1) and
accompanied by one `DrawElementsIndirectCommand` per non-empty mesh.
One `glMultiDrawElementsIndirect` call per model draws everything.

Cost: ~6 dot products per instance per frame. Fine up to ~100 k instances
per frame; above that the linear scan shows up in profiles, motivating
Phase 2.

### Phase 2 — BVH Acceleration + Sidecar Cache

**Status:** implemented.

For models exceeding ~32 instances, a bounding volume hierarchy groups
nearby placements into a binary tree and culls entire subtrees with a
single frustum test. This reduces per-frame work from O(N) to O(log N) in
the best case (camera zoomed to a corner) and remains well under 1 ms for
100 k instances in the worst case (everything on screen).

A BVH was chosen over an octree because BIM data is spatially non-uniform
— dense MEP risers in one zone, sparse open atria in another. An octree
subdivides space uniformly, wasting nodes on empty regions and creating
deep chains in dense ones. A BVH adapts its splits to the actual
placement distribution.

#### Activation

The BVH is optional and non-disruptive. Until it is built, the Phase 1
linear scan handles culling. The renderer checks for a BVH per model and
falls back to the scan for any model that doesn't have one.

It activates in one of two ways:

1. **Sidecar hit** — the `.ifcview` file next to the `.ifc` is found and
   valid; its instance data is uploaded and the BVH rebuilt on the fly
   from the restored AABBs (cheap — `< 100 ms` for 100 k placements).
2. **After streaming** — `finalizeModel()` builds the BVH synchronously
   once all chunks are in (instances already live on the GPU, so there's
   no EBO re-sort to do). The sidecar is written afterwards.

Models under 32 instances skip the BVH.

#### BVH node layout (32 B, two per cache line)

```cpp
struct BvhNode {
    float    aabb_min[3];      // 12 B
    float    aabb_max[3];      // 12 B
    uint32_t right_or_first;   // interior: right child index; leaf: first item index
    uint16_t count;            // 0 = interior, >0 = leaf
    uint16_t axis;             // 0/1/2 for interior; unused for leaf
};
```

Left child is always the next node (pre-order DFS). Leaf items are
indices into the per-model `instances` array; the parallel `bvh_items[]`
array carries the world AABBs.

#### Build: object-median split

1. Compute centroid of each item's AABB.
2. Pick the longest axis of the node's AABB.
3. `std::nth_element` partitions at the median on that axis — O(n).
4. Recurse until a leaf holds ≤ 8 items.

O(n log n) total. No SAH — for frustum culling (6-plane tests, early
subtree reject) the quality difference vs median is negligible.

#### Traversal: stack-based, no recursion

```
stack[64] = { 0 }                     // root
while stack not empty:
    node = nodes[stack.pop()]
    if node.aabb outside frustum: continue
    if leaf:
        for each item in node:
            if item.aabb in frustum: emit to visible list
    else:
        push right child, push left child   // left processed first (DFS)
```

Depth 64 is enough for billions of items on any balanced tree. The stack
is on the C++ stack, zero per-frame allocation.

#### Sidecar format (`.ifcview`, v8)

Raw memory dump, Blender-`.blend`-style — no serialisation, no parsing.
Stores everything needed to skip the `IfcGeom::Iterator` pass:

```
SidecarHeader            (magic "IFVW", version, endian, ...)
uint32_t + uint8_t[]     vertex data    (12 B/vert quantized; per-mesh basis in MeshInfo)
uint32_t + uint32_t[]    index data     (mesh-local)
uint32_t + MeshInfo[]    per-unique-mesh metadata (56 B each, incl. LOD1 slice)
uint32_t + InstanceCpu[] per-placement records (transform + AABB + ids)
uint32_t + PackedElementInfo[]   element tree records
uint32_t + char[]        string table
```

Sidecar path is the source stem + `.ifcview` — `foo.ifc` and `foo.ifcdb/`
both map to `foo.ifcview`, so the same cache serves either source format.
Staleness is user-managed: delete the sidecar to force a rebuild.
Endianness marker rejects cross-arch caches.

Sidecars store the raw `object_id` / `model_id` values from the session
that wrote them. On load they are rebased onto the current session's ID
space (`object_id += next_object_id_ - min_id_in_sidecar`, `model_id`
overwritten with the freshly-assigned handle) before the elements hit
`element_map_` or the viewport. Without this, two cached models loaded
back-to-back collide — both start at `object_id=1` and the second model's
property lookups return the first model's data.

### GPU Instancing pipeline (the central pillar)

Everything above plugs into a single data-flow, worth documenting on its
own because it's what makes the whole thing fast.

Per-model state on the GPU:

| Buffer | Contents | Lifetime |
|--------|----------|----------|
| `VBO` | Quantized local-coord vertex data (12 B/vert: u16x3 pos, oct i8x2 normal, RGBA8). One range per unique representation. | Grow-on-demand during streaming; static after finalize. |
| `MeshGpu SSBO` (binding 2) | Per-mesh dequant basis (`vec4 aabb_min`, `vec4 aabb_max`). | Grow-on-demand; static after finalize. |
| `EBO` | Mesh-local uint32 indices. One range per unique representation. | Same. |
| `SSBO` (binding 0) | `InstanceGpu[]` (80 B each: mat4 transform, object_id, color_override, pad). | Appended during streaming, static after finalize. |
| `visible SSBO` (binding 1) | `uint32[]` — flat list of visible instance indices, ordered by mesh, uploaded each frame. | Rewritten every frame. |
| Draw-indirect buffer | `DrawElementsIndirectCommand[]` — one per non-empty mesh, uploaded each frame. | Rewritten every frame. |

Draw command:

```c
struct DrawElementsIndirectCommand {
    uint32_t count;         // mesh.index_count
    uint32_t instanceCount; // visible-list length for this mesh
    uint32_t firstIndex;    // mesh.ebo_byte_offset / 4
    uint32_t baseVertex;    // mesh.vbo_byte_offset / 12
    uint32_t baseInstance;  // offset into the flat visible-index array
};
```

The vertex shader reads `visible[gl_BaseInstanceARB + gl_InstanceID]` to
get the real instance id, then indexes into the instance SSBO:

```glsl
uint slot = uint(gl_BaseInstanceARB) + uint(gl_InstanceID);
uint iid  = visible[slot];
InstanceRecord inst = instances[iid];
gl_Position = u_view_projection * inst.transform * vec4(a_position, 1.0);
```

`gl_BaseInstanceARB` requires `GL_ARB_shader_draw_parameters`, which is
available on all GL-4.6-capable drivers.

Reflection handling: at upload time we store a parallel
`instance_reflected[]` byte array (1 if the transform's upper-3×3 has
det < 0). The cull pass produces two flat visible-list slices — fwd
(non-reflected) first, rev (reflected) after — concatenated into one
buffer. The renderer issues MDI twice: fwd with `glFrontFace(GL_CCW)`,
rev with `glFrontFace(GL_CW)`. `GL_CULL_FACE` stays on and does the
right thing in both passes.

### Current bottleneck — draw-bound, not upload-bound

The original README's Phase 3 ("GPU-driven indirect draw") described
moving draw submission to the GPU via compute. In the meantime, GPU
instancing and MDI made the CPU-side draw cost essentially free (10
`glMultiDrawElementsIndirect` calls per frame for 10 models). **That
goal is met.** The real ceiling lies elsewhere, and it took a couple of
bad hypotheses to pin down.

#### Profiled scene

10 models / 379 k instances / 128 M triangles, everything in view, no
camera motion, GTX 1650 (PCIe dGPU, 4 GB VRAM):

| Metric | Value |
|--------|-------|
| FPS | 6.7 |
| Frame time | 149 ms |
| gl_draws | 10 |
| Sub-draws packed in indirect buffers | 67 037 |

`nvidia-smi` reports 95 % GPU utilisation during render — the GPU is
the thing that's pinned.

#### False lead: "the per-frame uploads are the bottleneck"

The first round of probes pointed at the two `glNamedBufferSubData`
calls per model per frame (visible list ~1.5 MB + indirect buffer
~1.3 MB):

| Probe | Result | Initial interpretation |
|-------|--------|------------------------|
| Camera off-screen (nothing visible) | 60 fps | GPU idle → CPU path cheap |
| Comment out the two `glNamedBufferSubData` | 60 fps, blank screen | Uploads are the bottleneck |

This led to an aborted Phase 3A implementation of persistent-mapped
triple-buffered rings (and then staging + VRAM-resident with
`glCopyNamedBufferSubData`). Neither moved the FPS needle — both still
sat at 6.7 fps.

The probe was wrong: **commenting out the uploads emptied the indirect
buffer, so MDI drew zero triangles. "No upload" and "no draw" were
indistinguishable in the test.**

#### What actually isolates the draw cost

Two diagnostic env vars now live in `render()`:

- `IFC_SKIP_MDI=1` — keep everything (cull, upload, binds) but skip the
  actual `glMultiDrawElementsIndirect` calls.
- `IFC_MAX_SUBDRAWS=N` — truncate each MDI's drawcount to N while still
  running the rest of the frame.

Results on the profiled scene:

| Probe | FPS | Frame time |
|-------|-----|-----------|
| baseline | 6.7 | 149 ms |
| `IFC_SKIP_MDI=1` | 62.5 | 16 ms |
| `IFC_MAX_SUBDRAWS=30000` | 6.7 | 149 ms |
| `IFC_MAX_SUBDRAWS=10000` | 7.5 | 133 ms |
| `IFC_MAX_SUBDRAWS=1000` | 20.2 | 49 ms |

Readings:

1. `SKIP_MDI` gives 62 fps with all upload/bind machinery still running
   — the non-draw path fits in ~16 ms easily. **Not upload-bound.**
2. Halving the sub-draw count (67 k → 30 k) saves 0 ms. If per-sub-draw
   command-processor overhead were material, dropping 37 k sub-draws
   would save measurable time no matter which sub-draws were dropped.
   It doesn't. **67 k sub-draws is not the bottleneck** — the long tail
   carries almost no triangles, and the heavyweights dominate.
3. Time only starts coming down once the cap is low enough to shed bulk
   triangle work (1000 sub-draws → 49 ms). The curve is consistent with
   a long-tailed distribution: a handful of very big meshes × instance
   counts do most of the rasterisation.

**Conclusion: the GTX 1650 is rasterising 128 M triangles at ~850 M
tri/s, and that eats ~133 ms of the 149 ms frame.** No CPU-side or
upload-side work will recover it. The only way forward is to draw
fewer triangles.

### Phase 3 (revised) — Shed triangles, not bytes

In order of effort/payoff for BIM workloads:

#### 3A. Screen-space contribution culling — ✅ done

Reject frustum-visible objects whose bounding-sphere projects below a
pixel-radius threshold. Applied both at BVH-node level (whole subtrees
pruned, so distant parts of the model never touch per-instance tests)
and per-instance level. Short-circuits when the camera is inside the
AABB so nothing-you're-standing-next-to is ever lost. Pick pass uses
threshold 0 so sub-pixel objects remain clickable.

Because the pick pass re-runs the cull with its own parameters (no
contribution cull, no HiZ) and writes into each model's shared
`visible_ssbo` / indirect buffer, `pickObjectAt()` must invalidate
`have_cached_cull_` on exit. Otherwise the next `render()` sees an
unchanged camera, skips the cull, and draws the pick-pass buffers —
the user sees obviously-wrong shading until they nudge the camera.

Sphere-based (centre = AABB midpoint, radius = half-diagonal,
r_px = focal_px · radius / distance). Loses a little precision on
very elongated bounds vs. 8-corner projection, but costs ~5× less per
test, and because BVH-node pre-cull handles the long tail in one shot
it doesn't matter.

Threshold defaults to 2 px radius, overridable via `IFC_MIN_PX` env
var. Measured on the 10-model / 128 M-tri test scene (GTX 1650):

| Threshold | FPS | Triangles drawn | Objects drawn |
|-----------|-----|-----------------|---------------|
| 0 px (off) | 6.7 | 128 M | 379 k |
| 2 px | 20.2 | 40 M (31 %) | 89 k (24 %) |
| 4 px | 30.3 | 15 M (12 %) | 29 k (8 %) |

At 4 px, frame time breakdown matches: ~16 ms non-draw baseline (from
`IFC_SKIP_MDI=1`) + ~18 ms of raster (15 M tris / 850 M tri/s) ≈ 34 ms
= observed 33 ms. The ceiling is now genuinely vertex/raster
throughput on the post-cull geometry — next steps (LOD, HiZ) attack
that directly.

#### 3B. Distance / contribution LOD — ✅ done

Decimate each unique representation once (at sidecar-build time), store
the reduced index slice in the same EBO, and switch to it per-instance
per-frame whenever the projected sphere radius is small enough that the
reduced silhouette is indistinguishable from the original.

##### Pipeline

1. **After streaming finishes**, `MainWindow` calls `buildLods(sd)` on
   the snapshotted `SidecarData`. Each eligible mesh's decimated index
   list is appended to `sd.indices`; the per-mesh `MeshInfo` gains two
   new fields:

   ```cpp
   uint32_t lod1_ebo_byte_offset;  // appended slice, same VBO
   uint32_t lod1_index_count;      // 0 = no LOD1 was built
   ```

   `MeshInfo` grew from 48 to 56 bytes, which also bumps the sidecar
   format to v5.

2. `viewport_->applyLodExtension(model_id, sd)` pushes the new index
   suffix onto the live EBO via `glNamedBufferSubData` and replaces the
   CPU-side `m.meshes` vector. The VBO and instance SSBO are untouched
   — LOD1 reuses the same vertices, only the indices differ.

3. The sidecar is then written with both LOD0 and LOD1 indices baked in,
   so subsequent loads of the same file pick up LOD1 for free.

##### Selection

The contribution-cull pass already computes each instance's projected
pixel radius. LOD1 is selected when that radius falls below
`IFC_LOD1_PX` (default 30 px) and the mesh has a non-empty LOD1 slice.
Camera-inside-AABB short-circuits select LOD0 (treated as "infinite
radius") so you never accidentally see the reduced mesh up close.

The visible-instance pipeline gains two more buckets (`fwd_lod1_`,
`rev_lod1_`), so the four-way split is now `{fwd, rev} × {LOD0, LOD1}`.
LOD0/LOD1 within a winding slice are contiguous — only winding requires
`glFrontFace` to flip between MDI calls, LOD does not. `firstIndex` /
`count` in the `DrawElementsIndirectCommand` pick which slice of the EBO
to walk; everything else (base vertex, base instance, SSBO bindings,
shader) is unchanged.

##### Decimator choice: `meshopt_simplifySloppy`

`meshopt_simplifySloppy` is a **voxel-clustering decimator** — it
quantises positions into cells and merges everything in a cell to a
single point. This is the only meshoptimizer decimator that works on
BIM brep output, which has per-triangle vertex duplication (hard-edge
normals) and non-manifold topology (T-junctions, coplanar slivers,
separate solids meeting at a plane). The edge-collapse decimator
(`meshopt_simplify`) needs 2-manifold edge pairs to score collapses;
on BIM geometry it returns the input unchanged.

`simplifySloppy` rounds off sharp corners and can produce slightly
degenerate triangles, so it doesn't look great at mid-screen size.
For a LOD1 that only activates below 30 px projected radius that's
invisible in practice.

##### Tuning knobs (env vars)

| Var | Default | Effect |
|-----|---------|--------|
| `IFC_LOD1_PX` | `30` | Projected sphere radius (px) below which LOD1 kicks in. `0` disables LOD1 entirely. |
| `IFC_LOD_ERROR` | `0.2` | Target relative error passed to meshopt. |
| `IFC_LOD_RATIO` | `0.25` | Target triangle-count ratio (LOD1 aims for 25 % of LOD0 tris). |
| `IFC_LOD_MIN_SAVINGS` | `0.25` | Reject the LOD1 result if it doesn't shave at least this fraction of triangles. |
| `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. |

##### Measured results

Same 10-model / 128 M-tri scene as Phase 3A (GTX 1650), 2 px contribution
threshold, overview camera, all models finalised with LOD1 built:

| Build | FPS | Frame time | Visible tris | Visible objs |
|-------|-----|-----------|--------------|--------------|
| Phase 3A alone (2 px) | 20.2 | 49 ms | 40 M | 89 k |
| Phase 3A + 3B (LOD1 ≤ 30 px) | **43.2** | **23 ms** | 14 M | 81 k |

Roughly half the remaining frame time, same object count (LOD is
lossless w.r.t. visibility — swapping index slice doesn't hide
anything). The triangle reduction on meshes that qualified for LOD1 is
~80 %: e.g. 4.17 M → 0.82 M tris for the 3618 eligible meshes of Model
1, 3.25 M → 0.65 M for Model 2, etc. Only about 20 % of unique meshes
qualify (the threshold is 500 tris — below that the indirect-command
overhead dominates), but those are the fat tail carrying most of the
rasterisation cost.

LOD build itself runs on the main thread inside `onStreamingFinished`;
typical cost is 100–600 ms per model, folded into the already-visible
"finalizing" step. Cached into the sidecar afterwards, so subsequent
opens skip it entirely.

#### 3C. Hierarchical-Z occlusion culling — ✅ done (v1, CPU-side)

Reject frustum-visible instances whose AABB is fully behind something
already drawn. The last drawn frame's depth buffer is the oracle — if a
region's deepest rasterised fragment is closer than an AABB's nearest
point, nothing in that AABB can win the depth test.

In dense BIM this matters most on interior views: standing inside a
building, 80–95 % of the model sits behind the walls of the current
room and contributes nothing to the frame. Phase 3A drops the
*distant-and-small* geometry, 3B drops its triangle count when kept,
and 3C drops the *close-and-big-but-hidden* bulk that neither of those
can touch. On an outdoor overview shot (nothing is occluded) 3C does
almost nothing — which is fine, 3A+3B already cover that case.

##### Pipeline (v1: CPU-side, 1-frame stale)

```
render():
  draw main scene into MSAA default fb
  axis gizmo
  buildHizPyramid():          <-- new
    glBlitFramebuffer MSAA depth → single-sample depth tex (256×128)
    glReadPixels  depth tex → CPU
    max-reduce mip chain on CPU (8–9 levels)
    store the VP that produced this frame
  swapBuffers

cullAndUploadVisible():
  per BVH node:     frustum ∧ contribution ∧ hiz  (subtree early-out)
  per instance:     frustum ∧ contribution ∧ hiz
```

The pyramid is always the *previous* frame's depth. On a newly loaded
scene or after a camera jump the cull is conservatively too permissive
for a frame or two (draws the occluded stuff by accident) and then
settles. No flicker because we never *wrongly reject* a visible
instance — the comparison is `aabb_near_depth > hiz_max`, so the
worst case is a kept instance that was actually occluded.

##### Why CPU-side?

Because the readback is cheap at this resolution (~128 KB / frame,
single glReadPixels ≈ 0.5 ms on PCIe) and the test itself is trivial
— ~100 k AABBs × 8 corners × a small mip lookup is well under a
millisecond on one thread. Phase 3D will port the cull to a compute
shader reading the pyramid as a texture, eliminating the readback; but
Phase 3C's CPU implementation was small enough to do first and
measure.

No MSAA complication on the write side: we just blit the default
framebuffer's multi-sample depth into a single-sample texture (GL
handles the resolve). No separate occluder pass either — we use the
previous completed frame's depth buffer directly, which is what a
temporal-reprojection HiZ reduces to when the "occluder set" is
"everything visible last frame".

##### The test

```cpp
project 8 AABB corners through hiz_vp  →  NDC rect + min z
if any corner has w ≤ 0:        return false  // crosses near plane
if rect is outside [-1, 1]²:    return false
pick mip level where rect ≤ 2×2 texels
hiz_max = max(pyramid[mip][covered texels])
return aabb_near_depth > hiz_max
```

Comparing the AABB's *closest* point against the pyramid's *deepest*
value is the conservative direction — it only rejects when the AABB
is strictly beyond everything we already drew in that region. We pick
the mip at which the rect covers ≲ 2 texels on each axis so the lookup
is O(1) regardless of AABB size.

##### BVH integration

The same test runs on interior BVH node AABBs before leaf expansion,
so an occluded subtree skips all its instances in one shot. This is
where most of the per-frame cost savings show up on interior shots —
rejecting a 500-instance BVH subtree costs one 8-corner projection.

##### Tuning knobs

| Var | Default | Effect |
|-----|---------|--------|
| `IFC_NO_HIZ` | unset | `1` disables HiZ entirely (forces the Phase-3B-only path). |
| `IFC_HIZ_SIZE` | `256` | Base pyramid width in texels; height tracks viewport aspect. Raise for more accurate near-silhouette occlusion, lower to shrink readback. |

The stats overlay gains one counter, `hiz_rej`, showing how many
instances per frame the HiZ test rejected. On outdoor overview shots
it hovers near zero; on indoor shots it climbs into the hundreds of
thousands and the frame time drops accordingly.

##### Known caveats

- **Optional during camera motion (`IFC_HIZ_MOTION=1`).**  The pyramid
  is aligned to the previous frame's VP.  On a moving camera the stale
  depth can falsely occlude objects, particularly thin geometry (pipes,
  railings) at oblique angles.  By default HiZ is disabled during motion
  (`hiz_vp_ == current_vp` check).  Setting `IFC_HIZ_MOTION=1` forces
  HiZ on during motion — benchmarks show this is the single biggest
  perf lever (2.9× speedup), and the artifacts are transient and minor
  during active orbiting.  When the camera stops, a settle recull fires
  with `hiz_vp_valid_ = false`, disabling HiZ for that one frame and
  re-culling the full scene.  This guarantees the stationary view is
  artifact-free.  See Phase 3G for benchmark data.
- **Conservative occlusion test.**  The original "max over coarse mip"
  test was too aggressive for BIM scenes where the entire depth range
  compresses into 0.99–1.00.  Replaced with "all fine-mip texels must
  agree" — sample at mip 1, reject only if every texel has depth less
  than the AABB's nearest point, early-out on the first non-occluding
  texel.  Queries covering >64 texels skip HiZ entirely.  Eliminates
  most false occlusions at the cost of fewer true rejections.
- **Depth blit replaced with shader downsample.**  The original
  `glBlitFramebuffer` for scaling the resolved depth to HiZ size
  produced `GL_INVALID_VALUE` on some drivers.  Replaced with a
  fullscreen-triangle shader writing `gl_FragDepth`.  The resolve
  texture uses `GL_DEPTH24_STENCIL8` to match Qt's default FBO format
  (which uses D24S8 even when only depth is requested).
- **Readback syncs the GPU.** `glGetTextureImage` is blocking.
  Measured cost is well under a millisecond at 256×128; not a
  bottleneck on the machines tested.
- **Transparent geometry would need special handling**, but the
  current renderer doesn't have any, so no-op for now.

#### 3D. Parallel per-model cull (CPU, done)

A cheaper intermediate step before going full-GPU: each model's cull is
independent (no shared mutable state beyond atomic timing counters), so
`render()` fans the per-model culls out to a `std::async` pool and joins
before the serial GL-upload pass. On the 18-model / 569 k-instance test
scene this took the cull from ~25 ms wall-clock to ~5 ms — roughly a 4×
speedup on an 8-core machine, tracking `std::thread::hardware_concurrency()`
up to the model count. Load balancing is static (one job per model); a
single massive model still bottlenecks to single-threaded speed and would
need intra-model partitioning, but in practice BIM projects are
multi-discipline so the coarse partition lands well.

The stats line now reports `cull[wall X | work: clr Y trv Z emt W upl U]`:
`wall` is frame-time impact, the `work` numbers are per-thread sums showing
where CPU cycles went. `IFC_CULL_THREADS=0` forces single-threaded mode
for comparison.

#### 3E. GPU compute culling — experiments and removal

##### What we tried

Five iterations of GPU compute culling were explored on a 1.06 M-instance
/ 111-model scene (GTX 1650):

1. **Full GPU-driven rendering** — compute shader doing frustum +
   contribution + HiZ + LOD + winding + indirect command building via
   `glMultiDrawElementsIndirectCount`. Worked but introduced code smells
   (extension loaders, ad-hoc readbacks). Reverted.

2. **GPU frustum-only validation** — minimal compute shader (64
   threads/workgroup), 0.82 ms for 1.06 M instances vs 10–15 ms CPU.
   Proved GPU brute-force beats CPU BVH for raw AABB-vs-frustum.

3. **Hybrid with synchronous readback** — added contribution culling,
   read survivors back with `glGetNamedBufferSubData`. The 4–7 ms
   pipeline stall negated all GPU savings.

4. **Async one-frame-late readback** — persistent-mapped buffer +
   fence. Zero stalls, ~5.5 ms total vs ~5.5 ms CPU-only. Matched
   but didn't beat.

5. **Dirty-mesh tracking** — reduced emit from O(total meshes) to
   O(dirty meshes). Helped the consume phase but didn't change the
   bottom line.

##### Why it was removed

Benchmark with motion-adaptive culling + HiZ active (Phase 3G):

| Path | FPS |
|------|-----|
| CPU BVH (parallel) | 51.0 |
| GPU cull + async readback | 52.0 |

The GPU cull added ~585 lines of code (compute shader, persistent-mapped
readback buffer, fence management, per-model AABB SSBOs, 8 profiling
counters, cleanup at 4 sites) for a 2% improvement that was within
measurement noise. With HiZ + motion culling reducing the visible set
to ~11 k objects, the CPU BVH path handles the work in ~2 ms — there's
nothing left for the GPU to win.

**Removed** in favour of keeping the codebase simple. The CPU BVH path
is now the only cull path.

##### Lessons learned

1. **GPU brute-force beats CPU BVH for raw frustum tests** (0.82 ms vs
   10–15 ms for 1 M instances) but the advantage disappears once
   higher-level culling (HiZ, contribution) reduces the working set.
2. **Synchronous readback kills GPU cull.** Persistent-mapped async
   readback works but adds complexity for negligible gain.
3. **Hybrid GPU/CPU paths are maintenance-heavy** for diminishing
   returns when the CPU path is already fast enough.

#### 3F. Sub-draw fragmentation analysis

##### The problem

With the culling pipeline mature (BVH + contribution + HiZ + motion
culling), the dominant cost shifts to the *drawing* side.  On the 1.06 M-instance / 111-model
scene, frame times are 48–63 ms despite only 24–47 M visible
triangles — well within the GTX 1650's throughput.  The culprit is
the number of indirect sub-draws (individual `DrawElementsIndirectCommand`
entries inside each `glMultiDrawElementsIndirect` call).

##### Measurement

Diagnostic instrumentation (`IFC_SUBDRAW_DIAG=1`) revealed:

**Mixed scene (111 models, 1.06 M instances):**

| instanceCount | sub_draws | % of total | instances | triangles |
|---------------|-----------|------------|-----------|-----------|
| 1 | 114,624 | **95.7%** | 114,624 | 16.9 M |
| 2 | 2,269 | 1.9% | 4,538 | 1.3 M |
| 3–4 | 1,127 | 0.9% | 3,873 | 1.6 M |
| 5–8 | 1,106 | 0.9% | 6,407 | 1.9 M |
| 9–16 | 376 | 0.3% | 4,315 | 0.8 M |
| 17–64 | 264 | 0.2% | 7,766 | 8.0 M |
| 65–256 | 29 | <0.1% | 3,331 | 2.0 M |
| 257+ | 8 | <0.1% | 9,732 | 0.4 M |

**Steel-only scene (18 models, 570 k instances):**

| instanceCount | sub_draws | % of total | instances | triangles |
|---------------|-----------|------------|-----------|-----------|
| 1 | 68,616 | **85.9%** | 68,616 | 12.5 M |
| 2 | 5,385 | 6.7% | 10,770 | 2.7 M |
| 3–4 | 2,581 | 3.2% | 9,100 | 1.3 M |
| 5+ | 3,324 | 4.2% | 66,407 | 7.0 M |

##### Consolidation potential

The mesh-level consolidation analysis found:

- **119,803 unique visible mesh IDs = 119,803 sub_draws** (perfect 1:1)
- **0 meshes split by winding or LOD buckets** — no mesh_id appears in
  more than one (fwd/rev × lod0/lod1) bucket
- **0% reduction** available from merging across winding/LOD
- **114,624 meshes (95.7%)** are genuinely unique geometry placed
  exactly once — instancing provides zero benefit for these

This is a fundamental property of the IFC data, not a pipeline
inefficiency.  BIM models contain thousands of unique parametric
shapes (custom brackets, unique beam profiles, one-off fittings) each
placed at a single location.  Only a minority of elements (standard
doors, windows, pipe fittings) share geometry across placements.

##### Conclusions

1. **Instancing is maxed out.**  The pipeline already groups all
   instances of each mesh into a single sub_draw.  With 96% of meshes
   having exactly one visible instance, there is nothing more to
   group.

2. **Per-draw overhead dominates frame time.**  95–120 k sub_draws at
   ~20 fps = 48–50 ms/frame, but only 24–33 M triangles.  A GTX 1650
   can shade 1+ billion triangles/sec; the GPU is starving on
   per-command overhead (command fetch, baseInstance lookup, draw
   setup), not vertex/fragment throughput.

3. **The path forward is static batching.**  Merge the vertex and
   index data of multiple distinct single-instance meshes into
   combined VBO/EBO ranges, each issued as one sub_draw.  Batches of
   256–1024 spatially-coherent meshes would collapse 91–115 k
   sub_draws into 100–450, a 200–1000× reduction.

4. **Trade-offs of static batching:**
   - Culling granularity degrades from per-mesh to per-batch.  Batches
     must be spatially coherent (e.g., BVH subtree leaves) or invisible
     geometry gets drawn.
   - Per-instance attributes (object_id, colour_override) must move
     into the vertex stream or a per-vertex SSBO lookup, since
     instancing no longer applies to merged meshes.
   - The VBO/EBO layout changes at finalize time; existing instancing
     stays for multi-instance meshes (the 4% that benefit from it).
   - The sidecar format needs a version bump to cache batch membership.

5. **The steel scene validates the hypothesis.**  It has better
   instancing reuse (86% single-instance vs 96%) and correspondingly
   better fps (49 vs 20).  The ~2.5× fps ratio tracks the sub_draw
   ratio (~80 k vs ~120 k), confirming per-draw overhead as the
   dominant cost.

#### 3G. Motion-adaptive culling + HiZ during motion — ✅ done

The bottleneck during camera orbit is the sheer number of visible
objects and sub_draws.  Two complementary strategies address this:

##### Motion-adaptive contribution culling (`IFC_MIN_PX_MOTION`)

During camera motion, use a larger pixel-radius threshold to hide
small objects that contribute little at interactive rates.  When the
camera stops, a settle recull restores the base threshold and full
detail within one frame.  No visual artifacts — objects below the
motion threshold are genuinely tiny on screen.

##### HiZ during motion (`IFC_HIZ_MOTION=1`)

Force the one-frame-stale HiZ pyramid to remain active during camera
motion.  The stale depth causes minor false occlusions on thin
geometry at oblique angles, but these are transient during active
orbit.  When the camera stops, the settle recull invalidates the HiZ
pyramid (`hiz_vp_valid_ = false`) and re-culls without HiZ,
guaranteeing the stationary view is artifact-free.

##### Benchmark results

Benchmarked on 1.06 M-instance / 111-model scene, 200-frame orbit
(103° arc, 0.5°/frame), GTX 1650:

| Configuration                    | avg ms | fps  | speedup | obj   | sub_draws | hiz_rej |
|----------------------------------|--------|------|---------|-------|-----------|---------|
| Baseline (no opts)               | 61.25  | 16.3 | 1.0×    | 254k  | 155k      | 0       |
| MIN_PX_MOTION=10                 | 37.67  | 26.5 | 1.6×    | 70k   | 56k       | 0       |
| HIZ_MOTION=1                     | 21.44  | 46.6 | 2.9×    | 33k   | 17.5k     | 28k     |
| HIZ_MOTION=1 + MIN_PX_MOTION=10  | 19.62  | 51.0 | 3.1×    | 11.4k | 8.7k      | 11.5k   |

##### Conclusions

1. **HiZ during motion is the biggest single lever** — 2.9× alone.
   Artifacts are minor and transient during orbit; the stationary view
   is guaranteed correct by the settle recull.

2. **Motion pixel culling is clean and effective** — 1.6× with zero
   artifacts.

3. **Combining both gives diminishing returns** — 3.1× vs 2.9× (HiZ
   alone) or 1.6× (MIN_PX alone).  They compete over the same objects.

4. **The ~19 ms floor is GPU rendering**, not culling.  At 8.6k
   sub_draws the bottleneck shifts to draw dispatch + triangle
   rasterization.  Further improvement requires reducing sub_draws
   (static batching) or moving to a more efficient draw model.

##### Benchmark CLI

Press **C** during interactive use to print the current camera as a
`--camera` argument.  Then benchmark reproducibly:

```bash
./IfcViewer --camera tx,ty,tz,dist,yaw,pitch --benchmark 200 files...
```

The benchmark orbits the camera (0.5°/frame yaw), measures N frames
after a 5-frame warmup, prints avg/median/p1/p99 frame times, then
exits.  Env vars control the test configuration.

### Planned follow-ups (post-Phase-3)

- **Mesh shaders / meshlets.** Ceiling-raising, but overkill until the
  above are exhausted and we've hit silicon limits on vertex/raster
  throughput.

## Summary table

```
Scene size                      Bottleneck              Fix
-----------                     ----------              ---
< 100k instances                CPU cull scan           Phase 1 only
100k–500k                       CPU cull scan           BVH (Phase 2) — done
500k+ tris / overview shot      GPU vertex + raster     Phase 3A contribution cull
                                                        + Phase 3B LOD (done)
multi-million + occluders       redundant rasterisation Phase 3C HiZ (done, CPU readback)
many models, serial cull        single-thread BVH trv   Phase 3D parallel cull (done)
orbit fps on 1M+ scenes         too many vis objects    Phase 3G motion culling + HiZ (done, 3.1×)
90k+ unique visible meshes      per-draw GPU overhead   Phase 3F static batching (next)
```

## Roadmap

- [x] Material colour support (per-vertex RGBA8)
- [x] Per-model GPU buffers (VAO/VBO/EBO per model, no cross-model copies)
- [x] Per-object frustum culling (Phase 1)
- [x] BVH acceleration with per-model trees (Phase 2)
- [x] Raw binary `.ifcview` sidecar cache
- [x] Non-blocking sidecar loading (background thread I/O)
- [x] Progressive GPU upload (VBO/EBO growth + streaming-time instance appends)
- [x] GPU instancing (unique meshes + per-placement SSBO)
- [x] `glMultiDrawElementsIndirect` draw path
- [x] Reflection-aware two-pass draw for mirrored placements
- [x] Backface culling (user-toggleable, default on)
- [x] `reorient-shells` enabled in iterator
- [x] Perf diagnostic env vars (`IFC_SKIP_MDI`, `IFC_MAX_SUBDRAWS`, `IFC_MIN_PX`, `IFC_LOD1_PX`, `IFC_NO_HIZ`, `IFC_HIZ_SIZE`, `IFC_CULL_THREADS`, `IFC_MIN_PX_MOTION`, `IFC_HIZ_MOTION`, `IFC_SUBDRAW_DIAG`)
- [x] Phase 3A — screen-space contribution culling
- [x] Phase 3B — distance / contribution LOD (meshoptimizer `simplifySloppy`)
- [x] Phase 3C — Hierarchical-Z occlusion culling (v1, CPU-side readback)
- [x] Phase 3D — Parallel per-model CPU cull (`std::async` fan-out)
- [x] Quantized VBO (12 B/vert: u16x3 pos + oct i8x2 normal + RGBA8, sidecar v7)
- [x] Event-driven rendering (zero idle CPU/GPU, cull skipped on still frames)
- [x] Phase 3E — GPU compute-shader culling (explored, removed — CPU BVH matches at ~585 fewer lines)
- [x] Phase 3G — Motion-adaptive culling + HiZ during motion (3.1× orbit speedup on 1M-instance scene)
- [x] Benchmark CLI (`--camera`, `--benchmark`, press C to capture camera)
- [ ] **Phase 3F — Static batching of single-instance meshes** (next; reduces 90k+ sub_draws to hundreds)
- [ ] Vulkan/MoltenVK backend for macOS
- [ ] Embedded Python scripting console