Earlier probes pointed at per-frame glNamedBufferSubData uploads as the
bottleneck (60 fps when those two calls were commented out). That was a
false reading — zeroing the uploads also emptied the indirect buffer, so
MDI drew nothing. "No upload" and "no draw" were indistinguishable.
Two new diagnostic env vars in render() isolate the real costs:
IFC_SKIP_MDI=1 keep cull + upload + binds, skip only the MDI
draws. Gives 62 fps with everything else running,
confirming the non-draw path fits in ~16 ms.
IFC_MAX_SUBDRAWS=N cap each MDI's drawcount. 67k -> 30k sub-draws
saves 0 ms, confirming sub-draw count itself is
not the bottleneck; the long tail of sub-draws
carries ~no triangles.
On a GTX 1650 with 128 M triangles in view, nvidia-smi sits at 95 %
GPU util and FPS scales with triangle work, not sub-draw count. The
card is simply rasterising at ~850 M tri/s. No CPU-side or upload
trick recovers it.
Revised Phase 3 is therefore shedding triangles, not bytes:
3A screen-space contribution culling (next)
3B LOD
3C HiZ occlusion
3D GPU-side compute culling
README Phase 3 section rewritten around the diagnosis, including the
false lead, so future work doesn't re-tread the upload path. The
aborted staging+resident ring-buffer implementation was reverted (the
uncommitted working tree is gone — pure glNamedBufferSubData retained
for the visible + indirect buffers, which we now know is fine).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
The previous README described a pre-instancing world (32-byte world-
coord vertices with per-vertex object_id, ObjectDrawInfo structs, EBO
reordering after BVH build, and a Phase 3 plan built around moving
draw submission to the GPU). Most of that is either gone or already
solved:
- Vertices are now 28 B local-coord; per-instance transforms live
in an SSBO read through a visible-index SSBO and gl_BaseInstanceARB.
- ObjectDrawInfo is replaced by MeshInfo + InstanceCpu + InstanceGpu.
- No EBO reorder on BVH build — the BVH is over instance AABBs and
the mesh/EBO layout is orthogonal.
- Draw-call submission is already one glMultiDrawElementsIndirect
per model; the old Phase 3 goal is met.
New content worth keeping:
- GPU instancing section documents the mesh/instance/visible/indirect
buffer contract the whole renderer hangs off of.
- Reflection-aware two-pass draw is documented (det<0 placements,
forward/reverse slice split, glFrontFace toggle).
- reorient-shells and backface culling are called out as correctness
+ perf levers with their tradeoffs.
- Phase 3 is rewritten around the actual bottleneck surfaced by
profiling: per-frame glNamedBufferSubData stalls on the visible
and indirect buffers. Includes the diagnostic methodology (empty-
screen jump to 60 fps, window/MSAA invariance, upload-comment-out
experiment) so future-me remembers why this is the next step.
- 3A (persistent mapped ring buffers, near-term) and 3B (GPU-side
compute cull, longer-term) split out with scope estimates.
- Roadmap updated: instancing / MDI / reflections / reorient-shells
/ backface cull all ticked; 3A surfaced as the next open item.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Phase 2 performance: BVH acceleration with median-split build, per-model
trees, and EBO re-sorting for GPU cache coherence. Raw binary .ifcview
sidecar stores full geometry + BVH for instant subsequent loads (skip
tessellation entirely).
Per-model GPU buffers (VAO/VBO/EBO per model) eliminate cross-model buffer
copies on growth. Sidecar reads happen on a background thread. Bulk GPU
uploads are progressive (48 MB/frame chunks) so the viewport stays
interactive while multi-GB models stream in.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Reflect current architecture: per-model streamers, glMultiDrawElements
with frustum culling, 32-byte vertex format with color, multiselect
file picker, settings/stats files.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Dion Moult