Document colmap_golden_bundle in llm.txt

llm.txt

@@ -96,6 +96,42 @@ Dense MVS (needs a Metal or CUDA build; after sparse):
 - Distortion / high-res robustness: any ETH3D scene (24 MP, fisheye).
 - Pose-accuracy evaluation: any scene (all ship a GT/reference model).
 
+## Golden MVS reference (colmap_golden_bundle/)
+
+Also in this repo: `colmap_golden_bundle/` = the CUDA `patch_match_stereo`
+golden output for the `south-building` scene (upstream COLMAP 4.0.4 + CUDA on an
+NVIDIA T4). Purpose: a "did I port the SAME algorithm faithfully" reference to
+diff a Metal/other MVS port against — NOT ground-truth accuracy (CUDA output is
+itself an approximation; for accuracy use ETH3D/DTU real GT).
+
+Layout:
+  colmap_golden_bundle/
+    note.md                              # full provenance + reader (read this)
+    colmap_cuda_golden_data.ipynb        # idempotent notebook that produced it
+    golden_mvs/dense/fused.ply           # 3,609,743 points (93 MB)
+    golden_mvs/dense/stereo/depth_maps/*.geometric.bin   # 128 depth maps (reference)
+    golden_mvs/dense/stereo/normal_maps/*.geometric.bin  # 128 normal maps
+    logs/                                # undistort / patch_match / fusion logs
+
+Map format: COLMAP dense binary — ASCII header "width&height&channels&" then
+little-endian float32 in Fortran (column-major) order; 0 = invalid (ignore
+zeros). Depth = HxW, normals = HxWx3. Reader:
+  import numpy as np
+  def read_colmap_array(path):
+      with open(path, "rb") as fid:
+          hdr, amp = b"", 0
+          while amp < 3:
+              ch = fid.read(1); hdr += ch
+              if ch == b"&": amp += 1
+          w, h, c = (int(x) for x in hdr.decode().split("&")[:3])
+          data = np.fromfile(fid, np.float32)
+      return np.transpose(data.reshape((w, h, c), order="F"), (1, 0, 2)).squeeze()
+
+Validate a port: run `colmap patch_match_stereo` on south-building (geometric
+consistency on, undistort cap 2000 px to match), then compare your
+*.geometric.bin against these on overlapping valid (non-zero) pixels within
+tolerance. See colmap_golden_bundle/note.md for exact pipeline + per-stage perf.
+
 ## Caveats
 
 - The ETH3D laser-scan ground-truth surface/mesh is NOT included — only the