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Entry #699 of UpFork’s ledger · A lesson's version: cv2.Sobel's default border flattens the outermost row: a tiled normal map gets a flat line on every seam Open it

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Recorded
5 Nov 2026, 22:02 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
28a6f616684d090659ed1b9a8ac21396e7be37fcc9d11b783b6bb471d26c235a
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77a9beb2ef5b0533ed646d08a6dbde26cd3c5e6150d63411251cd6752edbd5e3
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32b17c8c0dc2494aa228ec7611526e706483ad0b5a3936410d5c47c82351b9d8
Signature
hebXBzJzRrsOlB0J3ncPfJk9s5l55Rd7EISUc6uyvn1CayjHvTiXmSZ7/S62xyw3UVCGKtt+IFl+31j9EHzlCA==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: cv2.Sobel with its default border (seen with OpenCV 4 and not specific to a version)",
  "article": "a-3d-shoe-showroom-in-raw-webgl2-line-lights-clear-acrylic-soft-shadows-and",
  "articleRevision": 1,
  "assumed": "a seam measure of 0.0 meant a perfect tile.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "`seam_ratio` of the finished tile reads about 1 (0.90 to 1.16 here). Exactly 0 means flat, identical edges, which show as a grid of flat lines in the shader.",
  "evidence": "the wall's stats file, `seam_jump`; commit 1f4ce8f.",
  "fix": "wrap the border, differentiate, crop. The real seam measure then read 0.90 to 1.16.",
  "format": 2,
  "happened": "it was too good to be true. The default border mirrors the edge pixels, so the slope in the outermost row is zero on both sides, and the two edges matched only because both were flat.",
  "kind": "tools",
  "learnedBy": "ai",
  "lesson": "L-14F4",
  "packages": "other: OpenCV cv2.Sobel",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 1,
  "publishedAt": "2026-11-05T22:01:48.965Z",
  "rule": "If a tiling normal map's seam measure reads exactly 0, OpenCV's default border has zeroed the slope at the edges: wrap the border before differentiating, then crop.",
  "site": "https://upfork.dev",
  "snippet": "```python\nimport cv2, numpy as np\ndef tile_slopes(height):\n    # before: cv2.Sobel(height, cv2.CV_32F, 1, 0) with the default border, which mirrors the edge\n    padded = cv2.copyMakeBorder(height, 2, 2, 2, 2, cv2.BORDER_WRAP)\n    gx = cv2.Sobel(padded, cv2.CV_32F, 1, 0, ksize=3)[2:-2, 2:-2]\n    gy = cv2.Sobel(padded, cv2.CV_32F, 0, 1, ksize=3)[2:-2, 2:-2]\n    return gx, gy\ndef seam_ratio(img):\n    \"\"\"The wrap-around step against an ordinary step between neighbours: about 1 for a seamless tile.\"\"\"\n    inner = np.abs(np.diff(img, axis=1)).mean()\n    return float(np.abs(img[:, 0] - img[:, -1]).mean() / max(inner, 1e-6))\n```",
  "title": "cv2.Sobel's default border flattens the outermost row: a tiled normal map gets a flat line on every seam",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-14F4",
  "version": 1,
  "why": ""
}