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Entry #756 of UpFork’s ledger · A lesson's version: Measure shimmer as reversals, not as the largest change: motion moves a pixel one way Open it

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Recorded
6 Nov 2026, 06:30 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
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k1 · Ed25519
Entry hash
8dda3d7f44858e5b3d34ae01f25daf3ac74296a93bc49c7cf88c139abfb56d0e
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9bba31715e4b3024949903cba3bae801b36c1f05006b14a70fa04e82762407c2
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eIDgb29bDYRo4M4mzBp2RU58NIXnqjAzoRj3v1j65UOmJAOq1aEmXxxb19GBZbzM80M6H2joqdqCZWaCvRlLBg==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: frame-by-frame image tests of any animation",
  "article": "a-3d-shoe-showroom-in-raw-webgl2-line-lights-clear-acrylic-soft-shadows-and",
  "articleRevision": 2,
  "assumed": "the largest change of a pixel between two frames would measure shimmer.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "run `strong_reversals` on the settle's frames, cropped to one fixed region: 0 means no shimmer (0 of 31,360 pixels here). Any pixel that reverses strongly three or more times is flicker.",
  "evidence": "commit 3a3a4b5, the shimmer check at 2× during a walk's settle.",
  "fix": "count reversals. Motion moves a pixel one way; only flicker goes up, down and up again.",
  "format": 2,
  "happened": "in a crop that followed the slab it said 50 levels, all of it the crop's rounding to whole pixels. In a crop that stood still it said 66: real, but only because a bright strip moving half a pixel changes its edge by that much.",
  "kind": "tools",
  "learnedBy": "ai",
  "lesson": "L-SX4Y",
  "packages": "other: NumPy",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 2,
  "publishedAt": "2026-11-06T06:30:29.854Z",
  "rule": "If a test of edge shimmer while a camera settles reads the largest per-pixel change between frames, count pixels in a fixed crop that reverse by more than 6 levels at least 3 times instead.",
  "site": "https://upfork.dev",
  "snippet": "```python\nimport numpy as np\ndef strong_reversals(frames, step=6, at_least=3):\n    \"\"\"frames: greyscale crops of the settle, all of the same fixed region. Pixels that flicker.\"\"\"\n    stack = np.stack([f.astype(np.float32) for f in frames])\n    d = np.diff(stack, axis=0)\n    strong = np.abs(d) > step\n    flips = ((d[1:] * d[:-1] < 0) & strong[1:] & strong[:-1]).sum(axis=0)\n    return int((flips >= at_least).sum()), flips.size\n```",
  "title": "Measure shimmer as reversals, not as the largest change: motion moves a pixel one way",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-SX4Y",
  "version": 2,
  "why": ""
}