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Entry #692 of UpFork’s ledger · A lesson's version: Removing a coloured backdrop does not remove its bounced light: neutralise a black and white shoe Open it

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Recorded
5 Nov 2026, 19:07 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
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k1 · Ed25519
Entry hash
1fed955a5db8791193553f6a120cdbbc2e8b7e16825d3710074b5eaa172d29a7
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74589a15a1b4611e364cb3b42111029462b8769119e4bbdcfd36fedc803c5563
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kQgzZDXhQI5qWKfQkSFR8iL65gD7R+tgBx3AtmLwsGLO0+fgGNYXMYPuD2h1UBzRbuR10A1J7GQDt1kxbCMADw==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: OpenCV LAB colour",
  "article": "cut-shoes-out-of-photos-so-they-stand-exactly-on-a-shelf-a-background-remova",
  "articleRevision": 2,
  "assumed": "taking the backdrop away would take its colour away.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "On a pale wall the white reads white and not grey, with no orange in the black gloss.",
  "evidence": "`court-panel` in `scripts/photos/shoes.json`.",
  "fix": "this is white balance for one material, so never do it to a coloured shoe.",
  "format": 2,
  "happened": "a court shoe shot on orange came out salmon, with orange reflections in its black gloss.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-HVVE",
  "packages": "",
  "product": "shoe-store-product-grid-react",
  "productRevision": 2,
  "publishedAt": "2026-11-05T19:07:31.764Z",
  "rule": "If a white shoe stays pink after leaving a coloured backdrop, the light bounced onto it. For a shoe that is itself black and white, keep about 10% of its colour and raise exposure about 12%.",
  "site": "https://upfork.dev",
  "snippet": "```python\n# before: cut out, nothing else, and the white stays salmon\n# after, with \"gain\": 1.12 and \"desaturate\": 0.1 in this shoe's settings\ngain = shoe.get(\"gain\", 1.0)\nif gain != 1.0:\n    final_rgb = np.clip(final_rgb.astype(np.float32) * gain, 0, 255).astype(np.uint8)\nkeep = shoe.get(\"desaturate\")  # the share of its colour the shoe keeps\nif keep is not None:\n    lab = cv2.cvtColor(final_rgb.astype(np.float32) / 255, cv2.COLOR_RGB2LAB)\n    lab[..., 1:] *= keep\n    final_rgb = (np.clip(cv2.cvtColor(lab, cv2.COLOR_LAB2RGB), 0, 1) * 255).astype(np.uint8)\n```",
  "title": "Removing a coloured backdrop does not remove its bounced light: neutralise a black and white shoe",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-HVVE",
  "version": 2,
  "why": "a white surface shows the light that falls on it, and the exposure step is needed because a desaturated white looks grey on a pale wall."
}