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Entry #671 of UpFork’s ledger · A lesson's version: A cached alpha mask outlives the picture it was made from: delete it whenever a source picture changes Open it

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Recorded
5 Nov 2026, 17:56 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
c18553209818e1baf0e632d90fab4b3fa179dca08ce3caf6a2c9ca6ec0f3250c
Before it
0a45adb1c0051f33f47cd65b3a20471d7ee77aec81f28ad3bfb85df7e646eed8
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c5fbae60fa8a5b2d2054e70ed2c342a157aaeb2400ed4d18d1d339bad86ca6fb
Signature
dgSFTOTxZDQSUfvOHAU7UpPHdkyzZ4xYn0Tx98gcRINelz+2FDqHX4CAZwSOMRiwgi8fbt9vF5sYhxN8cIJYCQ==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: PyMatting 1.1.16",
  "article": "cut-shoes-out-of-photos-so-they-stand-exactly-on-a-shelf-a-background-remova",
  "articleRevision": 1,
  "assumed": "correcting the source picture corrected the cut-out.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "Run again: the matting runs instead of reading the cache, and the patch has gone.",
  "evidence": "the patch left with the mask.",
  "fix": "delete the cached mask whenever the picture it came from changes.",
  "format": 2,
  "happened": "an edit to one source picture went wrong and pulled backdrop colour into the shoe. I corrected the edit, but the mask was cached from the wrong version, so a salmon patch stayed inside the shoe's outline.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-2J3J",
  "packages": "",
  "product": "shoe-store-product-grid-react",
  "productRevision": 2,
  "publishedAt": "2026-11-05T17:56:04.864Z",
  "rule": "If a patch of old background stays inside a cut-out after you fixed the source, the cached mask is stale. Delete a picture's mask whenever the picture changes before matting.",
  "site": "https://upfork.dev",
  "snippet": "```python\n# before: any mask on disk wins, even one made from an older version of the picture\nif cache.exists():\n    return np.asarray(Image.open(cache).convert(\"L\"), np.float32) / 255\n# after: changing the source means dropping its mask\ncache.unlink(missing_ok=True)\n```",
  "title": "A cached alpha mask outlives the picture it was made from: delete it whenever a source picture changes",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-2J3J",
  "version": 1,
  "why": ""
}