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Recorded on 5 November 2026, unchanged since

Entry #696 of UpFork’s ledger · A lesson's version: A camera centred on the page sees the top row of shelves edge-on: put the eye at the middle of the shelves Open it

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Recorded
5 Nov 2026, 22:02 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
0f545d7188d5854e23b191bd929b2e24f9b55c635b0f777b81b5cf27e310a665
Before it
341384075e57e7747dea5e8843ceb40ad4230b8c462179940017ad4f30aa9b42
Record hash
f187b047968a04a7522c5f8649fc07df1b56124781bc62605b47961300482783
Signature
tp34qAOnthCFrkzt2bvi2TseXLcFZwByOOw+reYBUJn6W9X8MkF9t2kwA/XxrLF6oTApPEX1Ly3N3TAyRECTDQ==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: any perspective camera over a grid of 3D shelves (seen in WebGL2 and not specific to it)",
  "article": "a-3d-shoe-showroom-in-raw-webgl2-line-lights-clear-acrylic-soft-shadows-and",
  "articleRevision": 1,
  "assumed": "a camera centred on the page would treat every row fairly.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "project the slab of the first row and of the last row through the pose: the underside of the first and the top face of the last come out equally tall on screen, because a flat face's apparent height grows with the eye's vertical distance from it.",
  "evidence": "commit 9064927, the P1 stills.",
  "fix": "move the eye to the middle of the shelves, so the top row is seen from below and the bottom row from above by the same amount, and let the lens shift move the picture, not the camera.",
  "format": 2,
  "happened": "the top row sat only 50 px above the eye, so its slabs looked nearly edge-on, while the bottom row showed its whole top face. The two rows read as different shelves, which is the very thing one shared camera was meant to fix.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-28C4",
  "packages": "",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 1,
  "publishedAt": "2026-11-05T22:01:48.965Z",
  "rule": "If rows of 3D shelves under one camera look like two different kinds of shelf, put the eye level with the middle of the shelves and use a projection lens shift to keep the page where it was.",
  "site": "https://upfork.dev",
  "snippet": "```ts\nconst SHOE_Z = 24;\ntype Layout = { contentHeight: number; slots: { slab: { y: number } }[] };\n// before (first attempt, from the build log): the eye at the page's centre, y = contentHeight / 2, with no lens shift.\n// after:\nfunction restPose(layout: Layout, width: number, height: number, top = 0) {\n  const f = Math.min(1700, Math.max(900, 0.9 * width));\n  const room = Math.max(1, height - top);                  // the host's header keeps its own strip\n  const stepBack = Math.max(1, layout.contentHeight / room);\n  const first = layout.slots[0]?.slab.y ?? 0;\n  const last = layout.slots[layout.slots.length - 1]?.slab.y ?? 0;\n  const eye = (first + last) / 2;                           // level with the middle of the shelves\n  const pageCentre = layout.contentHeight / 2;\n  const screenEye = top + room / 2 + (eye - pageCentre) / stepBack; // where that row belongs on screen\n  const shiftY = -((screenEye / height) * 2 - 1);           // in clip units: puts it there without turning\n  return { x: width / 2, y: eye, z: f * stepBack + SHOE_Z, shiftX: 0, shiftY };\n}\n```",
  "title": "A camera centred on the page sees the top row of shelves edge-on: put the eye at the middle of the shelves",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-28C4",
  "version": 1,
  "why": ""
}