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Recorded on 6 November 2026, unchanged since

Entry #752 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
6 Nov 2026, 06:30 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
a20c18979ebf13e38775b741a20401dc895ab41d60aced895210fdc0030e3450
Before it
9cf6c3dfab6e928f20ee0f521595b75cbfd7c816873c60324b1c9741e42a5856
Record hash
62bf3e8e6d8f2a6ea1b6751244d845848ef5dc0fe67372562c9466f04e0b9cc4
Signature
GGU+oUp+ldoRwUiBoQ4OLPUTm5x0XkDFVseToYSeOBMnfPW5Q29mNzNWV0tyj3Mgrr7VwEeWKPrgU+oJrv91Dw==
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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": 2,
  "assumed": "a camera centred on the page would treat every row fairly.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "project the first row's slab underside and the last row's slab top face through `restPose` and print their heights on screen in px: they differ by under 1 px, 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": 2,
  "publishedAt": "2026-11-06T06:30:29.854Z",
  "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": 2,
  "why": ""
}