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Entry #753 of UpFork’s ledger · A lesson's version: A clear box drawn without a depth buffer must be cut at the plane of what stands inside it 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
3a0be41af440c5f9691ffe52d52c24b7153ac55ad6cad2b6af7b4c75673f280d
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a20c18979ebf13e38775b741a20401dc895ab41d60aced895210fdc0030e3450
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52efcd82331fc1a489e3bce7c6289a4baececa06372f336a6970d44c65f4cea6
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wXCx7D0mGIdCTPBBoBhTdCXbJLTmbvkrPaxuLiubxnGEOOtxtz7oYw9GbELkq88Kw+3TXtQNY1zsam8y1y+4DQ==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: WebGL2 transparent geometry drawn without depth testing",
  "article": "a-3d-shoe-showroom-in-raw-webgl2-line-lights-clear-acrylic-soft-shadows-and",
  "articleRevision": 2,
  "assumed": "drawing the whole slab after the shoe would be close enough.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "look down at a lit slab: its top face shows no bright line across it at the shoe's plane, and the shoe is in front of the top's back half and behind its front half, for a shelf above the eye and one below it.",
  "evidence": "commit 9064927, `engine.ts` lines 607-658.",
  "fix": "the order above, with the top split at z = 24 and the rim measured from the box's real edges.",
  "format": 2,
  "happened": "a clear box has no good single place in the order against something standing inside it. The part of the top behind the shoe must be drawn before it and the part in front after. Splitting the face would draw a false bright line at the cut if the rim came from the face's own coordinates.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-BRGZ",
  "packages": "",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 2,
  "publishedAt": "2026-11-06T06:30:29.854Z",
  "rule": "If a clear box has a cut-out standing inside it and you draw without a depth buffer, draw its far faces, then the cut-out, then its near faces, splitting any face it crosses at its plane.",
  "site": "https://upfork.dev",
  "snippet": "```js\n// box.ranges[name]: [first vertex, count] of one face of a unit cube; the vertex shader places it by uOrigin + aPos * uScale.\n// uBoxMin and uBoxMax are set to the whole slab first. World y points down; the camera is always in front.\n// before: the whole slab after the shoe, so the back half of its top covered the shoe\nfunction drawSlab(gl, u, box, min, max, cam, shoeZ, drawShoe) {\n  const face = (name, z0 = min[2], z1 = max[2]) => {\n    gl.uniform3f(u.uOrigin, min[0], min[1], z0);\n    gl.uniform3f(u.uScale, max[0] - min[0], max[1] - min[1], z1 - z0);\n    gl.drawArrays(gl.TRIANGLES, ...box.ranges[name]);\n  };\n  const below = cam.y > max[1]; // the eye is under the slab\n  if (!below) face(\"bottom\");\n  face(\"top\", min[2], shoeZ); // the part of the top behind the shoe\n  if (cam.x > min[0]) face(\"left\");\n  if (cam.x < max[0]) face(\"right\");\n  drawShoe(); // draws with its own program, then binds the slab's again\n  face(\"top\", shoeZ, max[2]); // the part in front of it\n  if (below) face(\"bottom\");\n  if (cam.x < min[0]) face(\"left\");\n  if (cam.x > max[0]) face(\"right\");\n  face(\"front\");\n}\n```\n```glsl\n// The rim from the whole box's edges, in world units, so the cut at the shoe's plane draws no line.\nvec3 dd = min(vWorld - uBoxMin, uBoxMax - vWorld);\nfloat edge = abs(N.z) > 0.5 ? min(dd.x, dd.y) : (abs(N.y) > 0.5 ? min(dd.x, dd.z) : min(dd.y, dd.z));\n```",
  "title": "A clear box drawn without a depth buffer must be cut at the plane of what stands inside it",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-BRGZ",
  "version": 2,
  "why": ""
}