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Entry #702 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
5 Nov 2026, 22:02 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
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k1 · Ed25519
Entry hash
9bd0ce9709131515ac70cd36650ff0daaf8e685688a51064252b2a6491f355a1
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28f47df09cf7ebcd6aacbc5445051edc5edbf382a3dd32f4980b9ce5d0f5a41a
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d8f1d808f9d56d2d07ef25b2401bfff6a17d7019be7644f87a6b410216dd2990
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37H5iL6Kdce+9JvYJYexLHNLx2gXS1sEH2Edo2K5ct2SL7NpWVFHJTJrSruIqARUiv0W6axbZs/zox0FYXeOBw==
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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": 1,
  "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": 1,
  "publishedAt": "2026-11-05T22:01:48.965Z",
  "rule": "If a clear box holds a cut-out and there is no depth buffer, draw far faces, the cut-out, then near faces; split any face it crosses at its plane and take rims from the box's real edges.",
  "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": 1,
  "why": ""
}