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Recorded on 6 November 2026, unchanged since

Entry #754 of UpFork’s ledger · A lesson's version: One white pixel at a box's corner under MSAA: an interpolated position fell outside its face Open it

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Recorded
6 Nov 2026, 06:30 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
e40533483acb4267fb8e584c8ee4e661fb079d09503f5331c673b016aeb9e861
Before it
3a0be41af440c5f9691ffe52d52c24b7153ac55ad6cad2b6af7b4c75673f280d
Record hash
73a0be246a1c8fe0567e95b87424f05b03155fc1ab243997e9e5881db82508d2
Signature
wcdRixepe+kmLkSZTeGjb+gcLo1owAi6ni/GfMYnq1F5VihGXqY8/Zw6WRARtb7wLLKcORQc9CI7TQmF+vR4Bw==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: WebGL2 multisampled targets (seen on ANGLE over Direct3D 11)",
  "article": "a-3d-shoe-showroom-in-raw-webgl2-line-lights-clear-acrylic-soft-shadows-and",
  "articleRevision": 2,
  "assumed": "a fragment's interpolated position always lies inside its triangle.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "render at 4× MSAA, read the pixels back with `gl.readPixels`, and compare each corner pixel's luminance with the brightest pixel of the rim beside it: none is brighter.",
  "evidence": "commit badceaf, `slab.frag.glsl`.",
  "fix": "clamp before the exponential and the power.",
  "format": 2,
  "happened": "one white pixel sat at a slab corner, only with MSAA. The position interpolated for that fragment was just outside the face, so `edge` went negative and `exp(−edge / k)` exploded, and `pow` got a negative base.",
  "kind": "tools",
  "learnedBy": "ai",
  "lesson": "L-2QDG",
  "packages": "",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 2,
  "publishedAt": "2026-11-06T06:30:29.854Z",
  "rule": "If a single bright pixel appears at the corner of a face only with multisampling, clamp every interpolated distance before an `exp` or a `pow`: a sample can be evaluated just outside the primitive.",
  "site": "https://upfork.dev",
  "snippet": "```glsl\n// before\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));\nfloat rim = exp(-edge / max(0.9, fwidth(edge) * 1.1));\nfloat fresnel = pow(1.0 - ndv, 4.0);\n// after\nedge = max(edge, 0.0);\nfloat px = max(fwidth(edge), 1e-3);\nfloat rim = exp(-edge / max(0.9, px * 1.1));\nfloat fresnel = pow(max(1.0 - ndv, 0.0), 4.0);\n```",
  "title": "One white pixel at a box's corner under MSAA: an interpolated position fell outside its face",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-2QDG",
  "version": 2,
  "why": ""
}