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Enhance subdivision algorithm to eliminate T-junctions with a two-step edge splitting approach

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CNCKitchen
2026-03-17 11:08:00 +01:00
parent 1a23d7d7fa
commit 0e20de00dc
2 changed files with 129 additions and 47 deletions
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js/decimation.js
+21 -23
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@@ -138,38 +138,36 @@ function isBoundaryEdge(faces, vertFaces, v1, v2) {
return shared < 2;
}
// ── Guard 2: Link condition (non-manifold / pinch prevention) ────────────────
// The set of common neighbours of v1 and v2 must equal exactly the apex
// vertices of their shared faces. Extra common neighbours mean collapsing
// would fuse disconnected regions → non-manifold edge.
// ── Guard 2: Duplicate-face / pinch prevention ───────────────────────────────
// After collapsing v2 into v1, every face of v2 that survives (i.e. does not
// share v1) gets v2 replaced by v1. If any such remapped face is identical to
// a face already incident to v1, the collapse would create a duplicate → reject.
// This is the actual harm the link condition guards against, without the
// false-positives that the strict set-equality test produces on interior edges.
function hasLinkViolation(faces, vertFaces, v1, v2) {
// Collect apex vertices of the shared faces
const apices = new Set();
// Build a set of face "signatures" already incident to v1 (excluding shared faces).
// A signature is the sorted triple of vertex indices, encoded as a string.
const v1Sigs = new Set();
for (const f of vertFaces[v1]) {
if (faces[f * 3] < 0) continue;
const fa = faces[f * 3], fb = faces[f * 3 + 1], fc = faces[f * 3 + 2];
if (fa === v2 || fb === v2 || fc === v2) {
if (fa !== v1 && fa !== v2) apices.add(fa);
if (fb !== v1 && fb !== v2) apices.add(fb);
if (fc !== v1 && fc !== v2) apices.add(fc);
}
if (fa === v2 || fb === v2 || fc === v2) continue; // shared face, will be deleted
const arr = [fa, fb, fc].sort((a, b) => a - b);
v1Sigs.add(`${arr[0]},${arr[1]},${arr[2]}`);
}
// Build neighbour sets for v1 and v2 from active faces
const nb1 = new Set(), nb2 = new Set();
for (const f of vertFaces[v1]) {
if (faces[f * 3] < 0) continue;
for (let k = 0; k < 3; k++) { const nb = faces[f * 3 + k]; if (nb !== v1) nb1.add(nb); }
}
// Check every surviving face of v2 (after remapping v2→v1) for duplicates.
for (const f of vertFaces[v2]) {
if (faces[f * 3] < 0) continue;
for (let k = 0; k < 3; k++) { const nb = faces[f * 3 + k]; if (nb !== v2) nb2.add(nb); }
}
// Check common neighbours match apices exactly
for (const nb of nb1) {
if (nb !== v2 && nb2.has(nb) && !apices.has(nb)) return true;
const fa = faces[f * 3], fb = faces[f * 3 + 1], fc = faces[f * 3 + 2];
if (fa === v1 || fb === v1 || fc === v1) continue; // shared face, will be deleted
// Remap v2 → v1
const ra = fa === v2 ? v1 : fa;
const rb = fb === v2 ? v1 : fb;
const rc = fc === v2 ? v1 : fc;
const arr = [ra, rb, rc].sort((a, b) => a - b);
if (v1Sigs.has(`${arr[0]},${arr[1]},${arr[2]}`)) return true;
}
return false;
}