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Share face boundary splits so meshes stay watertight (ENG-3826) - #28
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akhilles merged 2 commits intoOct 11, 2026
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refine_boundary splits a face's boundary segment where its own chart needs it, and only that face got the new point. The other face on the edge kept the segment whole, so every such split was a T-junction. pcb welds a model's vertices before meshopt simplification, and those open seams kept it from simplifying smooth surfaces split into faces. Splits now use the midpoint (a + b) / 2, which is the same point for both faces. After a shell's faces are meshed, each face also splits the boundary edges that other faces split, at the same midpoint, by fanning the one triangle on that edge in two. The midpoint lies on the edge, so the face's surface and error do not change, and no face is meshed twice. The checked-in closed solids are now watertight: every edge borders two triangles.
The shared-split pass hashes every boundary edge of every face, and the refinement loop hashes each triangle's edges several times per pass. Their keys are indices and exact float bits, so they need no keyed hash. A multiply-rotate hasher replaces SipHash in those maps and in the crossing-edge vertex map. Output is unchanged. Sharing splits adds about 3% to tessellation time on the board corpus, mostly in the fan triangles it adds; this pays it back.
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Left a non-blocking comment. This is a functional tessellation change, not a small fixup, so it still needs human review. Cursor Security Agent completed with no findings, Cursor Bugbot was not running, and no reviewers were assigned.
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Adjacent faces now split a shared boundary edge at the same midpoint, so the tessellated mesh has no T-junctions on shared edges and pcb can weld and simplify across smooth seams. On the 1,004-model board corpus, pcb's simplified output has 14.6% fewer triangles and 13.8% fewer GLB buffer bytes, with 0 failed faces and no change in the error bound. The summed tessellation time stays the same because a second commit removes SipHash from the hot edge maps. Fixes ENG-3826.