A three.js LatheGeometry faces outward only when its profile points run from bottom to top

Fact

LatheGeometry winds its triangles from the order of the profile points it is given. Points listed with y increasing (bottom to top) give outward-facing triangles; the same points listed top to bottom give a surface wound inside out. The documentation only says each x must be positive, so nothing warns about the order. Checked on r186: a four-point profile gave 48 outward faces one way round and 48 inward the other.

Why it matters

The two cases fail differently, which hides the cause. A closed profile (one that starts and ends on the axis) wound inside out still looks solid with front-face culling: the near wall is culled and the far wall's inner side now faces the camera, so you see a plausible shape with slightly wrong lighting. An open profile (a sleeve, a cuff, a cup) has no far wall to stand in for it and simply vanishes from outside, leaving only whatever decorates its edge. Shapes that hang from a joint, such as limbs written from the shoulder down, are the usual way to get the order backwards.

How to apply

• Write every lathe profile from its lowest point up, or reverse it at the call site: new LatheGeometry([...points].reverse()) for a top-down list.
• Test a new lathe the cheap way: for each triangle, the cross product of two edges should point away from the y axis (dot it with the face centre's horizontal offset).
• If an open lathe is missing but its rim decoration shows, suspect point order before materials or culling settings.