Mesh surfaces

MeshSurfaces is the half of a mesh that is not its shape: the texture coordinates on its corners and the smoothing groups on its faces. UvProjection is which of three ways coordinates are worked out — world, box or planar.

It is pure arithmetic over the mesh's own arrays. Nothing here knows what a material is; that is the editor's, in face materials.

Making a block-out readable. Grey on grey is a shape you cannot judge the size of, and a checker whose squares are a fixed number of metres everywhere is what turns "how wide is that corridor" into something you count. Smoothing groups are the other half: they are what stops a generated cylinder coming out as a polygon.

You do not want it for unwrapping. Every unwrapper answers "where on this surface am I" and needs the surface to have been laid out; a block-out has not been laid out and never will be, because it exists to be replaced.

World is the default and that is the decision. A block-out box scaled 8×3 must not stretch its texels, and the only projection whose squares are the same size on two objects of different scales is the one that ignores the object entirely.

⚠ Coordinates are per corner, not per position. A cube's corner is three corners in three faces with three different coordinates, which is exactly why the drawing structure and the position graph are different graphs — a projection that wrote per position could not map two faces of one box differently, which is the whole point of a box projection.

⚠ A face that is not named keeps the coordinates it had. Mapping a wall and finding the floor remapped is what makes a per-face tool useless.

⚠ A hard edge is the absence of a smoothing group, so zero is a value rather than a sentinel. AutoSmooth numbers its groups from one and leaves a face with no neighbours inside the angle at zero, because "smoothed with nobody" and "hard" are the same picture and the first would fill a saved mesh with a group per face.

Smoothing is a union-find over faces across shared edges, not a flag per edge:

⚠ It has to be transitive. Every neighbour round a cylinder is within the angle, so the whole wall is one surface — and a flag per edge would make a corner between two smooth faces depend on which of them the normal was computed from first.

⚠ Averaging is weighted by area rather than counted. A cap's fan puts a great many tiny triangles at the pole and one big quad beside it; an unweighted average there is a normal dominated by the tessellation rather than by the shape, which is the classic pinched highlight at the top of a sphere.

A per-face transform turns about the face's own centre:

⚠ About its own centre rather than about the origin of the mapping. Rotating a face's texture a quarter turn about a point somewhere off in UV space moves it out of the frame as well as turning it, which is what makes a rotate field feel broken — and is what a naive matrix multiply does.

• Editable meshes — where the corner layers and the face groups live.
• Block-out shapes — what generates the faces these map.
• Face materials — the editor verbs, and what a viewport does with them.
• Mesh operations — the verbs that carry smoothing through an edit.