Mesh operations

MeshOperations is the geometry verbs: extrude, inset, bevel, loop cut, knife, subdivide, bridge, fill hole, flip, weld, merge by distance, dissolve, delete, detach and append. Each takes an EditMesh and a set of element indices, changes the mesh, and returns the faces it made. MeshLoop is the small record each of them assembles the new face table out of.

Building a room out of a cube without leaving the viewport. A designer's blockout pass is almost entirely these verbs applied to a face selection, and every one of them is arithmetic with a right answer — which is what makes them testable against a cube instead of against a gesture.

You do not want them for a mesh you are only drawing. MeshData is the drawing structure; going through the kernel to change one costs two conversions and gains nothing.

Nothing here knows about a scene, a selection or an undo stack. Which faces are selected is the editor's, and so is recording the change — see BlockoutGeometry, which is the same verbs with a document and a command behind each.

Every verb takes what to act on and returns what it made:

⚠ The face table is renumbered and the positions are not. A position index is what a selection holds, what an undo entry records and what a drag in flight is writing to; renumbering those under a running gesture is the defect doc 24's D3 exists to prevent. Faces move freely, which is exactly why a topology change drops an element selection.

⚠ Positions no face uses are left behind rather than compacted. EditMesh.Validate reports them as orphans, and Compact is what removes them — run between gestures, when nothing holds an index, because it renumbers the position table and hands back the map.

Knife is doc 24 § P3's last row, and its primitive is *split this face between these two points on its boundary*. KnifeCut is one of those; a stroke is a list of them and goes in one call:

⚠ A stroke is one call because each cut renumbers the table the next would index into. Cutting face 0 and then face 3 is cutting whatever face 3 has become after a rebuild, which is usually a face the stroke never crossed. Everything is resolved before anything is written, so a stroke whose last face refuses does not leave the ones before it half cut.

⚠ It ends in Stitch, because a cut leaves T-junctions by construction. A point part-way along an edge is a corner on the face that was cut and the middle of a whole edge on the face beside it: the two stop sharing an edge, Validate calls both halves boundaries, and the surface draws with a crack that opens and closes as the camera moves. Nothing about the geometry is wrong, which is why it survives every check that is not that one.

⚠ Two faces meeting at one point get one position. A stroke leaves a face by the edge the next face enters by; inserting a position apiece would put two coincident corners on one edge, which Stitch cannot repair because each is already a corner of the face that made it. Every count-shaped assertion stays green over that, and the seam only closes when somebody welds by distance.

⚠ A chord that separates nothing is refused rather than made degenerate. Both ends on one edge, both at one corner, or two adjacent corners each describe a cut that divides the face into itself and a line — and a face table with a two-corner face in it is one every later verb walks wrongly.

⚠ A region and a set of individual faces are different answers, and both are wanted. Extruding four faces as a region gives one box; individually gives four boxes with walls between them. What decides it is what counts as the rim: an edge between two selected faces is interior when they are one region and a rim when they are not.

Bevel is the verb that looks small and is not:

⚠ A bevel on an edge that meets three other bevelled edges at a vertex is a miniature research problem. The honest first version bevels edges independently and reports where it could not resolve a corner, rather than producing a self-intersecting one silently.

Two rules worth knowing about the shapes these produce. A partial subdivision splits its neighbours' edges too — so they become n-gons rather than leaving a T-junction, which would draw as a crack the first time anything moved. And a loop cut runs only through quads, because "the opposite edge" is a phrase about a four-sided face.

• Editable meshes — the structure these change, and why it is not a half-edge.
• Element selection — what supplies the indices, in the editor.
• docs/plan/24 § P3 — the verb inventory these implement, and the bindings each of them has.