Mesh operations

What it is#

MeshOperations is the geometry verbs: extrude, inset, bevel, loop cut, 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.

What it is for#

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.

Using it#

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

var made = MeshOperations.Extrude(mesh, faces, distance: 2f);MeshOperations.Inset(mesh, made, amount: 0.2f);MeshOperations.Bevel(mesh, edges, width: 0.1f, segments: 3, out var unresolved);

⚠ 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.

⚠ 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.

Examples#

Bevel is the verb that looks small and is not:

var made = MeshOperations.Bevel(mesh, edges, width, segments, out var unresolved);if (unresolved > 0) {    // "seven corners were left square" — a sentence a designer can act on}

⚠ 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.

MeshOperations.LoopCut(mesh, edge, cuts: 3, slide: 0.5f);

See also#

• 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.