odf / elm-mesh

Indexed mesh data structure for Elm with arbitrary face sizes

elm-mesh

This Elm package allows you to create and manipulate indexed meshes with arbitrary face sizes. It was inspired by elm-triangular-mesh and generally works in a very similar way. A mesh contains an array of vertices that can be of any type; the faces of a mesh are defined by lists of integer indices specifying which vertices makes up the face.

There are however two important differences:

• Faces can have three or more vertices, not just three.
• Each edge that is not on the boundary of the mesh must appear in exactly two faces, and in opposite directions.

The second property is best explained by an example:

import Mesh exposing (Mesh)
import Array

mesh : Mesh ( Float, Float )
mesh =
    let
        vertices =
            Array.fromList
                [ ( 0, 0 )
                , ( 4, 0 )
                , ( 4, 3 )
                , ( 0, 3 )
                ]

        faceIndices =
            [ [ 0, 1, 2, 3 ]
            , [ 3, 2, 1, 0 ]
            ]
    in
    Mesh.fromOrientedFaces vertices faceIndices

This creates a mesh that contains two rectangles put together back to back like a pillow. The first face, [ 0, 1, 2, 3 ], contains four directed edges described by the index pairs ( 0, 1 ), ( 1, 2 ), ( 2, 3 ), and ( 3, 0 ), the last one connecting the final vertex back to the first one and thus closing up the face. Similarly, the second face contains the directed edges ( 3, 2 ), ( 2, 1 ), ( 1, 0 ), and ( 0, 3 ). Each pair in the first list is matched by one in the second list with its first and second value, or in other words its beginning and end, flipped.

We can then retrieve the list of (undirected) edges described by their indices like this

Mesh.edgeIndices mesh
--> [ ( 0, 1 )
--> , ( 0, 3 )
--> , ( 1, 2 )
--> , ( 2, 3 )
--> ]

or as vertices like this

Mesh.edgeVertices mesh
--> [ ( ( 0, 0 ), ( 4, 0 ) )
--> , ( ( 0, 0 ), ( 0, 3 ) )
--> , ( ( 4, 0 ), ( 4, 3 ) )
--> , ( ( 4, 3 ), ( 0, 3 ) )
--> ]