An edge connects two vertices in a 3D mesh. Vertices have x, y, and z coordinates; connected edges bound faces, which form the surface of a polygon mesh. An edge describes geometry, not a relationship between assets in a digital twin. section 7.1 section 5.3.1
A triangular face has three vertices and three edges. Larger planar polygons can use more. Normals serve a different purpose: they indicate a direction perpendicular to a vertex or face for lighting and rendering. section 7.1
Listing 7.1 Definition of a 3D cube in OBJ file format
# Vertices v -1.0 -1.0 1.0 v 1.0 -1.0 1.0 v 1.0 1.0 1.0 v -1.0 1.0 1.0 v -1.0 -1.0 -1.0 v 1.0 -1.0 -1.0 v 1.0 1.0 -1.0 v -1.0 1.0 -1.0 # Face normals vn 0.0 0.0 1.0 # Front face normal vn 1.0 0.0 0.0 # Right face normal vn 0.0 0.0 -1.0 # Back face normal vn -1.0 0.0 0.0 # Left face normal vn 0.0 1.0 0.0 # Top face normal vn 0.0 -1.0 0.0 # Bottom face normal # Faces (format: vertex//normal) f 1//1 2//1 3//1 4//1 # Front face f 2//2 6//2 7//2 3//2 # Right face f 6//3 5//3 8//3 7//3 # Back face f 5//4 1//4 4//4 8//4 # Left face f 4//5 3//5 7//5 8//5 # Top face f 5//6 6//6 2//6 1//6 # Bottom face
The OBJ cube lists vertex coordinates, normals, and faces. In the front-face entry, vertices 1 and 2 are consecutive corners, so their connection is an edge; the listing does not declare that edge separately. section 7.1
For a digital-twin model, edges are part of a mesh representation of a surface. Meshes balance visual fidelity, geometric accuracy, and rendering efficiency; point clouds instead use discrete points, while voxel models use volumetric cubes. section 7.1
