Split a mesh into two patches along a geodesic boundary

Connects a set of surface waypoints with geodesic paths to form a closed boundary loop, then splits the mesh into the two regions created by that loop.

Consecutive waypoints are joined by the geodesic (Dijkstra) shortest path along the mesh surface; the last waypoint connects back to the first.

Usage

vcg_mesh_patch(mesh, waypoints, seed_vertex = NULL, max_edge_length = NA)

Arguments

mesh

triangular mesh of class 'mesh3d'.

waypoints

numeric matrix with exactly 3 columns (x, y, z) and at least 3 rows; each row is a 3-D coordinate defining a boundary corner. After any mesh subdivision, each coordinate is snapped to the nearest vertex in the (possibly refined) mesh. Consecutive snapped vertices are joined by geodesic paths to form the closed boundary loop.

seed_vertex

integer (optional, 1-based). A vertex known to be inside the desired first patch. When NULL (default) the smaller of the two regions is returned first.

max_edge_length

numeric (optional). When positive and finite, the mesh is refined before patching so that no edge exceeds this length. Global edge subdivision (vcg_subdivision) is applied repeatedly until the average edge length falls below the threshold (fast), then vcg_subdivide_max_edge_length handles any remaining outlier edges. Waypoint coordinates are snapped to vertices after refinement, so finer vertices improve boundary accuracy. Default NA (no refinement).

Value

A length-2 list of mesh3d objects. Each contains:

$orig_vertex

1-based integer vector: new vertex index i corresponds to column orig_vertex[i] of the original mesh$vb.

The first element is the patch whose centroid is closest to the mean waypoint position; the second is the complementary connected patch. On a multi-manifold mesh, disconnected components not adjacent to the boundary loop appear in neither patch. When the boundary loop does not divide the mesh (degenerate waypoints), the second element is NULL.

Note

All waypoints must lie on the same connected component of the mesh. The mesh should be manifold; run vcg_fix_defects first if needed.

See also

dijkstras_surface_distance, surface_path, vcg_fix_defects

Examples

mesh <- vcg_sphere()
mesh <- vcg_uniform_remesh(mesh)
#> Resampling mesh using a volume of 38 x 38 x 38
#>   VoxelSize is 0.069282, offset is 0.000000
#>   Mesh Box is 2.000000 2.000000 2.000000

waypoints <- diag(1, 3)
patches <- vcg_mesh_patch(mesh, waypoints)

plot_mesh_polygon(
  patches,
  col = list("red", 'gray'),
  alpha = list(1, 0.5),
  eye = c(10, 10, 10)
)