Selectively subdivide mesh edges that exceed a length threshold

Up-sample a triangular mesh by iteratively splitting only edges longer than max_edge_len. Each long edge is split at its midpoint; the new vertex is connected to the opposite corner of every adjacent face. Iteration stops when no edge exceeds the threshold or max_iter passes are exhausted.

This is far cheaper than vcg_subdivision when most edges are already short and only a small fraction need splitting.

Usage

vcg_subdivide_max_edge_length(mesh, max_edge_len, max_iter = NULL)

Arguments

mesh: triangular mesh of class 'mesh3d'.
max_edge_len: maximum allowed edge length (same units as mesh coordinates).
max_iter: maximum number of refinement passes. When NULL (default), derived automatically from the current maximum edge length: ceiling(log2(current_max / max_edge_len)) + 1L, the minimum number of bisections needed in the worst case. The loop also terminates early once no edge exceeds the threshold.

Value

An object of class "mesh3d" with all edges at most max_edge_len long (provided max_iter was sufficient).

Note

The mesh must be manifold. Run vcg_fix_defects first if the mesh has boundary edges or non-manifold vertices.

Coercing Surface Inputs

The surface objects are converted to 'mesh3d' object before applying further calculations.

When surface is a surface ieegio object, the returned mesh3d$vb contains vertices that have been left-multiplied by surface$geometry$transforms[[1]] (the first transform stored in the geometry, typically the ScannerAnat or voxel-to-world transform).

Breaking change: Earlier versions (before 0.2.6) of ravetools returned the raw surface$geometry$vertices without applying any transform, so downstream code often multiplied by surface$geometry$transforms[[1]] (or an equivalent) manually before working in world space. Such code will now double apply the transform and produce incorrect coordinates. If you previously applied a transform from surface$geometry$transforms by hand after calling a ravetools mesh function on an 'ieegio_surface', remove that manual step.

Surfaces with an empty or missing geometry$transforms list (for example, surfaces produced by ieegio's volume_to_surface, which stores an identity transform) are unaffected.

If geometry$transforms contains multiple transforms targeting different coordinate spaces, only the first one is used. Callers that need a specific target space should select and apply that transform themselves before calling ravetools mesh functions.

Examples

if (is_not_cran()) {

  sphere <- vcg_sphere()
  cur_max <- vcg_max_edge_length(sphere)
  sphere2 <- vcg_subdivide_max_edge_length(sphere, max_edge_len = cur_max * 0.4)
  vcg_max_edge_length(sphere2)  # should be <= cur_max * 0.4

}
#> [1] 0.04116183