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Find boundaries of ROIs on a surface mesh

Source: R/roi_boundaries.R
find_roi_boundaries.Rd

This function identifies boundaries between regions of interest (ROIs) defined on a triangular surface mesh. It is a translation of the core parts of Stuart Oldham's findROIboundaries MATLAB function, adapted for use with neurosurf objects.

Usage

find_roi_boundaries(
  vertices,
  faces,
  vertex_id,
  boundary_method = c("faces", "edge_vertices"),
  verbose = FALSE,
  use_cpp = TRUE
)

Arguments

vertices

Numeric matrix of vertex coordinates (\(n \times 3\)).

faces

Integer matrix of face indices (\(m \times 3\), 1-based).

vertex_id

Integer vector of ROI labels for each vertex (length \(n\)).

boundary_method

One of "faces" or "edge_vertices".

verbose

Logical; if TRUE, print progress messages.

use_cpp

Logical; if TRUE, use optimized C++ implementation.

Value

A list with elements:

boundary

For "faces": logical vector of length nrow(faces) indicating boundary faces. For "edge_vertices": list of coordinate matrices (\(k \times 3\)) giving boundary polygons for each connected ROI component.

boundary_roi_id

Integer vector giving the ROI id associated with each boundary polygon (empty for "faces").

roi_components

Integer vector giving the number of connected components for each ROI (indexed parallel to sort(unique(vertex_id))).

boundary_verts

For "edge_vertices": list of integer vectors giving the vertex ids used for each boundary polygon; NULL for "faces".

Details

Two boundary representations are currently supported:

  • "faces": returns a logical vector indicating which faces lie on a boundary between ROIs.

  • "edge_vertices": returns boundary polygons traced through mesh vertices, suitable for drawing ROI outlines.

Examples

# \donttest{
# Simple cube mesh with two ROIs (bottom vs top)
vertices <- matrix(c(
  0,0,0, 1,0,0, 1,1,0, 0,1,0,
  0,0,1, 1,0,1, 1,1,1, 0,1,1
), ncol = 3, byrow = TRUE)

faces <- matrix(c(
  1,2,3, 1,3,4,
  5,6,7, 5,7,8,
  1,2,6, 1,6,5,
  3,4,8, 3,8,7,
  1,4,8, 1,8,5,
  2,3,7, 2,7,6
), ncol = 3, byrow = TRUE)

roi <- c(1,1,1,1, 2,2,2,2)

b <- find_roi_boundaries(vertices, faces, roi, boundary_method = "edge_vertices")
str(b$boundary)
#> List of 2
#>  $ : num [1:5, 1:3] 0 1 1 0 0 0 0 1 1 0 ...
#>  $ : num [1:5, 1:3] 0 1 1 0 0 0 0 1 1 0 ...
# }