Convenience shape generators for resampled_searchlight()

Helpers that return ready-to-use shape_fun callbacks for resampled_searchlight(), covering a few sensible non-spherical defaults.

Usage

ellipsoid_shape(scales = c(1, 1, 1), jitter = 0)

cube_shape()

blobby_shape(drop = 0.3, edge_fraction = 0.7)

Arguments

scales

Length-3 positive numeric vector scaling the x/y/z axes relative to a sphere (for ellipsoid_shape). Values >1 stretch; <1 compress.

jitter

Non-negative numeric; standard deviation of multiplicative Gaussian noise applied to scales each draw (ellipsoid).

drop

Numeric in [0,1]; probability of dropping a voxel (blobby).

edge_fraction

Numeric in (0,1]; fraction of farthest voxels (by Euclidean distance from the center, in voxel units) considered "edge" and eligible for random dropping (blobby).

Value

A function suitable for the shape_fun argument of resampled_searchlight().

Details

Each returned function has signature function(mask, center, radius, iter, nonzero) and should return an \(n \times 3\) integer coordinate matrix. The coordinates are later converted to a ROIVolWindow internally.

Examples

mask <- read_vol(system.file("extdata", "global_mask_v4.nii", package="neuroim2"))

# Ellipsoid stretched along z with modest per-iteration jitter
sl_ellip <- resampled_searchlight(mask, radius = 6,
                                   shape_fun = ellipsoid_shape(scales = c(1, 1, 1.4),
                                                              jitter = 0.1))

# Simple axis-aligned cube (Chebyshev ball)
sl_cube <- resampled_searchlight(mask, radius = 5, shape_fun = "cube")

# Blobby sphere with 40% dropout on boundary voxels
sl_blob <- resampled_searchlight(mask, radius = 6,
                                 shape_fun = blobby_shape(drop = 0.4, edge_fraction = 0.6))