affine_utils: Affine utility functions

In neuroim2: Data Structures for Brain Imaging Data

Affine utility functions

Description

Utilities for point transforms and affine decomposition/composition.

Applies a homogeneous affine transform to points where the coordinate axis is the last dimension.

Useful when expanding an affine to include additional dimensions.

For arrays 'A, B, C, ...', returns 'A

Returns per-output-axis obliquity relative to cardinal axes, in radians.

Preserves rotations/shears and world location of the center voxel.

Usage

apply_affine(aff, pts, inplace = FALSE)

to_matvec(transform)

from_matvec(matrix, vector = NULL)

append_diag(aff, steps, starts = numeric(0))

dot_reduce(...)

voxel_sizes(affine)

obliquity(affine)

rescale_affine(affine, shape, zooms, new_shape = NULL)

Arguments

aff	Base affine matrix.
pts	Points with last dimension 'N - 1'; shape may be vector, matrix, or higher-dimensional array.
inplace	Included for API compatibility; ignored in R.
transform	Homogeneous transform matrix.
matrix	Linear part of transform ('N x M').
vector	Optional translation vector of length 'N'.
steps	Diagonal values for appended dimensions.
starts	Optional translations for appended dimensions.
...	Matrices/vectors compatible with %*%.
affine	Square homogeneous affine matrix.
shape	Original grid shape (spatial axes).
zooms	New voxel sizes for spatial axes.
new_shape	Optional new grid shape; defaults to 'shape'.

Details

These functions mirror core NiBabel affine helpers while using R-first conventions and stronger argument checks.

Value

Points transformed by 'aff', with same leading shape as 'pts'.

A list with 'matrix' and 'vector'.

Homogeneous affine matrix of shape '(N+1) x (M+1)'.

Expanded affine matrix.

Matrix product.

Numeric vector of voxel sizes (column norms of linear block).

Numeric vector of obliquity angles.

Rescaled affine matrix.

Examples

aff <- diag(c(2, 3, 4, 1))
aff[1:3, 4] <- c(10, 20, 30)

pts <- rbind(c(1, 2, 3), c(4, 5, 6))
apply_affine(aff, pts)

mv <- to_matvec(aff)
from_matvec(mv$matrix, mv$vector)

neuroim2 documentation built on April 16, 2026, 5:07 p.m.