#' @title Two-Dimensional Kernel Density Estimation of Images
#' @description Two-dimensional kernel density estimation of
#' 2 nifti images
#'
#' @param img1 Object of class \code{nifti}, array, or filename of image
#' @param img2 Object of class \code{nifti}, array, or filename of image
#' @param mask Binary bject of class \code{nifti}, array, or filename of image.
#' If \code{NULL}
#' @param ... Additional arguments to pass to \code{\link{kde2d}}
#'
#' @return Matrix of the joint PDF
#' @export
#' @importFrom MASS kde2d
kde2d.nifti = function(
img1,
img2,
mask = NULL,
...) {
L = get_value_data(
img1 = img1,
img2 = img2,
mask = mask)
res = MASS::kde2d(L$img1, L$img2, ...)
nbins = length(res$y)
stopifnot( nbins == length(res$x))
N = length(L$img1)
est_bias = nbins / (2 * log(2) * N)
joint = res$z
joint = joint / sum(joint)
return(joint)
}
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