R/kernel_exponential.R

In LandSciTech/pfocal: Fast Convolution of Matrices

#' Compute an Exponential kernel
#'
#' Functions to compute an exponential kernel.
#'
#' @param beta **\[numeric\]** The kernel's beta parameter.
#' @param r0 **\[numeric\]** The kernel's r0 (exponential).
#'
#' @return
#' A `matrix` corresponding to the kernel.
#' 
#' @examples 
#' 
#' exponential_kernel(beta = 2, r0 = 0.7)
#'
#' @export
#' @rdname kernel-exponential
#' @aliases kernel-exponential
exponential_kernel <- function(beta = 0.2, r0 = 0.05) {
  .q_kernel_to_kernel(.exponential_quarter_kernel(beta = beta, r0 = r0))
}

# Helpers -----------------------------------------------------------------

.exponential_quarter_kernel <- function(beta = 0.2, r0 = 0.05) {
  r <- 1
  while (exp(-beta * r) * (-beta * r - 1) < -r0) {
    r <- r + 1
  }

  distance_qk <- .euclidean_distance_quarter_kernel(r)

  circle_qk <- .hard_uniform_circle_quarter_kernel(r)

  return(exp(-beta * distance_qk) * circle_qk)
}