R/histogramEqualizeImage.R

In stnava/ANTsR: ANTs in R: Quantification Tools for Biomedical Images

#' histogramEqualizeImage
#'
#' Histogram equalize image
#'
#' @param image image to undergo intensity transformation.
#' @param numberOfHistogramBins number of histogram levels.
#' @return equalized image.
#'
#' @author NJ Tustison
#'
#' @examples
#' image <- antsImageRead(getANTsRData("r16"), 2)
#' eqImage <- histogramEqualizeImage(image)
#'
#' @export histogramEqualizeImage
histogramEqualizeImage <- function(
    image,
    numberOfHistogramBins = 256) {
  imageArray <- as.array(image)
  imageVector <- as.vector(imageArray)
  breakVector <- seq(min(imageArray), max(imageArray), length.out = numberOfHistogramBins)
  imageHistogram <- hist(imageVector, breaks = breakVector, plot = FALSE)
  cdf <- cumsum(imageHistogram$density)
  cdf <- cdf / tail(cdf, n = 1)
  imageArrayEqualizedFlat <- approx(imageHistogram$breaks[1:(numberOfHistogramBins - 1)], cdf, imageVector, method = "linear", rule = 2)
  imageArrayEqualized <- array(data = imageArrayEqualizedFlat$y, dim = dim(imageArray))
  imageArrayEqualized <- (imageArrayEqualized - min(imageArrayEqualized)) / (max(imageArrayEqualized) - min(imageArrayEqualized))
  imageArrayEqualized <- imageArrayEqualized * (max(imageArray) - min(imageArray)) + min(imageArray)

  return(as.antsImage(imageArrayEqualized, reference = image))
}