Extend the basic bitmap storage defined on this page to support dealing with image histograms. The image histogram contains for each luminance the count of image pixels having this luminance. Choosing a histogram representation take care about the data type used for the counts. It must have range of at least 0..NxM, where N is the image width and M is the image height.

Test task

Histogram is useful for many image processing operations. As an example, use it to convert an image into black and white art. The method works as follows:

 Convert image to grayscale;
 Compute the histogram
 Find the median: defined as the luminance such that the image has an approximately equal number of pixels with lesser and greater luminance.
 Replace each pixel of luminance lesser than the median to black, and others to white.
Use  read/write ppm file, and grayscale image solutions.

type histogram = int array
 
let get_histogram ~img:gray_channel =
  let width = Bigarray.Array2.dim1 gray_channel
  and height = Bigarray.Array2.dim2 gray_channel in
  let t = Array.make 256 0 in
  for x = 0 to pred width do
    for y = 0 to pred height do
      let v = gray_get_pixel_unsafe gray_channel x y in
      t.(v) <- t.(v) + 1;
    done;
  done;
  (t: histogram)
;;