///|
/// Public histogram API. The luma histogram reuses the same 256-bin
/// BT.601 integer binning that Otsu and histogram equalization are built
/// on, so every consumer of the library sees one consistent definition.

///|
/// 256-bin luma histogram (BT.601 integer weights): `result[v]` counts the
/// pixels whose luma is exactly `v`.
pub fn Image::histogram_luma(self : Image) -> FixedArray[Int] {
  self.luma_histogram()
}

///|
/// Per-channel 256-bin histograms, returned as `(red, green, blue)`.
pub fn Image::histogram_rgb(
  self : Image,
) -> (FixedArray[Int], FixedArray[Int], FixedArray[Int]) {
  let hr = FixedArray::make(256, 0)
  let hg = FixedArray::make(256, 0)
  let hb = FixedArray::make(256, 0)
  let n = self.pixel_count()
  for p in 0..