Pixel sorting

module pixelSorting

import nanyx/math

-- Pixel sorting: a glitch-art effect where the pixels of each row of an
-- image are reordered by colour.

type Pixel = (r: float, g: float, b: float)   -- each channel in 0..1
type Image = list(list(Pixel))

-- Perceived luminance
def Pixel.luminance: Pixel -> float = { p ->
  Math.sqrt(0.241 * p.r + 0.691 * p.g + 0.068 * p.b)
}

-- Hue and value, which classify colours better than the raw channels
def Pixel.hsv: Pixel -> (h: float, s: float, v: float) = { p ->
  def max = [p.r, p.g, p.b] \max
  def min = [p.r, p.g, p.b] \min
  def delta = max - min

  def h =
    if | delta == 0.0 -> 0.0
       | max == p.r -> (((p.g - p.b) / delta) % 6.0) / 6.0
       | max == p.g -> ((p.b - p.r) / delta + 2.0) / 6.0
       | else -> ((p.r - p.g) / delta + 4.0) / 6.0

  (h = h, s = if max == 0.0 then 0.0 else delta / max, v = max)
}

-- A sort key that groups pixels into `repetitions` hue bands, alternating the
-- direction of the luminance in every other band so the result looks smoother
def stepKey: (Pixel, int) -> (int, float, int) = { p, repetitions ->
  def (h = h, v = v) = p.hsv
  def band = Math.floor(h * repetitions.toFloat)
  def value = Math.floor(v * repetitions.toFloat)

  if band % 2 == 1 then (band, repetitions.toFloat - p.luminance, repetitions - value)
  else (band, p.luminance, value)
}

def Pixel.isBlack: Pixel -> bool = { p -> p.r == 0.0 and p.g == 0.0 and p.b == 0.0 }

-- A row is sorted only if every pixel has some colour; rows containing black
-- pixels are left alone so that hard edges survive
def sortRow: list(Pixel) -> list(Pixel) = { row ->
  if row \any { p -> p.isBlack } then row
  else row \sortBy { p -> stepKey(p, 8) }
}

export def sortImage: Image -> Image = { image -> image \map(sortRow) }

-- The number of pixels that took part in the sort
export def measure: Image -> int = { image ->
  image
    \filter { row -> not (row \any { p -> p.isBlack }) }
    \map { .length }
    \sum
}

This program lives in the repository as examples/pixel-sorting.nyx.