pokemon-color-search/convert.py

# I could not find a single decent RGB -> CIELUV conversion library out there

def rescale_and_linearize(component: int) -> float:
  # takes an sRGB color component [0,255]
  # first rescales to [0,1]
  # then linearizes according to some CIEXYZ stuff I don't understand
  # then rescales to [0, 100]
  component /= 255
  linearized = component / 12.92 if component <= 0.04045 else ((component + 0.055) / 1.055) ** 2.4
  return 100 * linearized


# conversion values I also do not understand
# pulled from https://www.image-engineering.de/library/technotes/958-how-to-convert-between-srgb-and-ciexyz
# instead of easy rgb, since it seemed to give more accurate values
rgb_to_xyz_matrix = [
  [0.4124564, 0.3575761, 0.1804375],
  [0.2126729, 0.7151522, 0.0721750],
  [0.0193339, 0.1191920, 0.9503041],
]

# reference values I also also do not understand
# pulled from easy rgb
# note X and Y here have nothing to do with the X and Y metrics below
ref_x = 95.047
ref_y = 100.000
ref_z = 108.883
ref_denom = ref_x + 15 * ref_y + 3 * ref_z
ref_u = 4 * ref_x / ref_denom
ref_v = 9 * ref_y / ref_denom


def rgb_to_cieluv(r: int, g: int, b: int) -> tuple[float, float, float]:
  # accepts RGB (components [0, 255])
  # converts to CIE LUV (components [0, 1])
  # math taken from http://www.easyrgb.com/en/math.php
  
  # RGB (components [0, 255]) -> XYZ (components [0, 100])
  # X, Y and Z output refer to a D65/2° standard illuminant.
  sr, sg, sb = (rescale_and_linearize(c) for c in (r, g, b))
  x, y, z = (cr * sr + cg * sg + cb * sb for cr, cg, cb in rgb_to_xyz_matrix)

  # XYZ (components [0, 100]) -> LUV (components [0, 100])
  uv_denom = x + 15 * y + 3 * z
  u = 4 * x / uv_denom
  v = 9 * y / uv_denom

  if y > 0.8856:
    yprime = (y / 100) ** (1/3)
  else:
    yprime = (y / 100) * 7.787 + (16/116)

  lstar = 116 * yprime - 16
  lstar_factor = 13 * lstar
  return lstar, lstar_factor * (u - ref_u), lstar_factor * (v - ref_v)