Scale LH and tones

scalelh.py

# pragma: init
from coloraide.gamut import Fit
from coloraide.spaces import RGBish

class OkLChScale(Fit):
    """
    Gamut mapping by scaling.

    Expected gamut mapping spaces are RGB type spaces.
    For best results, linear light RGB spaces are preferred.
    """

    NAME = "oklch-scale"
    SPACE = "oklch"

    def fit(self, color, space, **kwargs):
        """Scale the color within its gamut but preserve L and h as much as possible."""

        # Requires an RGB-ish space, preferably a linear space.
        if not isinstance(color.CS_MAP[space], RGBish):
            raise ValueError("Scaling only works in an RGBish color space, not {}".format(type(color.CS_MAP[space])))

        # For now, if a non-linear CSS variant is specified, just use the linear form.
        if space in {'srgb', 'display-p3', 'rec2020', 'a98-rgb', 'prophoto-rgb'}:
            space += '-linear'

        orig = color.space()
        mapcolor = color.convert(self.SPACE, norm=False) if orig != self.SPACE else color.clone().normalize(nans=False)
        gamutcolor = color.convert(space, norm=False) if orig != space else color.clone().normalize(nans=False)
        self.scale(gamutcolor)
        gamutcolor.set(
            {
                self.SPACE + '.l': mapcolor['l'],
                self.SPACE + '.h': mapcolor['h']
            }
        )
        self.scale(gamutcolor)
        color.update(gamutcolor)

    def scale(self, color):
        """Scale the RGB color within its gamut."""

        deltas = [c - 0.5 for c in color[:-1]]
        max_distance = max(abs(c) for c in deltas)
        scalingFactor = max_distance / 0.5;
        color[:-1] = [c / scalingFactor + 0.5 for c in deltas]

from coloraide.everything import ColorAll as Base
class Color(Base): ...
Color.register(OkLChScale())
# pragma: init

K_1 = 0.173
K_2 = 0.004
K_3 = (1.0 + K_1) / (1.0 + K_2)

def toe_inv(x: float) -> float:
    """Inverse toe function for L_r."""

    return (x ** 2 + K_1 * x) / (K_3 * (x + K_2))

def reduce_oklch_tonal_palette(c):
    c = Color(c).convert('oklch')
    tones = [0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 100]
    return [c.clone().set('l', toe_inv(tone / 100)).fit('srgb', method='oklch-chroma') for tone in tones]

def scale_oklch_tonal_palette(c):
    c = Color(c).convert('oklch')
    tones = [0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 100]
    return [c.clone().set('l', tone / 100).fit('srgb', method='oklch-scale') for tone in tones]

Steps(scale_oklch_tonal_palette('red'))
Steps(reduce_oklch_tonal_palette('red'))