Converts colors from HSL to RGB and back.

hsl.py

"""Converts colors from HSL to RGB and back."""

import numpy as np
from scipy import optimize


def tstack(a):
    """Stacks arrays in sequence along the last axis (tail)."""
    a = np.asarray(a)
    return np.concatenate([x[..., np.newaxis] for x in a], axis=-1)


def tsplit(a):
    """Splits arrays in sequence along the last axis (tail)."""
    a = np.asarray(a)
    return np.array([a[..., x] for x in range(a.shape[-1])])


def hue_to_rgb_once(m1, m2, h):
    h %= 1
    if h * 6 < 1:
        return m1 + (m2 - m1) * h * 6
    if h * 2 < 1:
        return m2
    if h * 3 < 2:
        return m1 + (m2 - m1) * (2/3 - h) * 6
    return m1


def hue_to_rgb(m1, m2, h):
    m1m2h = tstack([m1, m2, h])
    return np.apply_along_axis(lambda a: hue_to_rgb_once(*a), -1, m1m2h)


def hsl_to_rgb(hsl):
    """Converts HSL colors to RGB. See
    https://www.w3.org/TR/2018/PR-css-color-3-20180315/#hsl-color.
    """
    h, s, l = tsplit(hsl)
    m1, m2 = np.zeros_like(h), np.zeros_like(h)
    m2 += (l <= 0.5) * l * (s + 1)
    m2 += (l > 0.5) * (l + s - l * s)
    m1 = l * 2 - m2
    r = hue_to_rgb(m1, m2, h + 1/3)
    g = hue_to_rgb(m1, m2, h)
    b = hue_to_rgb(m1, m2, h - 1/3)
    return tstack([r, g, b])


def rgb_to_hsl_once(rgb):
    def loss(hsl):
        rgb2 = hsl_to_rgb(hsl)
        return sum((rgb - rgb2)**2)
    x0 = np.array([0.5, 0.5, 0.5])
    opt = optimize.fmin_bfgs(loss, x0, disp=False)
    return opt


def rgb_to_hsl(rgb):
    """Converts RGB colors back to HSL."""
    rgb = np.asarray(rgb)
    hsl = np.apply_along_axis(rgb_to_hsl_once, -1, rgb)
    hsl[..., 0] %= 1
    return np.clip(hsl, 0, 1)