chroma-js/src/generator/cubehelix.js

JavaScript library for color conversions

// cubehelix interpolation
// based on D.A. Green "A colour scheme for the display of astronomical intensity images"
// http://astron-soc.in/bulletin/11June/289392011.pdf
import { type, clip_rgb, TWOPI } from '../utils/index.js';
import chroma from '../chroma.js';
const { pow, sin, cos } = Math;

export default function (
    start = 300,
    rotations = -1.5,
    hue = 1,
    gamma = 1,
    lightness = [0, 1]
) {
    let dh = 0,
        dl;
    if (type(lightness) === 'array') {
        dl = lightness[1] - lightness[0];
    } else {
        dl = 0;
        lightness = [lightness, lightness];
    }
    const f = function (fract) {
        const a = TWOPI * ((start + 120) / 360 + rotations * fract);
        const l = pow(lightness[0] + dl * fract, gamma);
        const h = dh !== 0 ? hue[0] + fract * dh : hue;
        const amp = (h * l * (1 - l)) / 2;
        const cos_a = cos(a);
        const sin_a = sin(a);
        const r = l + amp * (-0.14861 * cos_a + 1.78277 * sin_a);
        const g = l + amp * (-0.29227 * cos_a - 0.90649 * sin_a);
        const b = l + amp * (+1.97294 * cos_a);
        return chroma(clip_rgb([r * 255, g * 255, b * 255, 1]));
    };
    f.start = function (s) {
        if (s == null) {
            return start;
        }
        start = s;
        return f;
    };
    f.rotations = function (r) {
        if (r == null) {
            return rotations;
        }
        rotations = r;
        return f;
    };
    f.gamma = function (g) {
        if (g == null) {
            return gamma;
        }
        gamma = g;
        return f;
    };
    f.hue = function (h) {
        if (h == null) {
            return hue;
        }
        hue = h;
        if (type(hue) === 'array') {
            dh = hue[1] - hue[0];
            if (dh === 0) {
                hue = hue[1];
            }
        } else {
            dh = 0;
        }
        return f;
    };
    f.lightness = function (h) {
        if (h == null) {
            return lightness;
        }
        if (type(h) === 'array') {
            lightness = h;
            dl = h[1] - h[0];
        } else {
            lightness = [h, h];
            dl = 0;
        }
        return f;
    };
    f.scale = () => chroma.scale(f);
    f.hue(hue);
    return f;
}