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Color Manipulation

hue °
chroma %
lightness %

Demonstrates color manipulation with a raster source.

A raster source allows arbitrary manipulation of pixel values. In this example, RGB values on the input tile source are adjusted in a pixel-wise operation before being rendered with a second raster source. The raster operation takes pixels in in RGB space, converts them to HCL color space, adjusts the values based on the controls above, and then converts them back to RGB space for rendering.

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index.html<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Color Manipulation</title>
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
      table.controls td {
        text-align: center;
        padding: 2px 5px;
        min-width: 60px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <table class="controls">
      <tr>
        <td>hue</td>
        <td><span id="hueOut"></span>°</td>
        <td><input id="hue" type="range" min="-180" max="180" value="0"/></td>
      </tr>
      <tr>
        <td>chroma</td>
        <td><span id="chromaOut"></span> %</td>
        <td><input id="chroma" type="range" min="0" max="100" value="100"/></td>
      </tr>
      <tr>
        <td>lightness</td>
        <td><span id="lightnessOut"></span> %</td>
        <td><input id="lightness" type="range" min="0" max="100" value="100"/></td>
      </tr>
    </table>
    <script src="index.js"></script>
  </body>
</html>
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index.jsimport 'ol/ol.css';
import Map from 'ol/Map';
import View from 'ol/View';
import ImageLayer from 'ol/layer/Image';
import {Raster as RasterSource, Stamen} from 'ol/source';


/**
 * Color manipulation functions below are adapted from
 * https://github.com/d3/d3-color.
 */
var Xn = 0.950470;
var Yn = 1;
var Zn = 1.088830;
var t0 = 4 / 29;
var t1 = 6 / 29;
var t2 = 3 * t1 * t1;
var t3 = t1 * t1 * t1;
var twoPi = 2 * Math.PI;


/**
 * Convert an RGB pixel into an HCL pixel.
 * @param {Array<number>} pixel A pixel in RGB space.
 * @return {Array<number>} A pixel in HCL space.
 */
function rgb2hcl(pixel) {
  var red = rgb2xyz(pixel[0]);
  var green = rgb2xyz(pixel[1]);
  var blue = rgb2xyz(pixel[2]);

  var x = xyz2lab(
    (0.4124564 * red + 0.3575761 * green + 0.1804375 * blue) / Xn);
  var y = xyz2lab(
    (0.2126729 * red + 0.7151522 * green + 0.0721750 * blue) / Yn);
  var z = xyz2lab(
    (0.0193339 * red + 0.1191920 * green + 0.9503041 * blue) / Zn);

  var l = 116 * y - 16;
  var a = 500 * (x - y);
  var b = 200 * (y - z);

  var c = Math.sqrt(a * a + b * b);
  var h = Math.atan2(b, a);
  if (h < 0) {
    h += twoPi;
  }

  pixel[0] = h;
  pixel[1] = c;
  pixel[2] = l;

  return pixel;
}


/**
 * Convert an HCL pixel into an RGB pixel.
 * @param {Array<number>} pixel A pixel in HCL space.
 * @return {Array<number>} A pixel in RGB space.
 */
function hcl2rgb(pixel) {
  var h = pixel[0];
  var c = pixel[1];
  var l = pixel[2];

  var a = Math.cos(h) * c;
  var b = Math.sin(h) * c;

  var y = (l + 16) / 116;
  var x = isNaN(a) ? y : y + a / 500;
  var z = isNaN(b) ? y : y - b / 200;

  y = Yn * lab2xyz(y);
  x = Xn * lab2xyz(x);
  z = Zn * lab2xyz(z);

  pixel[0] = xyz2rgb(3.2404542 * x - 1.5371385 * y - 0.4985314 * z);
  pixel[1] = xyz2rgb(-0.9692660 * x + 1.8760108 * y + 0.0415560 * z);
  pixel[2] = xyz2rgb(0.0556434 * x - 0.2040259 * y + 1.0572252 * z);

  return pixel;
}

function xyz2lab(t) {
  return t > t3 ? Math.pow(t, 1 / 3) : t / t2 + t0;
}

function lab2xyz(t) {
  return t > t1 ? t * t * t : t2 * (t - t0);
}

function rgb2xyz(x) {
  return (x /= 255) <= 0.04045 ? x / 12.92 : Math.pow((x + 0.055) / 1.055, 2.4);
}

function xyz2rgb(x) {
  return 255 * (x <= 0.0031308 ?
    12.92 * x : 1.055 * Math.pow(x, 1 / 2.4) - 0.055);
}

var raster = new RasterSource({
  sources: [new Stamen({
    layer: 'watercolor'
  })],
  operation: function(pixels, data) {
    var hcl = rgb2hcl(pixels[0]);

    var h = hcl[0] + Math.PI * data.hue / 180;
    if (h < 0) {
      h += twoPi;
    } else if (h > twoPi) {
      h -= twoPi;
    }
    hcl[0] = h;

    hcl[1] *= (data.chroma / 100);
    hcl[2] *= (data.lightness / 100);

    return hcl2rgb(hcl);
  },
  lib: {
    rgb2hcl: rgb2hcl,
    hcl2rgb: hcl2rgb,
    rgb2xyz: rgb2xyz,
    lab2xyz: lab2xyz,
    xyz2lab: xyz2lab,
    xyz2rgb: xyz2rgb,
    Xn: Xn,
    Yn: Yn,
    Zn: Zn,
    t0: t0,
    t1: t1,
    t2: t2,
    t3: t3,
    twoPi: twoPi
  }
});

var controls = {};

raster.on('beforeoperations', function(event) {
  var data = event.data;
  for (var id in controls) {
    data[id] = Number(controls[id].value);
  }
});

var map = new Map({
  layers: [
    new ImageLayer({
      source: raster
    })
  ],
  target: 'map',
  view: new View({
    center: [0, 2500000],
    zoom: 2,
    maxZoom: 18
  })
});

var controlIds = ['hue', 'chroma', 'lightness'];
controlIds.forEach(function(id) {
  var control = document.getElementById(id);
  var output = document.getElementById(id + 'Out');
  control.addEventListener('input', function() {
    output.innerText = control.value;
    raster.changed();
  });
  output.innerText = control.value;
  controls[id] = control;
});
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package.json{
  "name": "color-manipulation",
  "dependencies": {
    "ol": "6.1.1"
  },
  "devDependencies": {
    "parcel": "1.11.0"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --experimental-scope-hoisting --public-url . index.html"
  }
}