Color interpolation (#26)

Support a `colorSpace` property on functions that allows
users to choose LAB or HCL color interpolation.

LICENSE

@@ -11,3 +11,35 @@ INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
 LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
 OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 PERFORMANCE OF THIS SOFTWARE.
+
+--------------------------------------------------------------------------------
+
+Contains a portion of d3-color https://github.com/d3/d3-color
+
+Copyright 2010-2016 Mike Bostock
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+* Neither the name of the author nor the names of contributors may be used to
+  endorse or promote products derived from this software without specific prior
+  written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

color_spaces.js

@@ -0,0 +1,98 @@
+// Constants
+var Kn = 18,
+    Xn = 0.950470, // D65 standard referent
+    Yn = 1,
+    Zn = 1.088830,
+    t0 = 4 / 29,
+    t1 = 6 / 29,
+    t2 = 3 * t1 * t1,
+    t3 = t1 * t1 * t1,
+    deg2rad = Math.PI / 180,
+    rad2deg = 180 / Math.PI;
+
+// Utilities
+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 xyz2rgb(x) {
+    return 255 * (x <= 0.0031308 ? 12.92 * x : 1.055 * Math.pow(x, 1 / 2.4) - 0.055);
+}
+
+function rgb2xyz(x) {
+    return (x /= 255) <= 0.04045 ? x / 12.92 : Math.pow((x + 0.055) / 1.055, 2.4);
+}
+
+// LAB
+function rgbToLab(rgbColor) {
+    var b = rgb2xyz(rgbColor[0]),
+        a = rgb2xyz(rgbColor[1]),
+        l = rgb2xyz(rgbColor[2]),
+        x = xyz2lab((0.4124564 * b + 0.3575761 * a + 0.1804375 * l) / Xn),
+        y = xyz2lab((0.2126729 * b + 0.7151522 * a + 0.0721750 * l) / Yn),
+        z = xyz2lab((0.0193339 * b + 0.1191920 * a + 0.9503041 * l) / Zn);
+
+    return [
+        116 * y - 16,
+        500 * (x - y),
+        200 * (y - z),
+        rgbColor[3]
+    ];
+}
+
+function labToRgb(labColor) {
+    var y = (labColor[0] + 16) / 116,
+        x = isNaN(labColor[1]) ? y : y + labColor[1] / 500,
+        z = isNaN(labColor[2]) ? y : y - labColor[2] / 200;
+    y = Yn * lab2xyz(y);
+    x = Xn * lab2xyz(x);
+    z = Zn * lab2xyz(z);
+    return [
+        xyz2rgb(3.2404542 * x - 1.5371385 * y - 0.4985314 * z), // D65 -> sRGB
+        xyz2rgb(-0.9692660 * x + 1.8760108 * y + 0.0415560 * z),
+        xyz2rgb(0.0556434 * x - 0.2040259 * y + 1.0572252 * z),
+        labColor[3]
+    ];
+}
+
+// HCL
+function rgbToHcl(rgbColor) {
+  var labColor = rgbToLab(rgbColor);
+  var l = labColor[0],
+    a = labColor[1],
+    b = labColor[2];
+  var h = Math.atan2(b, a) * rad2deg;
+  return [
+      h < 0 ? h + 360 : h,
+      Math.sqrt(a * a + b * b),
+      l,
+      rgbColor[3]
+  ];
+}
+
+function hclToRgb(hclColor) {
+  var h = hclColor[0] * deg2rad,
+    c = hclColor[1],
+    l = hclColor[2];
+  return labToRgb([
+      l,
+      Math.cos(h) * c,
+      Math.sin(h) * c,
+      hclColor[3]
+  ]);
+}
+
+module.exports ={
+  lab: {
+    forward: rgbToLab,
+    reverse: labToRgb
+  },
+  hcl: {
+    forward: rgbToHcl,
+    reverse: hclToRgb
+  }
+};

index.js

@@ -1,5 +1,11 @@
 'use strict';
 
+var colorSpaces = require('./color_spaces');
+
+function identityFunction(x) {
+    return x;
+}
+
 function createFunction(parameters, defaultType) {
     var fun;
 
@@ -27,10 +33,35 @@ function createFunction(parameters, defaultType) {
             throw new Error('Unknown function type "' + type + '"');
         }
 
+        var outputFunction;
+
+        // If we're interpolating colors in a color system other than RGBA,
+        // first translate all stop values to that color system, then interpolate
+        // arrays as usual. The `outputFunction` option lets us then translate
+        // the result of that interpolation back into RGBA.
+        if (parameters.colorSpace && parameters.colorSpace !== 'rgb') {
+            if (colorSpaces[parameters.colorSpace]) {
+                var colorspace = colorSpaces[parameters.colorSpace];
+                // Avoid mutating the parameters value
+                parameters = JSON.parse(JSON.stringify(parameters));
+                for (var s = 0; s < parameters.stops.length; s++) {
+                    parameters.stops[s] = [
+                        parameters.stops[s][0],
+                        colorspace.forward(parameters.stops[s][1])
+                    ];
+                }
+                outputFunction = colorspace.reverse;
+            } else {
+                throw new Error('Unknown color space: ' + parameters.colorSpace);
+            }
+        } else {
+            outputFunction = identityFunction;
+        }
+
         if (zoomAndFeatureDependent) {
             var featureFunctions = {};
             var featureFunctionStops = [];
-            for (var s = 0; s < parameters.stops.length; s++) {
+            for (s = 0; s < parameters.stops.length; s++) {
                 var stop = parameters.stops[s];
                 if (featureFunctions[stop[0].zoom] === undefined) {
                     featureFunctions[stop[0].zoom] = {
@@ -47,20 +78,24 @@ function createFunction(parameters, defaultType) {
                 featureFunctionStops.push([featureFunctions[z].zoom, createFunction(featureFunctions[z])]);
             }
             fun = function(zoom, feature) {
-                return evaluateExponentialFunction({ stops: featureFunctionStops, base: parameters.base }, zoom)(zoom, feature);
+                return outputFunction(evaluateExponentialFunction({
+                  stops: featureFunctionStops,
+                  base: parameters.base
+                }, zoom)(zoom, feature));
             };
             fun.isFeatureConstant = false;
             fun.isZoomConstant = false;
 
         } else if (zoomDependent) {
             fun = function(zoom) {
-                return innerFun(parameters, zoom);
+                return outputFunction(innerFun(parameters, zoom));
             };
             fun.isFeatureConstant = true;
             fun.isZoomConstant = false;
         } else {
             fun = function(zoom, feature) {
-                return innerFun(parameters, feature[parameters.property]);
+                return outputFunction(
+                  innerFun(parameters, feature[parameters.property]));
             };
             fun.isFeatureConstant = false;
             fun.isZoomConstant = true;

test/test.js

@@ -5,7 +5,7 @@ var MapboxGLFunction = require('../').interpolated;
 
 test('function types', function(t) {
 
-    t.test('contant', function(t) {
+    t.test('constant', function(t) {
 
         t.test('range types', function(t) {
 
@@ -107,6 +107,59 @@ test('function types', function(t) {
                 t.end();
             });
 
+            t.test('three elements', function(t) {
+                var f = MapboxGLFunction({
+                    type: 'exponential',
+                    colorSpace: 'lab',
+                    stops: [[1, [0, 0, 0, 1]], [10, [0, 1, 1, 1]]]
+                });
+
+                t.deepEqual(f(0), [0, 0, 0, 1]);
+                t.deepEqual(f(5).map(function (n) {
+                    return parseFloat(n.toFixed(3));
+                }), [0, 0.444, 0.444, 1]);
+
+                t.end();
+            });
+
+            t.test('rgb colorspace', function(t) {
+                var f = MapboxGLFunction({
+                    type: 'exponential',
+                    colorSpace: 'rgb',
+                    stops: [[0, [0, 0, 0, 1]], [10, [1, 1, 1, 1]]]
+                });
+
+                t.deepEqual(f(5).map(function (n) {
+                    return parseFloat(n.toFixed(3));
+                }), [0.5, 0.5, 0.5, 1]);
+
+                t.end();
+            });
+
+            t.test('unknown color spaces', function(t) {
+                t.throws(function () {
+                    MapboxGLFunction({
+                        type: 'exponential',
+                        colorSpace: 'unknown',
+                        stops: [[1, [0, 0, 0, 1]], [10, [0, 1, 1, 1]]]
+                    });
+                }, 'Unknown color space: unknown');
+
+                t.end();
+            });
+
+            t.test('interpolation mutation avoidance', function(t) {
+                var params = {
+                    type: 'exponential',
+                    colorSpace: 'lab',
+                    stops: [[1, [0, 0, 0, 1]], [10, [0, 1, 1, 1]]]
+                };
+                var paramsCopy = JSON.parse(JSON.stringify(params));
+                MapboxGLFunction(params);
+                t.deepEqual(params, paramsCopy);
+                t.end();
+            });
+
         });
 
         t.test('zoom + data stops', function(t) {