feat: add chroma.contrastAPCA (#353)

* feat: add chroma.contrastAPCA resolves #302 * docs: fix rendering of transparent css colors in docs * build
gka · Aug 19, 2024 · fb4bdf2 · fb4bdf2
1 parent 705310d
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diff --git a/dist/chroma.cjs b/dist/chroma.cjs
@@ -3687,6 +3687,71 @@
         return l1 > l2 ? (l1 + 0.05) / (l2 + 0.05) : (l2 + 0.05) / (l1 + 0.05);
     }
 
+    /**
+     * @license
+     *
+     * The APCA contrast prediction algorithm is based of the formulas published
+     * in the APCA-1.0.98G specification by Myndex. The specification is available at:
+     * https://github.com/raw/Myndex/apca-w3/master/images/APCAw3_0.1.17_APCA0.0.98G.svg
+     *
+     * Note that the APCA implementation is still beta, so please update to
+     * future versions of chroma.js when they become available.
+     *
+     * You can read more about the APCA Readability Criterion at
+     * https://readtech.org/ARC/
+     */
+
+    // constants
+    var W_offset = 0.027;
+    var P_in = 0.0005;
+    var P_out = 0.1;
+    var R_scale = 1.14;
+    var B_threshold = 0.022;
+    var B_exp = 1.414;
+
+    function contrastAPCA (text, bg) {
+        // parse input colors
+        text = new Color(text);
+        bg = new Color(bg);
+        // if text color has alpha, blend against background
+        if (text.alpha() < 1) {
+            text = mix(bg, text, text.alpha(), 'rgb');
+        }
+        var l_text = lum.apply(void 0, text.rgb());
+        var l_bg = lum.apply(void 0, bg.rgb());
+
+        // soft clamp black levels
+        var Y_text =
+            l_text >= B_threshold
+                ? l_text
+                : l_text + Math.pow(B_threshold - l_text, B_exp);
+        var Y_bg =
+            l_bg >= B_threshold ? l_bg : l_bg + Math.pow(B_threshold - l_bg, B_exp);
+
+        // normal polarity (dark text on light background)
+        var S_norm = Math.pow(Y_bg, 0.56) - Math.pow(Y_text, 0.57);
+        // reverse polarity (light text on dark background)
+        var S_rev = Math.pow(Y_bg, 0.65) - Math.pow(Y_text, 0.62);
+        // clamp noise then scale
+        var C =
+            Math.abs(Y_bg - Y_text) < P_in
+                ? 0
+                : Y_text < Y_bg
+                  ? S_norm * R_scale
+                  : S_rev * R_scale;
+        // clamp minimum contrast then offset
+        var S_apc = Math.abs(C) < P_out ? 0 : C > 0 ? C - W_offset : C + W_offset;
+        // scale to 100
+        return S_apc * 100;
+    }
+    function lum(r, g, b) {
+        return (
+            0.2126729 * Math.pow(r / 255, 2.4) +
+            0.7151522 * Math.pow(g / 255, 2.4) +
+            0.072175 * Math.pow(b / 255, 2.4)
+        );
+    }
+
     var sqrt = Math.sqrt;
     var pow = Math.pow;
     var min = Math.min;
@@ -3904,6 +3969,7 @@
         Color: Color,
         colors: w3cx11,
         contrast: contrast,
+        contrastAPCA: contrastAPCA,
         cubehelix: cubehelix,
         deltaE: deltaE,
         distance: distance,

diff --git a/dist/chroma.min.cjs b/dist/chroma.min.cjs
diff --git a/docs/index.html b/docs/index.html
@@ -256,6 +256,13 @@ <h4 id="-color1-color2-">(color1, color2)</h4>
 // contrast greater than 4.5 = high enough
 chroma.contrast(&#39;pink&#39;, &#39;purple&#39;);
 </code></pre>
+<h3 id="chroma-contrastapca">chroma.contrastAPCA</h3>
+<h4 id="-text-background-">(text, background)</h4>
+<p><strong>New (3.1):</strong> Computes the <a href="https://www.myndex.com/APCA/">APCA contrast</a> ratio of a text color against its background color. The basic idea is that you check the contrast between the text and background color and then use <a href="https://github.com/raw/Myndex/apca-w3/master/images/APCAlookupByContrast.jpeg">this lookup table</a> to find the minimum font size you&#39;re allowed to use (given the font weight and purpose of the text). </p>
+<pre><code class="lang-js">chroma.contrastAPCA(&#39;hotpink&#39;, &#39;pink&#39;);
+chroma.contrastAPCA(&#39;purple&#39;, &#39;pink&#39;);
+</code></pre>
+<p>Read more about how to interpret and use this metric at <a href="https://readtech.org/ARC">APCA Readability Criterion</a>. Please note that the APCA algorithm is still in beta and may change be subject to changes in the future.</p>
 <h3 id="chroma-distance">chroma.distance</h3>
 <h4 id="-color1-color2-mode-lab-">(color1, color2, mode=&#39;lab&#39;)</h4>
 <p>Computes the <a href="https://en.wikipedia.org/wiki/Euclidean_distance#Three_dimensions">Euclidean distance</a> between two colors in a given color space (default is <code>Lab</code>).</p>
@@ -1007,7 +1014,7 @@ <h3 id="1-0-0">1.0.0</h3>
                             [
                                 'background-color:' + (isCSS ? val : col.hex()),
                                 'color:' + (l < 0.7 ? 'white' : 'black'),
-                                'opacity:' + col.alpha()
+                                ...(isCSS ? [] : ['opacity:' + col.alpha()])
                             ].join(';')
                         );
 

diff --git a/docs/libs/chroma.cjs b/docs/libs/chroma.cjs
@@ -3687,6 +3687,71 @@
         return l1 > l2 ? (l1 + 0.05) / (l2 + 0.05) : (l2 + 0.05) / (l1 + 0.05);
     }
 
+    /**
+     * @license
+     *
+     * The APCA contrast prediction algorithm is based of the formulas published
+     * in the APCA-1.0.98G specification by Myndex. The specification is available at:
+     * https://github.com/raw/Myndex/apca-w3/master/images/APCAw3_0.1.17_APCA0.0.98G.svg
+     *
+     * Note that the APCA implementation is still beta, so please update to
+     * future versions of chroma.js when they become available.
+     *
+     * You can read more about the APCA Readability Criterion at
+     * https://readtech.org/ARC/
+     */
+
+    // constants
+    var W_offset = 0.027;
+    var P_in = 0.0005;
+    var P_out = 0.1;
+    var R_scale = 1.14;
+    var B_threshold = 0.022;
+    var B_exp = 1.414;
+
+    function contrastAPCA (text, bg) {
+        // parse input colors
+        text = new Color(text);
+        bg = new Color(bg);
+        // if text color has alpha, blend against background
+        if (text.alpha() < 1) {
+            text = mix(bg, text, text.alpha(), 'rgb');
+        }
+        var l_text = lum.apply(void 0, text.rgb());
+        var l_bg = lum.apply(void 0, bg.rgb());
+
+        // soft clamp black levels
+        var Y_text =
+            l_text >= B_threshold
+                ? l_text
+                : l_text + Math.pow(B_threshold - l_text, B_exp);
+        var Y_bg =
+            l_bg >= B_threshold ? l_bg : l_bg + Math.pow(B_threshold - l_bg, B_exp);
+
+        // normal polarity (dark text on light background)
+        var S_norm = Math.pow(Y_bg, 0.56) - Math.pow(Y_text, 0.57);
+        // reverse polarity (light text on dark background)
+        var S_rev = Math.pow(Y_bg, 0.65) - Math.pow(Y_text, 0.62);
+        // clamp noise then scale
+        var C =
+            Math.abs(Y_bg - Y_text) < P_in
+                ? 0
+                : Y_text < Y_bg
+                  ? S_norm * R_scale
+                  : S_rev * R_scale;
+        // clamp minimum contrast then offset
+        var S_apc = Math.abs(C) < P_out ? 0 : C > 0 ? C - W_offset : C + W_offset;
+        // scale to 100
+        return S_apc * 100;
+    }
+    function lum(r, g, b) {
+        return (
+            0.2126729 * Math.pow(r / 255, 2.4) +
+            0.7151522 * Math.pow(g / 255, 2.4) +
+            0.072175 * Math.pow(b / 255, 2.4)
+        );
+    }
+
     var sqrt = Math.sqrt;
     var pow = Math.pow;
     var min = Math.min;
@@ -3904,6 +3969,7 @@
         Color: Color,
         colors: w3cx11,
         contrast: contrast,
+        contrastAPCA: contrastAPCA,
         cubehelix: cubehelix,
         deltaE: deltaE,
         distance: distance,

diff --git a/docs/libs/chroma.min.cjs b/docs/libs/chroma.min.cjs
diff --git a/docs/src/footer.inc.html b/docs/src/footer.inc.html
@@ -180,7 +180,7 @@
                             [
                                 'background-color:' + (isCSS ? val : col.hex()),
                                 'color:' + (l < 0.7 ? 'white' : 'black'),
-                                'opacity:' + col.alpha()
+                                ...(isCSS ? [] : ['opacity:' + col.alpha()])
                             ].join(';')
                         );
 

diff --git a/docs/src/index.md b/docs/src/index.md
@@ -349,6 +349,19 @@ chroma.contrast('pink', 'hotpink');
 chroma.contrast('pink', 'purple');
 ```
 
+### chroma.contrastAPCA
+#### (text, background)
+
+**New (3.1):** Computes the [APCA contrast](https://www.myndex.com/APCA/) ratio of a text color against its background color. The basic idea is that you check the contrast between the text and background color and then use [this lookup table](https://github.com/raw/Myndex/apca-w3/master/images/APCAlookupByContrast.jpeg) to find the minimum font size you're allowed to use (given the font weight and purpose of the text). 
+
+```js
+chroma.contrastAPCA('hotpink', 'pink');
+chroma.contrastAPCA('purple', 'pink');
+```
+
+Read more about how to interpret and use this metric at [APCA Readability Criterion](https://readtech.org/ARC). Please note that the APCA algorithm is still in beta and may change be subject to changes in the future.
+
+
 ### chroma.distance
 #### (color1, color2, mode='lab')
 

diff --git a/index.js b/index.js
@@ -42,6 +42,7 @@ import scale from './src/generator/scale.js';
 // other utility methods
 import { analyze } from './src/utils/analyze.js';
 import contrast from './src/utils/contrast.js';
+import contrastAPCA from './src/utils/contrastAPCA.js';
 import deltaE from './src/utils/delta-e.js';
 import distance from './src/utils/distance.js';
 import { limits } from './src/utils/analyze.js';
@@ -65,6 +66,7 @@ Object.assign(chroma, {
     Color,
     colors,
     contrast,
+    contrastAPCA,
     cubehelix,
     deltaE,
     distance,
@@ -89,6 +91,7 @@ export {
     Color,
     colors,
     contrast,
+    contrastAPCA,
     cubehelix,
     deltaE,
     distance,

diff --git a/index.umd.js b/index.umd.js
@@ -58,6 +58,7 @@ import scale from './src/generator/scale.js';
 // other utility methods
 import { analyze } from './src/utils/analyze.js';
 import contrast from './src/utils/contrast.js';
+import contrastAPCA from './src/utils/contrastAPCA.js';
 import deltaE from './src/utils/delta-e.js';
 import distance from './src/utils/distance.js';
 import { limits } from './src/utils/analyze.js';
@@ -81,6 +82,7 @@ Object.assign(chroma, {
     Color,
     colors,
     contrast,
+    contrastAPCA,
     cubehelix,
     deltaE,
     distance,

diff --git a/src/utils/contrastAPCA.js b/src/utils/contrastAPCA.js
@@ -0,0 +1,68 @@
+import Color from '../Color.js';
+import mix from '../generator/mix.js';
+
+/**
+ * @license
+ *
+ * The APCA contrast prediction algorithm is based of the formulas published
+ * in the APCA-1.0.98G specification by Myndex. The specification is available at:
+ * https://github.com/raw/Myndex/apca-w3/master/images/APCAw3_0.1.17_APCA0.0.98G.svg
+ *
+ * Note that the APCA implementation is still beta, so please update to
+ * future versions of chroma.js when they become available.
+ *
+ * You can read more about the APCA Readability Criterion at
+ * https://readtech.org/ARC/
+ */
+
+// constants
+const W_offset = 0.027;
+const P_in = 0.0005;
+const P_out = 0.1;
+const R_scale = 1.14;
+const B_threshold = 0.022;
+const B_exp = 1.414;
+
+export default (text, bg) => {
+    // parse input colors
+    text = new Color(text);
+    bg = new Color(bg);
+    // if text color has alpha, blend against background
+    if (text.alpha() < 1) {
+        text = mix(bg, text, text.alpha(), 'rgb');
+    }
+    const l_text = lum(...text.rgb());
+    const l_bg = lum(...bg.rgb());
+
+    // soft clamp black levels
+    const Y_text =
+        l_text >= B_threshold
+            ? l_text
+            : l_text + Math.pow(B_threshold - l_text, B_exp);
+    const Y_bg =
+        l_bg >= B_threshold ? l_bg : l_bg + Math.pow(B_threshold - l_bg, B_exp);
+
+    // normal polarity (dark text on light background)
+    const S_norm = Math.pow(Y_bg, 0.56) - Math.pow(Y_text, 0.57);
+    // reverse polarity (light text on dark background)
+    const S_rev = Math.pow(Y_bg, 0.65) - Math.pow(Y_text, 0.62);
+    // clamp noise then scale
+    const C =
+        Math.abs(Y_bg - Y_text) < P_in
+            ? 0
+            : Y_text < Y_bg
+              ? S_norm * R_scale
+              : S_rev * R_scale;
+    // clamp minimum contrast then offset
+    const S_apc = Math.abs(C) < P_out ? 0 : C > 0 ? C - W_offset : C + W_offset;
+    // scale to 100
+    return S_apc * 100;
+};
+
+function lum(r, g, b) {
+    return (
+        0.2126729 * Math.pow(r / 255, 2.4) +
+        0.7151522 * Math.pow(g / 255, 2.4) +
+        0.072175 * Math.pow(b / 255, 2.4)
+    );
+}
diff --git a/test/contrast.test.js b/test/contrast.test.js
@@ -1,4 +1,5 @@
 import { describe, it, expect } from 'vitest';
+import contrastAPCA from '../src/utils/contrastAPCA.js';
 import chroma from 'chroma-js';
 
 const contrast = chroma.contrast;
@@ -24,3 +25,33 @@ describe('Testing contrast ratio', () => {
         expect(contrast('black', '#222').toFixed(2)).toBe('1.32');
     });
 });
+
+describe('Testing contrast ratio with APCA', () => {
+    it('maximum contrast', () => {
+        expect(+contrastAPCA('black', 'white').toFixed(1)).toBe(106);
+        expect(+contrastAPCA('white', 'black').toFixed(1)).toBe(-107.9);
+    });
+
+    it('minimum contrast', () => {
+        expect(+contrastAPCA('gray', 'gray').toFixed(1)).toBe(0);
+    });
+
+    it('contrast without alpha', () => {
+        expect(+contrastAPCA('#594d45', '#ffd4d4').toFixed(1)).toBe(69.0);
+        expect(+contrastAPCA('#b04646', '#d6d6d6').toFixed(1)).toBe(52.9);
+        expect(+contrastAPCA('#c2afaf', '#d6d6d6').toFixed(1)).toBe(17.0);
+    });
+
+    it('contrast with alpha', () => {
+        // todo: there's a slight difference to the values shown in the APCA demo
+        // when computing contrast between colors with alpha
+        expect(+contrastAPCA('#00000044', 'white').toFixed(1)).toBe(37.3); // 36.7
+        expect(+contrastAPCA('#ffffffc0', 'black').toFixed(1)).toBe(-68); // -68.6
+    });
+
+    it('inverse contrast', () => {
+        expect(+contrastAPCA('#f5f5b3', '#614f63').toFixed(1)).toBe(-81.7);
+        expect(+contrastAPCA('#67a7d6', '#4f6357').toFixed(1)).toBe(-30.5);
+        expect(+contrastAPCA('#d667cb', '#b04646').toFixed(1)).toBe(-17.9);
+    });
+});