Merge pull request #7 from mapbox/yiq-color

YIQ-based color metric

README.md

@@ -7,8 +7,8 @@
 The smallest, simplest and fastest JavaScript pixel-level image comparison library,
 originally created to compare screenshots in tests.
 
-Features accurate [anti-aliased pixels detection](http://www.ee.ktu.lt/journal/2009/7/25_ISSN_1392-1215_Anti-aliased%20Pxel%20and%20Intensity%20Slope%20Detector.pdf)
-and [perceptual color metrics](https://en.wikipedia.org/wiki/YUV).
+Features accurate **anti-aliased pixels detection**
+and **perceptual color difference metrics**.
 
 Inspired by [Resemble.js](https://github.com/Huddle/Resemble.js)
 and [Blink-diff](https://github.com/yahoo/blink-diff).
@@ -20,6 +20,11 @@ so it's **blazing fast** and can be used in **any environment** (Node or browser
 var numDiffPixels = pixelmatch(img1, img2, diff, 800, 600, {threshold: 0.1});
 ```
 
+Implements ideas from the following papers:
+
+- [Measuring perceived color difference using YIQ NTSC transmission color space in mobile applications](http://www.progmat.uaem.mx:8080/artVol2Num2/Articulo3Vol2Num2.pdf) (2010 Y. Kotsarenko, F. Ramos)
+- [Anti-aliased pixel and intensity slope detector](http://www.ee.ktu.lt/journal/2009/7/25_ISSN_1392-1215_Anti-aliased%20Pxel%20and%20Intensity%20Slope%20Detector.pdf) (2009 V. Vyšniauskas)
+
 ### Example output
 
 | expected | actual | diff |

index.js

@@ -8,8 +8,9 @@ function pixelmatch(img1, img2, output, width, height, options) {
 
     var threshold = options.threshold === undefined ? 0.1 : options.threshold;
 
-    // maximum acceptable square YUV distance between two colors
-    var maxDelta = 255 * 255 * 3 * threshold * threshold,
+    // maximum acceptable square distance between two colors;
+    // 35215 is the maximum possible value for the YIQ difference metric
+    var maxDelta = 35215 * threshold * threshold,
         diff = 0;
 
     // compare each pixel of one image against the other one
@@ -35,10 +36,10 @@ function pixelmatch(img1, img2, output, width, height, options) {
                     diff++;
                 }
 
-            } else {
+            } else if (output) {
                 // pixels are similar; draw background as grayscale image blended with white
-                var val = 255 - 0.1 * (255 - grayPixel(img1, pos)) * img1[pos + 3] / 255;
-                if (output) drawPixel(output, pos, val, val, val);
+                var val = blend(grayPixel(img1, pos), 0.1);
+                drawPixel(output, pos, val, val, val);
             }
         }
     }
@@ -107,32 +108,38 @@ function antialiased(img, x1, y1, width, height, img2) {
            (!antialiased(img, maxX, maxY, width, height) && !antialiased(img2, maxX, maxY, width, height));
 }
 
-// calculate either the squared YUV distance between colors,
-// or just the brightness differene (Y component) if yOnly is true
+// calculate color difference according to the paper "Measuring perceived color difference
+// using YIQ NTSC transmission color space in mobile applications" by Y. Kotsarenko and F. Ramos
 
-function colorDelta(img1, img2, i, j, yOnly) {
-    var a1 = img1[i + 3] / 255,
-        a2 = img2[j + 3] / 255,
+function colorDelta(img1, img2, k, m, yOnly) {
+    var a1 = img1[k + 3] / 255,
+        a2 = img2[m + 3] / 255,
 
-        r1 = img1[i + 0] * a1,
-        g1 = img1[i + 1] * a1,
-        b1 = img1[i + 2] * a1,
+        r1 = blend(img1[k + 0], a1),
+        g1 = blend(img1[k + 1], a1),
+        b1 = blend(img1[k + 2], a1),
 
-        r2 = img2[j + 0] * a2,
-        g2 = img2[j + 1] * a2,
-        b2 = img2[j + 2] * a2,
+        r2 = blend(img2[m + 0], a2),
+        g2 = blend(img2[m + 1], a2),
+        b2 = blend(img2[m + 2], a2),
 
-        y1 = 0.299 * r1 + 0.587 * g1 + 0.114 * b1,
-        y2 = 0.299 * r2 + 0.587 * g2 + 0.114 * b2,
+        y = rgb2y(r1, g1, b1) - rgb2y(r2, g2, b2);
 
-        yd = y1 - y2;
+    if (yOnly) return y; // brightness difference only
 
-    if (yOnly) return yd;
+    var i = rgb2i(r1, g1, b1) - rgb2i(r2, g2, b2),
+        q = rgb2q(r1, g1, b1) - rgb2q(r2, g2, b2);
 
-    var ud = 0.492 * (b1 - y1) - 0.492 * (b2 - y2),
-        vd = 0.877 * (r1 - y1) - 0.877 * (r2 - y2);
+    return 0.5053 * y * y + 0.299 * i * i + 0.1957 * q * q;
+}
+
+function rgb2y(r, g, b) { return r * 0.29889531 + g * 0.58662247 + b * 0.11448223; }
+function rgb2i(r, g, b) { return r * 0.59597799 - g * 0.27417610 - b * 0.32180189; }
+function rgb2q(r, g, b) { return r * 0.21147017 - g * 0.52261711 + b * 0.31114694; }
 
-    return (yd * yd) + (ud * ud) + (vd * vd);
+// blend semi-transparent color with white
+function blend(c, a) {
+    return 255 + (c - 255) * a;
 }
 
 function drawPixel(output, pos, r, g, b) {
@@ -142,8 +149,10 @@ function drawPixel(output, pos, r, g, b) {
     output[pos + 3] = 255;
 }
 
-function grayPixel(img, pos) {
-    return 0.30 * img[pos + 0] +
-           0.59 * img[pos + 1] +
-           0.11 * img[pos + 2];
+function grayPixel(img, i) {
+    var a = img[i + 3] / 255,
+        r = blend(img[i + 0], a),
+        g = blend(img[i + 1], a),
+        b = blend(img[i + 2], a);
+    return rgb2y(r, g, b);
 }

test/test.js

@@ -6,10 +6,10 @@ var PNG = require('pngjs2').PNG,
     path = require('path'),
     match = require('../.');
 
-diffTest('1a', '1b', '1diff', 0.03, false, 144);
-diffTest('2a', '2b', '2diff', 0.03, false, 12785);
-diffTest('3a', '3b', '3diff', 0.03, false, 212);
-diffTest('4a', '4b', '4diff', 0.03, false, 36383);
+diffTest('1a', '1b', '1diff', 0.05, false, 143);
+diffTest('2a', '2b', '2diff', 0.05, false, 12439);
+diffTest('3a', '3b', '3diff', 0.05, false, 212);
+diffTest('4a', '4b', '4diff', 0.05, false, 36089);
 
 function diffTest(imgPath1, imgPath2, diffPath, threshold, includeAA, expectedMismatch) {
     var name = 'comparing ' + imgPath1 + ' to ' + imgPath2 +