Improve differentiation of semitransparent pixels

* add failing unit test when white is compared with transparent

* fix failing unit test

* fix failing unit test

* increase sensitivity of transparent differenc detection by making pattern repeat with non-integer period.

* remove repeated comparison

* Add digits to color periods

* fix unit test after changing coefficients ever so slightly

---------

Co-authored-by: Volodymyr Agafonkin <[email protected]>

Author: NathanMOlson

index.js

@@ -188,9 +188,9 @@ function colorDelta(img1, img2, k, m, yOnly) {
     if (!da && !dr && !dg && !db) return 0;
 
     if (a1 < 255 || a2 < 255) { // blend pixels with background
-        const rb = 255;
-        const gb = 255;
-        const bb = 255;
+        const rb = 48 + 159 * (k % 2);
+        const gb = 48 + 159 * ((k / 1.618033988749895 | 0) % 2);
+        const bb = 48 + 159 * ((k / 2.618033988749895 | 0) % 2);
         dr = (r1 * a1 - r2 * a2 - rb * da) / 255;
         dg = (g1 * a1 - g2 * a2 - gb * da) / 255;
         db = (b1 * a1 - b2 * a2 - bb * da) / 255;

test/fixtures/5diff.png

@@ -20,10 +20,11 @@ diffTest('2a', '2b', '2diff', {
 }, 12437);
 diffTest('3a', '3b', '3diff', options, 212);
 diffTest('4a', '4b', '4diff', options, 36049);
-diffTest('5a', '5b', '5diff', options, 0);
+diffTest('5a', '5b', '5diff', options, 6);
 diffTest('6a', '6b', '6diff', options, 51);
 diffTest('6a', '6a', '6empty', {threshold: 0}, 0);
 diffTest('7a', '7b', '7diff', {diffColorAlt: [0, 255, 0]}, 2448);
+diffTest('8a', '5b', '8diff', options, 32896);
 
 test('throws error if image sizes do not match', () => {
     assert.throws(() => match(new Uint8Array(8), new Uint8Array(9), null, 2, 1), 'Image sizes do not match');