78: detect diffs in alpha channel

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Changelog
 
 [[Unreleased]]
+-  [diff is displayed if images only differ in their alpha channel](https://github.com/ehmkah/imgdiff/issues/78)
 
 ## [1.12.0]
 - Upgrade project to JDK 21 from JDK 11

src/main/kotlin/de/ehmkah/projects/imgdiff/DiffedImageCreator.kt

@@ -74,16 +74,18 @@ class DiffedImageCreator {
                     val r1 = rgb1 shr 16 and 0xff
                     val g1 = rgb1 shr 8 and 0xff
                     val b1 = rgb1 and 0xff
+                    val a1 = rgb1 shr 24 and 0xff
                     val r2 = rgb2 shr 16 and 0xff
                     val g2 = rgb2 shr 8 and 0xff
                     val b2 = rgb2 and 0xff
+                    val a2 = rgb2 shr 24 and 0xff
                     diff = Math.abs(r1 - r2)
                     diff += Math.abs(g1 - g2)
                     diff += Math.abs(b1 - b2)
                     diff /= 3
 
                     diffPixel = diff shl 16 or (diff shl 8) or diff
-                    if (diffPixel == 0) {
+                    if (diffPixel == 0 && a1 == a2) {
                         diffPixel = backgroundColor(currentWidth, currentHeight)
                     } else {
                         diffPixel = differentValue

src/test/kotlin/de/ehmkah/projects/imgdiff/DiffedImageCreatorTest.kt

@@ -69,6 +69,17 @@ class DiffedImageCreatorTest {
         assertTrue(compareImages(expected, actual))
     }
 
+    @Test
+    fun testDifferentAlphaChannel() {
+        val original = readImage("/black.png")
+        val changed = readImage("/blackAlpha.png")
+
+        val actual = sut.getDifferenceImageWhiteAsBackground(original, changed)
+        val expected = readImage("/expectedAlphaDiff.png")
+
+        assertTrue(compareImages(expected, actual))
+    }
+
     @Test
     fun testImagesHaveNoDiff() {
         val original = readImage("/identical.png")