Fix color.Premultiply

go.mod

@@ -1,3 +1,3 @@
 module git.tebibyte.media/sashakoshka/goutil
 
-go 1.21.0
+go 1.20.0

image/color/color.go

@@ -22,9 +22,9 @@ func ToRGBA (c color.Color) color.RGBA {
 // Premultiply applies alpha-premultiplication that colors must apply as per
 // color.Color.RGBA. This is an intrinsically lossy proccess.
 func Premultiply (ur, ug, ub, ua uint32) (r, g, b, a uint32) {
-	return (ur * a) / 0xFFFF,
-		(ug * a) / 0xFFFF,
-		(ub * a) / 0xFFFF,
+	return (ur * ua) / 0xFFFF,
+		(ug * ua) / 0xFFFF,
+		(ub * ua) / 0xFFFF,
 		ua
 }
 

image/color/color_test.go

@@ -0,0 +1,17 @@
+package ucolor
+
+import "testing"
+
+func TestPremultiply (test *testing.T) {
+	r, g, b, a := Premultiply(0xFFFF, 0xFFFF, 0xFFFF, 0x8888)
+	if r != 0x8888 || g != 0x8888 || b != 0x8888 || a != 0x8888 {
+		test.Fatalf("wrong value: %08x %08x %08x %08x", r, g, b, a)
+	}
+}
+
+func TestUnpremultiply (test *testing.T) {
+	r, g, b, a := Unpremultiply(0x8888, 0x8888, 0x8888, 0x8888)
+	if r != 0xFFFF || g != 0xFFFF || b != 0xFFFF || a != 0x8888 {
+		test.Fatalf("wrong value: %08x %08x %08x %08x", r, g, b, a)
+	}
+}