From bea63946fc4484c1ef08d7381cc9dbb673c93797 Mon Sep 17 00:00:00 2001
From: Hajime Hoshi <hajimehoshi@gmail.com>
Date: Tue, 27 Feb 2018 12:16:16 +0900
Subject: [PATCH] affine: Bug fix: ColorM.Apply was wrong when alpha is 0

---
 colorm_test.go            | 23 ++++++++++++++++-------
 internal/affine/colorm.go | 14 +++++++-------
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/colorm_test.go b/colorm_test.go
index 1937e5e65..40e995816 100644
--- a/colorm_test.go
+++ b/colorm_test.go
@@ -170,11 +170,11 @@ func TestColorMConcatSelf(t *testing.T) {
 	}
 }
 
-func absU32(x uint32) uint32 {
-	if x < 0 {
-		return -x
+func absDiffU32(x, y uint32) uint32 {
+	if x < y {
+		return y - x
 	}
-	return x
+	return x - y
 }
 
 func TestColorMApply(t *testing.T) {
@@ -184,6 +184,9 @@ func TestColorMApply(t *testing.T) {
 	shiny := ColorM{}
 	shiny.Translate(1, 1, 1, 0)
 
+	shift := ColorM{}
+	shift.Translate(0.5, 0.5, 0.5, 0.5)
+
 	cases := []struct {
 		ColorM ColorM
 		In     color.Color
@@ -214,14 +217,20 @@ func TestColorMApply(t *testing.T) {
 			Out:    color.RGBA{0xb0, 0xb0, 0xb0, 0xb0},
 			Delta:  1,
 		},
+		{
+			ColorM: shift,
+			In:     color.RGBA{0x00, 0x00, 0x00, 0x00},
+			Out:    color.RGBA{0x40, 0x40, 0x40, 0x80},
+			Delta:  0x101,
+		},
 	}
 	for _, c := range cases {
 		out := c.ColorM.Apply(c.In)
 		r0, g0, b0, a0 := out.RGBA()
 		r1, g1, b1, a1 := c.Out.RGBA()
-		if absU32(r0-r1) > c.Delta || absU32(g0-g1) > c.Delta ||
-			absU32(b0-b1) > c.Delta || absU32(a0-a1) > c.Delta {
-			t.Errorf("%v.Apply(%v) = %v, want %v", c.ColorM, c.In, out, c.Out)
+		if absDiffU32(r0, r1) > c.Delta || absDiffU32(g0, g1) > c.Delta ||
+			absDiffU32(b0, b1) > c.Delta || absDiffU32(a0, a1) > c.Delta {
+			t.Errorf("%v.Apply(%v) = {%d, %d, %d, %d}, want {%d, %d, %d, %d}", c.ColorM, c.In, r0, g0, b0, a0, r1, g1, b1, a1)
 		}
 	}
 }
diff --git a/internal/affine/colorm.go b/internal/affine/colorm.go
index 079ccbb2f..9661c993f 100644
--- a/internal/affine/colorm.go
+++ b/internal/affine/colorm.go
@@ -69,14 +69,14 @@ func (c *ColorM) Apply(clr color.Color) color.Color {
 		return clr
 	}
 	r, g, b, a := clr.RGBA()
-	if a == 0 {
-		return color.Transparent
-	}
+	rf, gf, bf, af := float32(0.0), float32(0.0), float32(0.0), float32(0.0)
 	// Unmultiply alpha
-	rf := float32(r) / float32(a)
-	gf := float32(g) / float32(a)
-	bf := float32(b) / float32(a)
-	af := float32(a) / 0xffff
+	if a > 0 {
+		rf = float32(r) / float32(a)
+		gf = float32(g) / float32(a)
+		bf = float32(b) / float32(a)
+		af = float32(a) / 0xffff
+	}
 	eb := c.body
 	et := c.translate
 	rf2 := eb[0]*rf + eb[4]*gf + eb[8]*bf + eb[12]*af + et[0]