MakeColor now indicates conversion errors by returning a second value

README.md

@@ -124,17 +124,16 @@ Because a `colorful.Color` implements Go's `color.Color` interface (found in the
 `image/color` package), it can be used anywhere that expects a `color.Color`.
 
 Furthermore, you can convert anything that implements the `color.Color` interface
-into a `colorful.Color` using the `MakeColorful` function:
+into a `colorful.Color` using the `MakeColor` function:
 
 ```go
-c := colorful.MakeColor(color.Gray16{12345})
+c, ok := colorful.MakeColor(color.Gray16{12345})
 ```
 
-**Caveat:** Be aware that for this latter conversion  (using `MakeColor`) hits a corner-case
-when alpha is exactly zero. Because `color.Color` uses pre-multiplied alpha colors,
-this means the RGB values are lost and it's impossible to recover them. The current
-implementation `panic()`s in that case, see [issue 21](https://github.com/lucasb-eyer/go-colorful/issues/21)
-for discussion and suggesting alternatives.
+**Caveat:** Be aware that this latter conversion (using `MakeColor`) hits a
+corner-case when alpha is exactly zero. Because `color.Color` uses pre-multiplied
+alpha colors, this means the RGB values are lost (set to 0) and it's impossible
+to recover them. In such a case `MakeColor` will return `false` as its second value.
 
 ### Comparing colors
 In the RGB color space, the Euclidian distance between colors *doesn't* correspond
@@ -456,16 +455,17 @@ but that is outside the scope of this library.
 Thanks to [@ZirconiumX](https://github.com/ZirconiumX) for starting this investigation,
 see [issue #18](https://github.com/lucasb-eyer/go-colorful/issues/18) for details.
 
-### Q: `MakeColor` panics when alpha is zero!
-A: Because in that case, the conversion is undefined. See
-[issue 21](https://github.com/lucasb-eyer/go-colorful/issues/21)
-as well as the short caveat discussion in the ["The `color.Color` interface"](README.md#the-colorcolor-interface) section above.
+### Q: Why would `MakeColor` ever fail!?
+A: `MakeColor` fails when the alpha channel is zero. In that case, the
+conversion is undefined. See [issue 21](https://github.com/lucasb-eyer/go-colorful/issues/21)
+as well as the short caveat note in the ["The `color.Color` interface"](README.md#the-colorcolor-interface)
+section above.
 
 Who?
 ====
 
 This library has been developed by Lucas Beyer with contributions from
-Bastien Dejean (@baskerville) and Phil Kulak (@pkulak).
+Bastien Dejean (@baskerville), Phil Kulak (@pkulak) and Christian Muehlhaeuser (@muesli).
 
 License: MIT
 ============

colors.go

@@ -22,8 +22,11 @@ func (col Color) RGBA() (r, g, b, a uint32) {
 }
 
 // Constructs a colorful.Color from something implementing color.Color
-func MakeColor(col color.Color) Color {
+func MakeColor(col color.Color) (Color, bool) {
 	r, g, b, a := col.RGBA()
+	if a == 0 {
+		return Color{0, 0, 0}, false
+	}
 
 	// Since color.Color is alpha pre-multiplied, we need to divide the
 	// RGB values by alpha again in order to get back the original RGB.
@@ -34,7 +37,7 @@ func MakeColor(col color.Color) Color {
 	b *= 0xffff
 	b /= a
 
-	return Color{float64(r) / 65535.0, float64(g) / 65535.0, float64(b) / 65535.0}
+	return Color{float64(r) / 65535.0, float64(g) / 65535.0, float64(b) / 65535.0}, true
 }
 
 // Might come in handy sometimes to reduce boilerplate code.

colors_test.go

@@ -475,31 +475,38 @@ func TestClamp(t *testing.T) {
 
 func TestMakeColor(t *testing.T) {
 	c_orig_nrgba := color.NRGBA{123, 45, 67, 255}
-	c_ours := MakeColor(c_orig_nrgba)
+	c_ours, ok := MakeColor(c_orig_nrgba)
 	r, g, b := c_ours.RGB255()
-	if r != 123 || g != 45 || b != 67 {
-		t.Errorf("NRGBA->Colorful->RGB255 error: %v became (%v, %v, %v)", c_orig_nrgba, r, g, b)
+	if r != 123 || g != 45 || b != 67 || !ok {
+		t.Errorf("NRGBA->Colorful->RGB255 error: %v became (%v, %v, %v, %t)", c_orig_nrgba, r, g, b, ok)
 	}
 
 	c_orig_nrgba64 := color.NRGBA64{123 << 8, 45 << 8, 67 << 8, 0xffff}
-	c_ours = MakeColor(c_orig_nrgba64)
+	c_ours, ok = MakeColor(c_orig_nrgba64)
 	r, g, b = c_ours.RGB255()
-	if r != 123 || g != 45 || b != 67 {
-		t.Errorf("NRGBA64->Colorful->RGB255 error: %v became (%v, %v, %v)", c_orig_nrgba64, r, g, b)
+	if r != 123 || g != 45 || b != 67 || !ok {
+		t.Errorf("NRGBA64->Colorful->RGB255 error: %v became (%v, %v, %v, %t)", c_orig_nrgba64, r, g, b, ok)
 	}
 
 	c_orig_gray := color.Gray{123}
-	c_ours = MakeColor(c_orig_gray)
+	c_ours, ok = MakeColor(c_orig_gray)
 	r, g, b = c_ours.RGB255()
-	if r != 123 || g != 123 || b != 123 {
-		t.Errorf("Gray->Colorful->RGB255 error: %v became (%v, %v, %v)", c_orig_gray, r, g, b)
+	if r != 123 || g != 123 || b != 123 || !ok {
+		t.Errorf("Gray->Colorful->RGB255 error: %v became (%v, %v, %v, %t)", c_orig_gray, r, g, b, ok)
 	}
 
 	c_orig_gray16 := color.Gray16{123 << 8}
-	c_ours = MakeColor(c_orig_gray16)
+	c_ours, ok = MakeColor(c_orig_gray16)
 	r, g, b = c_ours.RGB255()
-	if r != 123 || g != 123 || b != 123 {
-		t.Errorf("Gray16->Colorful->RGB255 error: %v became (%v, %v, %v)", c_orig_gray16, r, g, b)
+	if r != 123 || g != 123 || b != 123 || !ok {
+		t.Errorf("Gray16->Colorful->RGB255 error: %v became (%v, %v, %v, %t)", c_orig_gray16, r, g, b, ok)
+	}
+
+	c_orig_rgba := color.RGBA{255, 255, 255, 0}
+	c_ours, ok = MakeColor(c_orig_rgba)
+	r, g, b = c_ours.RGB255()
+	if r != 0 || g != 0 || b != 0 || ok {
+		t.Errorf("RGBA->Colorful->RGB255 error: %v became (%v, %v, %v, %t)", c_orig_rgba, r, g, b, ok)
 	}
 }