image/color: have CMYK.RGBA work in 16-bit color, per the Color interface.

Change-Id: I3621527c924a43724032f80a072505c60d929ab3
Reviewed-on: https://go-review.googlesource.com/8180Reviewed-by: Rob Pike <r@golang.org>

src/image/color/ycbcr.go

@@ -158,11 +158,11 @@ func RGBToCMYK(r, g, b uint8) (uint8, uint8, uint8, uint8) {
 
 // CMYKToRGB converts a CMYK quadruple to an RGB triple.
 func CMYKToRGB(c, m, y, k uint8) (uint8, uint8, uint8) {
-	w := uint32(0xff - k)
-	r := uint32(0xff-c) * w / 0xff
-	g := uint32(0xff-m) * w / 0xff
-	b := uint32(0xff-y) * w / 0xff
-	return uint8(r), uint8(g), uint8(b)
+	w := uint32(0xffff - uint32(k)*0x101)
+	r := uint32(0xffff-uint32(c)*0x101) * w / 0xffff
+	g := uint32(0xffff-uint32(m)*0x101) * w / 0xffff
+	b := uint32(0xffff-uint32(y)*0x101) * w / 0xffff
+	return uint8(r >> 8), uint8(g >> 8), uint8(b >> 8)
 }
 
 // CMYK represents a fully opaque CMYK color, having 8 bits for each of cyan,
@@ -174,8 +174,14 @@ type CMYK struct {
 }
 
 func (c CMYK) RGBA() (uint32, uint32, uint32, uint32) {
-	r, g, b := CMYKToRGB(c.C, c.M, c.Y, c.K)
-	return uint32(r) * 0x101, uint32(g) * 0x101, uint32(b) * 0x101, 0xffff
+	// This code is a copy of the CMYKToRGB function above, except that it
+	// returns values in the range [0, 0xffff] instead of [0, 0xff].
+
+	w := uint32(0xffff - uint32(c.K)*0x101)
+	r := uint32(0xffff-uint32(c.C)*0x101) * w / 0xffff
+	g := uint32(0xffff-uint32(c.M)*0x101) * w / 0xffff
+	b := uint32(0xffff-uint32(c.Y)*0x101) * w / 0xffff
+	return uint32(r), uint32(g), uint32(b), 0xffff
 }
 
 // CMYKModel is the Model for CMYK colors.

src/image/color/ycbcr_test.go

@@ -18,14 +18,34 @@ func delta(x, y uint8) uint8 {
 // TestYCbCrRoundtrip tests that a subset of RGB space can be converted to YCbCr
 // and back to within 1/256 tolerance.
 func TestYCbCrRoundtrip(t *testing.T) {
-	for r := 0; r < 255; r += 7 {
-		for g := 0; g < 255; g += 5 {
-			for b := 0; b < 255; b += 3 {
+	for r := 0; r < 256; r += 7 {
+		for g := 0; g < 256; g += 5 {
+			for b := 0; b < 256; b += 3 {
 				r0, g0, b0 := uint8(r), uint8(g), uint8(b)
 				y, cb, cr := RGBToYCbCr(r0, g0, b0)
 				r1, g1, b1 := YCbCrToRGB(y, cb, cr)
 				if delta(r0, r1) > 1 || delta(g0, g1) > 1 || delta(b0, b1) > 1 {
-					t.Fatalf("r0, g0, b0 = %d, %d, %d   r1, g1, b1 = %d, %d, %d", r0, g0, b0, r1, g1, b1)
+					t.Fatalf("\nr0, g0, b0 = %d, %d, %d\nr1, g1, b1 = %d, %d, %d", r0, g0, b0, r1, g1, b1)
+				}
+			}
+		}
+	}
+}
+
+// TestYCbCrToRGBConsistency tests that calling the RGBA method (16 bit color)
+// then truncating to 8 bits is equivalent to calling the YCbCrToRGB function (8
+// bit color).
+func TestYCbCrToRGBConsistency(t *testing.T) {
+	for y := 0; y < 256; y += 7 {
+		for cb := 0; cb < 256; cb += 5 {
+			for cr := 0; cr < 256; cr += 3 {
+				x := YCbCr{uint8(y), uint8(cb), uint8(cr)}
+				r0, g0, b0, _ := x.RGBA()
+				r1, g1, b1 := uint8(r0>>8), uint8(g0>>8), uint8(b0>>8)
+				r2, g2, b2 := YCbCrToRGB(x.Y, x.Cb, x.Cr)
+				if r1 != r2 || g1 != g2 || b1 != b2 {
+					t.Fatalf("y, cb, cr = %d, %d, %d\nr1, g1, b1 = %d, %d, %d\nr2, g2, b2 = %d, %d, %d",
+						y, cb, cr, r1, g1, b1, r2, g2, b2)
 				}
 			}
 		}
@@ -35,14 +55,36 @@ func TestYCbCrRoundtrip(t *testing.T) {
 // TestCMYKRoundtrip tests that a subset of RGB space can be converted to CMYK
 // and back to within 1/256 tolerance.
 func TestCMYKRoundtrip(t *testing.T) {
-	for r := 0; r < 255; r += 7 {
-		for g := 0; g < 255; g += 5 {
-			for b := 0; b < 255; b += 3 {
+	for r := 0; r < 256; r += 7 {
+		for g := 0; g < 256; g += 5 {
+			for b := 0; b < 256; b += 3 {
 				r0, g0, b0 := uint8(r), uint8(g), uint8(b)
 				c, m, y, k := RGBToCMYK(r0, g0, b0)
 				r1, g1, b1 := CMYKToRGB(c, m, y, k)
 				if delta(r0, r1) > 1 || delta(g0, g1) > 1 || delta(b0, b1) > 1 {
-					t.Fatalf("r0, g0, b0 = %d, %d, %d   r1, g1, b1 = %d, %d, %d", r0, g0, b0, r1, g1, b1)
+					t.Fatalf("\nr0, g0, b0 = %d, %d, %d\nr1, g1, b1 = %d, %d, %d", r0, g0, b0, r1, g1, b1)
+				}
+			}
+		}
+	}
+}
+
+// TestCMYKToRGBConsistency tests that calling the RGBA method (16 bit color)
+// then truncating to 8 bits is equivalent to calling the CMYKToRGB function (8
+// bit color).
+func TestCMYKToRGBConsistency(t *testing.T) {
+	for c := 0; c < 256; c += 7 {
+		for m := 0; m < 256; m += 5 {
+			for y := 0; y < 256; y += 3 {
+				for k := 0; k < 256; k += 11 {
+					x := CMYK{uint8(c), uint8(m), uint8(y), uint8(k)}
+					r0, g0, b0, _ := x.RGBA()
+					r1, g1, b1 := uint8(r0>>8), uint8(g0>>8), uint8(b0>>8)
+					r2, g2, b2 := CMYKToRGB(x.C, x.M, x.Y, x.K)
+					if r1 != r2 || g1 != g2 || b1 != b2 {
+						t.Fatalf("c, m, y, k = %d, %d, %d, %d\nr1, g1, b1 = %d, %d, %d\nr2, g2, b2 = %d, %d, %d",
+							c, m, y, k, r1, g1, b1, r2, g2, b2)
+					}
 				}
 			}
 		}