image/color: have NYCbCrA.RGBA work in 16-bit color.

This makes NYCbCrA consistent with YCbCr.

Fixes #13706.

Change-Id: Ifced84372e4865925fa6efef9ca2f1de43da70e0
Reviewed-on: https://go-review.googlesource.com/18115Reviewed-by: Rob Pike <r@golang.org>

src/image/color/ycbcr.go

@@ -98,7 +98,7 @@ func (c YCbCr) RGBA() (uint32, uint32, uint32, uint32) {
 	//	fmt.Printf("0x%04x 0x%04x 0x%04x\n", r0, g0, b0)
 	//	fmt.Printf("0x%04x 0x%04x 0x%04x\n", r1, g1, b1)
 	// prints:
-	//	0x7e18 0x808e 0x7db9
+	//	0x7e18 0x808d 0x7db9
 	//	0x7e7e 0x8080 0x7d7d
 
 	yy1 := int32(c.Y) * 0x10100 // Convert 0x12 to 0x121200.
@@ -144,13 +144,33 @@ type NYCbCrA struct {
 	A uint8
 }
 
-func (c NYCbCrA) RGBA() (r, g, b, a uint32) {
-	r8, g8, b8 := YCbCrToRGB(c.Y, c.Cb, c.Cr)
-	a = uint32(c.A) * 0x101
-	r = uint32(r8) * 0x101 * a / 0xffff
-	g = uint32(g8) * 0x101 * a / 0xffff
-	b = uint32(b8) * 0x101 * a / 0xffff
-	return
+func (c NYCbCrA) RGBA() (uint32, uint32, uint32, uint32) {
+	// The first part of this method is the same as YCbCr.RGBA.
+	yy1 := int32(c.Y) * 0x10100 // Convert 0x12 to 0x121200.
+	cb1 := int32(c.Cb) - 128
+	cr1 := int32(c.Cr) - 128
+	r := (yy1 + 91881*cr1) >> 8
+	g := (yy1 - 22554*cb1 - 46802*cr1) >> 8
+	b := (yy1 + 116130*cb1) >> 8
+	if r < 0 {
+		r = 0
+	} else if r > 0xffff {
+		r = 0xffff
+	}
+	if g < 0 {
+		g = 0
+	} else if g > 0xffff {
+		g = 0xffff
+	}
+	if b < 0 {
+		b = 0
+	} else if b > 0xffff {
+		b = 0xffff
+	}
+
+	// The second part of this method applies the alpha.
+	a := uint32(c.A) * 0x101
+	return uint32(r) * a / 0xffff, uint32(g) * a / 0xffff, uint32(b) * a / 0xffff, a
 }
 
 // NYCbCrAModel is the Model for non-alpha-premultiplied Y'CbCr-with-alpha

src/image/color/ycbcr_test.go

@@ -67,7 +67,31 @@ func TestYCbCrToRGBConsistency(t *testing.T) {
 // TestYCbCrGray tests that YCbCr colors are a superset of Gray colors.
 func TestYCbCrGray(t *testing.T) {
 	for i := 0; i < 256; i++ {
-		if err := eq(YCbCr{uint8(i), 0x80, 0x80}, Gray{uint8(i)}); err != nil {
+		c0 := YCbCr{uint8(i), 0x80, 0x80}
+		c1 := Gray{uint8(i)}
+		if err := eq(c0, c1); err != nil {
+			t.Errorf("i=0x%02x:\n%v", i, err)
+		}
+	}
+}
+
+// TestNYCbCrAAlpha tests that NYCbCrA colors are a superset of Alpha colors.
+func TestNYCbCrAAlpha(t *testing.T) {
+	for i := 0; i < 256; i++ {
+		c0 := NYCbCrA{YCbCr{0xff, 0x80, 0x80}, uint8(i)}
+		c1 := Alpha{uint8(i)}
+		if err := eq(c0, c1); err != nil {
+			t.Errorf("i=0x%02x:\n%v", i, err)
+		}
+	}
+}
+
+// TestNYCbCrAYCbCr tests that NYCbCrA colors are a superset of YCbCr colors.
+func TestNYCbCrAYCbCr(t *testing.T) {
+	for i := 0; i < 256; i++ {
+		c0 := NYCbCrA{YCbCr{uint8(i), 0x40, 0xc0}, 0xff}
+		c1 := YCbCr{uint8(i), 0x40, 0xc0}
+		if err := eq(c0, c1); err != nil {
 			t.Errorf("i=0x%02x:\n%v", i, err)
 		}
 	}