image: add image.CMYK and color.CMYK types.

Change-Id: Icf212a4b890725c803b16e76e1a88294b8b62cb8
Reviewed-on: https://go-review.googlesource.com/4800Reviewed-by: Rob Pike <r@golang.org>

src/image/color/ycbcr.go

@@ -97,3 +97,58 @@ func yCbCrModel(c Color) Color {
 	y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
 	return YCbCr{y, u, v}
 }
+
+// RGBToCMYK converts an RGB triple to a CMYK quadruple.
+func RGBToCMYK(r, g, b uint8) (uint8, uint8, uint8, uint8) {
+	rr := uint32(r)
+	gg := uint32(g)
+	bb := uint32(b)
+	w := rr
+	if w < gg {
+		w = gg
+	}
+	if w < bb {
+		w = bb
+	}
+	if w == 0 {
+		return 0, 0, 0, 255
+	}
+	c := (w - rr) * 255 / w
+	m := (w - gg) * 255 / w
+	y := (w - bb) * 255 / w
+	return uint8(c), uint8(m), uint8(y), uint8(255 - w)
+}
+
+// CMYKToRGB converts a CMYK quadruple to an RGB triple.
+func CMYKToRGB(c, m, y, k uint8) (uint8, uint8, uint8) {
+	w := uint32(255 - k)
+	r := uint32(255-c) * w / 255
+	g := uint32(255-m) * w / 255
+	b := uint32(255-y) * w / 255
+	return uint8(r), uint8(g), uint8(b)
+}
+
+// CMYK represents a fully opaque CMYK color, having 8 bits for each of cyan,
+// magenta, yellow and black.
+//
+// It is not associated with any particular color profile.
+type CMYK struct {
+	C, M, Y, K uint8
+}
+
+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
+}
+
+// CMYKModel is the Model for CMYK colors.
+var CMYKModel Model = ModelFunc(cmykModel)
+
+func cmykModel(c Color) Color {
+	if _, ok := c.(CMYK); ok {
+		return c
+	}
+	r, g, b, _ := c.RGBA()
+	cc, mm, yy, kk := RGBToCMYK(uint8(r>>8), uint8(g>>8), uint8(b>>8))
+	return CMYK{cc, mm, yy, kk}
+}

src/image/color/ycbcr_test.go

@@ -15,9 +15,9 @@ func delta(x, y uint8) uint8 {
 	return y - x
 }
 
-// Test that a subset of RGB space can be converted to YCbCr and back to within
-// 1/256 tolerance.
-func TestRoundtrip(t *testing.T) {
+// 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 {
@@ -31,3 +31,20 @@ func TestRoundtrip(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 {
+				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)
+				}
+			}
+		}
+	}
+}

src/image/image.go

@@ -826,6 +826,92 @@ func NewGray16(r Rectangle) *Gray16 {
 	return &Gray16{pix, 2 * w, r}
 }
 
+// CMYK is an in-memory image whose At method returns color.CMYK values.
+type CMYK struct {
+	// Pix holds the image's pixels, in C, M, Y, K order. The pixel at
+	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*4].
+	Pix []uint8
+	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
+}
+
+func (p *CMYK) ColorModel() color.Model { return color.CMYKModel }
+
+func (p *CMYK) Bounds() Rectangle { return p.Rect }
+
+func (p *CMYK) At(x, y int) color.Color {
+	return p.CMYKAt(x, y)
+}
+
+func (p *CMYK) CMYKAt(x, y int) color.CMYK {
+	if !(Point{x, y}.In(p.Rect)) {
+		return color.CMYK{}
+	}
+	i := p.PixOffset(x, y)
+	return color.CMYK{p.Pix[i+0], p.Pix[i+1], p.Pix[i+2], p.Pix[i+3]}
+}
+
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *CMYK) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
+}
+
+func (p *CMYK) Set(x, y int, c color.Color) {
+	if !(Point{x, y}.In(p.Rect)) {
+		return
+	}
+	i := p.PixOffset(x, y)
+	c1 := color.CMYKModel.Convert(c).(color.CMYK)
+	p.Pix[i+0] = c1.C
+	p.Pix[i+1] = c1.M
+	p.Pix[i+2] = c1.Y
+	p.Pix[i+3] = c1.K
+}
+
+func (p *CMYK) SetCMYK(x, y int, c color.CMYK) {
+	if !(Point{x, y}.In(p.Rect)) {
+		return
+	}
+	i := p.PixOffset(x, y)
+	p.Pix[i+0] = c.C
+	p.Pix[i+1] = c.M
+	p.Pix[i+2] = c.Y
+	p.Pix[i+3] = c.K
+}
+
+// SubImage returns an image representing the portion of the image p visible
+// through r. The returned value shares pixels with the original image.
+func (p *CMYK) SubImage(r Rectangle) Image {
+	r = r.Intersect(p.Rect)
+	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
+	// either r1 or r2 if the intersection is empty. Without explicitly checking for
+	// this, the Pix[i:] expression below can panic.
+	if r.Empty() {
+		return &CMYK{}
+	}
+	i := p.PixOffset(r.Min.X, r.Min.Y)
+	return &CMYK{
+		Pix:    p.Pix[i:],
+		Stride: p.Stride,
+		Rect:   r,
+	}
+}
+
+// Opaque scans the entire image and reports whether it is fully opaque.
+func (p *CMYK) Opaque() bool {
+	return true
+}
+
+// NewCMYK returns a new CMYK with the given bounds.
+func NewCMYK(r Rectangle) *CMYK {
+	w, h := r.Dx(), r.Dy()
+	buf := make([]uint8, 4*w*h)
+	return &CMYK{buf, 4 * w, r}
+}
+
 // Paletted is an in-memory image of uint8 indices into a given palette.
 type Paletted struct {
 	// Pix holds the image's pixels, as palette indices. The pixel at