image: introduce Gray and Gray16 types, and remove the named colors

except for Black and White.

R=rsc
CC=golang-dev, mpl, r
https://golang.org/cl/1903049

src/pkg/image/color.go

@@ -103,6 +103,27 @@ func (c Alpha16Color) RGBA() (r, g, b, a uint32) {
 	return a, a, a, a
 }
 
+// A GrayColor represents an 8-bit grayscale color.
+type GrayColor struct {
+	Y uint8
+}
+
+func (c GrayColor) RGBA() (r, g, b, a uint32) {
+	y := uint32(c.Y)
+	y |= y << 8
+	return y, y, y, 0xffff
+}
+
+// A Gray16Color represents a 16-bit grayscale color.
+type Gray16Color struct {
+	Y uint16
+}
+
+func (c Gray16Color) RGBA() (r, g, b, a uint32) {
+	y := uint32(c.Y)
+	return y, y, y, 0xffff
+}
+
 // A ColorModel can convert foreign Colors, with a possible loss of precision,
 // to a Color from its own color model.
 type ColorModel interface {
@@ -187,6 +208,24 @@ func toAlpha16Color(c Color) Color {
 	return Alpha16Color{uint16(a)}
 }
 
+func toGrayColor(c Color) Color {
+	if _, ok := c.(GrayColor); ok {
+		return c
+	}
+	r, g, b, _ := c.RGBA()
+	y := (299*r + 587*g + 114*b + 500) / 1000
+	return GrayColor{uint8(y >> 8)}
+}
+
+func toGray16Color(c Color) Color {
+	if _, ok := c.(Gray16Color); ok {
+		return c
+	}
+	r, g, b, _ := c.RGBA()
+	y := (299*r + 587*g + 114*b + 500) / 1000
+	return Gray16Color{uint16(y)}
+}
+
 // The ColorModel associated with RGBAColor.
 var RGBAColorModel ColorModel = ColorModelFunc(toRGBAColor)
 
@@ -204,3 +243,9 @@ var AlphaColorModel ColorModel = ColorModelFunc(toAlphaColor)
 
 // The ColorModel associated with Alpha16Color.
 var Alpha16ColorModel ColorModel = ColorModelFunc(toAlpha16Color)
+
+// The ColorModel associated with GrayColor.
+var GrayColorModel ColorModel = ColorModelFunc(toGrayColor)
+
+// The ColorModel associated with Gray16Color.
+var Gray16ColorModel ColorModel = ColorModelFunc(toGray16Color)

src/pkg/image/image.go

@@ -303,6 +303,78 @@ func NewAlpha16(w, h int) *Alpha16 {
 	return &Alpha16{pix}
 }
 
+// A Gray is an in-memory image backed by a 2-D slice of GrayColor values.
+type Gray struct {
+	// The Pixel field's indices are y first, then x, so that At(x, y) == Pixel[y][x].
+	Pixel [][]GrayColor
+}
+
+func (p *Gray) ColorModel() ColorModel { return GrayColorModel }
+
+func (p *Gray) Width() int {
+	if len(p.Pixel) == 0 {
+		return 0
+	}
+	return len(p.Pixel[0])
+}
+
+func (p *Gray) Height() int { return len(p.Pixel) }
+
+func (p *Gray) At(x, y int) Color { return p.Pixel[y][x] }
+
+func (p *Gray) Set(x, y int, c Color) { p.Pixel[y][x] = toGrayColor(c).(GrayColor) }
+
+// Opaque scans the entire image and returns whether or not it is fully opaque.
+func (p *Gray) Opaque() bool {
+	return true
+}
+
+// NewGray returns a new Gray with the given width and height.
+func NewGray(w, h int) *Gray {
+	buf := make([]GrayColor, w*h)
+	pix := make([][]GrayColor, h)
+	for y := range pix {
+		pix[y] = buf[w*y : w*(y+1)]
+	}
+	return &Gray{pix}
+}
+
+// A Gray16 is an in-memory image backed by a 2-D slice of Gray16Color values.
+type Gray16 struct {
+	// The Pixel field's indices are y first, then x, so that At(x, y) == Pixel[y][x].
+	Pixel [][]Gray16Color
+}
+
+func (p *Gray16) ColorModel() ColorModel { return Gray16ColorModel }
+
+func (p *Gray16) Width() int {
+	if len(p.Pixel) == 0 {
+		return 0
+	}
+	return len(p.Pixel[0])
+}
+
+func (p *Gray16) Height() int { return len(p.Pixel) }
+
+func (p *Gray16) At(x, y int) Color { return p.Pixel[y][x] }
+
+func (p *Gray16) Set(x, y int, c Color) { p.Pixel[y][x] = toGray16Color(c).(Gray16Color) }
+
+// Opaque scans the entire image and returns whether or not it is fully opaque.
+func (p *Gray16) Opaque() bool {
+	return true
+}
+
+// NewGray16 returns a new Gray16 with the given width and height.
+func NewGray16(w, h int) *Gray16 {
+	buf := make([]Gray16Color, w*h)
+	pix := make([][]Gray16Color, h)
+	for y := range pix {
+		pix[y] = buf[w*y : w*(y+1)]
+	}
+	return &Gray16{pix}
+}
+
 // A PalettedColorModel represents a fixed palette of colors.
 type PalettedColorModel []Color
 

src/pkg/image/names.go

@@ -4,29 +4,11 @@
 
 package image
 
-// Colors from the HTML 4.01 specification: http://www.w3.org/TR/REC-html40/types.html#h-6.5
-// These names do not necessarily match those from other lists, such as the X11 color names.
 var (
-	Aqua    = ColorImage{RGBAColor{0x00, 0xff, 0xff, 0xff}}
+	// Black is an opaque black ColorImage.
 	Black = ColorImage{RGBAColor{0x00, 0x00, 0x00, 0xff}}
-	Blue    = ColorImage{RGBAColor{0x00, 0x00, 0xff, 0xff}}
-	Fuchsia = ColorImage{RGBAColor{0xff, 0x00, 0xff, 0xff}}
-	Gray    = ColorImage{RGBAColor{0x80, 0x80, 0x80, 0xff}}
-	Green   = ColorImage{RGBAColor{0x00, 0x80, 0x00, 0xff}}
-	Lime    = ColorImage{RGBAColor{0x00, 0xff, 0x00, 0xff}}
-	Maroon  = ColorImage{RGBAColor{0x80, 0x00, 0x00, 0xff}}
-	Navy    = ColorImage{RGBAColor{0x00, 0x00, 0x80, 0xff}}
-	Olive   = ColorImage{RGBAColor{0x80, 0x80, 0x00, 0xff}}
-	Red     = ColorImage{RGBAColor{0xff, 0x00, 0x00, 0xff}}
-	Purple  = ColorImage{RGBAColor{0x80, 0x00, 0x80, 0xff}}
-	Silver  = ColorImage{RGBAColor{0xc0, 0xc0, 0xc0, 0xff}}
-	Teal    = ColorImage{RGBAColor{0x00, 0x80, 0x80, 0xff}}
+	// White is an opaque white ColorImage.
 	White = ColorImage{RGBAColor{0xff, 0xff, 0xff, 0xff}}
-	Yellow  = ColorImage{RGBAColor{0xff, 0xff, 0x00, 0xff}}
-
-	// These synonyms are not in HTML 4.01.
-	Cyan    = Aqua
-	Magenta = Fuchsia
 )
 
 // A ColorImage is a practically infinite-sized Image of uniform Color.