New image.A type, to represent anti-aliased font glyphs.

R=r, rsc
CC=golang-dev
https://golang.org/cl/199052

src/pkg/image/color.go

@@ -100,6 +100,18 @@ func (c NRGBA64Color) RGBA() (r, g, b, a uint32) {
 	return
 }
 
+// An AlphaColor represents an 8-bit alpha.
+type AlphaColor struct {
+	A uint8
+}
+
+func (c AlphaColor) RGBA() (r, g, b, a uint32) {
+	a = uint32(c.A)
+	a |= a << 8
+	a |= a << 16
+	return a, a, a, a
+}
+
 // A ColorModel can convert foreign Colors, with a possible loss of precision, to a Color
 // from its own color model.
 type ColorModel interface {
@@ -176,6 +188,14 @@ func toNRGBA64Color(c Color) Color {
 	return NRGBA64Color{uint16(r), uint16(g), uint16(b), uint16(a)}
 }
 
+func toAlphaColor(c Color) Color {
+	if _, ok := c.(AlphaColor); ok { // no-op conversion
+		return c
+	}
+	_, _, _, a := c.RGBA()
+	return AlphaColor{uint8(a >> 24)}
+}
+
 // The ColorModel associated with RGBAColor.
 var RGBAColorModel ColorModel = ColorModelFunc(toRGBAColor)
 
@@ -187,3 +207,6 @@ var NRGBAColorModel ColorModel = ColorModelFunc(toNRGBAColor)
 
 // The ColorModel associated with NRGBA64Color.
 var NRGBA64ColorModel ColorModel = ColorModelFunc(toNRGBA64Color)
+
+// The ColorModel associated with AlphaColor.
+var AlphaColorModel ColorModel = ColorModelFunc(toAlphaColor)

src/pkg/image/image.go

@@ -135,6 +135,36 @@ func NewNRGBA64(w, h int) *NRGBA64 {
 	return &NRGBA64{pixel}
 }
 
+// An Alpha is an in-memory image backed by a 2-D slice of AlphaColor values.
+type Alpha struct {
+	// The Pixel field's indices are y first, then x, so that At(x, y) == Pixel[y][x].
+	Pixel [][]AlphaColor
+}
+
+func (p *Alpha) ColorModel() ColorModel { return AlphaColorModel }
+
+func (p *Alpha) Width() int {
+	if len(p.Pixel) == 0 {
+		return 0
+	}
+	return len(p.Pixel[0])
+}
+
+func (p *Alpha) Height() int { return len(p.Pixel) }
+
+func (p *Alpha) At(x, y int) Color { return p.Pixel[y][x] }
+
+func (p *Alpha) Set(x, y int, c Color) { p.Pixel[y][x] = toAlphaColor(c).(AlphaColor) }
+
+// NewAlpha returns a new Alpha with the given width and height.
+func NewAlpha(w, h int) *Alpha {
+	pixel := make([][]AlphaColor, h)
+	for y := 0; y < h; y++ {
+		pixel[y] = make([]AlphaColor, w)
+	}
+	return &Alpha{pixel}
+}
+
 // A PalettedColorModel represents a fixed palette of colors.
 type PalettedColorModel []Color