image: add an Alpha16 type, for completeness.

R=rsc
CC=golang-dev, rog
https://golang.org/cl/1707042

src/pkg/image/color.go

@@ -93,6 +93,16 @@ func (c AlphaColor) RGBA() (r, g, b, a uint32) {
 	return a, a, a, a
 }
 
+// An Alpha16Color represents a 16-bit alpha.
+type Alpha16Color struct {
+	A uint16
+}
+
+func (c Alpha16Color) RGBA() (r, g, b, a uint32) {
+	a = uint32(c.A)
+	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 {
@@ -169,6 +179,14 @@ func toAlphaColor(c Color) Color {
 	return AlphaColor{uint8(a >> 8)}
 }
 
+func toAlpha16Color(c Color) Color {
+	if _, ok := c.(Alpha16Color); ok {
+		return c
+	}
+	_, _, _, a := c.RGBA()
+	return Alpha16Color{uint16(a)}
+}
+
 // The ColorModel associated with RGBAColor.
 var RGBAColorModel ColorModel = ColorModelFunc(toRGBAColor)
 
@@ -183,3 +201,6 @@ var NRGBA64ColorModel ColorModel = ColorModelFunc(toNRGBA64Color)
 
 // The ColorModel associated with AlphaColor.
 var AlphaColorModel ColorModel = ColorModelFunc(toAlphaColor)
+
+// The ColorModel associated with Alpha16Color.
+var Alpha16ColorModel ColorModel = ColorModelFunc(toAlpha16Color)

src/pkg/image/image.go

@@ -255,6 +255,54 @@ func NewAlpha(w, h int) *Alpha {
 	return &Alpha{pix}
 }
 
+// An Alpha16 is an in-memory image backed by a 2-D slice of Alpha16Color values.
+type Alpha16 struct {
+	// The Pixel field's indices are y first, then x, so that At(x, y) == Pixel[y][x].
+	Pixel [][]Alpha16Color
+}
+
+func (p *Alpha16) ColorModel() ColorModel { return Alpha16ColorModel }
+
+func (p *Alpha16) Width() int {
+	if len(p.Pixel) == 0 {
+		return 0
+	}
+	return len(p.Pixel[0])
+}
+
+func (p *Alpha16) Height() int { return len(p.Pixel) }
+
+func (p *Alpha16) At(x, y int) Color { return p.Pixel[y][x] }
+
+func (p *Alpha16) Set(x, y int, c Color) { p.Pixel[y][x] = toAlpha16Color(c).(Alpha16Color) }
+
+// Opaque scans the entire image and returns whether or not it is fully opaque.
+func (p *Alpha16) Opaque() bool {
+	h := len(p.Pixel)
+	if h > 0 {
+		w := len(p.Pixel[0])
+		for y := 0; y < h; y++ {
+			pix := p.Pixel[y]
+			for x := 0; x < w; x++ {
+				if pix[x].A != 0xffff {
+					return false
+				}
+			}
+		}
+	}
+	return true
+}
+
+// NewAlpha16 returns a new Alpha16 with the given width and height.
+func NewAlpha16(w, h int) *Alpha16 {
+	buf := make([]Alpha16Color, w*h)
+	pix := make([][]Alpha16Color, h)
+	for y := range pix {
+		pix[y] = buf[w*y : w*(y+1)]
+	}
+	return &Alpha16{pix}
+}
+
 // A PalettedColorModel represents a fixed palette of colors.
 type PalettedColorModel []Color