image: add PalettedImage interface, and make image/png recognize it.

R=golang-dev, rsc, bradfitz, nigeltao
CC=golang-dev
https://golang.org/cl/4956046

src/pkg/image/image.go

@@ -24,6 +24,17 @@ type Image interface {
 	At(x, y int) Color
 }
 
+// PalettedImage is an image whose colors may come from a limited palette.
+// If m is a PalettedImage and m.ColorModel() returns a PalettedColorModel p,
+// then m.At(x, y) should be equivalent to p[m.ColorIndexAt(x, y)]. If m's
+// color model is not a PalettedColorModel, then ColorIndexAt's behavior is
+// undefined.
+type PalettedImage interface {
+	// ColorIndexAt returns the palette index of the pixel at (x, y).
+	ColorIndexAt(x, y int) uint8
+	Image
+}
+
 // RGBA is an in-memory image of RGBAColor values.
 type RGBA struct {
 	// Pix holds the image's pixels, in R, G, B, A order. The pixel at
@@ -741,6 +752,9 @@ func (p PalettedColorModel) Index(c Color) int {
 		dr, dg, db := diff(cr, vr), diff(cg, vg), diff(cb, vb)
 		ssd := (dr * dr) + (dg * dg) + (db * db)
 		if ssd < bestSSD {
+			if ssd == 0 {
+				return i
+			}
 			ret, bestSSD = i, ssd
 		}
 	}

src/pkg/image/png/writer.go

@@ -336,9 +336,16 @@ func writeImage(w io.Writer, m image.Image, cb int) os.Error {
 				}
 			}
 		case cbP8:
-			paletted := m.(*image.Paletted)
-			offset := (y - b.Min.Y) * paletted.Stride
-			copy(cr[0][1:], paletted.Pix[offset:offset+b.Dx()])
+			if p, _ := m.(*image.Paletted); p != nil {
+				offset := (y - b.Min.Y) * p.Stride
+				copy(cr[0][1:], p.Pix[offset:offset+b.Dx()])
+			} else {
+				pi := m.(image.PalettedImage)
+				for x := b.Min.X; x < b.Max.X; x++ {
+					cr[0][i] = pi.ColorIndexAt(x, y)
+					i += 1
+				}
+			}
 		case cbTCA8:
 			// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
 			for x := b.Min.X; x < b.Max.X; x++ {
@@ -432,7 +439,12 @@ func Encode(w io.Writer, m image.Image) os.Error {
 	var e encoder
 	e.w = w
 	e.m = m
-	pal, _ := m.(*image.Paletted)
+
+	var pal image.PalettedColorModel
+	// cbP8 encoding needs PalettedImage's ColorIndexAt method.
+	if _, ok := m.(image.PalettedImage); ok {
+		pal, _ = m.ColorModel().(image.PalettedColorModel)
+	}
 	if pal != nil {
 		e.cb = cbP8
 	} else {
@@ -459,8 +471,8 @@ func Encode(w io.Writer, m image.Image) os.Error {
 	_, e.err = io.WriteString(w, pngHeader)
 	e.writeIHDR()
 	if pal != nil {
-		e.writePLTE(pal.Palette)
-		e.maybeWritetRNS(pal.Palette)
+		e.writePLTE(pal)
+		e.maybeWritetRNS(pal)
 	}
 	e.writeIDATs()
 	e.writeIEND()