image: add Paletted.Set, and a basic test of the concrete image types.

R=r
CC=golang-dev
https://golang.org/cl/4589045

src/pkg/image/image.go

@@ -563,14 +563,19 @@ func (p PalettedColorModel) Convert(c Color) Color {
 	if len(p) == 0 {
 		return nil
 	}
+	return p[p.Index(c)]
+}
+
+// Index returns the index of the palette color closest to c in Euclidean
+// R,G,B space.
+func (p PalettedColorModel) Index(c Color) int {
 	cr, cg, cb, _ := c.RGBA()
 	// Shift by 1 bit to avoid potential uint32 overflow in sum-squared-difference.
 	cr >>= 1
 	cg >>= 1
 	cb >>= 1
-	result := Color(nil)
-	bestSSD := uint32(1<<32 - 1)
-	for _, v := range p {
+	ret, bestSSD := 0, uint32(1<<32-1)
+	for i, v := range p {
 		vr, vg, vb, _ := v.RGBA()
 		vr >>= 1
 		vg >>= 1
@@ -578,11 +583,10 @@ func (p PalettedColorModel) Convert(c Color) Color {
 		dr, dg, db := diff(cr, vr), diff(cg, vg), diff(cb, vb)
 		ssd := (dr * dr) + (dg * dg) + (db * db)
 		if ssd < bestSSD {
-			bestSSD = ssd
-			result = v
+			ret, bestSSD = i, ssd
 		}
 	}
-	return result
+	return ret
 }
 
 // A Paletted is an in-memory image backed by a 2-D slice of uint8 values and a PalettedColorModel.
@@ -610,6 +614,13 @@ func (p *Paletted) At(x, y int) Color {
 	return p.Palette[p.Pix[y*p.Stride+x]]
 }
 
+func (p *Paletted) Set(x, y int, c Color) {
+	if !(Point{x, y}.In(p.Rect)) {
+		return
+	}
+	p.Pix[y*p.Stride+x] = uint8(p.Palette.Index(c))
+}
+
 func (p *Paletted) ColorIndexAt(x, y int) uint8 {
 	if !(Point{x, y}.In(p.Rect)) {
 		return 0

src/pkg/image/image_test.go

@@ -0,0 +1,71 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package image
+
+import (
+	"testing"
+)
+
+func cmp(t *testing.T, cm ColorModel, c0, c1 Color) bool {
+	r0, g0, b0, a0 := cm.Convert(c0).RGBA()
+	r1, g1, b1, a1 := cm.Convert(c1).RGBA()
+	return r0 == r1 && g0 == g1 && b0 == b1 && a0 == a1
+}
+
+func TestImage(t *testing.T) {
+	type buffered interface {
+		Image
+		Set(int, int, Color)
+		SubImage(Rectangle) Image
+	}
+	testImage := []Image{
+		NewRGBA(10, 10),
+		NewRGBA64(10, 10),
+		NewNRGBA(10, 10),
+		NewNRGBA64(10, 10),
+		NewAlpha(10, 10),
+		NewAlpha16(10, 10),
+		NewGray(10, 10),
+		NewGray16(10, 10),
+		NewPaletted(10, 10, PalettedColorModel{
+			Transparent,
+			Opaque,
+		}),
+	}
+	for _, m := range testImage {
+		b := m.(buffered)
+		if !Rect(0, 0, 10, 10).Eq(b.Bounds()) {
+			t.Errorf("%T: want bounds %v, got %v", b, Rect(0, 0, 10, 10), b.Bounds())
+			continue
+		}
+		if !cmp(t, b.ColorModel(), Transparent, b.At(6, 3)) {
+			t.Errorf("%T: at (6, 3), want a zero color, got %v", b, b.At(6, 3))
+			continue
+		}
+		b.Set(6, 3, Opaque)
+		if !cmp(t, b.ColorModel(), Opaque, b.At(6, 3)) {
+			t.Errorf("%T: at (6, 3), want a non-zero color, got %v", b, b.At(6, 3))
+			continue
+		}
+		b = b.SubImage(Rect(3, 2, 9, 8)).(buffered)
+		if !Rect(3, 2, 9, 8).Eq(b.Bounds()) {
+			t.Errorf("%T: sub-image want bounds %v, got %v", b, Rect(3, 2, 9, 8), b.Bounds())
+			continue
+		}
+		if !cmp(t, b.ColorModel(), Opaque, b.At(6, 3)) {
+			t.Errorf("%T: sub-image at (6, 3), want a non-zero color, got %v", b, b.At(6, 3))
+			continue
+		}
+		if !cmp(t, b.ColorModel(), Transparent, b.At(3, 3)) {
+			t.Errorf("%T: sub-image at (3, 3), want a zero color, got %v", b, b.At(3, 3))
+			continue
+		}
+		b.Set(3, 3, Opaque)
+		if !cmp(t, b.ColorModel(), Opaque, b.At(3, 3)) {
+			t.Errorf("%T: sub-image at (3, 3), want a non-zero color, got %v", b, b.At(3, 3))
+			continue
+		}
+	}
+}