image: change image representation from slice-of-slices to linear buffer,

stride and rect.

R=r
CC=golang-dev, rog
https://golang.org/cl/1849041

5 files changed, 301 insertions, 236 deletions

diff --git a/src/pkg/exp/draw/draw.go b/src/pkg/exp/draw/draw.go
index 636501cac5..30dfccdd90 100644
--- a/src/pkg/exp/draw/draw.go
+++ b/src/pkg/exp/draw/draw.go
@@ -167,14 +167,14 @@ func drawFillOver(dst *image.RGBA, r Rectangle, src image.ColorImage) {
 	x0, x1 := r.Min.X, r.Max.X
 	y0, y1 := r.Min.Y, r.Max.Y
 	for y := y0; y != y1; y++ {
-		dpix := dst.Pixel[y]
-		for x := x0; x != x1; x++ {
-			rgba := dpix[x]
+		dbase := y * dst.Stride
+		dpix := dst.Pix[dbase+x0 : dbase+x1]
+		for i, rgba := range dpix {
 			dr := (uint32(rgba.R)*a)/m + cr
 			dg := (uint32(rgba.G)*a)/m + cg
 			db := (uint32(rgba.B)*a)/m + cb
 			da := (uint32(rgba.A)*a)/m + ca
-			dpix[x] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
+			dpix[i] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
 		}
 	}
 }
@@ -183,25 +183,26 @@ func drawCopyOver(dst *image.RGBA, r Rectangle, src *image.RGBA, sp Point) {
 	x0, x1 := r.Min.X, r.Max.X
 	y0, y1 := r.Min.Y, r.Max.Y
 	for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
-		dpix := dst.Pixel[y]
-		spix := src.Pixel[sy]
-		for x, sx := x0, sp.X; x != x1; x, sx = x+1, sx+1 {
-			// For unknown reasons, even though both dpix[x] and spix[sx] are
+		dbase := y * dst.Stride
+		dpix := dst.Pix[dbase+x0 : dbase+x1]
+		sbase := sy * src.Stride
+		spix := src.Pix[sbase+sp.X:]
+		for i, rgba := range dpix {
+			// For unknown reasons, even though both dpix[i] and spix[i] are
 			// image.RGBAColors, on an x86 CPU it seems fastest to call RGBA
 			// for the source but to do it manually for the destination.
-			sr, sg, sb, sa := spix[sx].RGBA()
-			drgba := dpix[x]
-			dr := uint32(drgba.R)
-			dg := uint32(drgba.G)
-			db := uint32(drgba.B)
-			da := uint32(drgba.A)
+			sr, sg, sb, sa := spix[i].RGBA()
+			dr := uint32(rgba.R)
+			dg := uint32(rgba.G)
+			db := uint32(rgba.B)
+			da := uint32(rgba.A)
 			// The 0x101 is here for the same reason as in drawRGBA.
 			a := (m - sa) * 0x101
 			dr = (dr*a)/m + sr
 			dg = (dg*a)/m + sg
 			db = (db*a)/m + sb
 			da = (da*a)/m + sa
-			dpix[x] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
+			dpix[i] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
 		}
 	}
 }
@@ -211,15 +212,16 @@ func drawGlyphOver(dst *image.RGBA, r Rectangle, src image.ColorImage, mask *ima
 	y0, y1 := r.Min.Y, r.Max.Y
 	cr, cg, cb, ca := src.RGBA()
 	for y, my := y0, mp.Y; y != y1; y, my = y+1, my+1 {
-		dpix := dst.Pixel[y]
-		mpix := mask.Pixel[my]
-		for x, mx := x0, mp.X; x != x1; x, mx = x+1, mx+1 {
-			ma := uint32(mpix[mx].A)
+		dbase := y * dst.Stride
+		dpix := dst.Pix[dbase+x0 : dbase+x1]
+		mbase := my * mask.Stride
+		mpix := mask.Pix[mbase+mp.X:]
+		for i, rgba := range dpix {
+			ma := uint32(mpix[i].A)
 			if ma == 0 {
 				continue
 			}
 			ma |= ma << 8
-			rgba := dpix[x]
 			dr := uint32(rgba.R)
 			dg := uint32(rgba.G)
 			db := uint32(rgba.B)
@@ -230,7 +232,7 @@ func drawGlyphOver(dst *image.RGBA, r Rectangle, src image.ColorImage, mask *ima
 			dg = (dg*a + cg*ma) / m
 			db = (db*a + cb*ma) / m
 			da = (da*a + ca*ma) / m
-			dpix[x] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
+			dpix[i] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
 		}
 	}
 }
@@ -246,13 +248,16 @@ func drawFillSrc(dst *image.RGBA, r Rectangle, src image.ColorImage) {
 	// then use the first row as the slice source for the remaining rows.
 	dx0, dx1 := r.Min.X, r.Max.X
 	dy0, dy1 := r.Min.Y, r.Max.Y
-	firstRow := dst.Pixel[dy0]
-	for x := dx0; x < dx1; x++ {
-		firstRow[x] = color
+	dbase := dy0 * dst.Stride
+	i0, i1 := dbase+dx0, dbase+dx1
+	firstRow := dst.Pix[i0:i1]
+	for i, _ := range firstRow {
+		firstRow[i] = color
 	}
-	copySrc := firstRow[dx0:dx1]
 	for y := dy0 + 1; y < dy1; y++ {
-		copy(dst.Pixel[y][dx0:dx1], copySrc)
+		i0 += dst.Stride
+		i1 += dst.Stride
+		copy(dst.Pix[i0:i1], firstRow)
 	}
 }
 
@@ -260,8 +265,16 @@ func drawCopySrc(dst *image.RGBA, r Rectangle, src *image.RGBA, sp Point) {
 	dx0, dx1 := r.Min.X, r.Max.X
 	dy0, dy1 := r.Min.Y, r.Max.Y
 	sx0, sx1 := sp.X, sp.X+dx1-dx0
-	for y, sy := dy0, sp.Y; y < dy1; y, sy = y+1, sy+1 {
-		copy(dst.Pixel[y][dx0:dx1], src.Pixel[sy][sx0:sx1])
+	d0 := dy0*dst.Stride + dx0
+	d1 := dy0*dst.Stride + dx1
+	s0 := sp.Y*dst.Stride + sx0
+	s1 := sp.Y*dst.Stride + sx1
+	for y := dy0; y < dy1; y++ {
+		copy(dst.Pix[d0:d1], src.Pix[s0:s1])
+		d0 += dst.Stride
+		d1 += dst.Stride
+		s0 += src.Stride
+		s1 += src.Stride
 	}
 }
 
@@ -280,8 +293,9 @@ func drawRGBA(dst *image.RGBA, r Rectangle, src image.Image, sp Point, mask imag
 	for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
 		sx := sp.X + x0 - r.Min.X
 		mx := mp.X + x0 - r.Min.X
-		dpix := dst.Pixel[y]
-		for x := x0; x != x1; x, sx, mx = x+dx, sx+dx, mx+dx {
+		dbase := y * dst.Stride
+		dpix := dst.Pix[dbase+x0 : dbase+x1]
+		for i, rgba := range dpix {
 			ma := uint32(m)
 			if mask != nil {
 				_, _, _, ma = mask.At(mx, my).RGBA()
@@ -289,7 +303,6 @@ func drawRGBA(dst *image.RGBA, r Rectangle, src image.Image, sp Point, mask imag
 			sr, sg, sb, sa := src.At(sx, sy).RGBA()
 			var dr, dg, db, da uint32
 			if op == Over {
-				rgba := dpix[x]
 				dr = uint32(rgba.R)
 				dg = uint32(rgba.G)
 				db = uint32(rgba.B)
@@ -311,7 +324,8 @@ func drawRGBA(dst *image.RGBA, r Rectangle, src image.Image, sp Point, mask imag
 				db = sb * ma / m
 				da = sa * ma / m
 			}
-			dpix[x] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
+			dpix[i] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
+			sx, mx = sx+dx, mx+dx
 		}
 	}
 }
diff --git a/src/pkg/exp/draw/x11/conn.go b/src/pkg/exp/draw/x11/conn.go
index eb498cfb8d..7436e19d5e 100644
--- a/src/pkg/exp/draw/x11/conn.go
+++ b/src/pkg/exp/draw/x11/conn.go
@@ -99,7 +99,7 @@ func (c *conn) flusher() {
 				close(c.flush)
 				return
 			}
-			p := c.img.Pixel[y]
+			p := c.img.Pix[y*c.img.Stride : (y+1)*c.img.Stride]
 			for x := b.Min.X; x < b.Max.X; {
 				nx := b.Max.X - x
 				if nx > len(c.flushBuf1)/4 {
diff --git a/src/pkg/image/geom.go b/src/pkg/image/geom.go
index ecf0521cb4..ad0f987cb8 100644
--- a/src/pkg/image/geom.go
+++ b/src/pkg/image/geom.go
@@ -97,6 +97,12 @@ func (r Rectangle) Overlaps(s Rectangle) bool {
 		r.Min.Y < s.Max.Y && s.Min.Y < r.Max.Y
 }
 
+// Contains returns whether r contains p.
+func (r Rectangle) Contains(p Point) bool {
+	return p.X >= r.Min.X && p.X < r.Max.X &&
+		p.Y >= r.Min.Y && p.Y < r.Max.Y
+}
+
 // Canon returns the canonical version of r. The returned rectangle has
 // minimum and maximum coordinates swapped if necessary so that Min.X <= Max.X
 // and Min.Y <= Max.Y.
diff --git a/src/pkg/image/image.go b/src/pkg/image/image.go
index c352da25cc..b7e62bfae4 100644
--- a/src/pkg/image/image.go
+++ b/src/pkg/image/image.go
@@ -18,46 +18,48 @@ type Image interface {
 	At(x, y int) Color
 }
 
-// An RGBA is an in-memory image backed by a 2-D slice of RGBAColor values.
+// An RGBA is an in-memory image of RGBAColor values.
 type RGBA struct {
-	// The Pixel field's indices are y first, then x, so that At(x, y) == Pixel[y][x].
-	Pixel [][]RGBAColor
+	// Pix holds the image's pixels. The pixel at (x, y) is Pix[y*Stride+x].
+	Pix    []RGBAColor
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
 }
 
 func (p *RGBA) ColorModel() ColorModel { return RGBAColorModel }
 
-func (p *RGBA) Bounds() Rectangle {
-	if len(p.Pixel) == 0 {
-		return ZR
-	}
-	return Rectangle{ZP, Point{len(p.Pixel[0]), len(p.Pixel)}}
-}
+func (p *RGBA) Bounds() Rectangle { return p.Rect }
 
 func (p *RGBA) At(x, y int) Color {
-	// TODO(nigeltao): Check if (x,y) is outside the bounds, and return zero.
-	// Similarly for the other concrete image types.
-	return p.Pixel[y][x]
+	if !p.Rect.Contains(Point{x, y}) {
+		return RGBAColor{}
+	}
+	return p.Pix[y*p.Stride+x]
 }
 
 func (p *RGBA) Set(x, y int, c Color) {
-	// TODO(nigeltao): Check if (x,y) is outside the bounds, and return.
-	// Similarly for the other concrete image types.
-	p.Pixel[y][x] = toRGBAColor(c).(RGBAColor)
+	if !p.Rect.Contains(Point{x, y}) {
+		return
+	}
+	p.Pix[y*p.Stride+x] = toRGBAColor(c).(RGBAColor)
 }
 
 // Opaque scans the entire image and returns whether or not it is fully opaque.
 func (p *RGBA) 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 != 0xff {
-					return false
-				}
+	if p.Rect.Empty() {
+		return true
+	}
+	base := p.Rect.Min.Y * p.Stride
+	i0, i1 := base+p.Rect.Min.X, base+p.Rect.Max.X
+	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
+		for _, c := range p.Pix[i0:i1] {
+			if c.A != 0xff {
+				return false
 			}
 		}
+		i0 += p.Stride
+		i1 += p.Stride
 	}
 	return true
 }
@@ -65,261 +67,295 @@ func (p *RGBA) Opaque() bool {
 // NewRGBA returns a new RGBA with the given width and height.
 func NewRGBA(w, h int) *RGBA {
 	buf := make([]RGBAColor, w*h)
-	pix := make([][]RGBAColor, h)
-	for y := range pix {
-		pix[y] = buf[w*y : w*(y+1)]
-	}
-	return &RGBA{pix}
+	return &RGBA{buf, w, Rectangle{ZP, Point{w, h}}}
 }
 
-// An RGBA64 is an in-memory image backed by a 2-D slice of RGBA64Color values.
+// An RGBA64 is an in-memory image of RGBA64Color values.
 type RGBA64 struct {
-	// The Pixel field's indices are y first, then x, so that At(x, y) == Pixel[y][x].
-	Pixel [][]RGBA64Color
+	// Pix holds the image's pixels. The pixel at (x, y) is Pix[y*Stride+x].
+	Pix    []RGBA64Color
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
 }
 
 func (p *RGBA64) ColorModel() ColorModel { return RGBA64ColorModel }
 
-func (p *RGBA64) Bounds() Rectangle {
-	if len(p.Pixel) == 0 {
-		return ZR
+func (p *RGBA64) Bounds() Rectangle { return p.Rect }
+
+func (p *RGBA64) At(x, y int) Color {
+	if !p.Rect.Contains(Point{x, y}) {
+		return RGBA64Color{}
 	}
-	return Rectangle{ZP, Point{len(p.Pixel[0]), len(p.Pixel)}}
+	return p.Pix[y*p.Stride+x]
 }
 
-func (p *RGBA64) At(x, y int) Color { return p.Pixel[y][x] }
-
-func (p *RGBA64) Set(x, y int, c Color) { p.Pixel[y][x] = toRGBA64Color(c).(RGBA64Color) }
+func (p *RGBA64) Set(x, y int, c Color) {
+	if !p.Rect.Contains(Point{x, y}) {
+		return
+	}
+	p.Pix[y*p.Stride+x] = toRGBA64Color(c).(RGBA64Color)
+}
 
 // Opaque scans the entire image and returns whether or not it is fully opaque.
 func (p *RGBA64) 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
-				}
+	if p.Rect.Empty() {
+		return true
+	}
+	base := p.Rect.Min.Y * p.Stride
+	i0, i1 := base+p.Rect.Min.X, base+p.Rect.Max.X
+	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
+		for _, c := range p.Pix[i0:i1] {
+			if c.A != 0xffff {
+				return false
 			}
 		}
+		i0 += p.Stride
+		i1 += p.Stride
 	}
 	return true
 }
 
 // NewRGBA64 returns a new RGBA64 with the given width and height.
 func NewRGBA64(w, h int) *RGBA64 {
-	buf := make([]RGBA64Color, w*h)
-	pix := make([][]RGBA64Color, h)
-	for y := range pix {
-		pix[y] = buf[w*y : w*(y+1)]
-	}
-	return &RGBA64{pix}
+	pix := make([]RGBA64Color, w*h)
+	return &RGBA64{pix, w, Rectangle{ZP, Point{w, h}}}
 }
 
-// A NRGBA is an in-memory image backed by a 2-D slice of NRGBAColor values.
+// An NRGBA is an in-memory image of NRGBAColor values.
 type NRGBA struct {
-	// The Pixel field's indices are y first, then x, so that At(x, y) == Pixel[y][x].
-	Pixel [][]NRGBAColor
+	// Pix holds the image's pixels. The pixel at (x, y) is Pix[y*Stride+x].
+	Pix    []NRGBAColor
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
 }
 
 func (p *NRGBA) ColorModel() ColorModel { return NRGBAColorModel }
 
-func (p *NRGBA) Bounds() Rectangle {
-	if len(p.Pixel) == 0 {
-		return ZR
+func (p *NRGBA) Bounds() Rectangle { return p.Rect }
+
+func (p *NRGBA) At(x, y int) Color {
+	if !p.Rect.Contains(Point{x, y}) {
+		return NRGBAColor{}
 	}
-	return Rectangle{ZP, Point{len(p.Pixel[0]), len(p.Pixel)}}
+	return p.Pix[y*p.Stride+x]
 }
 
-func (p *NRGBA) At(x, y int) Color { return p.Pixel[y][x] }
-
-func (p *NRGBA) Set(x, y int, c Color) { p.Pixel[y][x] = toNRGBAColor(c).(NRGBAColor) }
+func (p *NRGBA) Set(x, y int, c Color) {
+	if !p.Rect.Contains(Point{x, y}) {
+		return
+	}
+	p.Pix[y*p.Stride+x] = toNRGBAColor(c).(NRGBAColor)
+}
 
 // Opaque scans the entire image and returns whether or not it is fully opaque.
 func (p *NRGBA) 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 != 0xff {
-					return false
-				}
+	if p.Rect.Empty() {
+		return true
+	}
+	base := p.Rect.Min.Y * p.Stride
+	i0, i1 := base+p.Rect.Min.X, base+p.Rect.Max.X
+	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
+		for _, c := range p.Pix[i0:i1] {
+			if c.A != 0xff {
+				return false
 			}
 		}
+		i0 += p.Stride
+		i1 += p.Stride
 	}
 	return true
 }
 
 // NewNRGBA returns a new NRGBA with the given width and height.
 func NewNRGBA(w, h int) *NRGBA {
-	buf := make([]NRGBAColor, w*h)
-	pix := make([][]NRGBAColor, h)
-	for y := range pix {
-		pix[y] = buf[w*y : w*(y+1)]
-	}
-	return &NRGBA{pix}
+	pix := make([]NRGBAColor, w*h)
+	return &NRGBA{pix, w, Rectangle{ZP, Point{w, h}}}
 }
 
-// A NRGBA64 is an in-memory image backed by a 2-D slice of NRGBA64Color values.
+// An NRGBA64 is an in-memory image of NRGBA64Color values.
 type NRGBA64 struct {
-	// The Pixel field's indices are y first, then x, so that At(x, y) == Pixel[y][x].
-	Pixel [][]NRGBA64Color
+	// Pix holds the image's pixels. The pixel at (x, y) is Pix[y*Stride+x].
+	Pix    []NRGBA64Color
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
 }
 
 func (p *NRGBA64) ColorModel() ColorModel { return NRGBA64ColorModel }
 
-func (p *NRGBA64) Bounds() Rectangle {
-	if len(p.Pixel) == 0 {
-		return ZR
+func (p *NRGBA64) Bounds() Rectangle { return p.Rect }
+
+func (p *NRGBA64) At(x, y int) Color {
+	if !p.Rect.Contains(Point{x, y}) {
+		return NRGBA64Color{}
 	}
-	return Rectangle{ZP, Point{len(p.Pixel[0]), len(p.Pixel)}}
+	return p.Pix[y*p.Stride+x]
 }
 
-func (p *NRGBA64) At(x, y int) Color { return p.Pixel[y][x] }
-
-func (p *NRGBA64) Set(x, y int, c Color) { p.Pixel[y][x] = toNRGBA64Color(c).(NRGBA64Color) }
+func (p *NRGBA64) Set(x, y int, c Color) {
+	if !p.Rect.Contains(Point{x, y}) {
+		return
+	}
+	p.Pix[y*p.Stride+x] = toNRGBA64Color(c).(NRGBA64Color)
+}
 
 // Opaque scans the entire image and returns whether or not it is fully opaque.
 func (p *NRGBA64) 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
-				}
+	if p.Rect.Empty() {
+		return true
+	}
+	base := p.Rect.Min.Y * p.Stride
+	i0, i1 := base+p.Rect.Min.X, base+p.Rect.Max.X
+	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
+		for _, c := range p.Pix[i0:i1] {
+			if c.A != 0xffff {
+				return false
 			}
 		}
+		i0 += p.Stride
+		i1 += p.Stride
 	}
 	return true
 }
 
 // NewNRGBA64 returns a new NRGBA64 with the given width and height.
 func NewNRGBA64(w, h int) *NRGBA64 {
-	buf := make([]NRGBA64Color, w*h)
-	pix := make([][]NRGBA64Color, h)
-	for y := range pix {
-		pix[y] = buf[w*y : w*(y+1)]
-	}
-	return &NRGBA64{pix}
+	pix := make([]NRGBA64Color, w*h)
+	return &NRGBA64{pix, w, Rectangle{ZP, Point{w, h}}}
 }
 
-// An Alpha is an in-memory image backed by a 2-D slice of AlphaColor values.
+// An Alpha is an in-memory image 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
+	// Pix holds the image's pixels. The pixel at (x, y) is Pix[y*Stride+x].
+	Pix    []AlphaColor
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
 }
 
 func (p *Alpha) ColorModel() ColorModel { return AlphaColorModel }
 
-func (p *Alpha) Bounds() Rectangle {
-	if len(p.Pixel) == 0 {
-		return ZR
+func (p *Alpha) Bounds() Rectangle { return p.Rect }
+
+func (p *Alpha) At(x, y int) Color {
+	if !p.Rect.Contains(Point{x, y}) {
+		return AlphaColor{}
 	}
-	return Rectangle{ZP, Point{len(p.Pixel[0]), len(p.Pixel)}}
+	return p.Pix[y*p.Stride+x]
 }
 
-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) }
+func (p *Alpha) Set(x, y int, c Color) {
+	if !p.Rect.Contains(Point{x, y}) {
+		return
+	}
+	p.Pix[y*p.Stride+x] = toAlphaColor(c).(AlphaColor)
+}
 
 // Opaque scans the entire image and returns whether or not it is fully opaque.
 func (p *Alpha) 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 != 0xff {
-					return false
-				}
+	if p.Rect.Empty() {
+		return true
+	}
+	base := p.Rect.Min.Y * p.Stride
+	i0, i1 := base+p.Rect.Min.X, base+p.Rect.Max.X
+	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
+		for _, c := range p.Pix[i0:i1] {
+			if c.A != 0xff {
+				return false
 			}
 		}
+		i0 += p.Stride
+		i1 += p.Stride
 	}
 	return true
 }
 
 // NewAlpha returns a new Alpha with the given width and height.
 func NewAlpha(w, h int) *Alpha {
-	buf := make([]AlphaColor, w*h)
-	pix := make([][]AlphaColor, h)
-	for y := range pix {
-		pix[y] = buf[w*y : w*(y+1)]
-	}
-	return &Alpha{pix}
+	pix := make([]AlphaColor, w*h)
+	return &Alpha{pix, w, Rectangle{ZP, Point{w, h}}}
 }
 
-// An Alpha16 is an in-memory image backed by a 2-D slice of Alpha16Color values.
+// An Alpha16 is an in-memory image 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
+	// Pix holds the image's pixels. The pixel at (x, y) is Pix[y*Stride+x].
+	Pix    []Alpha16Color
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
 }
 
 func (p *Alpha16) ColorModel() ColorModel { return Alpha16ColorModel }
 
-func (p *Alpha16) Bounds() Rectangle {
-	if len(p.Pixel) == 0 {
-		return ZR
+func (p *Alpha16) Bounds() Rectangle { return p.Rect }
+
+func (p *Alpha16) At(x, y int) Color {
+	if !p.Rect.Contains(Point{x, y}) {
+		return Alpha16Color{}
 	}
-	return Rectangle{ZP, Point{len(p.Pixel[0]), len(p.Pixel)}}
+	return p.Pix[y*p.Stride+x]
 }
 
-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) }
+func (p *Alpha16) Set(x, y int, c Color) {
+	if !p.Rect.Contains(Point{x, y}) {
+		return
+	}
+	p.Pix[y*p.Stride+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
-				}
+	if p.Rect.Empty() {
+		return true
+	}
+	base := p.Rect.Min.Y * p.Stride
+	i0, i1 := base+p.Rect.Min.X, base+p.Rect.Max.X
+	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
+		for _, c := range p.Pix[i0:i1] {
+			if c.A != 0xffff {
+				return false
 			}
 		}
+		i0 += p.Stride
+		i1 += p.Stride
 	}
 	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}
+	pix := make([]Alpha16Color, w*h)
+	return &Alpha16{pix, w, Rectangle{ZP, Point{w, h}}}
 }
 
-// A Gray is an in-memory image backed by a 2-D slice of GrayColor values.
+// An Gray is an in-memory image 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
+	// Pix holds the image's pixels. The pixel at (x, y) is Pix[y*Stride+x].
+	Pix    []GrayColor
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
 }
 
 func (p *Gray) ColorModel() ColorModel { return GrayColorModel }
 
-func (p *Gray) Bounds() Rectangle {
-	if len(p.Pixel) == 0 {
-		return ZR
+func (p *Gray) Bounds() Rectangle { return p.Rect }
+
+func (p *Gray) At(x, y int) Color {
+	if !p.Rect.Contains(Point{x, y}) {
+		return GrayColor{}
 	}
-	return Rectangle{ZP, Point{len(p.Pixel[0]), len(p.Pixel)}}
+	return p.Pix[y*p.Stride+x]
 }
 
-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) }
+func (p *Gray) Set(x, y int, c Color) {
+	if !p.Rect.Contains(Point{x, y}) {
+		return
+	}
+	p.Pix[y*p.Stride+x] = toGrayColor(c).(GrayColor)
+}
 
 // Opaque scans the entire image and returns whether or not it is fully opaque.
 func (p *Gray) Opaque() bool {
@@ -328,32 +364,36 @@ func (p *Gray) Opaque() bool {
 
 // 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}
+	pix := make([]GrayColor, w*h)
+	return &Gray{pix, w, Rectangle{ZP, Point{w, h}}}
 }
 
-// A Gray16 is an in-memory image backed by a 2-D slice of Gray16Color values.
+// An Gray16 is an in-memory image 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
+	// Pix holds the image's pixels. The pixel at (x, y) is Pix[y*Stride+x].
+	Pix    []Gray16Color
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
 }
 
 func (p *Gray16) ColorModel() ColorModel { return Gray16ColorModel }
 
-func (p *Gray16) Bounds() Rectangle {
-	if len(p.Pixel) == 0 {
-		return ZR
+func (p *Gray16) Bounds() Rectangle { return p.Rect }
+
+func (p *Gray16) At(x, y int) Color {
+	if !p.Rect.Contains(Point{x, y}) {
+		return Gray16Color{}
 	}
-	return Rectangle{ZP, Point{len(p.Pixel[0]), len(p.Pixel)}}
+	return p.Pix[y*p.Stride+x]
 }
 
-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) }
+func (p *Gray16) Set(x, y int, c Color) {
+	if !p.Rect.Contains(Point{x, y}) {
+		return
+	}
+	p.Pix[y*p.Stride+x] = toGray16Color(c).(Gray16Color)
+}
 
 // Opaque scans the entire image and returns whether or not it is fully opaque.
 func (p *Gray16) Opaque() bool {
@@ -362,12 +402,8 @@ func (p *Gray16) Opaque() bool {
 
 // 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}
+	pix := make([]Gray16Color, w*h)
+	return &Gray16{pix, w, Rectangle{ZP, Point{w, h}}}
 }
 
 // A PalettedColorModel represents a fixed palette of colors.
@@ -409,28 +445,41 @@ func (p PalettedColorModel) Convert(c Color) Color {
 
 // A Paletted is an in-memory image backed by a 2-D slice of uint8 values and a PalettedColorModel.
 type Paletted struct {
-	// The Pixel field's indices are y first, then x, so that At(x, y) == Palette[Pixel[y][x]].
-	Pixel   [][]uint8
+	// Pix holds the image's pixels. The pixel at (x, y) is Pix[y*Stride+x].
+	Pix    []uint8
+	Stride int
+	// Rect is the image's bounds.
+	Rect Rectangle
+	// Palette is the image's palette.
 	Palette PalettedColorModel
 }
 
 func (p *Paletted) ColorModel() ColorModel { return p.Palette }
 
-func (p *Paletted) Bounds() Rectangle {
-	if len(p.Pixel) == 0 {
-		return ZR
+func (p *Paletted) Bounds() Rectangle { return p.Rect }
+
+func (p *Paletted) At(x, y int) Color {
+	if len(p.Palette) == 0 {
+		return nil
+	}
+	if !p.Rect.Contains(Point{x, y}) {
+		return p.Palette[0]
 	}
-	return Rectangle{ZP, Point{len(p.Pixel[0]), len(p.Pixel)}}
+	return p.Palette[p.Pix[y*p.Stride+x]]
 }
 
-func (p *Paletted) At(x, y int) Color { return p.Palette[p.Pixel[y][x]] }
-
 func (p *Paletted) ColorIndexAt(x, y int) uint8 {
-	return p.Pixel[y][x]
+	if !p.Rect.Contains(Point{x, y}) {
+		return 0
+	}
+	return p.Pix[y*p.Stride+x]
 }
 
 func (p *Paletted) SetColorIndex(x, y int, index uint8) {
-	p.Pixel[y][x] = index
+	if !p.Rect.Contains(Point{x, y}) {
+		return
+	}
+	p.Pix[y*p.Stride+x] = index
 }
 
 // Opaque scans the entire image and returns whether or not it is fully opaque.
@@ -446,10 +495,6 @@ func (p *Paletted) Opaque() bool {
 
 // NewPaletted returns a new Paletted with the given width, height and palette.
 func NewPaletted(w, h int, m PalettedColorModel) *Paletted {
-	buf := make([]uint8, w*h)
-	pix := make([][]uint8, h)
-	for y := range pix {
-		pix[y] = buf[w*y : w*(y+1)]
-	}
-	return &Paletted{pix, m}
+	pix := make([]uint8, w*h)
+	return &Paletted{pix, w, Rectangle{ZP, Point{w, h}}, m}
 }
diff --git a/src/pkg/image/jpeg/reader.go b/src/pkg/image/jpeg/reader.go
index 55cc89aa31..5becf295b4 100644
--- a/src/pkg/image/jpeg/reader.go
+++ b/src/pkg/image/jpeg/reader.go
@@ -206,7 +206,7 @@ func (d *decoder) calcPixel(px, py, lumaBlock, lumaIndex, chromaIndex int) {
 	} else if b > 255 {
 		b = 255
 	}
-	d.image.Pixel[py][px] = image.RGBAColor{uint8(r), uint8(g), uint8(b), 0xff}
+	d.image.Pix[py*d.image.Stride+px] = image.RGBAColor{uint8(r), uint8(g), uint8(b), 0xff}
 }
 
 // Convert the MCU from YCbCr to RGB.