image: add an offset to Tiled.

R=r, r2
CC=golang-dev
https://golang.org/cl/2469041

src/pkg/image/geom.go

@@ -43,15 +43,13 @@ func (p Point) Div(k int) Point {
 func (p Point) Mod(r Rectangle) Point {
 	w, h := r.Dx(), r.Dy()
 	p = p.Sub(r.Min)
-	if p.X >= 0 {
-		p.X = p.X % w
-	} else {
-		p.X = w - 1 - (-1-p.X)%w
+	p.X = p.X % w
+	if p.X < 0 {
+		p.X += w
 	}
-	if p.Y >= 0 {
-		p.Y = p.Y % h
-	} else {
-		p.Y = h - 1 - (-1-p.Y)%h
+	p.Y = p.Y % h
+	if p.Y < 0 {
+		p.Y += h
 	}
 	return p.Add(r.Min)
 }

src/pkg/image/names.go

@@ -15,7 +15,7 @@ var (
 	Opaque = NewColorImage(Alpha16Color{0xffff})
 )
 
-// A ColorImage is a practically infinite-sized Image of uniform Color.
+// A ColorImage is an infinite-sized Image of uniform Color.
 // It implements both the Color and Image interfaces.
 type ColorImage struct {
 	C Color
@@ -43,11 +43,12 @@ func NewColorImage(c Color) *ColorImage {
 	return &ColorImage{c}
 }
 
-// A Tiled is a practically infinite-sized Image that repeats another Image in
-// both directions. Tiled{i}.At(x, y) will equal i.At(x, y) for all points
-// within i's Bounds.
+// A Tiled is an infinite-sized Image that repeats another Image in both
+// directions. Tiled{i, p}.At(x, y) will equal i.At(x+p.X, y+p.Y) for all
+// points {x+p.X, y+p.Y} within i's Bounds.
 type Tiled struct {
-	I Image
+	I      Image
+	Offset Point
 }
 
 func (t *Tiled) ColorModel() ColorModel {
@@ -57,10 +58,10 @@ func (t *Tiled) ColorModel() ColorModel {
 func (t *Tiled) Bounds() Rectangle { return Rectangle{Point{-1e9, -1e9}, Point{1e9, 1e9}} }
 
 func (t *Tiled) At(x, y int) Color {
-	p := Point{x, y}.Mod(t.I.Bounds())
+	p := Point{x, y}.Add(t.Offset).Mod(t.I.Bounds())
 	return t.I.At(p.X, p.Y)
 }
 
-func NewTiled(i Image) *Tiled {
-	return &Tiled{i}
+func NewTiled(i Image, offset Point) *Tiled {
+	return &Tiled{i, offset}
 }