draw: implement Copy; add an example_test.

Change-Id: Ia9cceac17c0326702530eac3a205308b02b85986
Reviewed-on: https://go-review.googlesource.com/7262
Reviewed-by: Rob Pike <r@golang.org>

draw/example_test.go

@@ -0,0 +1,66 @@
+// Copyright 2015 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 draw
+
+import (
+	"fmt"
+	"image"
+	"image/png"
+	"log"
+	"math"
+	"os"
+
+	"golang.org/x/image/math/f64"
+)
+
+func ExampleDraw() {
+	fSrc, err := os.Open("../testdata/blue-purple-pink.png")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer fSrc.Close()
+	src, err := png.Decode(fSrc)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	sr := src.Bounds()
+	dst := image.NewRGBA(image.Rect(0, 0, 400, 300))
+	qs := []Interpolator{
+		NearestNeighbor,
+		ApproxBiLinear,
+		CatmullRom,
+	}
+	c, s := math.Cos(math.Pi/3), math.Sin(math.Pi/3)
+	t := &f64.Aff3{
+		+2 * c, -2 * s, 100,
+		+2 * s, +2 * c, 100,
+	}
+
+	Copy(dst, image.Point{20, 30}, src, sr, nil)
+	for i, q := range qs {
+		q.Scale(dst, image.Rect(200+10*i, 100*i, 600+10*i, 150+100*i), src, sr, nil)
+	}
+	// TODO: delete the "_ = t" and uncomment this when Transform is implemented.
+	// NearestNeighbor.Transform(dst, t, src, sr, nil)
+	_ = t
+
+	// Change false to true to write the resultant image to disk.
+	if false {
+		fDst, err := os.Create("out.png")
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer fDst.Close()
+		err = png.Encode(fDst, dst)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+
+	fmt.Printf("dst has bounds %v.\n", dst.Bounds())
+	// Output:
+	// dst has bounds (0,0)-(400,300).
+}

draw/scale.go

@@ -13,6 +13,18 @@ import (
 	"golang.org/x/image/math/f64"
 )
 
+// Copy copies the part of the source image defined by src and sr and writes to
+// the part of the destination image defined by dst and the translation of sr
+// so that sr.Min translates to dp.
+func Copy(dst Image, dp image.Point, src image.Image, sr image.Rectangle, opts *Options) {
+	mask, mp, op := image.Image(nil), image.Point{}, Over
+	if opts != nil {
+		// TODO: set mask, mp and op.
+	}
+	dr := sr.Add(dp.Sub(sr.Min))
+	DrawMask(dst, dr, src, sr.Min, mask, mp, op)
+}
+
 // Scaler scales the part of the source image defined by src and sr and writes
 // to the part of the destination image defined by dst and dr.
 //
@@ -38,7 +50,7 @@ type Transformer interface {
 	Transform(dst Image, m *f64.Aff3, src image.Image, sr image.Rectangle, opts *Options)
 }
 
-// Options are optional parameters to Scale and Transform.
+// Options are optional parameters to Copy, Scale and Transform.
 //
 // A nil *Options means to use the default (zero) values of each field.
 type Options struct {