draw: alias the standard library's image/draw's exported types.

This relies on type aliases, a language feature new in Go 1.9.

The package documentation, in draw.go, explicitly gives the intent of
this package:

    This package is a superset of and a drop-in replacement for the
    image/draw package in the standard library.

Drop-in replacement means that I can replace all of my "image/draw"
imports with "golang.org/x/image/draw", to access additional features in
this package, and no further changes are required. That's mostly true,
but not completely true unless we use type aliases.

Without type aliases, users might need to import both "image/draw" and
"golang.org/x/image/draw" in order to convert from two conceptually
equivalent but different (from the compiler's point of view) types, such
as from one draw.Op type to another draw.Op type, to satisfy some other
interface or function signature.

Change-Id: Ice6d000d49b019c2d8761739a904232e9cd01cae
Reviewed-on: https://go-review.googlesource.com/36730
Run-TryBot: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

draw/draw.go

@@ -11,12 +11,12 @@
 // package in the standard library.
 package draw
 
-// This file just contains the API exported by the image/draw package in the
-// standard library. Other files in this package provide additional features.
+// This file, and the go1_*.go files, just contains the API exported by the
+// image/draw package in the standard library. Other files in this package
+// provide additional features.
 
 import (
 	"image"
-	"image/color"
 	"image/draw"
 )
 
@@ -32,13 +32,6 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
 	draw.DrawMask(dst, r, src, sp, mask, mp, draw.Op(op))
 }
 
-// Drawer contains the Draw method.
-type Drawer interface {
-	// Draw aligns r.Min in dst with sp in src and then replaces the
-	// rectangle r in dst with the result of drawing src on dst.
-	Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point)
-}
-
 // FloydSteinberg is a Drawer that is the Src Op with Floyd-Steinberg error
 // diffusion.
 var FloydSteinberg Drawer = floydSteinberg{}
@@ -48,32 +41,3 @@ type floydSteinberg struct{}
 func (floydSteinberg) Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point) {
 	draw.FloydSteinberg.Draw(dst, r, src, sp)
 }
-
-// Image is an image.Image with a Set method to change a single pixel.
-type Image interface {
-	image.Image
-	Set(x, y int, c color.Color)
-}
-
-// Op is a Porter-Duff compositing operator.
-type Op int
-
-const (
-	// Over specifies ``(src in mask) over dst''.
-	Over Op = Op(draw.Over)
-	// Src specifies ``src in mask''.
-	Src Op = Op(draw.Src)
-)
-
-// Draw implements the Drawer interface by calling the Draw function with
-// this Op.
-func (op Op) Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point) {
-	(draw.Op(op)).Draw(dst, r, src, sp)
-}
-
-// Quantizer produces a palette for an image.
-type Quantizer interface {
-	// Quantize appends up to cap(p) - len(p) colors to p and returns the
-	// updated palette suitable for converting m to a paletted image.
-	Quantize(p color.Palette, m image.Image) color.Palette
-}

draw/go1_8.go

@@ -0,0 +1,49 @@
+// 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.
+
+// +build !go1.9
+
+package draw
+
+import (
+	"image"
+	"image/color"
+	"image/draw"
+)
+
+// Drawer contains the Draw method.
+type Drawer interface {
+	// Draw aligns r.Min in dst with sp in src and then replaces the
+	// rectangle r in dst with the result of drawing src on dst.
+	Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point)
+}
+
+// Image is an image.Image with a Set method to change a single pixel.
+type Image interface {
+	image.Image
+	Set(x, y int, c color.Color)
+}
+
+// Op is a Porter-Duff compositing operator.
+type Op int
+
+const (
+	// Over specifies ``(src in mask) over dst''.
+	Over Op = Op(draw.Over)
+	// Src specifies ``src in mask''.
+	Src Op = Op(draw.Src)
+)
+
+// Draw implements the Drawer interface by calling the Draw function with
+// this Op.
+func (op Op) Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point) {
+	(draw.Op(op)).Draw(dst, r, src, sp)
+}
+
+// Quantizer produces a palette for an image.
+type Quantizer interface {
+	// Quantize appends up to cap(p) - len(p) colors to p and returns the
+	// updated palette suitable for converting m to a paletted image.
+	Quantize(p color.Palette, m image.Image) color.Palette
+}

draw/go1_9.go

@@ -0,0 +1,57 @@
+// Copyright 2016 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.
+
+// +build go1.9
+
+package draw
+
+import (
+	"image/draw"
+)
+
+// We use type aliases (new in Go 1.9) for the exported names from the standard
+// library's image/draw package. This is not merely syntactic sugar for
+//
+//	type Drawer draw.Drawer
+//
+// as aliasing means that the types in this package, such as draw.Image and
+// draw.Op, are identical to the corresponding draw.Image and draw.Op types in
+// the standard library. In comparison, prior to Go 1.9, the code in go1_8.go
+// defines new types that mimic the old but are different types.
+//
+// The package documentation, in draw.go, explicitly gives the intent of this
+// package:
+//
+//	This package is a superset of and a drop-in replacement for the
+//	image/draw package in the standard library.
+//
+// Drop-in replacement means that I can replace all of my "image/draw" imports
+// with "golang.org/x/image/draw", to access additional features in this
+// package, and no further changes are required. That's mostly true, but not
+// completely true unless we use type aliases.
+//
+// Without type aliases, users might need to import both "image/draw" and
+// "golang.org/x/image/draw" in order to convert from two conceptually
+// equivalent but different (from the compiler's point of view) types, such as
+// from one draw.Op type to another draw.Op type, to satisfy some other
+// interface or function signature.
+
+// Drawer contains the Draw method.
+type Drawer = draw.Drawer
+
+// Image is an image.Image with a Set method to change a single pixel.
+type Image = draw.Image
+
+// Op is a Porter-Duff compositing operator.
+type Op = draw.Op
+
+const (
+	// Over specifies ``(src in mask) over dst''.
+	Over Op = draw.Over
+	// Src specifies ``src in mask''.
+	Src Op = draw.Src
+)
+
+// Quantizer produces a palette for an image.
+type Quantizer = draw.Quantizer