vector: add example test

Fixes golang/go#45723

Change-Id: Id9131dd690a4496ed5ec8aae3bf4d54f44a53d79
Reviewed-on: https://go-review.googlesource.com/c/image/+/316069
Trust: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

font/sfnt/example_test.go

@@ -25,6 +25,7 @@ func Example_rasterizeGlyph() {
 		originY = 32
 	)
 
+	// Load the 'G' glyph from the Go Regular font.
 	f, err := sfnt.Parse(goregular.TTF)
 	if err != nil {
 		log.Fatalf("Parse: %v", err)
@@ -42,6 +43,7 @@ func Example_rasterizeGlyph() {
 		log.Fatalf("LoadGlyph: %v", err)
 	}
 
+	// Translate and scale that glyph as we pass it to a vector.Rasterizer.
 	r := vector.NewRasterizer(width, height)
 	r.DrawOp = draw.Src
 	for _, seg := range segments {
@@ -77,9 +79,12 @@ func Example_rasterizeGlyph() {
 		}
 	}
 
+	// Finish the rasterization: the conversion from vector graphics (shapes)
+	// to raster graphics (pixels).
 	dst := image.NewAlpha(image.Rect(0, 0, width, height))
 	r.Draw(dst, dst.Bounds(), image.Opaque, image.Point{})
 
+	// Visualize the pixels.
 	const asciiArt = ".++8"
 	buf := make([]byte, 0, height*(width+1))
 	for y := 0; y < height; y++ {

vector/example_test.go

@@ -0,0 +1,73 @@
+// Copyright 2021 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 vector_test
+
+import (
+	"image"
+	"image/draw"
+	"os"
+
+	"golang.org/x/image/vector"
+)
+
+func Example_draw() {
+	const (
+		width  = 30
+		height = 20
+	)
+
+	// Define a closed shape with three edges: two linear and one quadratic.
+	// One of its vertices is at the top-left corner of the (1, 2) pixel, which
+	// is also the bottom-right corner of the (0, 1) pixel.
+	//
+	// Co-ordinates can be floating point numbers, not just integers. They can
+	// also be outside the vector.Rasterizer's dimensions. The shapes will be
+	// clipped during rasterization.
+	r := vector.NewRasterizer(width, height)
+	r.DrawOp = draw.Src
+	r.MoveTo(1, 2)
+	r.LineTo(20, 2)
+	r.QuadTo(40.5, 15, 10, 20)
+	r.ClosePath()
+
+	// Finish the rasterization: the conversion from vector graphics (shapes)
+	// to raster graphics (pixels). Co-ordinates are now integers.
+	dst := image.NewAlpha(image.Rect(0, 0, width, height))
+	r.Draw(dst, dst.Bounds(), image.Opaque, image.Point{})
+
+	// Visualize the pixels.
+	const asciiArt = ".++8"
+	buf := make([]byte, 0, height*(width+1))
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x++ {
+			a := dst.AlphaAt(x, y).A
+			buf = append(buf, asciiArt[a>>6])
+		}
+		buf = append(buf, '\n')
+	}
+	os.Stdout.Write(buf)
+
+	// Output:
+	// ..............................
+	// ..............................
+	// .8888888888888888888+.........
+	// .+88888888888888888888+.......
+	// ..888888888888888888888+......
+	// ..+888888888888888888888+.....
+	// ...8888888888888888888888+....
+	// ...+8888888888888888888888+...
+	// ....88888888888888888888888+..
+	// ....+88888888888888888888888..
+	// .....88888888888888888888888..
+	// .....+8888888888888888888888..
+	// ......8888888888888888888888..
+	// ......+88888888888888888888+..
+	// .......8888888888888888888+...
+	// .......+88888888888888888.....
+	// ........888888888888888+......
+	// ........+88888888888+.........
+	// .........8888888++............
+	// .........+8+++................
+}