shiny/imageutil: new package.

Add an imageutil.Border function, for drawing outlined-but-not-filled
rectangles, such as widget borders, checkboxes, table cells, etc.

Change-Id: I1e55e5fe492a5d2bb4df6f03ab8d14c9cb9db67d
Reviewed-on: https://go-review.googlesource.com/24632
Reviewed-by: David Crawshaw <[email protected]>

shiny/example/basic/main.go

@@ -19,6 +19,7 @@ import (
 	"math"
 
 	"golang.org/x/exp/shiny/driver"
+	"golang.org/x/exp/shiny/imageutil"
 	"golang.org/x/exp/shiny/screen"
 	"golang.org/x/image/math/f64"
 	"golang.org/x/mobile/event/key"
@@ -85,8 +86,11 @@ func main() {
 				}
 
 			case paint.Event:
-				w.Fill(sz.Bounds(), blue0, screen.Src)
-				w.Fill(sz.Bounds().Inset(10), blue1, screen.Src)
+				const inset = 10
+				for _, r := range imageutil.Border(sz.Bounds(), inset) {
+					w.Fill(r, blue0, screen.Src)
+				}
+				w.Fill(sz.Bounds().Inset(inset), blue1, screen.Src)
 				w.Upload(image.Point{20, 0}, b, b.Bounds())
 				w.Fill(image.Rect(50, 50, 350, 120), red, screen.Over)
 

shiny/imageutil/imageutil.go

@@ -0,0 +1,101 @@
+// 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.
+
+// Package imageutil implements some image utility functions.
+package imageutil
+
+import (
+	"image"
+)
+
+// TODO: move Border into the standard library's package image?
+
+// Border returns four rectangles that together contain those points between r
+// and r.Inset(inset). Visually:
+//
+//	00000000
+//	00000000
+//	11....22
+//	11....22
+//	11....22
+//	33333333
+//	33333333
+//
+// The inset may be negative, in which case the points will be outside r.
+//
+// Some of the returned rectangles may be empty. None of the returned
+// rectangles will overlap.
+func Border(r image.Rectangle, inset int) [4]image.Rectangle {
+	if inset == 0 {
+		return [4]image.Rectangle{}
+	}
+	if r.Dx() <= 2*inset || r.Dy() <= 2*inset {
+		return [4]image.Rectangle{r}
+	}
+
+	x := [4]int{
+		r.Min.X,
+		r.Min.X + inset,
+		r.Max.X - inset,
+		r.Max.X,
+	}
+	y := [4]int{
+		r.Min.Y,
+		r.Min.Y + inset,
+		r.Max.Y - inset,
+		r.Max.Y,
+	}
+	if inset < 0 {
+		x[0], x[1] = x[1], x[0]
+		x[2], x[3] = x[3], x[2]
+		y[0], y[1] = y[1], y[0]
+		y[2], y[3] = y[3], y[2]
+	}
+
+	// The top and bottom sections are responsible for filling the corners.
+	// The top and bottom sections go from x[0] to x[3], across the y's.
+	// The left and right sections go from y[1] to y[2], across the x's.
+
+	return [4]image.Rectangle{{
+		// Top section.
+		Min: image.Point{
+			X: x[0],
+			Y: y[0],
+		},
+		Max: image.Point{
+			X: x[3],
+			Y: y[1],
+		},
+	}, {
+		// Left section.
+		Min: image.Point{
+			X: x[0],
+			Y: y[1],
+		},
+		Max: image.Point{
+			X: x[1],
+			Y: y[2],
+		},
+	}, {
+		// Right section.
+		Min: image.Point{
+			X: x[2],
+			Y: y[1],
+		},
+		Max: image.Point{
+			X: x[3],
+			Y: y[2],
+		},
+	}, {
+		// Bottom section.
+		Min: image.Point{
+			X: x[0],
+			Y: y[2],
+		},
+		Max: image.Point{
+			X: x[3],
+			Y: y[3],
+		},
+	}}
+}

shiny/imageutil/imageutil_test.go

@@ -0,0 +1,74 @@
+// 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.
+
+package imageutil
+
+import (
+	"image"
+	"testing"
+)
+
+func area(r image.Rectangle) int {
+	dx, dy := r.Dx(), r.Dy()
+	if dx <= 0 || dy <= 0 {
+		return 0
+	}
+	return dx * dy
+}
+
+func TestBorder(t *testing.T) {
+	r := image.Rect(100, 200, 400, 300)
+
+	insets := []int{
+		-100,
+		-1,
+		+0,
+		+1,
+		+20,
+		+49,
+		+50,
+		+51,
+		+149,
+		+150,
+		+151,
+	}
+
+	for _, inset := range insets {
+		border := Border(r, inset)
+
+		outer, inner := r, r.Inset(inset)
+		if inset < 0 {
+			outer, inner = inner, outer
+		}
+
+		got := 0
+		for _, b := range border {
+			got += area(b)
+		}
+		want := area(outer) - area(inner)
+		if got != want {
+			t.Errorf("inset=%d: total area: got %d, want %d", inset, got, want)
+		}
+
+		for i, bi := range border {
+			for j, bj := range border {
+				if i <= j {
+					continue
+				}
+				if !bi.Intersect(bj).Empty() {
+					t.Errorf("inset=%d: %v and %v overlap", inset, bi, bj)
+				}
+			}
+		}
+
+		for _, b := range border {
+			if got := outer.Intersect(b); got != b {
+				t.Errorf("inset=%d: outer intersection: got %v, want %v", inset, got, b)
+			}
+			if got := inner.Intersect(b); !got.Empty() {
+				t.Errorf("inset=%d: inner intersection: got %v, want empty", inset, got)
+			}
+		}
+	}
+}