op/clip: split Rect into pixel-aligned Rect and rounded RRect

The pixel-aligned Rect is more efficient and easier to use in the common case
of layout clipping.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

op/clip/shapes.go

@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package clip
+
+import (
+	"gioui.org/f32"
+	"gioui.org/op"
+)
+
+// RRect represents the clip area of a rectangle with rounded
+// corners.
+//
+// Specify a square with corner radii equal to half the square size to
+// construct a circular clip area.
+type RRect struct {
+	Rect f32.Rectangle
+	// The corner radii.
+	SE, SW, NW, NE float32
+}
+
+// op returns the op for the rectangle.
+func (rr RRect) op(ops *op.Ops) Op {
+	return roundRect(ops, rr.Rect, rr.SE, rr.SW, rr.NW, rr.NE)
+}
+
+// Add the rectangle clip.
+func (rr RRect) Add(ops *op.Ops) {
+	rr.op(ops).Add(ops)
+}
+
+// roundRect returns the clip area of a rectangle with rounded
+// corners defined by their radii.
+func roundRect(ops *op.Ops, r f32.Rectangle, se, sw, nw, ne float32) Op {
+	size := r.Size()
+	// https://pomax.github.io/bezierinfo/#circles_cubic.
+	w, h := float32(size.X), float32(size.Y)
+	const c = 0.55228475 // 4*(sqrt(2)-1)/3
+	var p Path
+	p.Begin(ops)
+	p.Move(r.Min)
+
+	p.Move(f32.Point{X: w, Y: h - se})
+	p.Cube(f32.Point{X: 0, Y: se * c}, f32.Point{X: -se + se*c, Y: se}, f32.Point{X: -se, Y: se}) // SE
+	p.Line(f32.Point{X: sw - w + se, Y: 0})
+	p.Cube(f32.Point{X: -sw * c, Y: 0}, f32.Point{X: -sw, Y: -sw + sw*c}, f32.Point{X: -sw, Y: -sw}) // SW
+	p.Line(f32.Point{X: 0, Y: nw - h + sw})
+	p.Cube(f32.Point{X: 0, Y: -nw * c}, f32.Point{X: nw - nw*c, Y: -nw}, f32.Point{X: nw, Y: -nw}) // NW
+	p.Line(f32.Point{X: w - ne - nw, Y: 0})
+	p.Cube(f32.Point{X: ne * c, Y: 0}, f32.Point{X: ne, Y: ne - ne*c}, f32.Point{X: ne, Y: ne}) // NE
+	return p.End()
+}