op/clip: expose LineTo and QuadTo

Using delta position with Line and Quad can drift over successive calls.
Also, in some cases it's much more convenient to use absolute
coordinates rather than relative.

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

internal/rendertest/clip_test.go

@@ -78,6 +78,26 @@ func TestPaintArc(t *testing.T) {
 	})
 }
 
+func TestPaintAbsolute(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		p := new(clip.Path)
+		p.Begin(o)
+		p.Move(f32.Pt(100, 100)) // offset the initial pen position to test "MoveTo"
+
+		p.MoveTo(f32.Pt(20, 20))
+		p.LineTo(f32.Pt(80, 20))
+		p.QuadTo(f32.Pt(80, 80), f32.Pt(20, 80))
+		p.Outline().Add(o)
+
+		paint.FillShape(o, red, clip.Rect(image.Rect(0, 0, 128, 128)).Op())
+	}, func(r result) {
+		r.expect(0, 0, colornames.White)
+		r.expect(30, 30, colornames.Red)
+		r.expect(79, 79, colornames.White)
+		r.expect(90, 90, colornames.White)
+	})
+}
+
 func TestPaintTexture(t *testing.T) {
 	run(t, func(o *op.Ops) {
 		squares.Add(o)

op/clip/clip.go

@@ -67,9 +67,14 @@ func (p *Path) Begin(ops *op.Ops) {
 	data[0] = byte(opconst.TypeAux)
 }
 
-// MoveTo moves the pen to the given position.
-func (p *Path) Move(to f32.Point) {
-	to = to.Add(p.pen)
+// Move moves the pen by the amount specified by delta.
+func (p *Path) Move(delta f32.Point) {
+	to := delta.Add(p.pen)
+	p.MoveTo(to)
+}
+
+// MoveTo moves the pen to the specified absolute coordinate.
+func (p *Path) MoveTo(to f32.Point) {
 	p.end()
 	p.pen = to
 	p.start = to
@@ -78,7 +83,7 @@ func (p *Path) Move(to f32.Point) {
 // end completes the current contour.
 func (p *Path) end() {
 	if p.pen != p.start {
-		p.lineTo(p.start)
+		p.LineTo(p.start)
 	}
 	p.contour++
 }
@@ -86,12 +91,13 @@ func (p *Path) end() {
 // Line moves the pen by the amount specified by delta, recording a line.
 func (p *Path) Line(delta f32.Point) {
 	to := delta.Add(p.pen)
-	p.lineTo(to)
+	p.LineTo(to)
 }
 
-func (p *Path) lineTo(to f32.Point) {
+// LineTo moves the pen to the absolute point specified, recording a line.
+func (p *Path) LineTo(to f32.Point) {
 	// Model lines as degenerate quadratic Béziers.
-	p.quadTo(to.Add(p.pen).Mul(.5), to)
+	p.QuadTo(to.Add(p.pen).Mul(.5), to)
 }
 
 // Quad records a quadratic Bézier from the pen to end
@@ -99,10 +105,12 @@ func (p *Path) lineTo(to f32.Point) {
 func (p *Path) Quad(ctrl, to f32.Point) {
 	ctrl = ctrl.Add(p.pen)
 	to = to.Add(p.pen)
-	p.quadTo(ctrl, to)
+	p.QuadTo(ctrl, to)
 }
 
-func (p *Path) quadTo(ctrl, to f32.Point) {
+// QuadTo records a quadratic Bézier from the pen to end
+// with the control point ctrl, with absolute coordinates.
+func (p *Path) QuadTo(ctrl, to f32.Point) {
 	data := p.ops.Write(ops.QuadSize + 4)
 	bo := binary.LittleEndian
 	bo.PutUint32(data[0:], uint32(p.contour))
@@ -236,7 +244,7 @@ func (p *Path) arc(alpha float64, c f32.Point, rx, ry, beg, delta float64) {
 			2*p1.X-0.5*(p0.X+p2.X),
 			2*p1.Y-0.5*(p0.Y+p2.Y),
 		)
-		p.quadTo(ctl, p2)
+		p.QuadTo(ctl, p2)
 	}
 }
 
@@ -289,7 +297,7 @@ func (p *Path) approxCubeTo(splits int, maxDist float32, ctrl0, ctrl1, to f32.Po
 	c := ctrl0.Mul(3).Sub(p.pen).Add(ctrl1.Mul(3)).Sub(to).Mul(1.0 / 4.0)
 	const maxSplits = 32
 	if splits >= maxSplits {
-		p.quadTo(c, to)
+		p.QuadTo(c, to)
 		return splits
 	}
 	// The maximum distance between the cubic P and its approximation Q given t
@@ -301,7 +309,7 @@ func (p *Path) approxCubeTo(splits int, maxDist float32, ctrl0, ctrl1, to f32.Po
 	v := to.Sub(ctrl1.Mul(3)).Add(ctrl0.Mul(3)).Sub(p.pen)
 	d2 := (v.X*v.X + v.Y*v.Y) * 3 / (36 * 36)
 	if d2 <= maxDist*maxDist {
-		p.quadTo(c, to)
+		p.QuadTo(c, to)
 		return splits
 	}
 	// De Casteljau split the curve and approximate the halves.