gpu,op/clip: implement stroked paths with miter joins

Signed-off-by: Sebastien Binet <s@sbinet.org>

gpu/gpu.go

@@ -162,8 +162,9 @@ func (op *clipOp) decode(data []byte) {
 		bounds: layout.FRect(r),
 		width:  math.Float32frombits(bo.Uint32(data[17:])),
 		style: clip.StrokeStyle{
-			Cap:  clip.StrokeCap(data[21]),
-			Join: clip.StrokeJoin(data[22]),
+			Cap:   clip.StrokeCap(data[21]),
+			Join:  clip.StrokeJoin(data[22]),
+			Miter: math.Float32frombits(bo.Uint32(data[23:])),
 		},
 	}
 }

gpu/stroke.go

@@ -423,6 +423,10 @@ func quadBezierSplit(p0, p1, p2 f32.Point, t float32) (f32.Point, f32.Point, f32
 // strokePathJoin joins the two paths rhs and lhs, according to the provided
 // stroke style sty.
 func strokePathJoin(sty clip.StrokeStyle, rhs, lhs *strokeQuads, hw float32, pivot, n0, n1 f32.Point, r0, r1 float32) {
+	if sty.Miter > 0 {
+		strokePathMiterJoin(sty, rhs, lhs, hw, pivot, n0, n1, r0, r1)
+		return
+	}
 	switch sty.Join {
 	case clip.BevelJoin:
 		strokePathBevelJoin(rhs, lhs, hw, pivot, n0, n1, r0, r1)
@@ -464,6 +468,47 @@ func strokePathRoundJoin(rhs, lhs *strokeQuads, hw float32, pivot, n0, n1 f32.Po
 	}
 }
 
+func strokePathMiterJoin(sty clip.StrokeStyle, rhs, lhs *strokeQuads, hw float32, pivot, n0, n1 f32.Point, r0, r1 float32) {
+	if n0 == n1.Mul(-1) {
+		strokePathBevelJoin(rhs, lhs, hw, pivot, n0, n1, r0, r1)
+		return
+	}
+
+	// This is to handle nearly linear joints that would be clipped otherwise.
+	limit := math.Max(float64(sty.Miter), 1.001)
+
+	cw := dotPt(rot90CW(n0), n1) >= 0.0
+	if cw {
+		// hw is used to calculate |R|.
+		// When running CW, n0 and n1 point the other way,
+		// so the sign of r0 and r1 is negated.
+		hw = -hw
+	}
+	hw64 := float64(hw)
+
+	cos := math.Sqrt(0.5 * (1 + float64(cosPt(n0, n1))))
+	d := hw64 / cos
+	if math.Abs(limit*hw64) < math.Abs(d) {
+		sty.Miter = 0 // Set miter to zero to disable the miter joint.
+		strokePathJoin(sty, rhs, lhs, hw, pivot, n0, n1, r0, r1)
+		return
+	}
+	mid := pivot.Add(normPt(n0.Add(n1), float32(d)))
+
+	rp := pivot.Add(n1)
+	lp := pivot.Sub(n1)
+	switch {
+	case cw:
+		// Path bends to the right, ie. CW.
+		lhs.lineTo(mid)
+	default:
+		// Path bends to the left, ie. CCW.
+		rhs.lineTo(mid)
+	}
+	rhs.lineTo(rp)
+	lhs.lineTo(lp)
+}
+
 // strokePathCap caps the provided path qs, according to the provided stroke style sty.
 func strokePathCap(sty clip.StrokeStyle, qs *strokeQuads, hw float32, pivot, n0 f32.Point) {
 	switch sty.Cap {

internal/opconst/ops.go

@@ -47,7 +47,7 @@ const (
 	TypePushLen           = 1
 	TypePopLen            = 1
 	TypeAuxLen            = 1
-	TypeClipLen           = 1 + 4*4 + 4 + 2
+	TypeClipLen           = 1 + 4*4 + 4 + 2 + 4
 	TypeProfileLen        = 1
 )
 

internal/rendertest/clip_test.go

@@ -211,6 +211,96 @@ func TestStrokedPathRoundRound(t *testing.T) {
 	})
 }
 
+func TestStrokedPathFlatMiter(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		const width = 10
+		sty := clip.StrokeStyle{
+			Cap:   clip.FlatCap,
+			Join:  clip.BevelJoin,
+			Miter: 5,
+		}
+
+		beg := f32.Pt(40, 10)
+		{
+			p := new(clip.Path)
+			p.Begin(o)
+			p.Move(beg)
+			p.Line(f32.Pt(50, 0))
+			p.Line(f32.Pt(-50, 50))
+			p.Line(f32.Pt(50, 0))
+			p.Quad(f32.Pt(-50, 20), f32.Pt(-50, 50))
+			p.Line(f32.Pt(50, 0))
+
+			p.Stroke(width, sty).Add(o)
+			paint.Fill(o, colornames.Red)
+		}
+
+		{
+			p := new(clip.Path)
+			p.Begin(o)
+			p.Move(beg)
+			p.Line(f32.Pt(50, 0))
+			p.Line(f32.Pt(-50, 50))
+			p.Line(f32.Pt(50, 0))
+			p.Quad(f32.Pt(-50, 20), f32.Pt(-50, 50))
+			p.Line(f32.Pt(50, 0))
+
+			p.Stroke(2, clip.StrokeStyle{}).Add(o)
+			paint.Fill(o, colornames.Black)
+		}
+
+	}, func(r result) {
+		r.expect(0, 0, colornames.White)
+		r.expect(40, 10, colornames.Black)
+		r.expect(40, 12, colornames.Red)
+	})
+}
+
+func TestStrokedPathFlatMiterInf(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		const width = 10
+		sty := clip.StrokeStyle{
+			Cap:   clip.FlatCap,
+			Join:  clip.BevelJoin,
+			Miter: float32(math.Inf(+1)),
+		}
+
+		beg := f32.Pt(40, 10)
+		{
+			p := new(clip.Path)
+			p.Begin(o)
+			p.Move(beg)
+			p.Line(f32.Pt(50, 0))
+			p.Line(f32.Pt(-50, 50))
+			p.Line(f32.Pt(50, 0))
+			p.Quad(f32.Pt(-50, 20), f32.Pt(-50, 50))
+			p.Line(f32.Pt(50, 0))
+
+			p.Stroke(width, sty).Add(o)
+			paint.Fill(o, colornames.Red)
+		}
+
+		{
+			p := new(clip.Path)
+			p.Begin(o)
+			p.Move(beg)
+			p.Line(f32.Pt(50, 0))
+			p.Line(f32.Pt(-50, 50))
+			p.Line(f32.Pt(50, 0))
+			p.Quad(f32.Pt(-50, 20), f32.Pt(-50, 50))
+			p.Line(f32.Pt(50, 0))
+
+			p.Stroke(2, clip.StrokeStyle{}).Add(o)
+			paint.Fill(o, colornames.Black)
+		}
+
+	}, func(r result) {
+		r.expect(0, 0, colornames.White)
+		r.expect(40, 10, colornames.Black)
+		r.expect(40, 12, colornames.Red)
+	})
+}
+
 func TestStrokedPathZeroWidth(t *testing.T) {
 	run(t, func(o *op.Ops) {
 		const width = 2

op/clip/clip.go

@@ -55,6 +55,7 @@ func (p Op) Add(o *op.Ops) {
 	bo.PutUint32(data[17:], math.Float32bits(p.width))
 	data[21] = uint8(p.style.Cap)
 	data[22] = uint8(p.style.Join)
+	bo.PutUint32(data[23:], math.Float32bits(p.style.Miter))
 }
 
 // Begin the path, storing the path data and final Op into ops.

op/clip/stroke.go

@@ -8,6 +8,11 @@ package clip
 type StrokeStyle struct {
 	Cap  StrokeCap
 	Join StrokeJoin
+
+	// Miter is the limit to apply to a miter joint.
+	// The zero Miter disables the miter joint; setting Miter to +∞
+	// unconditionally enables the miter joint.
+	Miter float32
 }
 
 // StrokeCap describes the head or tail of a stroked path.