internal/stroke: don't draw round join if angle is NaN

References: https://todo.sr.ht/~eliasnaur/gio/331
Signed-off-by: Andy Balholm <andy@balholm.com>

gpu/internal/rendertest/clip_test.go

@@ -207,6 +207,32 @@ func TestStrokedPathCoincidentControlPoint(t *testing.T) {
 	})
 }
 
+func TestStrokedPathBalloon(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		// This shape is based on the one drawn by the Bubble function in
+		// github.com/llgcode/draw2d/samples/geometry/geometry.go.
+		p := new(clip.Path)
+		p.Begin(o)
+		p.MoveTo(f32.Pt(42.69375, 10.5))
+		p.CubeTo(f32.Pt(42.69375, 10.5), f32.Pt(14.85, 10.5), f32.Pt(14.85, 31.5))
+		p.CubeTo(f32.Pt(14.85, 31.5), f32.Pt(14.85, 52.5), f32.Pt(28.771875, 52.5))
+		p.CubeTo(f32.Pt(28.771875, 52.5), f32.Pt(28.771875, 63.7), f32.Pt(17.634375, 66.5))
+		p.CubeTo(f32.Pt(17.634375, 66.5), f32.Pt(34.340626, 63.7), f32.Pt(37.125, 52.5))
+		p.CubeTo(f32.Pt(37.125, 52.5), f32.Pt(70.5375, 52.5), f32.Pt(70.5375, 31.5))
+		p.CubeTo(f32.Pt(70.5375, 31.5), f32.Pt(70.5375, 10.5), f32.Pt(42.69375, 10.5))
+		cl := clip.Stroke{
+			Path:  p.End(),
+			Width: 2.83,
+		}.Op().Push(o)
+		paint.Fill(o, black)
+		cl.Pop()
+	}, func(r result) {
+		r.expect(0, 0, transparent)
+		r.expect(70, 52, colornames.Black)
+		r.expect(70, 90, transparent)
+	})
+}
+
 func TestPathReuse(t *testing.T) {
 	run(t, func(o *op.Ops) {
 		var path clip.Path

internal/stroke/stroke.go

@@ -510,14 +510,18 @@ func strokePathRoundJoin(rhs, lhs *StrokeQuads, hw float32, pivot, n0, n1 f32.Po
 		// Path bends to the right, ie. CW (or 180 degree turn).
 		c := pivot.Sub(lhs.pen())
 		angle := -math.Acos(float64(cosPt(n0, n1)))
-		lhs.arc(c, c, float32(angle))
+		if !math.IsNaN(angle) {
+			lhs.arc(c, c, float32(angle))
+		}
 		lhs.lineTo(lp) // Add a line to accommodate for rounding errors.
 		rhs.lineTo(rp)
 	default:
 		// Path bends to the left, ie. CCW.
 		angle := math.Acos(float64(cosPt(n0, n1)))
 		c := pivot.Sub(rhs.pen())
-		rhs.arc(c, c, float32(angle))
+		if !math.IsNaN(angle) {
+			rhs.arc(c, c, float32(angle))
+		}
 		rhs.lineTo(rp) // Add a line to accommodate for rounding errors.
 		lhs.lineTo(lp)
 	}