works more or less

drawing/curve.go

@@ -1,8 +1,6 @@
 package drawing
 
-import (
-	"math"
-)
+import "math"
 
 const (
 	// CurveRecursionLimit represents the maximum recursion that is really necessary to subsivide a curve into straight lines
@@ -98,31 +96,60 @@ func SubdivideQuad(c, c1, c2 []float64) {
 	return
 }
 
+func traceWindowIndices(i int) (startAt, endAt int) {
+	startAt = i * 6
+	endAt = startAt + 6
+	return
+}
+
+func traceCalcDeltas(c []float64) (dx, dy, d float64) {
+	dx = c[4] - c[0]
+	dy = c[5] - c[1]
+	d = math.Abs(((c[2]-c[4])*dy - (c[3]-c[5])*dx))
+	return
+}
+
+func traceIsFlat(dx, dy, d, threshold float64) bool {
+	return (d * d) < threshold*(dx*dx+dy*dy)
+}
+
+func traceGetWindow(curves []float64, i int) []float64 {
+	startAt, endAt := traceWindowIndices(i)
+	return curves[startAt:endAt]
+}
+
 // TraceQuad generate lines subdividing the curve using a Liner
 // flattening_threshold helps determines the flattening expectation of the curve
 func TraceQuad(t Liner, quad []float64, flatteningThreshold float64) {
+	const curveLen = CurveRecursionLimit * 6
+	const curveEndIndex = curveLen - 1
+	const lastIteration = CurveRecursionLimit - 1
+
 	// Allocates curves stack
-	var curves [CurveRecursionLimit * 6]float64
+	curves := make([]float64, curveLen)
+
+	// copy 6 elements from the quad path to the stack
 	copy(curves[0:6], quad[0:6])
-	i := 0
-	// current curve
+
+	var i int
 	var c []float64
 	var dx, dy, d float64
 
 	for i >= 0 {
-		c = curves[i*6:]
-		dx = c[4] - c[0]
-		dy = c[5] - c[1]
+		c = traceGetWindow(curves, i)
+		dx, dy, d = traceCalcDeltas(c)
 
-		d = math.Abs(((c[2]-c[4])*dy - (c[3]-c[5])*dx))
+		// bail early if the distance is 0
+		if d == 0 {
+			return
+		}
 
 		// if it's flat then trace a line
-		if (d*d) < flatteningThreshold*(dx*dx+dy*dy) || i == len(curves)-1 {
+		if traceIsFlat(dx, dy, d, flatteningThreshold) || i == lastIteration {
 			t.LineTo(c[4], c[5])
 			i--
 		} else {
-			// second half of bezier go lower onto the stack
-			SubdivideQuad(c, curves[(i+1)*6:], curves[i*6:])
+			SubdivideQuad(c, traceGetWindow(curves, i+1), traceGetWindow(curves, i))
 			i++
 		}
 	}

drawing/curve_test.go

@@ -0,0 +1,35 @@
+package drawing
+
+import (
+	"testing"
+
+	assert "github.com/blendlabs/go-assert"
+)
+
+type point struct {
+	X, Y float64
+}
+
+type mockLine struct {
+	inner []point
+}
+
+func (ml *mockLine) LineTo(x, y float64) {
+	ml.inner = append(ml.inner, point{x, y})
+}
+
+func (ml mockLine) Len() int {
+	return len(ml.inner)
+}
+
+func TestTraceQuad(t *testing.T) {
+	assert := assert.New(t)
+
+	// Quad
+	// x1, y1, cpx1, cpy2, x2, y2 float64
+	// do the 9->12 circle segment
+	quad := []float64{10, 20, 20, 20, 20, 10}
+	liner := &mockLine{}
+	TraceQuad(liner, quad, 0.5)
+	assert.NotZero(liner.Len())
+}

drawing/flattener.go

@@ -23,10 +23,10 @@ type Flattener interface {
 // Flatten convert curves into straight segments keeping join segments info
 func Flatten(path *Path, flattener Flattener, scale float64) {
 	// First Point
-	var startX, startY float64 = 0, 0
+	var startX, startY float64
 	// Current Point
-	var x, y float64 = 0, 0
-	i := 0
+	var x, y float64
+	var i int
 	for _, cmp := range path.Components {
 		switch cmp {
 		case MoveToComponent:
@@ -43,6 +43,7 @@ func Flatten(path *Path, flattener Flattener, scale float64) {
 			flattener.LineJoin()
 			i += 2
 		case QuadCurveToComponent:
+			// we include the previous point for the start of the curve
 			TraceQuad(flattener, path.Points[i-2:], 0.5)
 			x, y = path.Points[i+2], path.Points[i+3]
 			flattener.LineTo(x, y)