diff --git a/internal/stroke/stroke.go b/internal/stroke/stroke.go
index 95025eb8..92caed32 100644
--- a/internal/stroke/stroke.go
+++ b/internal/stroke/stroke.go
@@ -543,7 +543,7 @@ func strokePathRoundCap(qs *StrokeQuads, hw float32, pivot, n0 f32.Point) {
 // An electronic version may be found at:
 //  http://spaceroots.org/documents/ellipse/elliptical-arc.pdf
 func ArcTransform(p, f1, f2 f32.Point, angle float32, segments int) f32.Affine2D {
-	c := f32.Point{
+	center := f32.Point{
 		X: 0.5 * (f1.X + f2.X),
 		Y: 0.5 * (f1.Y + f2.Y),
 	}
@@ -551,9 +551,9 @@ func ArcTransform(p, f1, f2 f32.Point, angle float32, segments int) f32.Affine2D
 	// semi-major axis: 2a = |PF1| + |PF2|
 	a := 0.5 * (dist(f1, p) + dist(f2, p))
 
-	// semi-minor axis: c^2 = a^2+b^2 (c: focal distance)
-	f := dist(f1, c)
-	b := math.Sqrt(a*a - f*f)
+	// semi-minor axis: c^2 = a^2 - b^2 (c: focal distance)
+	c := dist(f1, center)
+	b := math.Sqrt(a*a - c*c)
 
 	var rx, ry, alpha float64
 	switch {
@@ -567,21 +567,21 @@ func ArcTransform(p, f1, f2 f32.Point, angle float32, segments int) f32.Affine2D
 
 	var x float64
 	switch {
-	case f1 == c || f2 == c:
+	case f1 == f2:
 		// degenerate case of a circle.
 		alpha = 0
 	default:
 		switch {
-		case f1.X > c.X:
-			x = float64(f1.X - c.X)
-			alpha = math.Acos(x / f)
-		case f1.X < c.X:
-			x = float64(f2.X - c.X)
-			alpha = math.Acos(x / f)
-		case f1.X == c.X:
+		case f1.X > center.X:
+			x = float64(f1.X - center.X)
+			alpha = math.Acos(x / c)
+		case f1.X < center.X:
+			x = float64(f2.X - center.X)
+			alpha = math.Acos(x / c)
+		case f1.X == center.X:
 			// special case of a "vertical" ellipse.
 			alpha = math.Pi / 2
-			if f1.Y < c.Y {
+			if f1.Y < center.Y {
 				alpha = -alpha
 			}
 		}
@@ -593,14 +593,14 @@ func ArcTransform(p, f1, f2 f32.Point, angle float32, segments int) f32.Affine2D
 		rot f32.Affine2D // rotation matrix for each segment
 		inv f32.Affine2D // transform from ellipse-based frame to absolute one
 	)
-	ref = ref.Offset(f32.Point{}.Sub(c))
+	ref = ref.Offset(f32.Point{}.Sub(center))
 	ref = ref.Rotate(f32.Point{}, float32(-alpha))
 	ref = ref.Scale(f32.Point{}, f32.Point{
 		X: float32(1 / rx),
 		Y: float32(1 / ry),
 	})
 	inv = ref.Invert()
-	rot = rot.Rotate(f32.Point{}, float32(0.5*θ))
+	rot = rot.Rotate(f32.Point{}, 0.5*θ)
 
 	// Instead of invoking math.Sincos for every segment, compute a rotation
 	// matrix once and apply for each segment.