diff --git a/engine/shape.go b/engine/shape.go
index 44026a409..29a4ef194 100644
--- a/engine/shape.go
+++ b/engine/shape.go
@@ -46,7 +46,7 @@ func Ellipse(diamx, diamy float64) Shape {
 // 3D Cone with the vertex down.
 func Cone(diam, height float64) Shape {
 	return func(x, y, z float64) bool {
-		return z >= 0 && sqr64(x/diam)+sqr64(y/diam)-sqr64(z/height) <= 0.25
+		return z >= 0 && sqr64(x/diam)+sqr64(y/diam)+0.25*sqr64(z/height) <= 0.25
 	}
 }