Add tolerance argument

Signed-off-by: Arjo Chakravarty <[email protected]>

include/ignition/math/Plane.hh

@@ -126,13 +126,16 @@ namespace ignition
       /// given the line's gradient and a point in parametrized space.
       /// \param[in] _point A point that lies on a line.
       /// \param[in] _gradient The gradient of the line.
+      /// \param[in] _tolerance The tolerance for determining a line is
+      /// parallel to the plane. Optional, default=10^-16
       /// \return The point of intersection. std::nullopt if the line is
       /// parrallel to the plane.
       public: std::optional<Vector3<T>> Intersection(
         const Vector3<T> &_point,
-        const Vector3<T> &_gradient) const
+        const Vector3<T> &_gradient,
+        const double &_tolerance = 1e-6) const
       {
-        if(abs(this->Normal().Dot(_gradient)) < 1e-6)
+        if(abs(this->Normal().Dot(_gradient)) < _tolerance)
         {
           return std::nullopt;
         }