Medial axis
![An ellipse (red), its evolute (blue), and its medial axis (green). The symmetry set, a super-set of the medial axis is the green and yellow curves. One bi-tangent circle is shown.](/Images/godic/202501/26/Ellipse_symmetry_set.svg2211.png")
![(a) A simple 3d object. (b) Its medial axis transform. The colors represent the distance from the medial axis to the object's boundary.](/Images/godic/202501/26/MedialAxisTransformOf3dObject2211.png")
The medial axis of an object is the set of all points having more than one closest point on the object's boundary. Originally referred to as the topological skeleton, it was introduced by Blum as a tool for biological shape recognition. In mathematics the closure of the medial axis is known as the cut locus.