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| [[Image:Ellipse symmetry set.svg||right|thumb|240px|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.]]
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| 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 <ref>''A transformation for extracting new descriptors of shape'' H Blum, Models for the perception of speech and visual form, 1967 [http://pageperso.lif.univ-mrs.fr/~edouard.thiel/rech/1967-blum.pdf]
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| </ref> as a tool for biological [[shape]] recognition.
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| In mathematics the [[closure (topology)|closure]] of the medial axis is known as the [[cut locus]].
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| In 2D, the medial axis of a subset ''S'' which is bounded by planar curve ''C'' is the locus of the centers of circles that are tangent to curve ''C'' in two or more points, where all such circles are contained in ''S''. (It follows that the medial axis itself is contained in ''S''.)
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| The medial axis of a [[simple polygon]] is a tree whose leaves are the vertices of the
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| polygon, and whose edges are either straight segments or arcs of parabolas.
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| The medial axis together with the associated radius function of the maximally inscribed discs is called the '''medial axis transform''' ('''MAT'''). The medial axis transform is a complete shape descriptor (see also [[shape analysis]]), meaning that it can be used to reconstruct the [[shape]] of the original domain.
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| The medial axis is a subset of the '''[[symmetry set]]''', which is defined similarly, except that it also includes circles not contained in ''S''. (Hence, the symmetry set of ''S'' generally extends to infinity, similar to the [[Voronoi diagram]] of a point set.)
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| The medial axis generalizes to k-dimensional hypersurfaces by replacing 2D circles with k-dimension hyperspheres. The 2D medial axis is useful for [[optical character recognition|character]] and object recognition, while the 3D medial axis has applications in [[surface reconstruction]] for physical models, and for dimensional reduction of complex models.
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| If ''S'' is given by a unit speed parametrisation <math>\gamma:\mathbf{R}\to\mathbf{R}^2</math>, and <math>\underline{T}(t) = {d\gamma\over dt}</math> is the unit tangent vector at each point. Then there will be a bitangent circle with center ''c'' and radius ''r'' if
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| *<math>(c-\gamma(s))\cdot\underline{T}(s)=(c-\gamma(t))\cdot\underline{T}(t)=0,</math>
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| *<math>|c-\gamma(s)|=|c-\gamma(t)|=r.\,</math>
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| For most curves, the symmetry set will form a one dimensional curve and can contain [[cusp (singularity)|cusps]]. The symmetry set has end points corresponding to the [[Vertex (curve)|vertices]] of ''S''.
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| [[Image:MedialAxisTransformOf3dObject.png||right|thumb|240px| (a) A simple 3d object. (b) Its medial axis transform. The colors represent the distance from the medial axis to the object's boundary.]]
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| ==See also==
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| *[[Voronoi diagram]] - which can be regarded as a discrete form of the medial axis.
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| *[[Straight skeleton]]
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| *[[Local feature size]]
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| ==External links==
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| *[http://mecca.louisville.edu/~msabry/projects/cskel.htm Variational Symmetry Axis]
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| *[http://www.balintmiklos.com/scale-axis/theory_socg_2009.html The Scale Axis Transform] - a generalization of the medial axis
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| *[http://polyskeleton.appspot.com/ Straight Skeleton for polygon with holes] - Straight Skeleton builder implemented in java.
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| *[http://www.staff.science.uu.nl/~gerae101/motion_planning/navmesh.html Multi-layered medial axis] - a generalization of the medial axis (for e.g. representing an airport or a multi-storey building)
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| *[http://code.google.com/p/powercrust/ Powercrust] a portable implementation in C++ that generates an approximate medial axis.
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| *[http://www.cis.kit.ac.jp/~gert/medax.html An intuitive explanation of the medial axis transformation] provides a more readable definition of the subject for the layman.
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| ==References==
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| <references/>
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| *''From the Infinitely Large to the Infinitely Small: Applications of Medial Symmetry Representations of Shape'' Frederic F. Leymarie1 and Benjamin B. Kimia2 [http://www.doc.gold.ac.uk/morpholingua/LeymarieKimia05.pdf]
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| [[Category:Geometric shapes]]
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