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Convex Hulls

What is a Convex Hull?

A set of points C is called convex if for any two points p and q in C the entire segment is contained in C. The convex hull of a set of points is the smallest convex set containing S.

On the right you see a set of points in the plane, the points on the convex hull are drawn in red. The picture is a screenshot from the example of how to compute a convex hull

Example of Convex Hull

Example of how to compute a convex hull

The function

    list<POINT> CONVEX_HULL(const list<POINT>& L)
computes the convex hull of the points in L and returns its list of vertices. The cyclic order of the vertices in the result corresponds to the counter-clockwise order of the vertices on the hull.

We use the notation POINT to indicate that the algorithm works both for points and rat_points. See also Writing Kernel Independent Code.

Alternative Algorithms for Convex Hull

LEDA provides three different algorithms for computing the convex hull of a point set
  • Sweep Algorithm: list<POINT> CONVEX_HULL_S(const list<POINT>& L);
    Running time: worst and best case: O(nlogn)
  • Incremental Construction: list<POINT> CONVEX_HULL_IC(const list<POINT>& L);
    Running time:
    • worst case: O(n2)
    • average case: O(nlogn)
    • best case: O(n)
  • Randomized Incremental Construction: list<POINT> CONVEX_HULL_RIC(const list<POINT>& L);
    Expected running time: O(nlogn)

The randomized incremental construction is the default algorithm for convex hull. It is generally faster in practice than incremental construction. It is also faster than the sweep algorithm if there are only few hull vertices. If the input points are on the unit circle the sweep algorithm is faster. More details can be found in the LEDA Book.

Convex Hulls in 3D

The function

    void CONVEX_HULL(list<d3_rat_point> L, GRAPH<d3_rat_point,int>& H)

takes as argument a list of d3_rat_points and returns the (planar embedded) surface graph H of the convex hull of L. The algorithm is based on an incremental space sweep.

Running Time: O(n2) in the worst case and O(nlog n) for most inputs.

Example of computing convex hulls in 3D

See also:

Linear Lists

Data Types for 2-D Geometry

Parameterized Graphs

Writing Kernel Independent Code

The Width of a Point Set

Geometry Algorithms



Number Types

Manual Entries

Manual Page of Geometry Algorithms

Manual Page 3D Convex Hull Algorithms

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