Editing Convex hull (section)

===Upper and lower hulls===
In two dimensions, the convex hull is sometimes partitioned into two parts, the upper hull and the lower hull, stretching between the leftmost and rightmost points of the hull. More generally, for convex hulls in any dimension, one can partition the boundary of the hull into upward-facing points (points for which an upward ray is disjoint from the hull), downward-facing points, and extreme points. For three-dimensional hulls, the upward-facing and downward-facing parts of the boundary form topological disks.<ref>{{harvtxt|de Berg|van Kreveld|Overmars|Schwarzkopf|2008}}, p. 6. The idea of partitioning the hull into two chains comes from an efficient variant of [[Graham scan]] by {{harvtxt|Andrew|1979}}.</ref>