Exteriorleft

Exteriorleft is a term used in some branches of computational geometry and computer graphics to describe a region associated with the boundary of a polygon or polyline, defined with respect to a chosen direction of traversal. Informally, the exteriorleft side of an edge is the side of that edge that lies on the exterior of the shape when the boundary is followed in the specified order.

In practice, exteriorleft is not a universally standard notion. Its definition depends on the orientation of the boundary and on how exterior is distinguished from interior. For a closed polygon with a fixed orientation, the interior is traditionally on a particular side of each edge; exteriorleft is then the complementary side, and its status can change if the boundary orientation is reversed. Because of this dependence, implementations must fix an orientation before referencing exteriorleft.

Uses include algorithms that require separation of exterior and interior regions, such as certain polygon clipping, flood-fill variants, and ray-based point-in-polygon tests. Exteriorleft can be encoded in data structures as a property on directed edges, indicating whether the exterior region lies to the left of the edge.

Terminology: exteriorleft is a compound term, often written as exteriorLeft or ExteriorLeft in code, reflecting a camelCase naming convention. It is related to concepts such as interior, exterior, winding number, and the left-right orientation used in many geometric algorithms. The term is not universal and appears primarily in niche texts or code bases exploring boundary-based region partitioning.