Concave hull (alpha shape)
A hull that follows concavities more closely than a convex hull, controlled by a parameter (often called alpha).
Category: Geometry · Also known as: alpha shape, alpha-shape, concave hull polygon
Definition (expanded)
Concave hulls (often implemented via alpha shapes) try to produce a boundary that better matches the input points or geometry, including inward dents. The result depends heavily on the alpha/tolerance parameter: too small can create holes or disconnected parts; too large approaches the convex hull.
Common mistakes
- Picking an alpha/tolerance that creates gaps, tiny spikes, or self-intersections.
- Assuming concave hull is unique (it’s parameter-dependent).
Related terms
Convex hullThe smallest convex polygon that fully contains a geometry (think: a tight rubber band around it).PolygonA closed area geometry defined by an exterior ring and optional interior rings (holes).MultiPolygonA geometry containing multiple polygons (parts), each with its own exterior ring and optional holes.ToleranceA threshold distance used by simplify, snapping, and some repair operations.VertexA single coordinate point in a ring or line; polygons are made of many vertices.