Surfaces

A surface is a two-dimensional closed shape, usually stored as a sequence of points. A simple surface may be defined as consisting of a single contiguous area that is associated with a single exterior boundary and 0 or more interior boundaries, or holes. All boundaries consist of consecutive points.

Polygon

A polygon is a planar surface, defined by 1 exterior and 0 or more interior boundaries, where each interior boundary defines a hole in the polygon.

The following rules define a valid polygon:

• Polygons are closed.
• The boundary of a polygon consists of a set of linear rings that make up its exterior and interior boundaries.
• No two rings in the boundary cross, but they may meet at a point.
• All lines constituting the polygon must be continuous.
• The interior boundaries are always connected point sets.
• No two boundary points may be the same.

Ellipse

An ellipse is a curved planar surface defined by a single coordinate defining its centre and further properties describing the radius of its axes in the x and y directions.

An ellipse also has a property defining its rotation from the horizontal.

Rectangle

A rectangle is a planar surface defined by two coordinates that constitute the bottom left and top right corners of the rectangle in coordinate space. Two further coordinates, defining the intermediate points of the rectangle, are determined geometrically by extrapolating lines horizontally and vertically from the base coordinates and placing points at the intersection of these lines.