A polygonal region within a 2-dimensional Frame
Within a SkyFrame, neighbouring vertices are always joined using the shortest path. Thus if an edge of 180 degrees or more in length is required, it should be split into section each of which is less than 180 degrees. The closed path joining all the vertices in order will divide the celestial sphere into two disjoint regions. The inside of the polygon is the region which is circled in an anti-clockwise manner (when viewed from the inside of the celestial sphere) when moving through the list of vertices in the order in which they were supplied when the Polygon was created (i.e. the inside is to the left of the boundary when moving through the vertices in the order supplied).
SimpVertices: Simplify by transforming the vertices?
AST_DOWNSIZE: Reduce the number of vertices in a Polygon.
AST_CONVEX$<$X$>$: Create a Polygon giving the convex hull of a pixel array
AST_OUTLINE$<$X$>$: Create a Polygon outlining values in a pixel array