Region is the base type for everything
Abstract region type
The fill_factor (value between 0 and 1) indicates the fraction of the region that is actually included; possible application: large scale resource coverage areas
Shape is the abstract type that is the building block of regions; in real life it needs to be replaced by a concrete shape
Abstract Shape type
Circle shape: center and radius
The coordinates of the circle's center
The radius of the circle
Ellipse shape: adds semi-minor axis and position angle to circleType
Half the minor axis of the ellipse, in radius_unit
Position angle of major axis (Radius).
smallCircleType indicates in polygons that side is along small circle; with optional pole
Vertex is a position with optional SmallCircle element; the SmallCircle element indicates that the polygon side formed by that vertex and its predecessor vertex is a small circle, rather than a great circle; SmallCircle has no meaning in Cartesian coordinates
Polygon: one or more vertices; counter-clockwise encircled area is enclosed; sides should span less than 180 deg in each coordinate if spherical
In order to form polygons, vertices are to be connected with straight line segments. In the case of spherical coordinates: greatcircle segments; if a smallCircle element si present, the vertex and its predecessor are to be connected with a smallcircle, by default in the CoordSys that is referenced; optionally, a pole may be specified (other than the CoordSys pole) that defines the smallcircle system
Box shape: a rectangle defined by its center and size on both dimensions; since it is a polygon, it is redundant, but simple rectangles with great circle sides are awkward to define in spherical coordinates
The coordinates of the box's center
The lengths of the box's sides
A sector is the counter-clockwise area between two half-lines
The vertex position of the sector
The area ccw from this position angle is included
The area cw from this position angle is included
An area on the unit sphere defined by the intersection with a plane
This needs to be a spherical coordinate vector; it is the unit vector that is normal to the plane that forms a constraint for a convex
The distance along the normal vector where the constraint plane intersects that vector; if positive, the spherical sector on the far side (seen from the center) is selected; if negative, the point of intersection is in the opposite direction of the vector, resulting in more than a hemisphere; the valid range is -1.0 to +1.0
A convex polygon defined by one or more Constraints
A convex hull
This is a hook for regions defined in sky indexing schemes
The union of two or more regions is a region
The intersection of two or more regions is a region
The negation of a region is a region
Head element of the Region substitution group
The intersection of two or more regions is a region
The union of two or more regions is a region
The negation of a region is a region
AllSky is just a convenience, meaning no bounds
The circle is defined by a center and a radius
Ellipse is an extension of the circle shape, with a minor axis radius and position angle
A polygon is a shape delineated by a list of vertices connected with lines, great circles, or small circles; the inside of the polygon is circumscribed counter-clockwise by the list of vertices; the polygon may be concave but not self-intersecting; the last vertex in the list is the predecessor of the first. For celestial coordinates it is assumed that we are looking at the sphere from the inside, with Right Ascension increasing to the left. Sides should span less than 180 deg in each coordinate.
A box is a rectangle; the midpoints of its sides are the endpoints of a cross centered on the center position with arms that extend half the size for each coordinate in both directions, and the sides are lines or great circles that intersect the cross at its end points at right angles; the arms of the cross are parallel to the coordinate axes at the center point.
A sector selects the area between two half great circles or half lines meeting in a specified point
A convex is defined as the intersection of one or more half-plane constraints with the unit sphere; this may also be described as the union of one or more convex polygons bounded by one or more constraint planes
A convex hull is the convex polygon that contains all of the specified positions; the points have to be constrained to lie in one hemisphere
A shape defined through a sky indexing scheme; this is really a substitutino group for concrete implementations