convex != intersection

[Thomas McGlynn]

Ed Shaya wrote:

> 
> Why is an intersection of coverage constraints called "convex"?
> 
> The intersection of two rectangles may be another rectangle.  What is 
> convex about that?
> 
> Ed
> 
> 
Well rectangles are certainly convex.
In general the intersection of any two convex regions
is convex, i.e., any straight line drawn between two points in the
region is contained in the region.

When we deal with regions on the sky there can be substantial
distortions compared to the projections, such that a region which is
convex in the projection is not convex on the sky.    E.g.,
    88<dec<89 0<ra<360 is convex in a Cartesian projection
but it's a ring on the sphere.

So the frame in which convexity is determined needs to be determined.

	Tom