Re: button-hole problem

On Sat, 23 Apr 2005, Peter Vanroose wrote:

> > For example, if the projection point is on the target plane,
> > which happens to be disjoint from the circle, then the projected
> > circle is the EMPTY SET.

> The only situation where the projection is empty, is when the
> projection point is in the same plane as the circle, and the projection
> plane is parallel to this plane.

This is interesting.  Objects that can't be projected are culled in
`Picture::output()' in 3DLDF.  I will have to test this case.

> (Actually, in that case the projection is a line interval at infinity.
> So in projective space the projection is again non-empty. And again a
> conic section, now a degenerate ellipse.)

Thanks again for your help.

Laurence