Re: [help-3dldf] Re: button-hole problem

[L. Nobre G.]

On Sat, 23 Apr 2005, Laurence Finston wrote:

> On Sat, 23 Apr 2005, L. Nobre G. wrote:
>
> > >From the experimental physicist point of view the perspective projection
> > of a circle on a plane is "allways" an ellipse.
> > But
> > the projection of the center of the circle is "not" the center of the
> > ellipse.
>
> How can this be reconciled with what Laurent S. and Peter have written?
> Again, in words of one syllable, if possible, please.

The projection of a circle will not be an ellipse *only* when the circle
touches (parabola) or crosses (hyperbola) the plan, parallel to
the projection plan, containing Focus::position. Everything on this plan
projects at infinty. This kind of projection is not computable. Avoid it.
Forget it.
Now, the center of the projected ellipse is the projection of the circle's
center *only* when the circle is parallel to the projection plan and when
the circle's center is on the line --Focus::position--Focus::direction--.

Greetings

Lu\'{\i}s Nobre Gon\c{c}alves - http://matagalatlante.org