Thank you for this information. As I was trying to draw a trunc of a cone I was also trying to have a tangent of two ellipses. But if I project the top of the cone I should be able to contruct this tangent.
But I found now an other strange thing. I can construct a tangent to an ellips result of projection. But if the line starts at the end of a line that is the result of a projection, (the heigh of the cone) it does not work. Either the tangent goes to the center of the ellips or goes in an orthogonal direction or do not draw at all. It happens that this start point is also on the small axis of the ellips by construction.
So I construct an other point on this axis and got the same problem.
I did the same with an ellips directly drawn as (not a projection) and got the same problem.
The tangent to an ellips does not work if the start point is on an axis of the ellips.
Last edited by plgarcia
on Sat May 19, 2012 2:25 pm, edited 1 time in total.