I'm a little beginner.. I need some help, please !

Hi! Sorry for my poor english, I’ll try to be as clear as possible.
How can I determine the size on the screen of a segment given by its two vertices in the scene?

Tu ne serais pas francais des fois???
For init the OpenGL Panel, you have specified the size of the window and the type: projection or persceptive…
you have to make projection with cos and sin with these window parameters
Le Bayonnais