How I capture the visible objects (triangles, vertex) in the scene after clipping,
I think you have to try and explain again what it is exactly that you want to do… Maybe it's the fact that I dont speak English as my first language… but I did not quite get it 
Most importantly what do you mean by 'capture'?
object identification (object-visibility list) in the screen and the object's visible part.
There's a method in the camera class, "contains" which checks if an object is inside the frustum or not.
I need since only the active object visible in the screen to compute the object-visibility list (objects or the object's visible part are inside line sight), different to checks if an object is inside the frustum (object visible and objects are completely hidden).