You see this in a lot of movies and such but how do you do it?
I never found a tutorial in that subject.
lets say I want someone to hold a sword or any other kind of object or weapon that is a 3d object.
or another example I want that someone would have instead of an arm, to lets say have a robotic arm but only from the elbow.
any idea? even a link to some information could be helpful,
Tracking, match moving. Some things can be easily done in AE, other things require that you use other software to do the tracking and matchmoving. Basically you need to track the hand in 3D then if the camera is moving track the camera, then combine the track of the hand with the track of the camera so you can orient your CGI element with the hand and match the movement of the camera. With careful planning on set this is fairly easy to do, if you just grab a shot without knowing how you are planning to add the 3D object then it can become an arduous task requiring many hours of doing things by hand.