Transforms position from world space into viewport space.
Viewport space is normalized and relative to the camera. The bottom-left of the camera is (0,0); the top-right is (1,1). The z position is in world units from the camera.
// Finds out whether target is on the left or right side of the screen
var target : Transform;
function Update () {
var viewPos : Vector3 = camera.WorldToViewportPoint (target.position);
// viewport coordinates range from zero to one
if( viewPos.x > 0.5 )
print ("target is on the right side!");
else
print ("target is on the left side!");
}