ENTITY * ent = some_entity;
function trace()
{

   var distance_from_camera = 0;
   VECTOR mouse_to_world;
   vec_set(mouse_to_world,vector(ent.x,ent.y,distance_from_camera));
   vec_for_screen (mouse_to_world, camera);

   c_trace(mouse_to_world,vec_to_somewhere,IGNORE_ME);

}