I'm not clear what you are asking. Why should that even be a problem? A camera is a camera is a camera and how you add your 3D layers is totally up to you. You are seriously over-thinking and over-complicating this. It might be a good idea to lean back, read the general AE help on 3D layers and then clear your head. You are simply winding yourself up too much over "3D tracking". It's a tool associated with AE's 3D space, nothing more. It is neither required to be used to just dump some text in a comp nor has it any inherent magic that solves problems you couldn't tackle otherwise.
but what if I want to select a different region when selecting the dots, create another null again, and then track different elements to that null?
So go right ahead and select some other points and then right click -> create null. what seems to be the problem with 2 or more tracks in one shot? The real advantage of Tracking the Camera instead of point track or planar track is that you get the whole information in one go and can choose where you want your attach points based on that information. if you need to understand more about tracking try here:
maybe I asked it a bit wrong. I did exactly that Roei, en then I have 2 or 3 nulls just perfectly going where I want them to be. but, anything I put in my scene will only go along with the first tracked null. if I parent it to the second null it still only goes with the first one. thats pretty much the problem.
You are missing the basic point of camera tracking. When you get a camera track it adds keyframes to the camera, you don't get moving 3D layers. Adding multiple 3d elements to a scene and parenting them to each other will do nothing because they don't move. There's a basic flaw in your expectations.
The steps for camera tracking are
- Track the camera in a scene that has a bunch of fixed geometry that does not move (shots with waves crashing on a beach or a field of wheat blowing in the wind or a crowded sidewalk full of people are nearly impossible to camera track with any software)
- Set an origin and ground plane where you want comp world center to be.
- Add as many 3D objects as you need in your scene
- If you need additional 3D objects then you must either place them using the tracker, or match their position to one of the objects you placed in your scene and them move them in relation to the 3D scene you have created
Once again, and check this by selecting all your layers and pressing the U key twice to reveal all modified properties, there are NO keyframes added to the Nulls, solids or text layers you place in a scene using camera tracker.
Maybe these videos will help. I threw them together a long time ago just for quick demonstration of how to use the 3D tracker before set origin and ground plane was an option.