3 Replies Latest reply on Nov 19, 2015 12:29 AM by andrewm78418379

    Track and zoom green screen object?


      I have two shots I am trying to merge.  One is a handheld zoom and tracking shot of a person wearing a mask, and I am trying to put an orb I green screened onto the front of the mask.  I had been trying to animate the sequence manually, but decided to try out After Effects CC as it seemed like there would be a straightforward solution (like anchoring to a 3D point).  I haven't been able to find a solution that works well yet, the closest I've come is creating a camera tracker, but it does not seem to compensate for the zoom when I attach the orb layer to the solid I create.  Hope this is just something simple I'm missing! thanks for your help - Andrew

        • 1. Re: Track and zoom green screen object?
          Mathias Moehl Adobe Community Professional

          Have you tried to track it in mocha (a great tracker that is bundled with AE)?


          This recording of a live webinar helps you to get started with mocha:


          Learn mocha AE - Online Training Webinar | mamoworld

          • 2. Re: Track and zoom green screen object?
            Rick Gerard Adobe Community Professional & MVP

            The easiest solution can't be explained unless we see the shots. Here are a few things to consider. Perspective is controlled by camera position, focal length controls the framing. You are trying to blend a moving camera with a changing focal length. This means that there is a problem right off the top with matching perspective. This may not be important because I'm not sure what your shots look like. So there is this hand held shot of a person wearing a mask and they are walking so the camera and the person are both moving. Then there is this orb that was shot on a green screen that you want to attach to the mask. You have zoomed in on the shot of the person so as the perspective changes in the whole shot because the camera is moving and the perspective of the person moving in the shot changes because they are moving in the scene and the scale or framing of the shot changes because you are zooming in. Then you have  an orb (something spherical) that you shot with a locked down camera so the perspective and framing does not change. When you say that you want to put the orb onto the front of the mask I'm picturing the final composite as a shot of an actor wearing a mask that has some kind of trinket that looks like an orb and the actor is walking toward the camera while the camera zooms in. If that's it let's break down what is happening in the shot and the challenges that you face.


            Let's take the orb first. This is a locked down shot so there are no issues of changing perspective or changing scale (zooming in with the camera) do deal with. The only thing that you can do is hope that the perspective of the orb is close enough to the perspective of the actors face that it does not matter.


            Now let's talk about the hand held shot of the actor. As the camera moves there is a change in perspective in the whole scene. Because the actor is also moving, there is a change in perspective of the actor. Since the orb is going to be attached to the mask the actor is wearing you really don't need to worry about perspective changes in the background, you only need concern yourself with the perspective change of the actor. If the distance between the camera and the actor remains fairly constant then the perspective change is probably not going to be much of a factor. If the actor starts 10 feet from the camera and ends up 2 feet from he camera then you may have to deal with the perspective change as well. Now let's consider the zoom.


            A zoom does not change perspective, it changes only the framing or scale of the shot. You are going to have to figure out how to measure that change in scale and apply that change to the scale of the orb layer. Fortunately, that part may not be too hard.


            Let's now talk about attaching the orb to the mask. If the distance between the camera and the actor is fairly constant then you don't have to worry much about a change in perspective. This means that the easiest thing to do would be to stabilize the mask so that it is not moving. If the Actor does not turn their head very much then this is fairly easy. If the actor does turn their head then this becomes much more difficult. Let's assume that the actor does not move their head. You'll need to track position of the actors face and mask and scale and rotation. Then you'll need to apply that position, scale and rotation to the orb layer. You could do that kind of tracking by tracking the eye holes in the mask or by tracking the top of the head and the chin or two other parts of the mask that have a fixed geometry. Once the head and therefore the mask is tracked for position, scale and rotation you can apply that data to the orb. There are basically two ways to do that. You can either stabilize the shot so that the mask does not move in the frame, then position the orb on the stabilized mask, and then put the motion back in the shot and apply the tracking data to the layer with the orb so that it moves with the actors head. The other option is to just apply the tracking data directly to the orb layer to position it on the mask and match the scale and rotation changes to the actors head. You can use AE's tracker or the tracker in Mocha for either. It easiest and most effective method depends entirely on the shot.


            If the actor turns their head during the shot then everything in the preceding paragraph applies plus you need to track the perspective change in the actors head. This may be where you use camera tracking since AE and Mocha AE do not have any real 3D tracking tools that can track individual 3D elements like the actors face in a scene. Depending on the shot there may be a way to use AE's camera tracker to just track the movement of the face and then add a 3D layer of the sphere so that it will track with the movement of the actor. You could even use a 3D object. You would do that by first stabilizing the actors head in the shot so that the position, scale and of the actors head was locked down. You would then duplicate that layer and mask the layer so that only the actors head was visible in the shot. You would then pre-compose the masked shot of just the actors head and run the Camera tracker on the pre-comp. By doing this you may be able to fool the camera into thinking that the actors head does not move but the camera is moving around the actors head. If you can get a good track then you can add a null and a camera to the scene and then make the orb layer 3D and match i't position to the null. If the orb stays attached to the actors mask as it should then you could turn off the footage layer leaving only the orb and camera in the pre-comp.


            Now all you have to do is to put the movement back in the main shot by reversing the stabilization and apply that tracking data to the pre-comped orb.


            Here's a tutorial from Mikey that's pretty good and gives you an idea of how to camera track a moving head.


            I don't know if this helps at all, but it should point you in the right direction. The explanation also points out the need for pre-production planning when you do a shot like this. Just runnin' and gunnin' can get you in a lot of trouble.

            • 3. Re: Track and zoom green screen object?
              andrewm78418379 Level 1

              Thanks so much Rick for helping out.  It took a while to adapt to each clip, but I finally got the image to a good place using your tips.  Now I know how to be prepared next time, so thanks so much for the lesson. A