Motion Graphics by Jeremy Clark

Experimenting with Planar Tracking

Another skill which I have set out to learn this year is compositing virtual elements into live action sequences.This technique can be referred to as “Tracking”, “Match Moving”, and “Planar Tracking”.

First off, I wanted to learn how to master planar tracking.

“Planar Tracking” is a fairly new technology and gains it’s name from how the system analyzes the source video – It seeks out different ‘planes’, isolating surfaces that can be followed through a shot. The user can define a plane for the computer to follow, and if tracked successfully, the movement of the ‘tracked’ object can be used to drive the motion of newly composited elements, or inverseley to stabilize footage within a frame.

A powerful Planar Tracking program is Mocha, by Imagineer Systems. Once tracking information is derrived from a videoclip within Mocha, it can be used in After Effects to animate the motion of any composited layer. Virtual elements  can use this tracking information to control what is essentially a camera move that mimics that of the original shot, so that the virtual and live action elements appear to have been shot by the same camera.

My Planar Tracking Workflow took this shape:

1. Live action footage was shot with my digital camera and imported  into Mocha as an image sequence.

2. Using “X-Splines” I defined the rough shape of the plane I wanted to be tracked. In this example i used 5 splines, as despite being a square object, 5 points would form a shape that would better describe the movement of the plane throughout the entire shot.

3.  With “Splines” in place I then had to set “Corner Pins”. I only set 4 of these, one in each corner of the square plane I was tracking.

4. With these in place I set Mocha to “Track Forward” from a point in the centre of my footage. With this track completed, I returned to the same point and set Mocha to “Track Backwards”. This centre point that I speak of need not strictly be at the centre the timeline, but rather a point throighout the footage when the plane you want to track is at it’s most flat with the camera.

5. WIth the footage succesfully tracked I then clicked to “Export Tracking Data” from Mocha.

6. In After Effects I opened a new composition that matched the dimensions of my original footage. It’s really important to make sure that Clip Dimensions, Pixel Aspect Ratio and Frame Rate are all the same between your Mocha Track and you After Effects project.

7. I placed my original raw footage (From my digital camera) onto one layer in After Effects, then created a new layer, which would include the elements I wished to “Track” into the shot. In the case of this example I opted for a Text layer.

8. With the tracking information copied to my clipboard from Mocha’s “Export Tracking Data” window, I then simply highlighted my new text layer and “Pasted” the information, making sure that the timeline was set to the beginning of my After Effetcs comp.

And that was it! My new text (virtual object ) now appeared to move with my original (live action) footage. Check out my video above for examples of how this process unfolded. From here I hope to further experiment with this technique, and then eventually move into placing 3D object into a live action scenes through a technique which is referred to as “Match Moving”. Stay tuned, folks!


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