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Next, we are going to tackle a more difficult tracking assignment, a corner pin track, where we need to track four objects in screen all at the same time. If you have access to the Exercise Files, in the Comps folder open up 04-Corner Pin*starter. I am going to RAM Preview this quick. What we have is a computer in an MRI facility, and let's say for some reason we wanted to replace that display. In this case just tracking one object isn't going to be good enough, because there is some slight perspective shift during the course of this shot.
Therefore, we're going to need to perform some type of perspective track where we are looking at the corners of the screen individually so that we can warp a brand new layer to fit the perspective of this original display. I'll stop my Preview. Fortunately, the woman does not move in front of the monitor at any point of this shot, so we can pick anywhere to begin. I am going to go ahead and just for fun pick the end of the shot, just to give you experience doing something slightly different. Now, the one other thing you need to do before you go too far is add into this composition the new footage or layer you want to put over that monitor.
In the Exercise Files, go into the Sources folder, and we have a movie called Heart Monitor, which is a very simple animated display. This is something you might want to try your own hand in creating just for fun. Anyway, we are going to drag Heart Monitor into our composition with the MRI computer. It initially obscures the MRI computer shot, but don't worry, we are about to fit it onto the monitor in that underlying shot. Again, it's best to open up the layer you want to track in the Layer panel, but if you forget, don't panic, as long as you have the Tracker panel open and click Track Motion, After Effects will automatically open the Layer panel for you with the selected clip loaded in.
And to confirm that, the Motion Source is indeed the MRI computer movie. In the next movie we'll show you how to perform an actual perspective track, but first I have to warn you, this is the type of shot that After Effects is not very good at, because it has to do four different tracks independently and accurately. After Effects CS4 or later comes bundled with another application called a mocha from Imagineer Systems, which is far better at this type of tasks, but it's also more difficult to use. Fortunately, there's lots of resources out there on how to use mocha, but for now we are going to attempt this with After Effects' own built-in Tracker.
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