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Refining the track

From: After Effects CS6 New Features

Video: Refining the track

You see all these colorful dots around here? These are track points that After Effects has found to follow throughout the scene. A lot of them may indeed work. Some of them may not work. It's always a good idea to drag the Current Time Indicator through the shot and see if any of those track points don't make sense. For example, anything that's stuck to that Artbeats burn-in logo would not be a good track point. If I found any of these X's stuck on that burning, I would want to delete that track point.

Refining the track

You see all these colorful dots around here? These are track points that After Effects has found to follow throughout the scene. A lot of them may indeed work. Some of them may not work. It's always a good idea to drag the Current Time Indicator through the shot and see if any of those track points don't make sense. For example, anything that's stuck to that Artbeats burn-in logo would not be a good track point. If I found any of these X's stuck on that burning, I would want to delete that track point.

If you have other things moving through the scene that are not nailed down--car driving through the scene, a boat in the harbor, bird flying by, a person walking across the scene--you would also want to select any track points that follow those rogue elements and delete those. Again, you are not tracking an object in the scene; you are tracking what the camera did to move through the scene. And deleting a point is merely as simple as going ahead and selecting one of these X's or Shift+Clicking multiple X's or even lassoing around multiple points and hitting the Delete key to get rid of them.

You will have to re-solve the camera whenever you delete points, but that usually means you will get a more accurate solve, because you've removed bad track points. The size of these colorful X's that After Effects chose to be track points are based on how near or far away it thinks the object is. It may be hard to see, but on some of these distant buildings are really tiny little X track points. If you have trouble seeing them, you can change Show Track Points to 2D Source and now they will all be the same size.

That makes it a bit easier to follow what's going on. You will only see these track points if you have the 3D Camera Tracker effect selected. Deselect the effect, the track points go away. If you want to see those track points even when the effect is not selected--for example, during a RAM Preview--you can indeed turn on Render Track Points. But I will leave that off for now, and I will switch back to 3D Solve to give us some relative perspective on how near or far away those track points are. Before you start setting up layers that follow features in this footage that you have a good track, in addition to dragging the Current Time Indicator through and making sure these track points are indeed staying stuck on the buildings, there are a couple of other things you can do to help After Effects out before you move any further. One is the shot type.

If you know that the lens had a fixed length to it--no one was doing a zoom during the shot--Fixed Angle of View is a good choice. On the other hand, if you know that someone was zooming in--they had a zoom lens and they were changing the focal length during the shot, or had a cheap camcorder and were on the telephoto zoom button--you are going to change this to Variable Zoom, then have After Effects re-solve based on that. If you were so fortunate as to have a camera person who documented what lens was on the camera, even better is to choose that and enter a specific number.

Now, unfortunately, it's defined as Angle of View, as opposed to Lens Length. It would be nice if you could just enter a camera lens length in here, and that was possible in the days when most people shot high-end projects in 35 millimeter film. But now that there are so many different cameras, with so many different sensor sizes which interact with a lens's focal length, unfortunately, Angle of View is how you have to define these things. In addition to this little helper--and I will go back to Fixed Angle of View and let After Effects do its thing-- there is an Advanced box down here that gives you some additional clues and things you can set.

The Average Error is the result of After Effects guessing where all those points in those buildings are, constructing a mythical 3D world, then projecting them back onto this scene and seeing whether or not things stay aligned during the course of the shot. This is the average error in its attempt to reconstruct the scene. On a standard-def shot, to have anything under half pixel is a really low average error. I'm very happy with that. However, if this was unusually high, I might try different solve methods. The default is to auto-detect what the solve method should be, let After Effects figure out what's going on, but there are some other options.

Typical is, cameras moving through the scene, either on foot, attached to car, on a helicopter, et cetera. If you have a very simple scene with very few details--for example, if you're flying over, say, the plains out in the west, telling After Effects there aren't a lot of details to go looking for and choosing Mostly Flat Scene will help to get a better solve. Another common shot is when you do happen to have the camera on a tripod, or you're holding the camera but standing in one place and then just rotating and panning and tilting the camera around.

Unfortunately, that's one of the worst scenes to try to solve. There aren't as many parallax clues, which makes it harder to decide what's close and what's far away. It's best to leave this on Auto Detect, but if you have a high Average Error and if you know more about the scene, you can choose one of these other options and see if your error goes down with a different solve. You can see I got a slightly smaller error there, but it chose to use different points. And actually, from my purposes, that's a less attracted set of points, because I wanted to put things on these buildings and on these buildings back here.

So I'll go back to Auto Detect. Hide Warning Banner is pretty self-explanatory. Detailed Analysis basically says, create and track even more points than you normally would. If you are having trouble getting a good track on a scene because there is not a lot of detail in it, you might try checking that and trying again. Most of the time you won't need it. Anyway, now that After Effects has tracked this scene, we have deleted any points that we know are bad, in the next movie, we can move on to actually creating brand-new objects in After Effects that attach to these points in space.

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After Effects CS6 New Features

31 video lessons · 11772 viewers

Chris Meyer
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