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Explore how to use the motion tracker and stabilizer built into After Effects and shows how to handle a variety of shots. Author Chris Meyer leads a quick tour of the third-party software mocha and demonstrates the workflow for The Foundry's KEYLIGHT, both bundled with After Effects. The course also covers tracking a greenscreen shot with a handheld camera and replacing its background.
The After Effects Apprentice videos on lynda.com were created by Trish and Chris Meyer and are designed to be used on their own and as a companion to their book After Effects Apprentice. We are honored to host these tutorials in the lynda.com Online Training Library®.
Now that I've added a Track Point to my layer, I need to find a good feature inside this clip to track. I am looking for a collection of pixels that have high contrast, either in luminance values or in color, to the area around it. Candidates include these little bright highlights up in the clouds or these clumps of grass down here along the horizon line. The next thing I need to do is scrub through the length of my clip that I want to stabilize and make sure this feature I've identified stays on screen for preferably the entire shot and that nothing obscures it, in other words, walks in front of it.
Because these wildebeests are walking across the horizon for the length of this shot, those clumps of grass probably wouldn't work so well, because the wildebeests will get in the way, and After Affects might think its leg is actually a part of the grass. Far better are those highlights along the top of the screen. They are staying in the shot. There is a chance that the clouds are drifting slowly during the shot and I could reintroduce that animation later on if I wanted to, but they will work fine for this shot. Now, the Tracker Stabilizer in After Effects can track both back in time and forward in time.
And if I want to, I can go ahead and do both. However, all things being equal, it's better for me to start at the end or to start the clip, then track in one continuous direction throughout the length of the clip. I am going to go ahead and start at the very beginning of this clip. Next, I need to move the Track Point into place. The Track Point has a few distinctive sections. The inner plus symbol is known as the Attach Point. If I am doing motion tracking, where I want to attach one layer onto a feature that's moving in another layer, the placement of the Attach Point is very important.
In this case, where I am doing stabilizing, it's less important. It is the center for any stabilization that takes place, such as scaling a rotation, but it's not something I need to worry about so much in this shot which has fairly minor motion. The insert rectangle is known as the Feature Region. This is the collection of pixels at high contrast area in the shot that After Effects is going to be looking at and trying to match from frame to frame. The outer rectangle is the Search Region. That says how far and why should I go looking for this collection of featured pixels in every subsequent frame while I'm tracking? To move the entire Track Point at once, place your cursor over it until you see this four-directional arrow appear at its space.
That means you can pick up and move the whole thing. I am going to go ahead and move it up in the area of these little highlights in the clouds. Notice while I am doing so, the Feature Region is magnified to give me a better idea of what detail exists inside those pixels I am selecting to be my tracked feature. I can choose this area here, this high contrast area here, or just because I want to practice resizing the Feature and Search Regions, I want to go for this slightly longer area right here. Now, your goal with this Feature Region is that it must enclose this contrasted area.
If it's a little bit cut off like this, it's very hard for After Effects to say, where is the sharp end for me to look at? What you want is a nicely defined corner, end of the stick, or other point, or to go ahead and grab one of these corners and wind it out a little bit so that it encloses an entire bright area and is surrounded by pixels of the dark area. That gives it a nice feature to go ahead and isolate and track. So I've set up my Feature Region, I need to set up my Search Region.
Since this shot has fairly little motion, I can make it fairly tight. My main thing is worrying about, how much does this shot move in its biggest jump from frame to frame, not during the entire length of the clip, just between one frame and the next, because After Effects will automatically reset these regions on every frame. I don't want to make it too big. For example, if I put it out to here to enclose these other spots as well, After Effects may get confused while searching. Is this the featured ones, or is this the feature, or is this the feature? It's better to make it as tight as I can get away with.
Now, normally, picking up and moving one of these corners moves all four corners symmetrically. If I want to move just one of these corners, I need to hold Command on Mac or Ctrl on Windows and click and drag that corner. This is particularly handy if the motion is all in one direction during the shot. For example, if I know that this region that I want After Effects to track is moving towards the right throughout this shot, I'll bias my Search Region more towards that direction of travel.
I don't need to worry about these trailing pixels if the Feature Region is not moving in that direction. But in this case, I am going to go ahead and center my Search Region around my Feature Region, because this shot is randomly bouncing from frame to frame. Now, remember, this four-headed arrow means I'm going to move both of these regions together. If I want to move just the Search Region, I move my cursor over it until a little box appears at the foot of my cursor. That tells me I am going to move this region's box, like so.
Now, I cannot emphasize how important it is to choose a good feature to track and to tighten up your Search Region around that feature. This is the information After Effects is going to be using for the track. If you get a bad track, you may need to undo and either tweak this region or go find something else and track in the shot. We'll spend a little bit of time on this before we move on.
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