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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®.
In the previous movie, we set up a composition, select the footage that we wish to track and applied the tracker to it. Now we need to set it up for a perspective track. Underneath Track Type, you have a few additional choices we haven't played with. You've already worked with Stabilize and you've already worked with Transform, Raw just creates plain old tracking data that you're going to later copy and paste or use expressions to assign to other parameters, but the two of interest to us now are Parallel corner pin and Perspective corner pin.
Parallel corner pin assumes you have a parallelogram with the top and bottom edges of what you're trying to track are supposed to say parallel to each other as well as the left and right sides. Perspective corner pins as well maybe camera is skewed and you can't count on those edges being exactly parallel. If you go ahead and look at this monitor over here, these points are closer to each other than these points, so this line is not parallel to this line. In that case, you need to do a Perspective corner pin where you're looking at each corner of this rectangle independently. Your goal is to eventually map a new rectangle over the one you define here.
So I'll select Perspective corner pin and now four track points will appear. I need to assign one onto each corner of this monitor. I'm going to hold Command in Mac, Ctrl on Windows and press plus to zoom in, then hold to Spacebar to pan around. It's very easy to have good intentions with this track, but to go wrong. For example, on my first pass, I said oh, well, let's just tighten things up just in those little bright corners like this. Just pan down quickly, show you what I'm up to and say let's just track these very nearly focused regions, because we've nice cleanly defined corners.
And I'll tighten up my search regions as well so that After Effects doesn't have to look too far. However, with experimentation, I found that this doesn't work very well and I'll quickly demonstrate that to you. I'll click Options under the Luminance track, no need to process, no Fields, Subpixel Positioning fine. No need to Adapt Feature. It doesn't move that much, but if it does change a lot, let's stop so we know something is going wrong. I think this is good so far, I'll zoom out a step, re-center and analyze backwards so that it started on the last frame to the shot.
But watch these four corners. You'll see there is a quite a bit of jumping going on in these shots. The part of that is due to the fact that while tracking, After Effects is displaying the track points for the previous frame while it's analyzing the next frame. So we'll seem to be behind. As I'll drag through here, you can see a lot of jittering going on in those corners where there should not be any movement at all. Well, it's quite common with motion tracking that your first attempt is not going to work, and you will need to try alternate ideas.
In this case, I'm going to undo to get rid of my previous work that I done on this. I can keep undoing or I can just twirl open the layer inside the Timeline panel and delete that Tracker. I start over, Track Motion, type Perspective corner pin. After a lot of trial and error, I've actually found on this shot that I get better results if I just go ahead and use the original size feature regions and place it over those corners. In this case, I'm picking up more information, not just the flickering C or T, but also some additional hints in the shadows and the corners of the Bezel monitor, etcetera.
I will, however, tighten up the search regions a little bit just to cut down on false triggering, like that and like that. Now on a Perspective corner pin shot, the attach points are particularly important. I'm going to zoom in another step here. I really want to make sure that those attach points are beyond the area I want to obscure. Now if this attach point is inside, well, this is where my brand-new layer is going to get pinned and some of the old layers are going to peek out behind it.
I really need to make sure that one, I see the attach point icon and those it also changes to a white arrow, and then put my attach point just outside of that corner, so I make sure I cover up the old display. I hold down the Spacebar and drag to pan around, tweak this out a little bit, come down this corner, tweak that out a little bit and then go over to this corner, and make sure I'm covering the corners well. Now you will notice that due to the bowing of the monitor, the lines in between those corners are not perfectly covering the old display. Don't worry; we're going to tackle that refinement in the next movie.
Right now, we're just trying to get a good track. I zoomed out so I can see the whole display. I double-check my Options and I'm pretty sure luminance is right; I've got good contrast. Well, let's just go ahead and analyze backwards because I am starting again this clip again. With these larger feature regions, I do get a better track. However, as I drag through here, I still see there is a little bit of jittering going on and this is indeed a weakness of the After Effects corner pin tracker.
It does not know that this relationship is supposed to stay somewhat fixed. The shape defined by these corners should not be changing. This is where that third-party tracking tool, mocha, which is bundled with After Effects CS4 and later comes in handy and in the sidebar movie, I will show you how to use mocha on this particular shot. But anyway, we now have our four corner track, now let's look at our Motion Target; the new screen we want to put over this existing screen. In tracking situations, After Effects will look above for layers; there is only one, the Heart Monitor, which you put in earlier.
But if you did not select it correctly, you can select Edit Target and pick which layer you indeed want to corner pin into these four track points. I'll click OK, click Apply. In this case, I don't get an Options dialog, because there is no separation of X and Y, I need to do all four corners accurately, and I'll press 0 to RAM Preview, and now you can see our new display in context over the old monitor. And in fact, it doesn't look anywhere near as bad as we thought is going to look when we are assumed in looking just the track points.
It is very easy to obsess about track quality and frankly, you should obsess about track quality. However, before you toss out your work, at least, preview and see if the shot works. This shot does basically work, but I have some issues, like it could be the old display, peeking out around on corners here, and it could be integrated better into the shot and that's what we'll tackle in the next movie.
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