Dealing with obscuration
Video: Dealing with obscurationI've chosen which two dots to track with my two track points to stabilize Position, Rotation, and Scale, but I have a problem where one of actors walks in front of one of those dots. So I need to be careful with how it set up the After Effects options as well as the search region around this second track point. First, I'll go into the Options dialog. I always need to look at Channel. In this case, I've light green versus dark green. So Luminance is my best choice. I've removed any fields or pull down from this footage.
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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®.
- Understanding motion stabilization and keying
- Performing a track
- Applying tracking to effects
- Keying with KEYLIGHT
- Replacing images
- Improving the composite
- Garbage masking
- Dealing with interlaced footage
Dealing with obscuration
I've chosen which two dots to track with my two track points to stabilize Position, Rotation, and Scale, but I have a problem where one of actors walks in front of one of those dots. So I need to be careful with how it set up the After Effects options as well as the search region around this second track point. First, I'll go into the Options dialog. I always need to look at Channel. In this case, I've light green versus dark green. So Luminance is my best choice. I've removed any fields or pull down from this footage.
No need to Track Fields, always leave Subpixel Positioning on. Since the back of this green screen was towards the back of the stage and therefore out of focus, he starts a little bit on the fuzzy side. If you've trouble getting a good track, you might want to try Process Before Match and Enhance or sharpen the footage, but I can leave that off for starters. The real important thing is how to set up these parameters down here. I don't need to Adapt the Feature On Every Frame, because it's keeping pretty much the same shape and isn't really changing size that much. But what's really important is this pop-up.
What happens If my Confidence goes Below 80%? Well, when the actor goes in front of that dot, I'll pretty much will lose the track and lose therefore any Confidence that I founded. I don't want to Continue Tracking, because I've lost the shot. I don't want to Stop Tracking. I need to figure out a way to get around this. What I want to do is Extrapolate the Motion. Tell After Effects, okay, I know you can't find the dot right now, but keep going in the same direction at the same speed you were before and let's hope that it reappears. So I'm going to choose this option. I'll click OK.
The next very important thing is what is the size of this search region? If it's too big or too small you'll have problems. And let me demonstrate. I'm going to start off by placing it very large, because I'm saying, you know, this actor does walk in front of this dot, let's give a larger search region so that After Effects has a better chance of finding the dot afterwards. This sounds like a solid idea. I'm starting at the very beginning of the clip, I'm tracking forward, and watch what happens to this track point. It's initially good, because it's not obscured.
Here comes the actor though. You see the actor pushes it out of the way and it never finds the dot again. That didn't work at all. So I'll undo and let's try again. Let's go to the opposite extreme. Let's make a very tight search region so you don't go looking off into space of this dot, but stay more tightly focused on where you need to search. I'll click Analyze forward again, you will see it's much faster with a small search region. It found the dot, but it loses it again. What's going on here? Well, the small search region helped After Effects extrapolate back and find that dot.
However, the camera is moving so violently that in some of these subsequent frames like right around there, the dot has moved so far, it's gone outside of my now smaller search region, and that's where we lose the track. That doesn't work either. I need a compromise where my search region isn't so big that I completely lose the track, but isn't so small that I lose it just because the camera movement. I'm going to undo. Since I notice my problem was with the dot going off in this direction, I can go ahead and hold the Command key on Mac or Ctrl key on Windows and pull out these dots just in the direction of movement, saying here is where my motion is, worry about that, I can stay tight on this side.
Now let's Analyze forward. It's still finding the dot, the actor comes in, found the dot again, and now during the violent camera movement we continue to find the dot. We kept an accurate track. This is the most graphic example of where you really need to coordinate your track points with your options to get a good track. Just to hammer that home I'm going to undo for a second, go to my Options, and say I decided it might be a good idea to Adapt the Feature On Every Frame since the camera is moving. If I perform the track again you'll see that After Effects again has trouble finding this particular dot.
Since After Effects was making microscopic changes from frame to frame, because it was adapting the feature in every frame, it couldn't refind that original shape. So I want to emphasize; you will not have success your first time out motion tracking unless you're very lucky or very good. You'll need to play with different parameter settings and different track points until you do get a good track. Let's go ahead and get that track now. I'm going to scrub through to verify that I've indeed followed those dots, and I'm fairly happy with that track.
I do have a little bit area right in here where I'm lost in the wilderness. So in the next movie we're going to focus on cleaning up these problems and also applying the stabilization to the original footage.
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