The 2D Tracker
In After Effects, there is a standard tracker that will track parts of a video clip. See the image below. Basically, Box A looks for what ever pattern is in Box B across every frame, then exports this movement to the crosshair C.
Box A is call the
Search Area
Box B is the
Pattern Area
and the
Crosshair C is where the data is exported to.
Once you hit play, After Effects follows the Pattern Area by scanning through the search area for the same pattern every frame.
That's all fine and dandy when you pick good tracking points, but what makes a good tracking point and how does this tracker work?
Good Tracking Points
For the most part, you want to select points with high contrast. This is all well and good, but what if you can't find any? as it turns out there are a few solutions:
Only track points that actually exist
Don't try to track the intersections of two objects because no mater what you try, that point is not going to stay the same as the camera's perspective changes.
Don't track the edges of objects
Often there is high contrast on images with a building and the sky, but if this was a vide, we do't want to track that edge. For the most part, these edges allow the tracker to slide across it because of the similarity in the texture of the building and the texture of the sky. This results in an inaccurate track.
Never take your eyes off the track
When the computer is tracking your point, don't let yourself become distracted. Often when you aren't watching it, the track will veer off target. By watching it, zoomed in, you will know how accurate this track is going to be before it even finishes.
Use the right size tracker
As in most Tracking programs, you can designate the size of the pattern area, and the search area for your tracker in after effects. By changing the size to only encompass what you're trying to track you have a better chance of catching it every frame. Also if there is a lot of motion in your frame, perhaps it makes sense to have a larger search area.
What points to avoid
When tracking your points of high contrast make sure to pick points that do not have:
- Motion Blur
- Soft Focus/Rack Focus
- Occlusion
- Noise
- Low Contrast
- no features /single color
- Interlacing
Some Ways of Correcting the above issues
Motion Blur
Expand the search area. The extreme motion of the camera maybe causing the pattern you were trying to track the movie on the default search area and this will cause a tiger to stop or slip.
Expand the pattern size. This gives this offer more information to work with and might help it to continue through the motion blur.
Sometimes manually tracking the point at the parts where the clip is most blurry can keep it on track.
Soft Focus/Rack Focus
Use a bigger pattern size. Bigger pattern sizes can sometimes compensate for the noise in the soft focus features because the program is simply more pixels. This causes a sort of averaging effect on the track.
Slightly blurry the image. It seems to go against common sense to blur the plate when blurriness is the problem you're trying to solve. This technique sometimes is more effective because it will help get rid of the graininess and produce a smoother track.
Sometimes tracking from the in focus part of the shot to the out-of-focus part gives you a better result too.
Occlusion
In order to solve occlusion, you can track it by hand or for the frames your tracking point is occluded find another tracking point that could potentially work for a little while. Make sure, when you move the pattern and the search area, that you leave the origin (Crosshair C) right where it was. It should still follow the motion, relative to the position of the pattern and search area.
Noise
Whatever you do, try to remove the noise. Noise really confuses it to the tracking system. If even buy D noising your image, you still have a lot of green, try tracking only one channel.All video is composed of R G and B values, and perhaps there is less grain in one or two of those three channels.If all else fails's, do the track on the noisy shot, and use the smoother to all on the keyframes.
Low Contrast
Most 2D trackers will use the luminance value of the pixels they are tracking. If you can try tracking only in our, G, or be color channel instead. Sometimes one of these channels has more contrast than the others.Another solution is to add an effect of the clip that makes features more "legible." If you add a sharpen effects to your clip perhaps you can get better results.
no features /single color
If your clip has almost no features and is more or less a single color there is not many options. You could try tracking an edge, but the tracker may slip. You can try playing with the color channel your tracking, but you may not get any more information there either. Ultimately if you have to track a shot like this you are better off tracking it manually.
Interlacing
The easiest way to deal with this problem, is to Deinterlace the image before you begin your tutti track. Most compositing programs have a Deinterlacer.
I hope that you found some of these tips helpful in the projects you're working on, I know that they have been helpful for me and my green screening project!!
