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Bisecting Animation

Animating garbage masks in Adobe After Effects By Stephen Schleicher
When keying footage there will be times when you have a poorly lit set, imperfections in the background, etc. and you only really need to key that area immediately around the foreground subject. During these times you would use a garbage mask to isolate the foreground element and get rid of any background crud. But what happens if you need to animate the mask?

The rough mask that you create to define a region that you want to mask is called a garbage mask (or matte). Garbage masks are especially useful when you are keying because not only does it allow you to remove all the junk in the background, but because those areas of the background have been removed, After Effects doesnt have to perform keying calculations to those areas, thus speeding up screen refresh times and rendering.


The original image would be difficult to key with all of the clutter off to the side (top). By using a garbage mask (bottom), only the area that will be keyed remains.



As a general rule of thumb, I try to use garbage masks anytime I am working with green or blue screen footage.

At some point during motion graphics creation you will need to move or animate the garbage mask over time. It could be that your subject is moving across the screen, and in order to keep the subject in the shot, the garbage mask must follow the subject. This means you will have to animate the mask, changing the control points over time.

New users might think that animating a garbage mask requires animating frame-by-frame, making changes to the mask when needed. This is not the best way to animate a garbage mask (or any mask for that matter), as it will create an unnecessarily large amount of keyframes. Remember the great thing about computer animation is that it creates all of the in-between frames automatically based on the keyframes you set. The fewer the keyframes the better.

In the case of animating a garbage mask, you should use a process called bisecting animation. Bisecting animation is quite easy to understand. It begins when you create an initial keyframe for the mask at a key moment near the beginning of the Timeline. Well call this keyframe Keyframe A.

The initial keyframe for the garbage mask.


Next, you move the Timeline Indicator to the end of the composition and reposition the garbage mask around the subject. Because an initial keyframe has already been created, After Effects creates this keyframe for you. Well call this keyframe Keyframe B.
The ending position of the garbage mask.


To animate the garbage mask as easily as possible, move the Timeline Indicator halfway between Keyframe A and Keyframe B. Make any adjustments needed to the mask. Again, After Effects will automatically create a new keyframe for you (Keyframe C). You have bisected the timeline between Keyframes A and B.

The third keyframe bisects the first two. Using bisecting animation helps prevent unecessary keyframes.


Repeat the procedure by going halfway between Keyframe A and Keyframe C to create another keyframe (Keyframe D), and so on and so on. When you have bisected the clip between Keyframe A and Keyframe C to the point where the garbage mask moves smoothly, move the Timeline Indicator between Keyframe C and Keyframe B and repeat the procedure until the garbage mask for the entire clip has been adjusted appropriately.

The garbage mask keyframes created for this two second animation.


Based on experience this is a great way to keep from polluting your composition with keyframes that are not needed.

It may seem like a big hassle to spend the time following the bisecting animation process when it comes to animating masks, but once you get the hang of it, you will see how important it is, and how much time it will save you.

When not working deep in the labs of the DMN Central Division testing the latest and greatest software/hardware products Stephen Schleicher can be found at the local university teaching a few courses on video and web production. He can be reached at schleicher@mindspring.com. You can also visit him on the web at www.mindspring.com/~schleicher

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Stephen Schleicher is a well known writer, visual effects artist and media guru! You can see more of Stephen at
www.majorspoilers.com and www.stephenschleicher.com
Related Keywords:garbage mask, stephen schleicher, bisecting animation, after effects, mask, matte, keying

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