In Motion, in the Censor effect project, if you turn off the Effect Source (lowest layer) you'll see the "circle" used to "limit" the effect.
Actually, this is just an illusion:
The original Effect Source (in FCPX, that's whatever clip this effect is applied to) is Cloned (the layer is called "Mask"): a clone is basically a "rasterized copy" of the original -- what I mean by that is that Clones not only copy original objects, but also all their filters and other layers/effects if you clone a group of objects -- reducing them all down to a single layer copy. This copy can be treated just like any other object in Motion (with an interesting exception*): you can add masks and other filter effects and/or behaviors to create new actions while the original source of the clone is left intact.
In this effect, there is a Circle Mask applied to the Clone, as well as three different Filters: Pixellate, Gaussian Blur, and Brightness. These three different filters are turned on and off separately by use of a "pop-up" rig so that only one of the filters is applied to the clone at a time. The gear shaped objects are Behaviors, in this case Link behaviors that "link" to the Circle Blur filter properties that is attached to the Effect Source which control the location in the canvas and the scale of the mask (Radius to Scale-X and another to Scale-Y). The Circle Blur filter has Onscreen Controls of a center point and a "radius" and those property values can be shared to other objects in a project via these link behaviors. The Circle Blur filter effect is turned off in this project because it *only* needed as a *source* for the onscreen control properties (and those can be turned on for use in FCPX [Publish OSC] whether the filter type is used or not.) All of the visible action in this project is created by applying other filter effects to the Clone copy and masking the area so that the rest of the source is uneffected while this effect (Censor) is active in FCPX.
The *interesting exception: Clones can be "retimed" -- even timed to go in reverse. No other object (other than actual video) can be made to do the same. What does this mean? It means that you can create, basically, an "in-project" video of other objects in animation (without exporting the animation first) and retime them to loop over and over, go in reverse, speed up, slow down, etc... Clones are a very useful tool in Motion (not to mention "light-weight"). Their only drawback: they *flatten* 3D animations, so you can't apply "more 3D" effects to them because they'll just look flattened on a plane if you do. So any time you feel you need to duplicate something, ask yourself if you would be better served by a clone instead.
Thanks! I missed the "Clone" part. That's what I get for not being more methodical, and also for making stupid assumptions. I should know better.
So basically, other than some plumbing and fixtures, it does the same thing I would do in FCPX by hand. I would drag a copy of the source clip up above the source clip, and apply the mask and effect there. In Motion, this is accomplished by using a "Clone". Cool.
Thanks for the detailed explanation. I very much appreciate it.