Detection of objects inside a 3d trigger area?

Hi Folks,

I am curious if you know how one can use a 3d object (a cube for example) as a trigger region for particles or other objects (like other cubes) going inside the original trigger cube.

So once a particle gets inside the cube, it lets you know.

A good example of this is this project that is already done in vvvv: http://vvvv.org/contribution/kinect-virtual-hit-boxes

I wanted to create something that works a bit similar to that one.

Thank you all for any advice you might have.
ygr