Hi Harvey,
This feature currently only works with the built in Sphere object. You can disable the sphere so that it's invisible and the crop will still work, so it's a convenient way of cropping the render to an animated object. Sorry I missed this thread until now.
Cool compositing trick:
This feature was originally added as a way of optimising the rendering of animated mattes of imported objects, including the holdout caused by parts of the terrain in front of the object. For example, an aircraft alpha channel could be rendered in Terragen, with black terrain that would cut out parts of the alpha whenever the object was behind the terrain. That could later be used to composite the aircraft rendered in another app onto the Terragen background. You don't need the "crop to object" feature to do that, but it's wasteful to render the parts of the image that don't contain the aircraft. Therefore a sphere that had the same position and size as the aircraft was used to crop the render.
In future we should be able to make this feature work with any object and not need a dummy sphere with duplicate motion.
Matt
