Okay. I'm making a short animation and part of it is a van from the side driving across a desert. I want to make a dust cloud emanating from the wheels, kind of made from circles growing to form a cloud. Thing is, I want the separate circles to form a single mass so that I can treat them with a transparency gradient and still keep the individual movement (almost like a moving silhouette). I've figured I need to make a pre-comp and mask out the bottom where it meets the ground. Any ideas how I could do this?