Object detection by camera

I'm trying to find how can I make a program

to detemine an object (say a circle) by digital camera and calculate its dimension and when the object move near or far detect the new dimension and calculate the difference between them as a ratio...this is the idea

is this possible by java<<?>>

I think I can take a pic to the object in the two positions but how can

I distinguish the object from it?

[442 byte] By [tatom2000a] at [2007-10-3 11:35:57]