Zooming to Center

[Mark Greenberg]

I have an image that is much larger than the stack.  I have two  
buttons, ZoomIn and ZoomOut.  They do their jobs as far as zooming in  
and out by resizing the image, but I would like the part of the image  
that is in the center of the screen to stay there during the zoom  
(when possible).  Right now it moves around and I can't figure out  
the math to make it seem to zoom to (or from) the part of the picture  
that is in the center of the card.

	Thanks in Advance,

		Mark Greenberg