Flex/AIR pinch zoom centreing image

I am developing an application with flex/air and I need to centre an image in the view port while pinch zooming.

I start with an image of 3008x2000 at 1:1 scale, I then want to be able to zoom in and out on a certain area of the image with a combination of scaling and repositioning on the screen.

Does anybody have a working method of doing this smoothly? I keep messing with the math but am hav开发者_如何学编程ing no luck.