WPF: Best way to get a snapshot of what is under a Canvas control

Question

Hi, I have a WPF app, and I use a Canvas with 50% opacity as a cropping rect that can be resized and moved over an Image, and every time it moves, I use a CroppedBitmap to show a live preview of the image, but it makes the app become slow as I create a new CroppedBitmap every time...

What's the best way to get an image of the area the Canvas is?

thanks!