Windows phone 8 viewport control question

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I want to create image zoom an pan control. I need to have a rectangular area which will represent the smallest size of image, i can pinch zoom my image and the areas of image outside the rectangle should have an oppacity mask. I tried using viewport control

I have viewport control, and i placed image in it, i can pan it in the viewport area. The thing is when i pinch zoom image, the zones outside of viewport are not visible.
Is there a way to make zones that are outside the bounds of viewport visible