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I'm curious to see how designers have addressed this problem on a touch-screen device given the size and technical constraints of mobile.
Facebook has this. When you use an image for your profile you use the "pinch" gesture to crop.
I have seen others do a box that has one edge draggle (to expand the box larger) then you move the box (tap and drag the center) over the area.
I actually like the second option myself.
Required, but never shown