This app has two purposes: First, you can select a quadrilateral region in a photo and extract a rectangular photo from it. This is particularly useful when you want a rectangular picture of something, but for some reason (e.g. wrong angle of view of the camera) it is not a perfect rectangle or when you want to zoom in a non-rectangular region of your photo. It can also extract general shaped regions in photos and/or blur them.
Second, it can detect the edges of the objects in photos and draw up to 4 tangent lines with different colors on them. Why would someone want to do this? Imagine you want to see if something is a straight line in a photo and you can't tell that by the naked eye. Or if you just want to have fun drawing tangents on the curves in your photos by simply tapping on them!
