Category on UIImage to create full-resolution cropped image from UIImagePickerController info dictionary.

Background

The UIImagePickerController class includes a parameter (allowsEditing) that lets the user move and scale the image and crop it to a square shape.

The info dictionary returned as part of the UIImagePickerControllerDelegate protocol includes an "edited" version of the image, but its resolution is limited to a relatively small size.

However, the dictionary also contains a pointer to the full-resolution image along with the the value for the UIImagePickerControllerCropRect key, which is a CGRect corresponding to the area selected by the user.

Unfortunately grabbing a chunk of a UIImage from the device camera is not completely trivial, as the image data is always stored in the device's native orientation with a flag indicating that it should be rotated before being displayed.

So a bit of low-level CoreGraphics-fu is required to rotate and translate the image before selecting the relevant area and drawing it to a CGContext.

This category on UIImage does that in order to create a cropped image at the maximum possible resolution.