When you correct the perspective the app crops too much the image, I tried to pinch the image to zoom out it but the image was croped so much that made my photo useless... I cant use it. This is a bug not a bad understanding how the app works. Developers I can show you images of whats happening...
silvio.tanaka about Perspective Correct - Fix distortion and skewed pictures with keystone and straighten correction from camera or photo library, v1.7