This is a known area and OpenCV might well be involved, but still to start from the scratch.

How has something like Evernote's scannable app been developed. I mean, how does it automatically recognize a document using a camera and then extract it. 

What are the UIKit frameworks involved here and what are the libraries that may have been used. Or any nice articles or blogs. How does one go about understanding this.

![enter image description here](https://i.stack.imgur.com/CSweP.jpg)