Enhancement on page detection

[allgood]

In current state, the search for the "biggest quadrangular" is problematic because:

• It only works on surfaces that have a "solid color contrasting with paper"
• It doesn't work if paper have drawings up to the edge

Finding and implementing a new border detection algorithm is something that would benefit the app

This approach may be interesting: https://docs.opencv.org/3.4/d4/d7d/tutorial_harris_detector.html

[boi4]

Another problem is that if you have a document composed of stapled pages you don't have a perfect rectangle but a rectangle missing a small triangle at the bottom left which OpenNoteScanner has problems to detect.

[danteserrano]

Same problem occurs if you want to scan notebook pages.

[yaomtc]

I'm using a Pixel 4a. I've found that at least half of the receipts I try to scan just don't get recognized, even though they're straightened out fairly flat, they're against a contrasting surface, and the lighting is pretty good. Sometimes it's helped to fold over a portion of the top to hide a logo, but half the time I try that it doesn't do anything so it might not be the logo.