Allow adjusting the quadrilateral manually

[bgusach]

I'd be a great enhancement to be able to adjust the detected quadrilateral manually after taking the picture.

The algorithm for detection works pretty well IMO, but there are and always will be weird edge cases. For instance, if you're using stapled pages, you'll get a non-quadrilateral polygon like this:

Which in turn produces a distorted scan:

I understand that you're pursuing a very minimalist interface, but these kind of corner cases are unavoidable, IMO.

I'd love to have this, even as an an opt-in feature.

And thanks for the great job.

[pynicolas]

Thanks for sharing that, especially with an image!
Stapled documents should be better handled since version 1.6.0 (more precisely: 5b7d76e) but it can probably be improved.
In your case, the fact that the document appears significantly concave doesn't help to find a quadrilateral: I should probably look into applying some kind of perspective corrections before that step.

[bgusach]

Obviously, the better the auto-detection works, the better for everybody... but I'd still love the ability to adjust by hand.

[nekohayo]

If you need some implementation inspiration, the MakeACopy app in F-Droid (GitHub repo) already has this feature, and it's pretty close to what Adobe Scan does, with an automatic magnifying glass for precision adjustments etc.

[nekohayo]

I understand that you're pursuing a very minimalist interface, but these kind of corner cases are unavoidable, IMO.

In case it helps, here's a photo of sample someone shared with me, where the app really struggles with unstable stable edge detection:

This is best demonstrated with the video below that shows how much this kind of situation (which is not that uncommon, when you consider torn or "not cut straight" roll receipts for example) freaks out the edge detection:

FairScan.-.torn.envelope.edge.sample.mp4