State of the art: https://safjan.com/tools-for-doc-deskewing-and-dewarping/#what-to-use-for-unwarping-and-deskewing
vFlap: android app
List of apps: https://graphicdesign.stackexchange.com/questions/30948/best-way-to-flatten-a-curled-photographed-book-photograph
Process used: https://discourse.devontechnologies.com/t/three-cheers-for-the-free-vflat-app-for-de-warping-iphone-photos-of-book-pages/72523
Scan tailor, a QT app to stragithened scanned pages. https://github.com/scantailor/scantailor
Command line tool: https://github.com/Flameeyes/unpaper Interesting documentation