How to detect a pattern (such as a QR code) on a scanned paper so that it can be correctly rotated/scaled/identified?

Question

I have PDF files or JPG images that I need to print (these are formulars that will be filled in by hand), and I need that:

• after scan, the image should be rotated / scaled perfectly with Python, so that I can access to certain parts of the formular with x, y coordinates

• the scanned images should be recognized with a number ID.

What kind of pattern should I add on the formular paper, so that a Python library (which one?) will be able to crop it / rotate it / identify it with an ID?

I was thinking about adding a QR-code (containin a number ID) on top left of paper, and a QR-code on bottom right of the paper, or maybe also "hirondelles" symbols:

The QR code would be generated with qrcode library:

import qrcode
img = qrcode.make('ID1138934')

and added on top the PDF with this method.

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Note:

• I've read the question How to decode a QR-code image in (preferably pure) Python?, but this does not detect the QR-code on a scanned paper; it only takes a PNG containing the QR-code only as input.

• Here is a JPG, example of scanned document:

Answer 1

This task needs the combination of two techniques:

• First step: detect the x,y coordinates of the 4 corners of each QR code. The Python library zbar is useful for this. The code in this question + the answer shows how to do it.

• Second step: now a deskewing / perspective correction / "homography" is needed. Here is how to do it with Python + OpenCV.