General information:
On the english Wiki you can find a lot of information about how a QR code work.
In a raw QR code the "bits" are black for 1 and white for 0.
But thats not the case in the finished QR code.
If you get to many white/black areas it can be hard to scan.
To fix this problem a mask is applied to the bits to make it more readable.
In specific parts of the QR code the scanner can read what mask patton to use.
For reliability this information are stored two places in the QR.
One place to identify the mask are these 3 bits (marked with green).
111 correspond to mask 2 (j % 3 = 0).
How to find the cache:
Get the coordinate from the bad QR.
You can get a clean 2 color png version of the bad QR here:
https://tirsdagsklubben.rocks/GCB6ZWJ/BadQR4b.png
(QR21x21, 210x210 pixel, each bit 10x10 pixel)
You task is to get the QR code from the url, add the right mask to the databits to make the QR readable, read it to get the cache location.
Dette er en letterbox - det betyder, der er et stempel i cachen.
Dette er ikke en bytteting, men du kan stemple i din egen bog med det.
This is a letterbox - that means that there is a stamp in the cache.
This is not a trading item, but you are welcome to use it to stamp your own book.