Do not use writeRawBytes, but QDataStream's methods to serialize the integer data of the Haar signature. Only this ensures that the signature can be used cross-platform!
(think about accessing the database over a network from a little and a big endian machine)

Set the QDataStream binary version explicitly (following tip from <a href="http://www.purinchu.net/wp/2008/06/03/programming-tips/">http://www.purinchu.net/wp/2008/06/03/programming-tips/</a>;)

Note: This changes the binary format of the Haar matrix; you need to rebuild the fingerprints.

It seems there is a crash if you dont rebuild, so you will notice it ;-)