[solved] Resolution problem
I observed that the resolution set in "General Properties" was not the one used during the experiment. In fact the resolution used during the experiment is the one of the windows desktop.
I observed that under XP and a CRT screen, with OS 0.27.1 and psychopy as a backend (for the moment I had always problems with the xpyriment backend so I don't use it).
Btw, it would be nice to have the refresh rate used in the results by default. This would permit to know the precise timing display years after the experiment has been run.