xinput -> core pointer - Transformation?

[roman]

Hi!

I have a tablet and i want to use an external monitor that covers just a 
part of the screen. This is no problem concering xrandr as i can set a 
scale and a position.

The problem is with the pen. The pen core pointer is automatically 
postioned the right way - even in this situation. But no transformation 
matrix whatsoever is set in the xinput settings for the device.

Sounds nice BUT I want to use direct xinput for using xournal.

So i would like to modify the Transformation matrix. If i do so the 
xinput can be set to be in line with the monitors - but then the core 
pointer is wrong.

How is the xinput transformed into postions on screen for the core pointer?

Is there any way to get infos on the transformation that takes place?

Is it possible to modify it?

cheers
roman