draws a cross-hair in each corner of the screen in turn (clockwise from the lower left hand corner),
and waits each time for the user to touch the centre of the cross-hair with the stylus.
It then prompts with a final cross-hair in the centre of the screen, and waits once
more for the user to touch its centre with the stylus.
The process is repeated until
can calculate a transformation matrix that consistently maps the touch panel to screen
It then writes corresponding calibration commands for
on its standard output, which
can be saved in a file on the device, perhaps
provided in NVRAM by
or a file system in
can be used both inside and outside the
allowing calibration when the system is initialised, and whilst
the window system is running.