beginners question about incremental redisplay

[sauthier%elma.epfl.ch@CUNYVM.CUNY.EDU]

I tried to use the incremental redisplay capabilities of genera (7.2)
without success. I want to have a program able to handle some graphical
output. Each piece of graphic is also intended to be selectable and therefore
corresponds to a presentation type. I also want to be able to interactively
move this pieces on the screen.
I tried to implement this after an example found in the genera documentation
and the result was not exactly what I expected. For example if I select a
piece of the "drawing" and I move it, it is the only things which is redrawn,
but under certain circumstances (unfortunately, I don't know what they are,
but I suppose they have something to do with the overlapping of the highlighting

boxes) other part of my output are disturbed or shifted (the highlighting of
the presentation does not overlap the presentation anymore).

Before going into details, I would know if it is possible to do this, if
someone has had the same troubles, or if I am doing something wrong.

thanks for help.

    Eric Sauthier
    AI lab, Swiss Federal Institute of Technology, Lausanne

    EMAIL : sauthier@elma.epfl.ch