[PD] How can I mirror the canvas into a GEM window?

[joan]

Hello,
I'm starting to use puredata recently,
and I'd like to know if there exists a simple way to put a real-time 
mirror of the canvas into a gem window, for example, to overlay the 
images I want to display with the puredata objects I use to generate them.

Someone told me in max/msp there is an object called DesktopJitter that 
does that.
In theory it should be easy, as both the GEM windows and the canvas are 
displayed by the same graphic card.
However, someone told me the graphic objects are managed in a weird way 
in PD so -even if it's perhaps not related-, I am not sure I can find 
how to do it.
And I have no idea of how the canvas is managed in PD. Some advice?
Thanks,
Joan


PS. Unfortunately, I'm mainly working on Windows. However, if a solution 
exists only for mac or linux, I'll be happy to hear about it.