nanogui: Thread: How to draw image with transparency over another image ?

Hi,

I write nano-x based daemon, that should display battery indicator on
main screen.
Battery indicator is a series of PNG images with transparency.
Main screen (root window) has a picture as background.

And the question is - how to draw the PNG image over the window ?
I mean that if the battery state is changed that I should draw another
image on the same position, but of course the background will not
repaint by itself.

Is there any easy way to do so ?

Or I should save the image behind the battery indicator to some
offscreen pixmap and after each change of its state I should restore
the background and then draw the battery image ?

: And the question is - how to draw the PNG image over the window ?
: I mean that if the battery state is changed that I should draw another
: image on the same position, but of course the background will not
: repaint by itself.

There's a function GrClearWindow, which could force
the root window to redisplay the picture...  but I'm
not sure that's a good idea here, since the implementation
of this function would like cause the entire picture to
be re-displayed, clipped to your small battery display
area, which would waste tons of CPU time.

If your battery area was in fact a window, with no background
or border, then unmapping the window would have the same 
effect.  If the battery were a background image on that
window, it might be drawn with transparency and the whole
thing handled by unmap/map of the battery window... ?
I'm not sure whether we correctly handle transparency with
background pixmaps...

: Or I should save the image behind the battery indicator to some
: offscreen pixmap and after each change of its state I should restore
: the background and then draw the battery image ?

That's probably a pretty good idea. I'm not sure how to handle
the problem of when the root window image changes though. 
You'd likely have to select for root window UPDATE events.

Regards,

Greg