Manipulating Pixmap

hi,
i think this is not the right ml to post for a question on
writing an application for X but i didn't find anything like a xfree-dev
ml, so forgive me.
<intro>
i have mad a funny application which display animation file on the rootwindow
each frame of the animation (which are in Xpm format) is converted 
in a Pixmap (with XpmCreatePixmapFromData() ) and then "printed" on 
the rootwindow with XCopyArea.
</intro>

Because of the transparent Background of each frame i decided to clear 
the frame before showing the next one (with XClearArea), but it meand
"flickering", so i decided to make a trick: 
first i "XCopyArea()" into a Pixmap of the Area of the root window in
which i'll display the frame .
then i would like to "merge" the frame and the area following these rules
( if the pixel of the frame is "transparent" 
	then take the pixel of the Pixmap of the area of the root window, 
  else  take the pixel from the frame Pixmap ).

The problem is that i don't knwo how to do this, i've looked to XFree86
Library documentation but i didn't find anything to help me, i also
started to think that is not possible doing this with Pixmap structure
(i even didn't find where's defined a Pixmap structure).
Am i right or wrong ? any link, suggestion, tip ?

thanks in advance
Samuele 

-- 
Samuele Giovanni Tonon  <[email protected]>   http://www.linuxasylum.net/~samu/
          	Acid -- better living through chemistry.
			       Timothy Leary