render pixmap problem

[<bon...@gm...>]

Hello everyone:
       In one of my projects, i decide to use render pixmap as a pixmap to
draw on the background of my drawing program.What the problem is, when i us=
e
renderpixmap to get the pixmap, i found out the pixmap is not exact the
drawing. Data drawing is scaled in the center and there are blanks around
the data drawing. I just want to get the drawing without the blank. How can
I get exact data drawing size and position so i can use renderpixmap to get
the data drawing? Did i say it clearly? Any suggestion is my preciation.
thanks a lot.