[cairo] Shifting (scrolling) a translucent surface?

[Stefan Salewski]

I just tried to make scrolling faster for a small toy application,
(http://www.ssalewski.de/PetEd.html.en). Indeed I was successful, I am
reusing the background grid (with correct offset of course) instead of
redrawing, and I draw the foreground some pixes larger, so that I can
copy it with some offset to the GTK drawing area, for small scrolls. For
larger scrolls I have to shift the cairo surface, before I paint to the
uncovered stripe. Works fine, but now I have to cleanup (and better
understand) the code...

For translating/shifting the foreground I have currently code like this

$ cat pet_canvas.rb
[...]
>elsif ((offx.abs < $OFFSET) and (offy.abs < $OFFSET))
>  return
>else
>  t = cr.target
>  cr.identity_matrix
>  cr.push_group
>  cr.set_source(t, offx, offy)
>  cr.paint
>  cr.pop_group_to_source
>  cr.set_operator(Cairo::OPERATOR_CLEAR)
>  cr.paint
>  cr.set_operator(Cairo::OPERATOR_OVER)
>  cr.paint
>  offx = 0
>  offy = 0
>end

I guess overlapping copies are not allowed for cairo, see

http://old.nabble.com/is-self-copy-supposed-to-work--td28410811.html

But is my code a good solution? My foreground picture is translucent, so
I need that paint with operator_clear to erase all before copying back
the translated content. Or is there an operator available which does the
same with only one paint? And, I wonder how the OPERATOR_CLEAR depends
on the source surface? First I tried cr.set_source_rgba(1,1,1,1) before
the clear operation, to get a solid source for clearing operation. But
it seems to be not necessary. Is the source ignored for clear operation?
May it be a better approach to allocate an new cairo context with empty
surface, copy the picture to that surface, discard the old and use the
new one? 

Best regards,

Stefan Salewski