background offsets

[Jens-Uwe Morawski]

Hi,

for a screen-document i draw an overlay under the text-area.
The overlay is a simple MP-figure:

fill unitsquare yxscaled(\overlaywidth+6pt,\overlayheight+6pt) 
        withcolor transparent (1,0.7,white) ;

This overlay is centered under the text-area giving a 3pt offset
to every side.

I would prefer to specify the offset in the \setupbackgrounds
command:
\setupbackgrounds[text][text][offset=3pt,background=<overlay-name>]

and use in the MP-figure only:
fill unitsquare yxscaled(\overlaywidth,\overlayheight) 
        withcolor transparent (1,0.7,white) ;

but this only works for \framed, not for \setupbackgrounds. Why?

Thanks.

Best,
  Jens