[cairo] Problem clearing edge of a clip region

[Ian Britten]

Hi all,
   [ cairo v1.8.8, pixman 0.14.0 ]
I'm having a problem when trying to clear the contents of a
clip region - Specifically the border pixels - and was wondering
if anyone can offer any insight?

In the attached sample, I define a clip region, draw to it, then
clear it.  However, after doing the clear, the edge pixels are
still (partially) visible, and seemingly weren't cleared.

Am I doing something wrong?  Might this be a bug?  Or is this
somehow due to subpixel/AA rendering?
If it's a problem with my technique/understanding, can anyone
offer any suggestions about how to proceed?  Or point me to
something to read?  My code is based on
http://cairographics.org/FAQ/#clear_a_surface, but that doesn't
discuss this particular case.

Many thanks for any info!
Ian
-------------- next part --------------
A non-text attachment was scrubbed...
Name: clip_clear.c
Type: text/x-csrc
Size: 1158 bytes
Desc: not available
Url : http://lists.cairographics.org/archives/cairo/attachments/20090925/14a71d52/attachment.c