[cairo] Problem clearing edge of a clip region

Bill Spitzak spitzak at gmail.com
Tue Sep 29 11:26:17 PDT 2009

I'm pretty certain this is antialiasing causing it. Clearing an 
antialised clip region will only partially clear the edge pixels.

You should see if you can round up your clip region to pixel boundaries.

Ian Britten wrote:
>        [ bump ]
> Any comments?  Sorry to re-ask - I know everyone is busy...  :P
> Thanks,
> Ian
>> 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
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