[cairo] Pattern rendering questions

Ian Britten britten at caris.com
Thu Aug 21 06:36:02 PDT 2008

Hi all,
I hope I'm not overwhelming this list with questions, but I've got
some more, regarding my "Polygon fill styles?" work.  For now, I've
ignored my outstanding questions, roughed in a basic implementation,
and attached some preliminary results.
[ I hope small attachments are acceptable on this list ]

Each of these was generated using the approach described by Kalle
Vahlman yesterday.  I didn't include my code, as the basic mechanics
seem to be working.  I think my questions are more general rendering
questions, and not necessarily specific to my attempt at fill styles.

In the first one (diagonal.png):
I generate a small surface (w, h), stroke a line to it (0,0), (w,h),
make the pattern, CAIRO_EXTEND_REPEAT, etc, etc.
- Note the missing pixels, presumably from where the tiles are
   assembled after the pattern is clipped to the extent of the
   -> Can anyone suggest any technique which might fill in these
      missing pixels?

In the second one (cross.png)
As with the previous pattern, I generate this one by drawing an X
from (0,0), (w,h), etc, but at 50% alpha.
- Note the darker pixels, presumably from where the pattern
   overlaps, and the alphas of the individual strokes is combined.
   (Or, maybe it's from where the tiles overlap.  I'm not sure...)
   -> Can anyone suggest a technique that would result in a
      consistent alpha for the entire pattern?

In both of these cases, I'm pretty sure I understand what is
happening, and the result is arguably correct.  It's just not
quite what I want...  :(

Thanks for any help/suggestions!

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