[cairo-bugs] [Bug 20091] Polygon clipping to surface fails for certain values of vertices

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Fri Feb 13 02:13:28 PST 2009


--- Comment #5 from M Joonas Pihlaja <jpihlaja at cc.helsinki.fi>  2009-02-13 02:13:26 PST ---

I'm sorry I didn't have my brain switched on while looking at your test case
earlier so completely missed the obvious problem: The input coordinates
overflow cairo's internal fixed point numbers used to represent polygon
geometry, and the resulting overflowed coordinates aren't to be trusted
anymore.  The current format uses 24 bits to address the pixel and eight bits
to address fractions of a pixel, leaving the valid coordinate range in the +-8M
range.  This is up from the +32k range we had earlier, but obviously not the
full double precision range.  So this is an expected failure after all. :|

I'm leaving this bug report open as a reminder until we prominently document
the input coordinate restrictions and fail in a meaningful way in the face of
large numbers.

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