[cairo-bugs] [Bug 10531] cairo nquartz backend floating point precision issues

[bugzilla-daemon at freedesktop.org]

http://bugs.freedesktop.org/show_bug.cgi?id=10531





------- Comment #2 from Brian.Ewins at gmail.com  2007-04-05 15:23 PST -------
Good point Carl. And after 2048 pixels-width of Lorem Ipsum, I still couldn't
reproduce the bug.

_cairo_atsui_font_text_to_glyphs uses the glyph positions from ATSUI pretty
directly, but that isn't the case with Gecko - see eg
http://lxr.mozilla.org/seamonkey/source/gfx/thebes/src/gfxFont.cpp#191
> // Multiplying by the reciprocal may introduce tiny error here,
> // but we assume cairo is going to round coordinates at some stage
> // and this is faster
> glyph->x = ToDeviceUnits(x, devUnitsPerAppUnit);

So, to test this I made _cairo_atsui_font_text_to_glyphs increase all glyph
advances by 0.0001 pixels. This reproduces the bug - one glyph over the 2048
pixels gets enough cumulative shift to have a visible effect, and the patch
fixes it.

I could implement this as a test rather than a hack (by explicitly
mis-positioning a few hundred glyphs, then calling show_glyphs). I'm not sure
how worthwhile that is.


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