[cairo] Possible bug in font hinting metrics?
george.wright at collabora.co.uk
Fri Sep 7 09:03:36 PDT 2007
Can someone explain to me how font hinting is supposed to work?
We are rendering text on a cairo context which can be scaled by the user. If
the scale factor of the context is 1.0 then the text renders fine. However,
if hinting is enabled and the context is scaled, then the text has awkward
gaps between the letters. We are getting the glyph extents through
cairo_scaled_font_glyph_extents and rendering a buffer of glyphs using
calculations based on those reported glyph extents.
From the cairo API documentation at
"If you call cairo_scale(cr, 2.0, 2.0), text will be drawn twice as big, but
the reported text extents will not be doubled. They will change slightly due
to hinting (so you can't assume that metrics are independent of the
transformation matrix), but otherwise will remain unchanged."
We are obviously not getting the hinted extents.
To see what I mean I've made a screenshot at
http://vm.gwright.org.uk/cairo_text.png - in particular, I mean things like
the unnaturally large gap between 'w' and 'e' in "the web".
Is there a way of getting the true (hinted) extents or is this a bug in cairo?
George Wright, http://www.gwright.org.uk
Collabora Ltd - http://www.collabora.co.uk
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