[cairo] Wrong extents when rendering glyphs to non-integer positions
psychon at znc.in
Tue Oct 19 03:02:39 PDT 2010
thanks to a bug in the XCB backend, I noticed that
_cairo_composite_rectangles_init_for_glyphs returns a wrong extent for
non-integer-aligned glyphs (see the result of xcb's overlapping-glyphs test).
The problem is that the xcb and image backends (and others?) round the pixel
coordinates before drawing, e.g. cairo-image-surface.c, line 3977:
x = _cairo_lround (info->glyphs[i].x -
y = _cairo_lround (info->glyphs[i].y -
However, the function that does all the extents-work
(_cairo_scaled_font_glyph_device_extents) doesn't do this rounding. Instead, it
generates a cairo_box_t with the exact bounding rectangle and then rounds this
via _cairo_box_round_to_rectangle to a cairo_rectangle_int_t.
I did a quick-and-dirty hack that makes _cairo_scaled_font_glyph_device_extents
use _cairo_lround. The test suite then said that this fixed text-rotate in
the test suite (and only broke Xlib's clip-text due to a wrong text postion).
Andrea Canciani suggested that this change might break hinting. Looking at the
font face code, I need someone else to confirm this. ;)
Could somehow who has a better understanding of cairo's glyph rendering take a
look at this and perhaps come up with a nice(r) fix? IMHO, all backends that can
only draw glyphs to integer positions and thus use _cairo_lround might be
affected by this problem in one way or another.
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-- Faber in Fahrenheit 451
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