[cairo] Subpixel Positioning w/FreeType
freddie at witherden.org
Tue Oct 6 12:58:32 PDT 2009
I am wondering about any current or future proposals for supporting
subpixel positioning with FreeType. Subpixel positioning -- not to be
confused with subpixel rendering/anti-aliasing -- allows for more
accurate glyph positioning.
Much like subpixel rendering, subpixel positioning takes advantage of
the fact that LCD screens are made up of three subpixels in a specific
order. This order is usually RGB. The predictable subpixel order
allows for three times the horizontal resolution. The same technique
can be applied to glyph positions. Instead of having to start on a
pixel boundary glyphs are instead able to start on a subpixel
boundary. This allows for spacing accurate to 1/3 of a pixel in most
It is my understanding (from looking over the source) that Cairo does
not currently support subpixel positing under FreeType. Further, I
also understand that the FreeType code has been recently re-written
(there was a note on the 1.10 roadmap) to take advantage of the
subpixel filtering API exported by FreeType.
I believe that basic, 1/3 of a pixel, subpixel positing could be
implemented in Cairo without a huge amount of work. It would of course
only be applicable when subpixel rendering is enabled. Since Cairo and
Pango both already use fractional co-ordinates this should not break
Both Quartz (OS X) and ClearType (Windows) support this already.
Polemically yours, Freddie.
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