[Fontconfig] ClearType-like results (Was:Xft - (incorrect ?) subpixel filtering. )

Yaroslav Rastrigin yarick at relex.ru
Fri May 30 18:35:30 EST 2003

Hello .

Well. First, Keith, I want to thank you for your reply to my message - it was 
very encouraging and motivated me enough to continue investigation (I'm still 
convinced WinXP ClearType provides much more readable text output on 
not-so-highres LCDs than any other technology). Your comments were also very 
What I have now : 'reference' (as seen on http://grc.com/cleartype.htm) 
implementation of 5-level filter. Results are quite promising, although still 
not on-par with CT (text is a bit more blurry/slightly bolder (sp?) than CT's 
output).  (I could provide results, if somebody is interested).
I tend to think this is because either 
a) MS core TTF fonts supplied with XP probably have some more additional info 
with proper weighting coefficients (ClearType hints ?). Could someone 
confirm/deny it ?
b) Incorrect/too simple formulae to calculate each subpixel's value 
distribution. Is there any patent/techdoc info about MS ClearType's used 
algorithms ? Would it be called 'patent infringement' if such a patent exists 
and similar technique will be implemented in Xft  ? 

And one more question, this one must probably go to freetype-devel ml, but in 
case somebody knows - is there any way to use freetype2's grayscale renderer 
in monochrome mode - without 5-level gray smoothing ? I've looked through 
code, and simplest solution seems to just modify gray levels table to have 
0x00 as first one or two entries and 0xff in others, but would it be 
correct/enough  ? With subpixeled AA there's no need to gray-smooth diagonal 
outlines - they will be blurred with subpixel filtering anyway. Rendering 
with 3x horizontal resolution is essential, though, so ft's simple monochrome  
renderer won't suffice.

With all the best, yarick at relex dot ru.

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