[cairo] Sub-pixel Font Filtering in 1.10

Freddie Witherden freddie at witherden.org
Fri Jan 15 01:30:03 PST 2010

On Tuesday 12 January 2010 21:52:34 Brandon Wright wrote:
> Lance Hepler's patch
> (http://lists.freedesktop.org/archives/cairo/2009-February/016494.html)
> doesn't require any more external support than cairo needs now, and
> this is the same approach Qt has taken.

I would object the committal of these patches. How the font back-end chooses 
to filter the text it renders is its prerogative --- it is none of Cairo's 

The above patch is a nightmare for several reasons.

 1. It makes it harder for distributions to disable sub-pixel rendering --- a 
patented technology. Hence distributions wishing to respect software patents 
need a simple way to remove all infringing code. If libraries use the FreeType 
API then this only needs to be done in one place. (When compiling FreeType --- 
no API breakage either.)

 2. If the filtering coefficients (which are empirical) change in FreeType these 
patches will also need to be changed. It is code duplication at best.

Please see David Turner's Cairo patches which make use of the FreeType API. 
(And out of all of the patch sets I've come across are the only ones I 
consider to be correct from a technical standpoint.)

The reason we are in this mess at the moment is because the Xft developers 
decided to perform sub-pixel rendering themselves (as opposed to helping 
design an API for FreeType). Lets not make the same mistake twice :)

> Apple has options for linear 3 and 5-tap FIR filters, and calls them
> "Light" and "Medium." I believe the early patches for Cairo had
> similar names in their implementations as well. Regardless, Fontconfig
> already has these options and calls them "lcddefault" and  "lcdlight"
> with the config Cairo currently uses called "lcdlegacy." It's
> important to understand that no one's talking about exposing these
> settings in the Cairo API--just providing complete configuration for
> the font backend. It's irrelevant what you call them inside Cairo
> because these settings are already named and widely used with
> Fontconfig, Qt, and third-party patches. The only person who's going
> to see what they're called is whoever is working on the Cairo
> internals.

Apple no longer provide such options --- they were removed with the release of 
Snow Leopard (10.6). Moreover, I am not totally convinced that the options 
were 3- and 5-tap filters respectively (or what the coefficients were). Since no 
behaviour was ever formally specified (just listed as "Font smoothing" it would 
be fool-hardly to depend on it.

Finally, I do not believe that there options were ever exposed by a per-
application public-API.

Polemically yours, Freddie.

More information about the cairo mailing list