Comment 68 for bug 164640

Patents related to subpixel antialiasing are listed on http://david.freetype.org/cleartype-patents.html and http://www.microsoft.com/about/legal/intellectualproperty/search/details.mspx?ip_id=IDARAQHD&techType=Any&ipCat=Any&feeStructure=Any&keywords=cleartype&ipVenture=false

They were filled between 1998 and 1999 and have an issue date from 2001 to 2003. Adding 20 years to the latest issue date makes them expire around 2023 (if they are not renewed). Not quite in time for Cairo 1.6 ;-)

I tried to search for the filtering algorithms in these patents (intra-pixel or FIR), but didn't see the specific filter algorithms or FIR kernels described. They seem to cover the whole subpixel process rather than specific algorithms (which is what is quoted from David in comment 0).

I checked an OpenSuse live-cd. They patch Cairo with this patch and do not compile FreeType with subpixel support. The fonts are rendered in gray-antialiasing even when you choose subpixel in the font properties dialog.

My feeling is that applying this patch this is the safest way to go for Cairo regarding patents. Unfortunately, not having subpixel antialiasing available by default is the price to pay for avoiding patents infringements (in the countries where software patents apply). OpenSuse is already in this situation today, so that may be an example that a major distribution can ship without this feature by default.