I will make a test with xBR. Unlike HQx however I couldn't find a standard repository of the source code. After some search I found http://paintown.svn.sourceforge.net/viewvc/paintown/trunk/src/util/sdl/xbr.cpp wich is marked as "Hyllian's 2xBR v3.3b". If somebody knows a better source for xBR let me know! Complicating the implementation, this source code deals with 16 bit 565 RGB, while I need 32 bit ARGB.