The licensing deal between the original game developer and the porting house may be a flat one-time payment, a percentage of the profits from the Mac game's sale, or both. While this license gives the porting house access to artwork and source code, it does not normally cover middleware such as third-party game engines. Modifying the source code to the Macintosh platform may be difficult as code for games is often highly optimized for the Windows operating system and Intel-compatible processors. The latter presented an obstacle in previous years when the Macintosh platform utilized PowerPC processors due to the difference in endianness between the two types of processors, but as today's Macintosh computers employ Intel processors as well, the obstacle has been mitigated somewhat. One example of common work for a porting house is converting graphics instructions targeted for Microsoft's DirectX graphics library to instructions for the OpenGL library; DirectX is favored by most Windows game developers, but is incompatible with the Macintosh.