1999.12.08 18:03 "RE: Testing: [was: Test Adobe ZIP TIFF needed]", by Martí Maria
You could do it without any patents, probably, but trade secrets might be as big or bigger an obstacle. Adobe doesn't publish the details of the CMYK separation methods used in Photoshop, for example. AFAIK none of the big prepress players likes to talk about how they do it.
One nice method to do this accurately and without time penalty is using ICC profiles. I have put on public domain a package for doing that conversion. The test program does use Sam's library for low level I/O and then, the cmm engine Translates from/To RGB<->CMYK on a device-dependent way. More details on http://www.abaforum.es/martim/lcms.htm