- What is this?

PXLJR (http://sourceforge.net/projects/hp-pxl-jetready/) is an IJS-based driver for the HP JetReady 4.x protocol printer family (Color Laserjet 3500 and 3550) under linux and other unix-like systems. This effort is not endorsed by nor affiliated with HP.
It is licensed under the terms of GNU General Public License.

- Compiling and Run-time requirements?

About the same as HPIJS. PXLJR needs a C compiler rather than a C++ compiler, and also need automake/autoconf (this may be improved in later release).

- HPIJS/HPLIP also supports these two printers. Why are you "reinventing the wheel"?

HPIJS (http://hpinkjet.sourceforge.net) provides 2 printing modes: one color and one gray, corresponding to the max-compression/worst-quality setting of the HP driver under Windows. There are two more settings for better quality in the Windows driver. PXLJR provides these two additional settings, so there are a total of 6 printing modes. Also, the max-compression gray mode of PXLJR is 1.5 to 3 times faster.
One the other hand, PXLJR doesn't provide a full set of facilities like printer status report, etc. For other bells and whistles, please use HPLIP's toolbox instead. There is no conflict installing both HPIJS and PXLJR simultaneously.

- Why did you not improve HPIJS instead?

There are professional reasons that I need to maintain autonomy; on the other hand, the two addtional quality settings are already in the Windows driver. The HPIJS people can consult their Windows driver people and there is no need for me to contribute; the other improvement - the speed for the gray modes - would require substantial modification, and affects and changes code paths that I cannot test due to not have the other printers.

- Any other improvement I can do to my prints?

There is a file 'gs-gamma.ps' which changes the gamma curve and boasts the shadow details if one prepends it to a print-job.

- Known problems, Limitations?

The right-hand border for PXLJR is about 0.5mm wider than HPIJS. I haven not been able to track it down, but the difference is only noticeable if one prints all the way to the edge of the printerable area, and compare side-by-side.
Standard page size (A4, letter, envelope, etc) works, but custom arbitrary size only works on 32-bit small endian. (i.e. x86). HPIJS has a similiar and worse problem - and both will be resolved in the corresponding next releases.
"make install" installs too many unnecessary things, and does not install the ppd. Only really only need
to copy the ijs_pxljr into /usr/bin and the ppd into a CUPS specific place.

- Upcoming improvements?

Making dependence on libjpeg optional.
Make Multiple-copies option work. Currently if one wants X identical copies, one needs to send job X times.
Make the gamme curve change a more user-friendly option.
See if a 1-bit B/W version can go even faster.