The next assignment will deal with developing an X11 driver complete with color and polygons as well as lines and text. First I'll send you some of the key code you will need along with some references.
The basic idea will be that you can replace your current device dependent file - perhaps you called it liblpr.c - with the new one I will send you - called libx11.c. Compiling your program with libx11.c instead of liblpr.c will create a graphics program that uses real X11 graphics primitives (ie they work with the real pixels on the screen) rather than the simple text primitives that we used in assignment 1-3 (which treated a text character as a pixel).