Comment 9 for bug 288570

Printers such as HP's Deskjet series have very limited image processing capabilities. For images, the host computer is providing the rasterising. Each dot of ink on the page is defined by a bit in the file being sent to the printer. An HP Deskjet 970Cxi is a CMYK printer (four inks) while an HP Photosmart is a CcMmYK printer (six inks). Each color of ink will require it's own array of bits.

When printing at 1200 dpi, each square inch of printing will require 1,440,000 bits per ink color. For graphics printing, the temporary spool file will by necessity be large because it is the raster image in PCL of what is to be printed.

Here is HP's PCL manual. Chapter 15 covers raster images and compression techniques:
http://h20000.www2.hp.com/bc/docs/support/SupportManual/bpl13210/bpl13210.pdf