Hardware Reference
In-Depth Information
printers accept commands that describe the pages to be printed (as opposed to sim-
ply accepting bit maps prepared by the main CPU). These commands are given in
languages such as HP's PCL and Adobe's PostScript or PDF, which are complete,
albeit specialized, programming languages.
Laser printers at 600 dpi and up can do a reasonable job of printing black and
white photographs but the technology is trickier than it might at first appear. Con-
sider a photograph scanned in at 600 dpi that is to be printed on a 600-dpi printer.
The scanned image contains 600
600 pixels/inch, each one consisting of a gray
value from 0 (white) to 255 (black). The printer can also print 600 dpi, but each
printed pixel is either black (toner present) or white (no toner present). Gray val-
ues cannot be printed.
The usual solution to printing images with gray values is to use
halftoning
, the
same as commercially printed posters. The image is broken up into halftone cells,
each typically 6
×
6 pixels. Each cell can contain between 0 and 36 black pixels.
The eye perceives a cell with many pixels as darker than one with fewer pixels.
Gray values in the range 0 to 255 are represented by dividing this range into 37
zones. Values from 0 to 6 are in zone 0, values from 7 to 13 are in zone 1, and so
on (zone 36 is slightly smaller than the others because 37 does not divide 256 ex-
actly). Whenever a gray value in zone 0 is encountered, its halftone cell on the
paper is left blank, as illustrated in Fig. 2-37(a). A zone-1 value is printed as 1
black pixel. A zone-2 value is printed as 2 black pixels, as shown in Fig. 2-37(b).
Other zone values are shown in Fig. 2-37(c)-(f). Of course, taking a photograph
scanned at 600 dpi and halftoning this way reduces the effective resolution to 100
cells/inch, called the
halftone screen frequency
, conventionally measured in
lpi
(
lines per inch
).
×
(a)
(b)
(c)
(d)
(e)
(f)
Figure 2-37.
Halftone dots for various grayscale ranges. (a) 0-6. (b) 14-20.
(c) 28-34. (d) 56-62. (e) 105-111. (f) 161-167.
Color Printing
Although most laser printers are monochrome, color laser printers are starting
to become more common, so some explanation of color printing (also applicable to
inkjet and other printers) is perhaps useful here. As you might imagine, it is not
trivial. Color images can be viewed in one of two ways: transmitted light and
reflected light. Transmitted-light images, such as those produced on monitors, are
