PC Hardware A Beginner’s Guide

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Chapter 17: Printers^417


WordPerfect, a graphics package like Adobe Illustrator or Photoshop, or a desktop pub-
lishing package like Microsoft Publisher or Adobe PageMaker) generates a print-image
file, which can be anything from a plain text document to a complex full-color photo-
graph. The application communicates to the operating system that it would like to send
its file to the printer. The operating system places the print file in the system print queue,
where it awaits the availability of the printer.
When the printer is available, the operating system and the printer’s device driver be-
gin transferring the print file to the printer, which involves translating the print file into
commands and information that the printer can interpret into a printed document or im-
age. The following sections describe the process used to convert the application’s print
file into a printed document or image on an inkjet printer.


Halftoning


The first step of the inkjet print process is called halftoning. You may not know it, but you
are most likely very familiar with this technique of producing a graphic image—or at
least the outputs of this process, anyway. Halftoning is the technique used to produce
graphics and photographs in most newspapers. If you look very closely at a newspaper
picture, you will see thousands of small dots of various shades of gray, black, and white
that your eye and brain blend to form a picture. The image’s halftones are created by the
arrangement of the dots on the page.
To print an image in halftones requires that the page be divided into a number of
cells. Each cell is a rectangular matrix of dots (a.k.a. pixels), as shown in Figure 17-11. A
solid black cell has all of its dots printed black and a white cell has no printed dots. Print-
ing only some of the dots in the cell black produces shades of gray. Lighter grays have
fewer printed dots; darker grays have more printed dots. For example, a 10 percent
grayscale has one-tenth of the dots in a cell printed black, and a 50 percent grayscale has
half of its dots printed. The number of dots in a cell determines the number of grayscale
shades available. A cell made u pof 4 dots by 4 dots can produce 17 (4x4+1)shades of
gray. An8x8cell is capable of 65 shades of gray. The cells are then used like tiles across
and down the page to create an image.


Compression and Decompression


The output from the halftoning process is a bitmapped version of the image to be printed.
In addition to the bitma pimage, the file now also contains some additional bits that indi-
cate which of the four CMYK (cyan, magenta, yellow, black) colors are on or off for each
dot. The bitmap image file is compressed to minimize its space and the amount of data
that is transmitted from the computer to the printer. The printer’s device drivers perform
the compression of the file. The compressed file is sent to the printer one line at a time,
which means one pass of the printhead across the page. Since an inkjet printer does not
print an entire line of text in one pass, it can take many passes to complete one line of 12-point
text. The computer can send the data to the printer much faster than the printer can print
each line. Therefore, the print data is received into a print buffer that the printer uses as

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