Chapter 17: Printers^421
The determination of whether a color is to print or not is actually a mathematical ac-
tivity. Each cell has a binary value associated with it that indicates which of its dot loca-
tions are in use. The threshold matrix has a binary value for each cell location. Each cell is
compared to the threshold, and if the value in the halftone cell is greater than that in the
screen, the cell’s dots are printed. Otherwise, if the screen’s value is equal to or greater
than the halftone cell, the dots are not printed.Image Diffusion Image diffusion, also called diffuse dithering and error diffusion, is the
technique used by virtually all inkjet printers. It is preferred because it creates realistic
images without distinguishable patterns.
This process treats each dot in the image as if it could be printed in one of 255 shades of a
color or grayscale. An inkjet printer is only capable of printing the dot in one of its four col-
ors. For discussion purposes, let’s assume the color being printed is black, which has a value
of 255 (no dot, or white space, has a value of 0). The image diffusion process determines a
color (grayscale) value for each dot in the image. It then calculates an error value that repre-
sents the difference of what will actually be printed at the dot’s location (either a black dot or
no dot) and the grayscale value it determined for that location.
For example, if the drivers determined that a dot should have a grayscale value of 128
and no dot is to be printed, the error for that dot would be 128 minus 0, or 128. If a dot
were to be printed at that location, the error would be 128 minus 255, or an error of nega-
tive 127. The error values are used to diffuse the color of the adjacent dots. If the error is a
negative number, then black dots are less likely to be printed in adjacent pixels. If the er-
ror is positive, black dots are more likely to be printed in the adjacent cells. The final de-
termination depends on the error diffusion applied to the neighboring dots.Inkjet Ink
The ink used in an inkjet printer is chosen very carefully, especially in thermal
drop-on-demand inkjet printers. The research into inkjet inks has been ongoing since the
first inkjet printer was introduced. The ink used impacts the design of the entire
printhead, including the ink reservoir, the nozzles, the ink channels, and particularly the
heaters. Ink is an integral part of the printer’s overall design. Should the ink clog the noz-
zles, change color when heated, or not dry fast enough, the page produced by the printer
will not be usable and will most likely not be readable.
Inkjet printer ink has a number of very important characteristics: Quick drying The ink, particularly color ink, must dry very quickly after it
hits the paper. If it doesn’t, it can overlap neighboring ink droplets and cause
degradation of both the image’s resolution and the color quality.
Water resistance Water-resistant ink stays on the paper even after it gets wet.
This is an important characteristic because printed documents will be handled
and smudged by sweat or body oils as fingers move over the ink.
Light-fast Light-fast ink doesn’t fade when exposed to sunlight.
Thickness The ink must be thick enough to produce quality images on the
paper but not so thick that it clogs the nozzles, even when heated.