PC Hardware A Beginner’s Guide

(^420) PC Hardware: A Beginner’s Guide
control how much of a color is sprayed. Four bits are used for each dot to indicate which
of the four colors are on or off. In contrast, a monochrome (black-ink only) printer requires
a single bit for each dot.
Where monochrome halftoning creates an image to be printed using cells that pro-
duce shades of gray, color halftoning is able to produce a wide range of colors with only
four ink colors. Color halftoning works very much like monochrome halftoning except
that four halftone layers are created: one for each color with a dot anywhere that color is
used. When the layers are logically superimposed on the printed page, the actual colors
of the image emerge.
The challenge of the halftone process is to hide the dots used to create an image and
presentasmoothblendingofcolorsthatcreatesarealisticlookingimagetotheviewer.This
requires very sophisticated software, which is why color qualities vary by manufacturer,
andaprocessthatallowstheviewer’seyetosmooththedotpatternsonthepage.Thisisac-
complished using one of two halftoning methods: ordered dithering or image diffusion.
Most inkjet printers use image diffusion as their halftoning method because it creates
more uniform dot patterns. However, some manufacturers, most notably Lexmark, offer
both halftone methods and allow the user to choose which to use on a given project.
Ordered Dithering Ordered dithering creates the transition from one color to another
by evenly spacing pixels of each color along the common edge of the two colors (see Fig-
ure17-12).Thishigh-endmethodisusedonprofessional-levelgraphicsthatrequiremore
accuratecolorrepresentations.Orderedditheringisalsofastertocreatethanimagediffusion.
Like monochrome halftoning, ordered dithering divides each color plane into cells.
Each cell uses a separate pattern of dots, depending on the size of the cell. The number of
pixels in a cell is a function of the print resolution of the printer, but the more dots in the
cells, the more shades of a color that can be represented.
A separate threshold screen or matrix is used for each layer to determine the cells that
are to be printed. The threshold matrix allows only the cells in certain locations to be
printed and blocks any other cells from being printed to create the dithering (or transition)
foreachcolor.Rememberthateachlayerofthecolorhalftonerepresentsonlyasinglecolor.
The other color involved with the ordered dithering is handled on that color’s layer.
Figure 17-12. Examples of an image section showing no dithering applied, ordered dithering, and
image diffusion methods of color transition