User`s guide

Chapter 1:Introduction to Image Processing
ScanShop User’s Guide - 3
per pixel, the more information there is that can be used to represent an image,
and the larger the resulting file size. Black and white line art, for instance, has
only 1 bit per pixel, that is, a bit in the image can be either 1 or 0 (black or white).
Gray scale output, on the other hand, may have 4, 6 or 8-bits per pixel, yielding
from 16 to 256 shades of gray. Color images typically have 24 bits per pixel (8
each for red, green and blue) and can represent up to 16.7 million colors.
Matching Scan to Output
For best results, it is important to scan images at the dpi which your output device
will support. As a general rule, when a resolution value is specified for a scanning
operation, it should be made the same as, or as close as possible to, the resolution
of the output device that will be used for reproducing the image, i.e., the printer
resolution.
When importing images into desktop publishing applications, any needed size
modification of the image or the printout is generally done by the application
itself to the user’s specifications. When working with image editing applications
however, the user is directly responsible for the image sizing. You need to be very
much aware of the differences between input and output resolution because they
can affect the size of the image dramatically. For instance, if you scan an image
at 600 dpi but print it on a laser printer with only 300 dpi, your image will be
twice as large as the original.
Common Image Processing Features
This section describes the fundamental ideas behind certain image processing
functions. The scanners that offer these features implement them differently.
These implementations are to be found in another section, “Appendix A: Scanner
and Printer Specific Features” on page 59 for specific information on scanners.
Dithering
Dithering allows you to create the appearance of a continuous-tone image for line
art (black and white) images (one bit/pixel scan). Dithering is similar to
halftoning but dithering uses only dots of uniform size. Dithering is a method by
which the scanner organizes the image into geometric configurations of black
and white dots that give the illusion of gray. These dots can seem random, as seen
in error diffusion dithering, or can be displayed in patterns of dots. Dithering
options are particular to each scanner.