Apple Macintosh PhotoFlash - Manual (Page 82)

Changing the number of colors used by an image
A digitized image consists of a grid of pixels, or dots. You can use the
Number of Colors command in the Image menu to control how many
different colors each pixel is capable of representing. This number directly
affects the fidelity of the image, especially in the case of photographic or
other continuous-tone images.
If a single pixel can represent only black or white, it occupies very little space
on disk, but it can give only a rough approximation of any grays or colors in
the original image. At the other extreme, if a single pixel can represent one of
millions of colors, it occupies much more space on disk, but it is likely to give
a very good approximation of any colors or grays in the original.
IMPORTANT The number of colors used by an image may differ from the
number of colors you can actually see on screen, which depends on the kind
of monitor you have and the current setting for that monitor in the Monitors
control panel. For example, on a black-and-white monitor an image that uses
millions of colors displays only white or black pixels, because that’s all
the monitor can display. The Number of Colors command controls only
the number of colors used internally to represent an image’s pixels, not the
number of colors used by your monitor.
The next section describes how to use the Number of Colors command.
“Deciding How Many Colors to Use,” later in this chapter, describes some of
the issues you should consider if you’re not sure how many colors to use for a
particular image.
Using the Number of Colors command
Before you change the number of colors used by an image, it’s a good idea
to use the Duplicate command in the File menu to create a copy of the image
in a new window. This ensures that you can retrieve the original version of
the image no matter what you do to the copy. Otherwise, if you reduce the
number of colors used by the original image and use the Save command
to save the new version, the original information about the image’s colors is
gone forever.
72 Chapter 4