Technical data

92 Color and Input/Output Options
Bit depth
A bitmap is basically a bunch (literally a “map”) of numbers that tell
each dot (pixel) on a computer monitor what color it should be. And
since computer numbers consist of binary digits (1’s and 0’s, or “bits”),
each pixel in effect has one or more bits backing it up, telling it what to
do. From this fact arises the concept of bit depth (also known as “pixel
depth”), one of the essential attributes of any bitmap image. Bitmaps
not only have height and width, they have depth. The more bits
assigned to each pixel, the more possible color states the pixel can be
told to take—the greater its “color depth.”
For example: If you’re only using 1 bit per pixel, the pixel can only be
ON or OFF, in other words “1” or “0,” the two states of the bit—hence
white or black (monochrome). By comparison, a bit depth of 4 bits per
pixel can store 16 values; 8 bits per pixel, 256, and so on. 16-bit images
have roughly “thousands” of values to describe each pixel’s color, and
24-bit images have “millions.” Not
surprisingly, the file size of an image is
basically the product of its linear
dimensions (number of pixels) times its bit
depth, so a picture saved as a 24-bit image
would take up three times as much disk
space as an 8-bit version.
Of course, the appearance of a bitmap on a screen depends not only on
the bit depth of the picture but on that of the computer screen displaying
it. Just a few years ago (in the “old days”), many monitors were limited
to 16 colors, and 256 was a big deal. There were “VGA” and “SVGA,”
and today the choices include “High Color” (16 bit) and “True Color.”
Just because you may have a higher-end system, don’t forget that many
others do not. A 24-bit image with millions of colors may look abysmal
on a 256-color monitor—a key consideration when it comes to creating
graphics for the Web, as opposed to CMYK separations for a print
publication. In print publishing, designers must worry about whether the
colors specified in their electronic images will produce “true” output
when reproduced in ink, under standard lighting conditions. In Web or
CD-ROM publishing, the main worry is how to optimize or reduce the
file size as far as possible, while maintaining some semblance of quality
in the image (more about optimizing below). Fortunately, PhotoPlus
includes tools to support all these needs.