Understanding Application Software - Computing Fundamentals - page 145

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Raster (Photo) Editing Programs
A raster graphic , also called a bitmap image, is one that's composed of
a grid of colored pixels . When you use a scanner or digital camera to
capture an image or take a photo, the result is a raster graphic. Raster
graphics can also be created from scratch using a program such as Paint
(the free raster image-editing program that comes with Windows) or,
on the higher end, Adobe Photoshop (or the consumer-level version,
Photoshop Elements, shown in Figure 6.6).

raster graphic A graphic that consists of a grid
of colored pixels that collectively form an image.

pixel An individual colored dot in a raster graphic
or on a display screen.

A raster graphic has a native size, known as its resolution . Resolution is
measured in number of pixels horizontally by number of pixels vertically,
like this: 1024 × 768. This is the same way monitor screen resolution
is measured, as you may remember from Chapter 3, “Input, Output,
and Storage.” The higher the resolution, the better the picture will look
when printed at a large size.

resolution The number of pixels that comprise
an image horizontally and vertically.

A raster image also has a color depth , which means the number of bits
required to describe each pixel's color. A standard color depth is 24-bit,
which uses 8 bits for red, 8 bits for green, and 8 bits for blue. There
are 2 24 (that is, 2 to the 24th power, or about 16 million) unique colors
possible in 24-bit color. The human eye can only detect about 10 million
different colors, so this is more than enough for full photorealism.

color depth The number of bits needed to
describe the color of each pixel in a certain
display mode, such as 16-bit or 32-bit.

One disadvantage of raster graphics is that they take up a lot of disk space.
Each pixel requires 24 bits to describe it, at a minimum, so a 1024 × 768
image would take up at least 2,359,296 bytes (about 2.3 MB) in an uncom-
pressed file—maybe more, depending on the file format. Because raster
images can be so large, some file formats employ compression .

compression The process of reducing the size
of a data file by encoding information using fewer
bits than the original file.