Absolute Beginner's Guide to Digital Photography

CHAPTER 7 FILES AND FILE FORMATS 87

FIGURE 7.3
High compres-
sion in JPEG
files creates visi-
ble banding
and image arti-
facts in areas
with no tonal
variation.

■ Excellent image fidelity.The JPEG format was designed

to be fast and provide high fidelity in compressed images.

This format was also designed to be

adjustable so that users could “dial in” the

desired level of compression.

■ Platform independent. The format is

simple enough to be included among

many different operating systems and soft-

ware programs.

■ Flexible.Almost any continuous-tone still

image can be compressed in JPEG regard-

less of color, color space, or size. JPEG also

supports grayscale. JPEG also is not limited

by a specific range of colors, such as GIF,

with its 256-color palette.

■ Compression or no compression. JPEG provides several compression

methods, including a user-defined, scalable compression method.

■ Sequential or progressive load. Aside from traditional sequential open-

ing, images can be saved in Progressive mode. This popular format for the

Web loads the image in a series of steps, with each step adding detail.

The JPEG image compression scheme includes two compression methods: lossy and

lossless. The lossy method uses a method called DCT (Discrete Cosine Transform) to

compress specific size blocks of an image (explained later). The lossless portion of

JPEG uses a predictive method for lossless compression, but is rarely implemented.

Do not use the JPEG for-

mat for computer-gener-

ated images, such as

those used for 3D com-

puter graphics. The com-

pression algorithm will introduce

artifacts and noise in the image.

JPEG should only be used for true

continuous tone images.

caution