JPEG File Format
JPEG (Joint Photographic Experts Group) is the favored format for compressing color
photographs and other multicolor images, and has become a standard format on
the Internet. JPEG can compress images to as little as two to five percent of their
original file size, but as the degree of compression increases, there is a corresponding
loss of image quality.
JPEG is called a lossycompression process because visual information is lost (dis-
carded) in the compression process. The compression algorithm extracts essential
information from an image and discards unnecessary information (see Figure 7.2).
Because of the lost information, highly compressed images show visible distortions,
calledcompression artifacts.86 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY
FIGURE 7.2
JPEGs saved
with 12 quality
(670KB) and 0
quality (39KB)
in Photoshop.
The original
TIFF file was
1.3MB.Images that include large areas of single colors or slight gradients, such as the sky,
display these image artifacts prominently (see Figure 7.3).
JPEG compression is primarily designed for photographs. The reason is that JPEG
sacrifices colors for brightness. The human eye is very sensitive to brightness
changes, but not color changes. For this reason, the JPEG compression algorithm can
remove or average color information in an image without affecting brightness infor-
mation.
The JPEG format has specific advantages among file formats: