Chapter 30. JPEG Format

Developed by and taking its name from the Joint Photographic Experts Group, JPEG is a compression algorithm used by files in the JFIF format, commonly referred to as "JPEG files." JPEGs use either the .jpg or .jpeg suffix. Like any graphics file format in widespread use on the Web, JPEGs are platform-independent.

JPEG files have the following characteristics :

• They are 24-bit color images.

• They use JPEG's "lossy" compression scheme.

• They may be "progressive" (interlaced), displaying in a number of passes on download.

Any image can be saved in JPEG format, but due to its true color capacity and the way JPEG compression works, the format is ideal for photographic and other continuous tone images, such as paintings, watercolor illustrations, and grayscale images with the 256 shades of gray (see Figure 30-1). JPEGs do not support transparency or animation .

JPEGs are notably not good at compressing graphical images with large areas of solid color, such as logos, line art, type, and cartoon-like illustrations. Not only could the image end up blotchy, but the file usually will be quite a bit larger than a GIF file of the same image. JPEG compression is also not good at sharp edges or typography because it tends to leave artifacts that "ripple" the edges.

This chapter begins with further explanations of the JPEG features listed earlier. It also discusses strategies for keeping JPEG file sizes at a minimum.