 and the ITU-T as a technique for the compression of still images while still retaining varying degrees of quality as required by the user's application. Here's how it works. Please refer to Figures 2-16a and b, which are photographs of my daughter, Cristina. Figure 2-16a shows the original photograph, a reasonably good qual- ity picture that has, in fact, been substantially compressed using JPEG. Figure 2-16b is a small portion of the first image, specifically Cristina's right eye. Notice the small boxes that make up the image. Those boxes are called picture elements, or pixels. Each pixel requires substantial computer memory and processing resources: eight bits for storage of the red components of the image, eight bits for green, and eight bits for blue -the three primary colors (and the basis for the well-known RGB color scheme). That's 24-bit color, and every pixel on a computer screen requires them. Furthermore, there are a lot of pixels on a screen: Even a relatively low-resolution monitor that operates at 640x480 has 307,200 pixels, with 24 bits allocated per pixel. That equates to 921,600 bytes of information, or roughly one megabyte. And that's just color information. Just for fun, let's see what happens when we make the image move, as we will do if we're transporting video. Since typical video generates 30 frames per second, that's 221,184,000 bits that have to be allocated per second-a 222 Mbps signal. That's faster than a 155 Mbps SONET OC 3c signal! The message is that we'd better be doing some kind of com- pression! JPEG uses an ingenious technique to reduce the bit count in still images. First, it clusters the pixels in the image (look at Figure 2-16b) into 16-pixel-by-16-pixel groups, which it then reduces to eight-by-eight groups by eliminating every other pixel. The JPEG software then calcu- lates an average color, hue, and brightness value for each eight-by-eight block, which it encodes and transmits to the receiver. In some cases the Downloaded from Digital Engineering Library @ McGraw-Hill (w