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Group (JPEG), and a variety of others; and encryption, including Pretty Good Privacy (PGP), Public Key Cryptography (PKC), and so on. Note that these services can be used on any form of data: Spreadsheets, word processing documents, and rock music can all be compressed and encrypted. Compression is typically used to reduce the number of bits required to represent a file through a complex manipulative mathematical process that identifies redundant information in the image, removes it, and sends the resulting smaller file off to be transmitted or archived. To explain how compression works, let s examine JPEG. JPEG was developed jointly by ISO 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 quality 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 OC3c signal! The message is that we d better be doing some kind of compression! 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 calculates 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
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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Figure 2-16a My daughter, Cristina
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Figure 2-16b Cristina s eye showing pixels
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image can be further compressed, but the point is that the number of bits required to reconstruct a high-quality image is dramatically reduced by using JPEG.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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More About Compression
Compression schemes do a good job of compressing and faithfully reconstituting images, particularly when the image being compressed is a photograph or video clip. To understand the dynamics of this relationship, let s take a moment to consider what it takes to create a digital photograph displayed on a computer screen. A laptop computer display is often referred to as being 640 by 480, 800 by 600, or 1024 by 768. These numbers refer to the number of picture elements, more commonly called pixels, that make up the display. Look closely at the screen of your computer and you will find that it is made up of thousands of tiny spots of light (the pixels), each of which can take on whatever characteristics are required to correctly and faithfully paint the image on the screen. These characteristics include color components (sometimes called chrominance), black and white components (sometimes called luminance), brightness (the intensity of the signal), and hue (the actual wavelength of the color). These characteristics are important in video and digital imaging systems because they determine the quality of the final image. The image, then, is a mosaic of light; the tiles that make up the mosaic are light-emitting diodes that create the proper light at each pixel location. Each pixel has a red, green, and blue light generator, as shown in Figure 2-17. Red, green, and blue are called the primary colors, because as colors they form the basis for the creation of all other colors. It is a well known fact that if three white lights are covered with red, green, and blue color gels respectively, and the lights are shined at roughly the same spot as shown in Figure 2-18, the result will be a light spot for each color, but where the three colors intersect the result will be white light. The combination of the three primary colors creates white. Each primary color also has a complimentary color in the overall spectrum. As Figure 2-19 shows, the complementary color for red is cyan, while the complimentary colors for green and blue are magenta and yellow, respectively. Table 2-2 shows the relationships that exist between the primary and complementary colors. For full-color, uncompressed images, each of the red, green, and blue elements requires eight bits, or 24 bits per pixel. This yields what is known as 256-bit color (28). Now consider the storage requirements for an image that is 640 by 480 pixels in size.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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