Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [164]
Some bitmap images are created "manually" by someone using a paint program designed for a graphical operating system. Other bitmap images are created algorithmically by computer code. These days, however, bitmaps are very often used for images from the real world (such as photographs), and there are several different pieces of hardware that allow you to move images from the real world into the computer. These devices generally use something called a charge-coupled device (CCD), which is a semiconductor that releases an electrical charge when exposed to light. One CCD cell is required for each pixel to be sampled.
The scanner is the oldest of these devices. Much like a photocopy machine, it uses a row of CCDs that sweep along the surface of a printed image, such as a photograph. The CCDs generate electrical charges based on the intensity of light. Software that works with the scanner translates the image into a bitmap that's stored in a file.
Video camcorders use a two-dimensional array of CCD cells to capture images. Generally these images are recorded on videotape. But the video output might be fed directly into a video frame grabber, which is a board that converts an analog video signal to an array of pixel values. These frame grabbers can be used with any common video source, such as that from a VCR or a laser disc player, or even directly from a cable television box.
Most recently, digital cameras have become financially viable for the home user. These often look very much like normal cameras. But instead of film, an array of CCDs is used to capture an image that's stored directly in memory within the camera and later transferred into the computer.
A graphical operating system often supports the storage of bitmaps in files in a particular format. The Macintosh uses the Paint format, the name of which is a reference to the MacPaint program that inaugurated the format. (The Macintosh PICT format that combines bitmaps and vector graphics is actually the preferred format.) In Windows, the native format is referred to as BMP, which is the filename extension used for bitmaps.
Bitmaps can be quite large, and it's beneficial to figure out some way to make them smaller. This effort falls under an area of computer science known as data compression.
Suppose we were dealing with an image with 3 bits per pixel such as I described earlier. You have a picture of sky and a house and a lawn. This picture probably has large patches of blue and green. Maybe the very top row of the bitmap has 72 blue pixels in a row. The bitmap file could be made smaller if there were some way to actually encode the number 72 in the file to mean that the blue pixel repeats 72 times. This type of compression is known as run-length encoding, or RLE.
The common office fax machine uses RLE compression to reduce the size of an image before sending it over the telephone line. Because a fax interprets an image as black and white with no gray shades or colors, there are generally long stretches of white pixels.
A bitmap file format that's been popular for over a decade is the Graphics Interchange Format, or GIF (pronounced jif like the peanut butter), developed by CompuServe in 1987. GIF files use a compression technique called LZW, which stands for its creators, Lempel, Ziv, and Welch. LZW is more powerful than RLE because it detects patterns of differently valued pixels rather than just consecutive strings of same-value pixels.
Both RLE and LZW are referred to as lossless compression techniques because the original file can be entirely recreated from the compressed data. In other words, the compression is reversible. It's fairly easy to prove that reversible compression doesn't work for every type of file. In some cases, the "compressed" file is actually larger than the original file!
In recent years, lossy compression techniques have become popular. A lossy compression isn't reversible because some of the original data is effectively discarded. You wouldn't want to use lossy compression on your spreadsheets or word processing