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by Dinesh Thakur

A bitmap is an image or shape of any kind-a picture, a text character, a photo-that's composed of a collection of tiny individual dots. A wild landscape on your screen is a bitmapped graphic, or simply a bitmap. Remember that whatever you see on the screen is composed of tiny dots called pixels. When you make a big swipe across the screen in a paint program with your computerized "brush," all that really happens is that you turn some of those pixels on and some off. You can then edit that bitmapped swipe dot by dot; that is, you can change any of the pixels in the image. Bitmaps can be created by a scanner, which converts drawings and photographs into electronic form, or by a human artist (like you) working with a paint program.

A computer screen is made up of thousands of dots of light, called pixels (short for picture elements). A single pixel is composed of up to three rays of light, red, blue, and green, blended into a single dot on-screen. By combining these rays and changing their intensity, virtually any color can be displayed on-screen. The number of bits required to display a single pixel onscreen varies by the total number of colors a particular monitor can display. The larger the number of possible colors, the larger the number of bits' required to describe the exact color needed. Regardless of the actual number of bits required, a bit map is a series of these bits stored in memory, which form a pattern when read left to right, top to bottom. When decoded by the computer and displayed as pixels on-screen, this pattern forms the image of a picture.

The simplest bitmaps are monochrome, which have only one color against a background. For these, the computer needs just a single bit of information for each pixel (remember, a bit is the smallest unit of data the computer recognizes). One bit is all it takes to turn the dot off (black) or on (white). To produce the image you see, the bits get "mapped" to the pixels on the screen in a pattern that displays the image.

In images containing more than black and white, you need more than one bit to specify the colors or shades of gray of each dot in the image. Multicolor images are bitmaps also. An image that can have many different colors or shades of gray is called a "deep bitmap," while a monochrome bitmap is known as a "bilevel bitmap." The "depth" of a bitmap-how many colors or shades it can contain - has a huge impact on how much memory and/or disk space the image consumes. A 256-color bitmap needs 8 times as much information, and thus disk space and memory, as a monochrome bitmap.

The resolution of a bitmapped image depends on the application or scanner you use to create the image, and the resolution setting you choose at the time. It's common to find bitmapped images with resolutions of 72 dots per inch (dpi), 144 dpi, 300 dpi, or even 600 dpi. A bitmap's resolution is permanently fixed-a bitmapped graphic created at 72 dpi will print at 72 dpi even on a 300 dpi printer such as the LaserWriter. On the other hand, you can never exceed the resolution of your output device (the screen, printer, or what have you); even though you scanned an image at 600 dpi, it still only prints at 300 dpi on a LaserWriter, since that's the LaserWriter's top resolution.

You can contrast bitmapped images with vector or object-oriented images, in which the image is represented by a mathematical description of the shapes involved. You can edit the shapes of an object graphic, but not the individual dots. On the other hand, object-oriented graphics are always displayed or printed at the maximum resolution of the output device. But keep in mind that an object-oriented graphic is still displayed as a bitmap on the screen.

Bit-mapped fonts and bit-mapped graphics use pixels to form pictures or letters. However, because of the number of bits required to encode a single pixel, bit-mapped fonts and graphics consume a great deal of memory. In comparison, trying to create a perfect circle by coloring the squares on a piece of graph paper demonstrates the problems inherent with this method of displaying text and graphics. Because a computer screen is layed out in a grid of dots (pixels) like graph paper, a distortion will show up along the angled and curved lines in an image. This distortion is called "jaggies" or "aliasing."