What is a Bitmap Image?

A bitmap is one of many types of file formats for images stored in a computerized form. It carries the extension .BMP. Computers use bits of 1 and 0 to store data. A bitmap is literally a map of bits that form a particular picture when rendered to a display like a computer monitor.

To understand how a bitmap image displays, it’s important to understand the computer display screen. The display is made up of rows and columns of tiny blocks, or pixels. In a bitmap image, each pixel is assigned at least one bit to indicate whether the pixel should reflect the background color, the foreground color, or some other color.

In the case of a page of black and white text, let’s consider a single letter. The many pixels that make up that letter only require one bit of data each. Either the pixel will be black or white: 1 or 0.

When a bitmap displays a colored image, such as a lake scene, there are several shades of gradation in colors and lighting. In this case, each pixel in the bitmap might have 16, 24, or 48 bits of information associated with it. The more bits, the greater the resolution of the bitmap – and the larger the file.

Because bitmaps store so much information in the highest resolutions, they make very beautiful images. However, a bitmap image doesn’t rescale well. If blown up using a graphics program, the bitmap image becomes blocky and blurred. If reduced, it loses clarity.

Compression techniques are used to shrink the file size of the bitmap while maintaining as much data as is necessary to render a good picture. One such format is the 8-bit .GIF format, which uses a pallette of 256 colors. The advantage of the compressed .GIF is that it is a smaller file that can be resized with satisfactory results, as it uses lossless compression. The disadvantage is that it cannot faithfully reproduce images containing more than 256 colors.

Photographs saved in the bitmap format that need to be reduced for Internet use can be converted to the JPEG (JAY-peg) format. While this is a lossy compression format, it is capable of displaying more than 256 colors and does a better job of rendering photographs.

Because bitmap images are built pixel-by-pixel, they can be easily edited. Zooming in on a bitmap image in any graphics program allows the user to add, delete, or change the color of individual pixels. For this reason, logos, favicons, and icon images are frequently built in the bitmap format.