Bitmap Basics

Both bitmaps and metafiles have their place in computer graphics. Bitmaps are often used for very complex images originating in the real world, such as digitized photographs or video captures. Metafiles are more suitable for human-generated or machine-generated images, such as architectural drawings. Both bitmaps and metafiles can exist in memory or be stored on a disk as files, and both can be transferred among Windows applications through the clipboard.

The difference between bitmaps and metafiles is the difference between raster graphics and vector graphics. Raster graphics treats output devices in terms of discrete pixels; vector graphics treats the output device as a Cartesian coordinate system upon which lines and filled objects can be drawn. Most graphics output devices these days are raster devices. These include video displays, dot-matrix printers, laser printers, and ink-jet printers. A pen plotter, however, is a vector output device.

Bitmaps have two major drawbacks. First, they are susceptible to problems involving device dependence. The most obvious device dependency is color. Displaying a color bitmap on a monochrome device is often unsatisfactory. Another problem is that a bitmap often implies a particular resolution and aspect ratio of an image. Although bitmaps can be stretched or compressed, this process generally involves duplicating or dropping rows or columns of pixels, and this can lead to distortion in the scaled image. A metafile can be scaled to almost any size without distortion.

The second major drawback of bitmaps is that they require a large amount of storage space. For instance, a bitmap representation of an entire 640-by-480-pixel, 16-color Video Graphics Array (VGA) screen requires more than 150 KB; a 1024-by-768 image with 24 bits per pixel requires more than 2 MB. Metafiles usually require much less storage space than bitmaps. The storage space for a bitmap is governed by the size of the image and number of colors it contains, whereas the storage space for a metafile is governed by the complexity of the image and the number of individual GDI instructions it contains.

One advantage of bitmaps over metafiles, however, is speed. Copying a bitmap to a video display is usually much faster than rendering a metafile. In recent years, compression techniques have allowed the shrinking of bitmaps to a size where they can be effectively transmitted over telephone lines and used extensively in World Wide Web pages on the Internet.

Where Do Bitmaps Come From?

Bitmap images can be created "manually," for example, by using the Paint program included with Windows 98. Someone using a raster "paint" program rather than a vector "draw" program is working under the assumption that the eventual image will be too complex to be rendered with lines and filled areas.

Bitmap images can also be created algorithmically by computer code. Although most algorithmically generated images can be stored in a vector graphics metafile, highly detailed surfaces or fractals usually require bitmaps.

These days, bitmaps are often used for images from the real world, and various hardware devices let you move images from the real world into the computer. This hardware generally uses something called a charge-coupled device (CCD), which releases an electrical charge when exposed to light. Sometimes these CCD cells are arranged in an array, one CCD per pixel; to keep costs down, a single row of CCDs can be used to scan an image.

The scanner is the oldest of these computer-based CCD devices. 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. Analog-to-digital converters (ADCs) convert the charges into numbers, which then can be arranged in a bitmap.

Video camcorders also use arrays of CCD cells to capture images. Generally these images are recorded on videotape. However, the video output can be fed directly into a video frame grabber, which converts an analog video signal into an array of pixel values. These frame grabbers can be used with any compatible video source, such as that from a VCR or a laserdisc or DVD player, or even directly from a cable decoding 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, and an internal ADC allows the digital image to be stored directly in memory within the camera. Generally, the camera interfaces to the computer through the serial port.