Glossary of Terms

Bit Depth: This refers to the number of bits that are assigned to each pixel in an image. The more bits you have, the more photo-realistic the screen image will be. Here are some of the typical choices in bit depth for computer screens:

1-bit: If a computer has the ability to display 1-bit per pixel, each pixel can either be black or white.

4-bit: Some computers, especially laptops, offer 4-bit video capability, which translates into 16 shades of gray or color.

8-bit: With an 8-bit color depth, you can see 256 colors or levels of gray. An 8-bit system can work well for black and white photographs, but is just barely adequate for critical evaluation of a color photograph.

16-bit: This bit depth has the potential to display 32,000 different colors. At 16-bits and above, the video signal must split into thirds, providing one each for the red, blue, and green channels. Your computer devotes 15 bits to color (5 bits per color channel) and the one remaining bit is used to overlay all these colors.

24-bit: Each pixel on a screen can handle up to 256 colors, which lets systems display 16.7 million colors. A 24-bit model provides true photographic quality.

32-bit: Often when someone is talking about 32-bit color, they really mean 24-bit. Only a few computers offer 32-bit capabilities.

Since 256 levels of gray are displayed on an 8-bit system, that’s all you really need when working with black and white digital photographs, but if you plan on working with color images, you should use a computer that has a 24-bit display.

Bitmap: There are three classes of graphic files: bitmap, metafile, and vector. A bitmap (sometimes known as “raster”) is any graphic image composed of a collection of tiny individual dots or pixels–one for every point or dot on a computer screen.

BMP: Short for bitmap but don’t be confused with the generic term described above. BMP is a Win-dows based digital image format, BMP–often pronounced “bump”– is really a file extension for a specific kind of bitmapped graphics file.

Byte: Each electronic signal is one bit, but to represent more complex numbers or images, computers combine these signals into larger 8-bit groups called bytes. When 1024 (not 1000) bytes are combined, you get a kilobyte, often called K.

CCD: A Charge-Coupled Device is the same kind of light gathering device used in flat-bed scanners, digital cameras, and even video camcorders to convert the light passing through the lens into an electronic equivalent of the original image. These images become digitized by the CCD device.

Dither: A graphics display or printing process that uses a combination of dots or textures to create the impression of a continuous tone gray scale or color image.