Types of Digital Images
Binary or Bilevel Images
Binary images use only a single bit to represent each pixel. Since a bit can only exist in two states—on or off, every pixel in a binary image must be one of two col¬ors, usually black or white. This inability to represent intermediate shades of gray is what limits their usefulness in dealing with photographic images.
Indexed Color Images
Some color images are created using a limited palette of colors, typically 256 dif¬ferent colors. These images are referred to as indexed color images because the data for each pixel consists of a palette index indicating which of the colors in the palette applies to that pixel. There are several problems with using indexed color to repre¬sent photographic images. First, if the image contains more different colors than are in the palette, techniques such as dithering must be applied to represent the missing colors and this degrades the image. Second, combining two indexed color images that use different palettes or even retouching part of a single indexed color image creates problems because of the limited number of available colors.