Bitonal images are images that use only two colors, black and white, to build up the image. Bitonal images are commonly encountered in document imaging, because text information is readily stored in bitonal format. Bitonal format provides excellent readability and efficient lossless compression of document images, but also results in a number of limitations.

Documents are not always readily converted into clean bitonal images, and information that is presented in varying colors and shades of gray are often lost. Photographic content must be dithered or halftoned, resulting in much information loss. Bitonal images are also prone to produce artifacts when manipulated; rotating a line, for example, will result in a stair-step appearance, because it is not possible to interpolate the brightness of the pixels.

Some image processing operations, particularly ones that deal with grouping pixels together, work best in the domain of bitonal images. Bitonal images provide clearly delineated boundaries for pixel groups, and operations that remove such groups, such as despeckle, blob, line, and comb removal, are limited to the bitonal domain in ScanFix® Xpress.