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What is a Bitonal Image?

A bitonal image uses exactly two tones, most commonly black and white. Because the two states can be encoded as zero and one, an uncompressed representation may require only one bit per pixel.

How Bitonal Images work

A bitonal image is a two-tone raster intended to preserve marks and background without intermediate shades. Although often used interchangeably with binary image, “bitonal” emphasizes the visual or document representation, while binary masks may encode nonvisual classes. Its restricted palette enables efficient document-oriented compression and deterministic threshold operations. Scan workflows create it after deskewing and tonal cleanup, then feed OCR, archival storage, printing, or fax delivery.

Key facts

  1. One-bit pixels can be packed eight per byte before compression, but scan formats may add row padding, strip or tile structures, metadata, and compression overhead.
  2. Lossless bi-level compression exploits long runs and repeated document patterns; photographic compression designed for continuous tones is usually a poor structural match.
  3. Antialiased text contains intermediate edge levels, so converting it to bitonal output requires a threshold or halftone decision that can change apparent stroke weight.

When Bitonal Images matter

Bitonal output suits scanned text, fax data, signatures, and high-contrast archival documents. It minimizes storage but loses grayscale detail, making it unsuitable for photographs or shaded illustrations.

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