Image subtraction or pixel subtraction is a process whereby the digital numeric value of one pixel or whole image is subtracted from another image. This is primarily done for one of two reasons – levelling uneven sections of an image such as half an image having a shadow on it, or detecting changes between two images. This detection of changes can be used to tell if something in the image moved. This is commonly used in fields such as astrophotography

Astrophotography

Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...

 to assist with the computerized search for asteroids or Kupier belt objects

Kuiper belt

The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 in which the target is moving and would be in one place in one image, and another from an image one hour later and where using this technique would make the fixed stars in the background disappear leaving only the target. For an example see.

See Also

• Dark frame subtraction
  Dark frame subtraction
  In digital photography, dark-frame subtraction is a way to minimize image noise for pictures taken with long exposure times. It takes advantage of the fact that a component of image noise, known as fixed-pattern noise, is the same from shot to shot: noise from the sensor, dead or hot pixels...
  
   – where a neutral "blank" frame is subtracted to reduce noise
• Palomar Transient Factory
  Palomar Transient Factory
  The Palomar Transient Factory is an astronomical wide-field survey designed to search for optical transient and variable sources. It is fully automated, including a wide-field survey camera, an automated realtime data reduction pipeline, a dedicated photometric follow up telescope, and a full...
  
  – a wide-field survey that uses image subtraction