Image Basics _Contributed by Wim Bakker (bakker@itc.nl)_
   
What is an image?

   A digital image is a collection of digital samples.
   The real world scene is measured at regular distances (=digital). One
   such measurement is limited in
     * Space
       One sample covers only a very small area from the real scene.
     * Time
       The sensor needs some integration time for one measurement (which
       is usually very short).
     * Spectral coverage
       The sensor is only sensitive for a certain spectral range.
       
   Furthermore, the sample is quantized, which means that the physical
   measure in the real world scene is represented by a limited number of
   levels only. Usually 256 levels of "grey" are sufficient for digital
   images; 256 levels can be represented by an eight bit unsigned Digital
   Number (DN). "Unsigned" because the amount of light is always
   positive. More levels will need more bits; the quantization determines
   the amount of bits per pixel on the image storage.
   
   Image samples are usually called _pixel_ or _pel_ after the
   combination of "picture" and "element". A pixel is the smallest unit
   of a digital image. The size of this unit determines the resolution of
   an image. The term _resolution_ is used for the detail that can be
   represented by a digital image. As discussed before the resolution is
   limited in four ways:

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