The image is composed of rectangular regions, called pixels, and each pixel is assigned a number. Each number corresponds to a color or shade of grey. In order to reconstruct an image for viewing, one needs both the values for each pixel and a legend which maps the numerical values to a color. Image files on computers typically contain both the array of pixel values and the color map. Digital images usually contain less information than a corresponding image captured with conventional photographic methods.
Photographic film is an excellent repository for information.
One 35mm slide can easily hold one hundred million bytes of data. This makes the digital storage of images somewhat problematic. Even if the cost of storage were not a factor, the time involved in retrieving and displaying very large stored images would be prohibitive. All common conversion of photographic data to digital data involves the loss of some information. Once the images are digitized, they are often compressed.