The word pixel is first introduced by Frederic C. Billingsley to refer to elements of pictures in images. The word is formed by combining two words i.e. the word pix which signifies pictures and the word element. A digital image is a combination of a number of minute elements. A group of small elements together forms an image or a picture. These elements are in the form of dots and different coloured dots or similar coloured dots groups together to form a big image. These dots are like grains which occupy a small area on a display screen. In raster image, these minute dots or pixel is regarded as a physical point.