A pixel refers to a small square that constitutes an image. Each square has a definite position and an assigned color value, and the color and position of these squares determine the appearance of the image. All digital images are composed of pixels, which are often divided into two categories according to different coordinate origins. Each pixel is denoted as I(r,c) or f(x,y). For a grayscale image, the range of I is a univariate scalar: I=greylevel; for a color image, the range of I is a multivariate vector, for example I=(R,G,B).

A pixel can be regarded as an indivisible unit or element of the entire image. Indivisible means that it cannot be further divided into smaller units or elements, existing as a small cell with a single color.

Each raster image contains a certain number of pixels, and these pixels determine the size of the image displayed on the screen.