A digital image a[m,n] described in a **2D discrete space** is derived from an analog image a(x,y) in a **2D continuous space** through a sampling process that is frequently referred to as **digitization**

The **2D continuous image** a(x,y) is divided into N rows and M columns. The intersection of a row and a column is termed a pixel. The value assigned to the integer coordinates [m,n] with {m=0,1,2,…,M–1} and {n=0,1,2,…,N–1} is a[m,n]. In fact, in most cases a(x,y) – which we might consider to be the physical signal that impinges on the face of a 2D sensor – is actually a function of many variables including depth (z), color (λ), and time (t)