As discussed in more detail in chapter 4, functions whose area under the curve is finite can be represented in terms of sines and cosines of various frequenceies. The sine/cosine component with highest frequency determines the highest “frequency content” of the function. Suppose that this highest frequency is finitr and that the function is of unlimite duration (these functions are called band-limited functions). Then, the shnnon sampling theorem theorem {bracewell (1955)}tells us that, if the function is samples at a rate equal to or greater than twice its highest frequency, it is possible to recover completely the orginal function from its sampled image. The corruption is in the original function is undersamplrd, then a phenomenon called aliasing corrupts the sampled image. The corruption is in the A5.0from of additional frequency components being introduced into the sampled function. These are called aliased frequencies, note that the sampling rate in images is the number of samples taken (in both spatial directions) per unit distance.
As it turns out, except for a special case discussed in the following paragraph, it is impossible to satisfy the sampling theorem in practice. We can only work with sampled data that are finite in duration. We can model the process of converting a function