3.1 - Basic Linear Filtering with Application to Image Enhancement

Abstract

Linear filters are used for generic tasks such as image/video contrast improvement, denoising, and sharpening, as well as for more object- or feature-specific tasks such as target matching and feature enhancement. The term “image enhancement” has come to mean specifically a process of smoothing irregularities or noise that has somehow corrupted the image while modifying the original image information as little as possible. The noise is usually modeled as an additive noise or as a multiplicative noise. This chapter discusses the systems that are linear and shift invariant (LSI). The basic approach to linear image enhancement is low-pass filtering. There are different types of low-pass filters that can be used, of which several are studied in the book. For a given filter type, different degrees of smoothing can be obtained by adjusting the filter bandwidth. A narrower bandwidth low-pass filter may reject more of the high frequency content of a white or broadband noise, but it may also degrade the image content by attenuating important high frequency image details. The chapter analyzes why this tradeoff is difficult to balance.