Introduction

Images are often considered as records of the physical scenes that we have seen. Therefore, we wish to have images that reproduce the visual impressions of the original scenes as we remember them. Among the various attributes that contribute to the total visual impression, tone and color are two of the most important factors. Tone reproduction is the process of reproducing the visual brightness/lightness impression of the original scene in an image. Similarly, color reproduction refers to the process of reproducing the visual color impression. Although color perception involves brightness/lightness perception, the two topics will be discussed separately, with the implied, narrower definitions that tone reproduction deals with luminance perception and color reproduction chrominance perception. However, it should be understood that there are interactions and trade-offs between the two processes. The criteria and goals of tone reproduction vary from application to application, and we will mainly be interested in consumer imaging applications.

Since the success of a tone reproduction is finally judged by human observers, there are at least two separate systems involved in a tone reproduction task, i.e., the imaging system and the human visual system. Therefore, it is convenient to divide any tone reproduction into three processes: (1) the subjective process that specifies what a desired reproduction should be in terms of visual impression, (2) the psychophysical (translation) process that converts the perceptual criteria as specified in the subjective process into physically quantifiable criteria, and (3) the objective process that deals with calibrating and controlling image devices to achieve the desired reproduction in terms of physical quantities.