What are the two quantitative approaches used for the evaluation of image features?

The theory of histogram modification of continuous real-valued pictures is developed. It is shown that the transformation of gray levels taking a picture's histogram to a desired histogram is unique under the constraint that the transformation be monotonic increasing. Algorithms for implementing this solution on digital pictures are discussed. A gray-level transformation is useful for increasing visual contrast, but may destroy some of the information content. It is shown that solutions to the problem of minimizing the sum of the information loss and the histogram discrepancy are solutions to certain differential equations, which can be solved numerically.