Image Enhancement by S-Shaped Curves Using Successive Approximation for Preserving Brightness

For image contrast enhancement, an S-shaped transfer function has been developed to better match the human vision system with an associated median brightness subtraction and addition method to preserve the median brightness of an image. However, this median brightness preservation method will cause clipping of the dynamic range of images resulting in a loss of contrast. Moreover, preserving the mean or median image brightness will sometimes produce a different result without a consistent preference. In this letter, we have developed a method to preserve either the mean or median image brightness by shifting the intersection point of S-shaped curves along the y = x line by successive approximation. To improve computational efficiency, two newly derived equations have been proposed to reduce the number of optimum control parameters from four to two for solving the S-shaped curves. In addition, our proposed method can alter the image brightness without causing clipping at either ends of the intensity range. Our proposed brightness preservation method can also be applied to a video sequence to change its brightness without causing any flickering. It has been shown that our proposed method can produce visually pleasing enhanced images and videos.