Color Tone-Mapping Circuit for a Focal-Plane Implementation

In this article, we present a review of the driving principles and parameters of a previously reported focal-plane tone-mapping operator. We then extend it in order to include color information processing. The signal processing operations required for handling color images are white balance and demosaicing. Neither white balance nor demosaicing are carried out in the focal plane, in order to avoid increasing circuit size and complexity. Since, in this case, white balance is carried out after tone mapping, multiplication of red and blue channels by constant gains may lead to wrong color results. An alternative approach is proposed, in which different gains are assigned for every red and blue pixel of the matrix. Because of the introduction of color, a modification in the original circuit is proposed, which affects the integration time of red and blue pixels. This modification leads to a reduction in the number of photodiodes required in the pixel array, and hence to a reduction of the sensing circuit area. The results produced by the operator are compared to those obtained from two other digital tone-mapping operators.