Adaptive sky detection and preservation in dehazing algorithm

Single image defogging and/or dehazing algorithms usually emphasize the small intensity variations of the sky, when it is heterogeneous. This leads to a dramatic aspect of the restored image. This sky problem is probably one of the biggest defects of all the state-of-the-art dehazing algorithms. It was proposed to handle the sky problem by hard or smooth threshold of the sky area and to process the detected sky area differently not to emphasize too much the contrast of the sky heterogeneity. But the threshold value needed to be set by the user. We thus here propose a rule for selecting the threshold value as a function of the histogram of the image to process. This rule shows nice improvements on different kinds of images as shown in our experiments on a database of 1500 images.