Skin segmentation using multiple thresholding

The segmentation of skin regions in color images is a preliminary step in several applications. Many different methods for discriminating between skin and non-skin pixels are available in the literature. The simplest, and often applied, methods build what is called an "explicit skin cluster" classifier which expressly defines the boundaries of the skin cluster in certain color spaces. These binary methods are very popular as they are easy to implement and do not require a training phase. The main difficulty in achieving high skin recognition rates, and producing the smallest possible number of false positive pixels, is that of defining accurate cluster boundaries through simple, often heuristically chosen, decision rules. In this study we apply a genetic algorithm to determine the boundaries of the skin clusters in multiple color spaces. To quantify the performance of these skin detection methods, we use recall and precision scores. A good classifier should provide both high recall and high precision, but generally, as recall increases, precision decreases. Consequently, we adopt a weighted mean of precision and recall as the fitness function of the genetic algorithm. Keeping in mind that different applications may have sharply different requirements, the weighting coefficients can be chosen to favor either high recall or high precision, or to satisfy a reasonable tradeoff between the two, depending on application demands. To train the genetic algorithm (GA) and test the performance of the classifiers applying the GA suggested boundaries, we use the large and heterogeneous Compaq skin database