A number of bottom-up saliency detection algorithms have been proposed in the literature. Since these have been developed from intuition and principles inspired by psychophysical studies of human vision, the theoretical relations among them are unclear. In this paper, we present a unifying perspective. Saliency of an image area is defined in terms of divergence between certain feature distributions estimated from the central part and its surround. We show that various, seemingly different saliency estimation algorithms are in fact closely related. We also discuss some commonly used center-surround selection strategies. Experiments with two datasets are presented to quantify the relative advantages of these algorithms.