Method for hiding secret image

There exist two shortcomings in the method of gray difference replacement for hiding secret image: (1) the visual effect of the dazzle image is not perfect; (2) some information of the secret image may be lost. A method which combines the methods of gray transform and gray difference replacement is presented to hide secret image. The secret image and cover image are respectively transformed via gray transform to avoid data overflow caused by following difference operators, and then the gray difference value of the transformed cover image is replaced by that of the transformed secret image and produces a dazzle image similar to the cover image. Since the gray distribution of the transformed secret image is much similar to that of the cover image after difference, the number of cover image pixels, whose gray value needs to be adjusted during the course of replacement, is reduced. The visual effect of the produced dazzle image is vivid, and the secret image resumed from the dazzle image has higher fidelity. Simulations show that this method not only effectively improves the visual effect of the dazzle image but also reduces the distortion of the secret image. Both the dazzle image and the resumed secret image have higher PSNR (Peak Signal-to-Noise Ratio).