Optimal Image Watermarking UsingHybridFirefly Algorithm for Selecting Blocks and threshold Values

Watermarking is a method for embedding a watermark in a digital image to preserve its copyright. Watermark should have two contradictory properties of transparency and robustness. Locations of embedding watermark in the image and threshold values play an important role in improving these two properties. In this paper, a novel robust image watermarking scheme in hadamard transform domain is proposed which uses Hybrid Firefly Algorithm (HFA) for selecting suitable blocks and threshold values to balance transparency and robustness. HFA is the modified version of firefly algorithm which is proposed and used in this paper for first time.This algorithmgenerates distinct and discrete values for selecting blocks and simultaneously continues values for selecting threshold values. Hadamard transform applies on each selected block of the cover image and four watermark bits are embedded in one block using selected threshold values. Watermark detection is done blindly and objective function of the optimization algorithm is a combination of transparency and robustness. This scheme is applicable to color and gray scale images. Experimental results show that the proposed scheme is highly robust against different attacks and gives better results in terms of transparency compared to other similar methods.