Imperceptibility Metric for DWT Domain Digital Image Watermarking

The human visual system is very complicate and able to handle a voluminous amount of visual information. It is composed of a receiver with a preprocessing stage, the eye and the retina, a transmission channel, the optic nerve, and a processing engine, the visual cortex. Though one has attempted to understand and model the human visual system process a stimulus is progressed through its huge neural network has not well known because of our lack of knowledge about brain behavior. As well known, the discrete wavelet transform is widely used in signal processing applications such as signal analysis, de-noising, compression etc. It has also played a key role in the field of digital image and/or audio watermarking. A general metric to measure perceptible transparency is peak signal to signal ratio which is the ratio between the maximum possible power of a signal and the power of error that affects the fidelity of its representation. But peak signal to noise ratio does not consider the characteristics of human visual system. In this paper we proposed new metric to measure the invisibility of discrete wavelet transform domain watermarked image based on human visual system using 2- dimensional contrast sensitivity function. The proposed metric was applied to a spread spectrum image watermarking scheme in discrete wavelet transform domain and evaluated by the Minkowski pooling. The experimental results were compared with peak signal to noise ratio.