Image gray or intensity value attacks

including the absolute modification of gray values

contrast adjustment

parameter histogram equalization

and combined attacks can make many kinds of watermark detection approaches unsuccessful. Addressing these problems in this paper

a robust zero-watermark algorithm using the direction stability of the first principal component vector is proposed

which can effectively solve the conflict between invisibility and robustness. The direction stability of the first principal component vector is mathematically analyzed

and experiments are performed to observe the difference of the stability in the spatial domain and the frequency domain. On this basis

the input image is decomposed by the dual-tree complex wavelet transform(DT-CWT)and the output of the two low frequency bands is segmented into non-overlapping sub-blocks

respectively. After computing the first principal component vector for each sub-block

some vector angles between that of each sub-block and a secret reference vector can be obtained. By comparing the angles from two sub-blocks with the same position in the left and right low frequency bands

zero-watermark feature extraction is accomplished. In addition

we analyze the security and robustness for the proposed scheme. Sufficient experiments with six tested images

twenty kinds of common attacks and fifty kinds of combination attacks are carried out. Simulation results show the proposed scheme is superior in the comprehensive performance composed of robustness

withstanding diversity attack and practical application.