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where ti are the transformed coefficients, ti’ are the watermarked coefficients and xi is a random sequence of Gaussian distribution, used as the watermark. The a parameter has to do with the strength of the casting. We use two different values for it, one for the LLband and a different one for all other bands, that is a= for the LL band and for the others. The reason for this is that the low frequency band is more vulnerable to changes, meaning that slight changes are easily noticeable. The i parameter ranges from 1 to 20200, thus leading in a satisfactory number of 80000 coefficients that are altered. In each band we start from coefficient 5000 in the zigzag scanning order. Image watermarking using block site selection and DCT domain constraints Abstract: In this paper we propose an image watermarking algorithm based on constraints in the Discrete Cosine Transform (DCT) domain. An image watermarking algorithm has two stages: signature casting (embedding) and signature detection. In the first stage it embeds an identifying label in the image. This is recognized in the second stage. The proposed algorithm has two processing steps. In the first step certain pixel blocks are selected using a set of parameters while in the second step a DCT coefficient constraint is embedded in the selected blocks. Two different constraint rules are suggested for the parametric modification of the DCT frequency coefficients. The first one embeds a linear constraint among certain selected DCT coefficients and the second defines circular detection regions according to the given parameters. The watermarks cast by the proposed algorithm are resistant to JPEG pression and filtering. 1. Introduction Digital watermarks in the general context of TV broadcasting and cryptology were discussed in. To avoid the unauthorized distribution of images or other multimedia property, various solutions have been proposed. Most of them make unobservable modifications to images, that can be detected afterwards . Such image changes are called watermarks. The watermark should not alter visibly the image and it should be robust to alterations which may be caused by various image processing techniques. Algorithms proposed for watermarking have been reported in various papers. They are either stochastic or determinis