Inverting signal bits is a basic operation for digital signal processing such as watermarking. A transformation from an input level to an output level that performs all the following three functions has been proposed in the previous papers: first, inverting a chosen bit; second, minimizing level change caused by the inversion; and last, varying the resultant transformed levels randomly by a stochastic process.In the application of the transformation to watermarking image bit-planes this paper deals with distortions appearing on the transformed images in detail. First, the way how pixel levels are changed by the transformation to generate distortions is described. Second, the results of the subjective quality measurement that was carried out for human subjects are presented. The measurement indicates that subjective quality of low-detail areas shows a correlation with level change, and subjective quality of high-detail areas reveals a correlation with change of frequencies of level occurrence.
A novel method of watermarking in image spatial domains is proposed in this paper. The level transformation previously proposed for embedding watermark bits into each pixel of a source image is used adaptively with estimating the resulting subjective image quality. Thus, a desired subjective quality can be achieved on the watermarked image.First, the measurements of image quality by subjective evaluations are analyzed from the viewpoint of the correlation with the objective quality measures. Then, the estimating function that can yield an estimate of the subjective quality from the objective quality values is determined.By using the estimating function, the level transformation is determined in each image region so as to achieve a desired subjective image quality while increasing the capacity of embedding watermark bits. The simulation result demonstrates that the transformation is carried out in every image region adaptively according to the image texture.
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