Measurement of subjective quality of watermarked images made by inverting bits

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.