A region-specific QIM-based watermarking scheme for digital images

Angle Quantization Index Modulation (AQIM) is a watermarking technique that achieves provably good capacity-distortion-robustness performance. The scheme embeds each bit of a watermark message on an image by quantizing the angle formed by a vector of pixels or frequency coefficients from the host image with the origin of a hyperspherical coordinate system. While it has been demonstrated that AQIM is insensitive to intensity scaling attacks, the quantization step employed throughout the embedding process is set to a constant value regardless of the nature of the image. Because of this, the perceptual quality of the watermarked image cannot be regulated. In this paper, we propose a straightforward yet powerful AQIM-based watermarking scheme that considers the statistical behavior of the region where a message bit is to be embedded before settling on the size of the quantization step. Experimental results show that, for the same watermark capacity and image fidelity parameters, our proposed region-specific scheme exhibits a lower bit error rate than the regular AQIM approach.