Angle QIM: a novel watermark embedding scheme robust against amplitude scaling distortions

Quantization index modulation (QIM) watermarking has received a great deal of attention ever since the rediscovery of Costa´s result on codes with host-interference rejecting properties. While such embedding schemes exhibit considerable improvement in watermark capacity over their earlier predecessors, (e.g. spread-spectrum), their fragility to even the simplest attacks soon became apparent. Among such attacks, amplitude scaling has received special attention. We introduce a quantization scheme, named angle QIM (AQIM), that is provably insensitive to amplitude scaling attacks. Instead of embedding information by quantizing the amplitude of pixel values, AQIM works by quantizing the angle formed by the host-signal vector with the origin of a hyperspherical coordinate system. Hence, AQIM´s invariance to amplitude scaling can be shown by construction. Experimental results are presented for the bit error rate performance of AQIM under additive white Gaussian noise attacks.