An improved AQIM watermarking method with minimum-distortion angle quantization and amplitude projection strategy

About ten years ago, angle QIM (AQIM) was proposed by Ourique et al. for the purpose of resisting gain attacks. Later, to improve AQIM, Nezhadarya et al. proposed absolute AQIM (AAQIM) and further applied AAQIM to their gradient-based watermarking framework. Both of AQIM and AAQIM have the ability, in principle, to resist gain attacks, but neither of them can achieve optimal performance in terms of angular distortion. In this paper, we propose an improved AQIM watermarking method (IAQIM) with minimum-distortion angle quantization and amplitude projection strategy. IAQIM can not only obtain an optimal performance in terms of angular distortion by using the minimum-distortion angle quantization, but also further reduce the embedding distortion of host signals with the amplitude projection strategy. A theoretical analysis is presented for the performance of the proposed method. Experiments have been conducted to validate the theoretical analysis and to demonstrate the superiority of the proposed IAQIM watermarking method over the existing gain-invariant watermarking methods, including AQIM and AAQIM.