Blind watermarking for color images using EMMQ based on QDFT

•EMMQ incorporated EPA, MPSAM, and MM within a QDFT-based blind watermarking.•EPA is first pre-worked to ensure that the watermark is correctly embedded.•MPSAM is used to enable the mapping of multiple bits within a single QDFT block.•MM allows watermarked image superior image quality and strong resistance to attacks.•PSO makes it possible to optimize the EMMQ parameters. In this study, we developed a novel scheme for the blind watermarking of color images. The proposed scheme incorporates extreme pixel adjustment (EPA), multi-bit partly sign-altered mean modulation (MPSAM), mixed modulation (MM), and particle swarm optimization (PSO) within a scheme based on crisscross inter-block quaternion discrete Fourier transform (QDFT). Accordingly, the proposed scheme employing EPA, MPSAM, MM, and QDFT is referred to as EMMQ. The image is separated into non-overlapping 8 × 8 pixel blocks, whereupon MPSAM is used to map multiple bits within a single block using multiple coefficients in one of the four transformed components of QDFT. The use of MM to embed the watermark allows for superior image quality and strong resistance to image processing attacks. Our use of PSO also makes it possible to optimize the EMMQ parameters, thereby enabling outstanding robustness without compromising imperceptibility. Experiment results demonstrate the efficacy of the proposed scheme in resisting a variety of image-processing attacks, with performance superior to that of existing watermarking schemes in terms of imperceptibility and robustness for a given payload capacity.