“…To show the superiority of the scheme, we compare it with other four kinds of watermarking schemes. (1) three representative similar image null-watermarking schemes [8], [9], [14], (2) quaternion theory-based color medical image watermarking scheme [48], (3) lossless medical image watermarking scheme [49], and (4) five representative geometrically invariant quaternion momentsbased watermarking schemes [29], [32]- [34], [37]. Based on these four considerations, the following nine references are selected for comparison: reference [8], which proposed a wavelet domain based image public copyright certification scheme, which could effectively resist interpretation attacks and was highly robust in resisting various malicious attacks; reference [9], which proposed a novel adaptive image copyright protection scheme, in which the verification information was constructed by using Sobel edge detection scheme to detect the edge features of the image, achieving good effects; reference [14], which proposed a QEM based color image null-watermarking scheme, which could effectively resist various geometric attacks and common image processing attacks; reference [48], which constructed quaternion Hadamard transform (QHT) based on quaternion theory and proposed a color image watermarking scheme on the basis thereof; reference [49], which proposed a robust lossless medical image watermarking scheme applicable to e-health systems, with embedding factors K = 10 and K = 20; references [29], [32]- [34], [37], which are representative geometrically invariant watermarking schemes based on five quaternion moments, i.e., QEM, QPHTs, QRHFM, QLFM, and QRSCM, respectively.…”