NSCT-UPEMD-PHT based robust zero-watermarking algorithm and system

A computer-implemented copyright protection method, comprising:

• obtaining a first image to be subjected to copyright verification;
• extracting watermark from the first image, comprising:
• step 1:
• performing full-scale non-subsampled contourlet transform (NSCT) on the first image I(i, j) to obtain one first low-frequency sub-band and four first high-frequency sub-bands, I representing the first image;
• step 2:
• performing Hilbert-curve dimensionality reduction on the first low-frequency sub-band to obtain a first one dimensional (1D) low-frequency sub-band;
• step 3:
• for the first 1D low-frequency sub-band, decomposing a signal S′(t) of the first 1D low-frequency sub-band into different bands with wavelet packet decomposition (WPD), each band carrying a signal component, and determining a last intrinsic mode of the first 1D low-frequency sub-band after WPD;
• step 4:
• performing inverse Hilbert-curve dimensionality augmentation on the last intrinsic mode of the first 1D low-frequency sub-band after WPD, subjecting to a polar coordinate domain transformation and extracting values in a unit circle to obtain a modal matrix X;
• step 5:
• performing a polar harmonic transform (PHT) on the modal matrix X′ to obtain a first PHT moment, and performing following calculation on the first PHT moment:
• Y
• j
• ′
• =
• ∑
• i
• +
• 3
• i
• X
• i
• ′
• +
• N
• wherein Y′j is a processed PHT value, and X′i is an original value of an element of the first PHT moment;
• and
• step 6:
• subjecting a first processed PHT moment Y′(i, j) to mean normalization, followed by XORing with a stored key Wy(i, j) to obtain an encrypted watermark;
• performing watermark decryption, comprising:
• XORing a stored decryption key W0 and the encrypted watermark obtained in the step 6 to obtain a first watermark image;
• comparing the first watermark image with a second watermark image stored and containing copyright information;
• in response to a comparison result indicating that the first watermark image and the second watermark image are identical, determining that a copyright of the first image is legal.