A hybrid encoding method is used to assemble the double-random phase-encoding technique and the binary encoding method. Because the double-random phase-encoding technique is robust for noises and the binary encoding method is free of using external keys, the proposed hybrid encoding method has their advantages. The hybrid encoding method first encodes a covert image to form a complex-number matrix by using the double-random phase-encoding technique, where two random real-number matrices are used to increase the security of the encoding work. Then the elements of the two random real-number matrices and the elements of the complex-number matrix are encoded to form a binary-bit string by using the binary encoding method. Finally, the binary data in the binary-bit string are encoded into a host image to form an overt image with hidden information by using a gray-value modulation method. The decoding work is easy for authorized people, but it is very difficult for unauthorized people. Therefore, the proposed hybrid encoding method is a very useful encoding method.
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