Digital watermark is used to protect digital image against any illegal reproduction and tampering. In the selective block assignment process, the image is divided into N X N pixel blocks and each block is Discrete Cosine Transformed (DCT). The set of blocks will be then selectively chosen to encode the copyright message. Each selective block will be incremented by a value, in order to maintain the invisibility of the watermarking image, the incremented value should be within a range. The selection of blocks is based on measurement of the content. Depends on the amount of messages stored and the signal to noise ratio (SNR) of the resultant image required, a threshold is decided. In practice, the threshold will be set such that the duplicated message or an error correction mechanism can also be included in order to increase its robustness. The decoding process should be carried out by using the threshold values to get back the locations that have watermark information. Then the watermarked image is subtracted from the original image to obtain the secret data. Simulation results show that the watermarked image looks visually identical to the original and with an SNR of 44.7 dB for Lenna and with SNR 43 dB for airplane with size 256 X 256 pixels.

Date Published: 9 April 1999
PDF: 7 pages
Proc. SPIE 3657, Security and Watermarking of Multimedia Contents, (9 April 1999); doi: 10.1117/12.344668

Published in SPIE Proceedings Vol. 3657:
Security and Watermarking of Multimedia Contents
Ping Wah Wong; Edward J. Delp III, Editor(s)