An efficient and secure robust watermarking scheme for document images using Integer wavelets and block coding of binary watermarks

A novel, efficient and robust watermarking scheme for protection of document image contents is proposed in this work. An integer wavelet-based watermarking scheme for embedding the compressed version of the binary watermark logo has been developed for robust watermarking. At the sender side, the source document image is divided into empty and non-empty segments depending on the absence or presence of the information. Watermarking is applied for non-empty segments and thus the amount of embedding capacity is reduced. A binary watermark logo is compressed using binary block coding technique of appropriate block-size. A level-2 integer wavelet transformation is applied on the non-empty segment of the source document image. LL-sub-band of level-2 of the transformed image is subdivided into blocks of uniform size and compressed watermark bitstream is embedded into it. The compressed watermark is redundantly embedded into blocks using quantization technique. Thus, multiple copies of compressed watermark are available and each block of the source document image segment need not include the entire compressed watermark stream. At the receiver side, the extracted segments from each set of blocks are merged to obtain a single extracted bitstream. The bitstream is further decoded to get the binary watermark. The extracted and embedded watermarks are compared and authentication decision is taken based on majority voting technique. Based on the quantization step size, size of the logo and the level of wavelet transform, the watermarks are extracted without accessing the original image. The experimental results show that the proposed technique is highly robust. The performance of the proposed approach is measured in parameters Peak Signal to Noise Ratio (PSNR) and Normalized Correlation Coefficient (NCC). Results show that the proposed approach is better than the existing methods. In the proposed scheme for decompression of watermark, the level of block coding technique is the key, which provides an additional layer of security.