A Restorable Semi-fragile Watermarking Combined DCT with Interpolation

In this paper, a semi-fragile watermarking with interpolation method is proposed to improve recovery performance with less recovery watermark capacity. The original image is 2-fold down sampled to reduce the watermark payload and DCT (Discrete Cosine Transformation) is calculated on each 4 x 4 block in the down-sampled image. The DC coefficient and first two AC coefficients in each 4 x 4 block are quantized by selected quantized steps respectively and encoded with 11 bits to generate the recovery watermark corresponding to 8 x 8 block in the original image. The recovery watermark of each 8 x 8 block is embedded in the quantized DCT coefficients of other blocks. At the recovery side, the low resolution image is firstly reconstructed by the extracted valid recovery watermark and the high resolution one is reconstructed by the interpolation method based on the low resolution one. The tampered blocks are recovered by the corresponding blocks in the high resolution image. The image inpainting method is also used to recover the tampering coincident blocks. Experimental results show that the proposed restorable semi-fragile watermarking method can achieve better recovery performance under JPEG (Joint Photographic Experts Group) compression with superior invisibility.