Most previous DWT-based watermarking algorithms belong to either private watermarking algorithms or fragile watermarking algorithms, but there are few DWT-based robust public watermarking techniques for copyright protection. By taking full advantage of the masking characteristics of the human visual system, first a JND (just noticed difference) threshold matrix based on block is given in this paper, and then a robust image-adaptive public watermarking technique operating in DWT domain is presented. Firstly, the 8? blocks of the original image are rearranged into a 1-D Hilbert sequence in Hilbert scanning order. Then two neighboring blocks are selected from the Hilbert sequence of the host image blocks in turn, and 1-level DWT is applied to the two chosen blocks. Finally, a corresponding detail subband is chose from three detail subbands of the two neighboring blocks at a time, respectively. A binary watermark with visually recognizable patterns is embedded into the host image by modifying the polarity of the average value of the two corresponding subbands. The embedded watermark is invisible to human eyes and adapted to the original image by exploiting the HVS masking characteristics. The experimental results show that the proposed algorithm is effective and robust to common image processing operations and some geometric distortions such as cropping, pinching, pixel-shift, and so on, especially, it receives better robustness under signal enhancement operations. So a conclusion can be made that the proposed technique is practical.