In this paper, a selective weighting method is used for data embedding to achieve blind watermark detection. In the proposed system, block polarity and activity index modulation are used for the selective weighting. The block polarity is determined based on the number of coefficients that are larger than the median value. The block activity index is the pseudo-quantized block activity that is represented by the sum of absolute differences (SAD) of each coefficient to the median value. The block activity index modulation is performed based on the XOR operation of the randomized watermark and the randomized wavelet blocks polarity. In the block activity index modulation, if any coefficient is located very close to the median, it is vulnerable to attacks because its polarity can easily be changed. In such cases, the coefficient is forced to shift, by the just-noticeable-difference (JND) amount, toward the positive or negative end to enhance the robustness. The watermark embedding is actually performed by the activity index modulation that will modify each coefficient value by a small amount to force the activity to be quantized into a specific region. Simulation results show that the proposed method performs extremely well for Checkmark with non-geometric attacks, such as linear filtering, remodulation, denoising, and compression. The proposed scheme is also robust against image cropping, downsampling, rotation, and columns removal attacks.