Blind Watermarking Algorithm Based on DCT for Color Images

Based on luminance and texture masking and frequency sensitivity of human visual system (HVS), a blind watermarking algorithm based on discrete cosine transform (DCT) for color image was proposed. The blue color difference component (Cb) of the color image was 2:1 horizontal down-sampled, and watermark was embedded in the DCT domain by dithering the DCT quantization coefficients. Watermark was first coded by (15,7) Reed-Solomon code, scrambled by spread spectrum code and then embedded in the DCT intermediate frequency coefficients of the host image´s Cb component. The experimental results show good results both in watermark transparency and robustness. The algorithm can not only extract watermark blindly but also resist some digital image attacks, such as JPEG compression, noise, filtering, rotating, cropping and rescaling.