  Multiple watermarking technique, embedding several watermarks in one carrier,
has enabled many interesting applications. In this study, a novel multiple
watermarking algorithm is proposed based on the spirit of spread transform
dither modulation (STDM). It can embed multiple watermarks into the same region
and the same transform domain of one image; meanwhile, the embedded watermarks
can be extracted independently and blindly in the detector without any
interference. Furthermore, to improve the fidelity of the watermarked image,
the properties of the dither modulation quantizer and the proposed multiple
watermarks embedding strategy are investigated, and two practical optimization
methods are proposed. Finally, to enhance the application flexibility, an
extension of the proposed algorithm is proposed which can sequentially embeds
different watermarks into one image during each stage of its circulation.
Compared with the pioneering multiple watermarking algorithms, the proposed one
owns more flexibility in practical application and is more robust against
distortion due to basic operations such as random noise, JPEG compression and
volumetric scaling.
