We have proposed an “Illumination Watermarking” technology with which the images of objects without copyright protection can contain invisible digital watermarking. This technology uses spatially modulated illumination possibly using an orthogonal transform, such as discrete cosine transform (DCT), as the method of embedding watermarks, and it can be applied to objects that do not have electronically embedded watermarking such as pictures painted by artists. We conducted a new series of experiments where one-bit binary data were embedded in one block that consisted of 8$\times$8 pixels using the phase of the highest frequency component generated by a Walsh–Hadamard transform (WHT) as well as DCT. The experimental results revealed that embedded data could be read out with almost 100% accuracy in both cases, where the embedded watermarked image could hardly be observed. We also found that the influence of JPEG compression, which is commonly used in digital cameras, was very small.
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