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This paper presents an optical watermarking technique that is robust against geometrical distortion in images by using spatially modulated illumination. It can protect “analog” objects like pictures painted by artists from having photographs taken of them illegally in museums. Illegally captured images in practical situations may contain various distortions, and embedded watermarks may be incorrectly detected. Geometrical distortion caused by the shooting angle that the objects are captured at is a particularly major problem. We carried out experiments to evaluate the robustness of watermarking images that were geometrically distorted, in which distortions were intentionally created by moving the position of the projector and the digital camera from right in front of the object. The accuracy of the extracted watermarking data was almost 100%, even if the shooting angle was inclined by about 20 degrees between the projector and digital camera, in both cases when a Discrete Cosine Transform (DCT) and a Walsh-Hadamard Transform (WHT) were used as the methods of embedding watermarks. We introduced rectangular mesh fitting and a technique of “bi-linear interpolation” based on the four nearest points to correct the distortions.