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The problem of digital image watermarking invariant to planar projective transformation is considered in this paper. Methods that are invariant to certain spatial transformations, such as 2D geometric, and a more general affine transformation, are now widely available. However, those methods fail if a possibly watermarked image has undergone a projective transformation. A method based on a well-known projective invariant property of triangle area ratios, constructed from a set of four co-planar feature points, is described in this paper. The proposed method makes use of such invariant property to compute watermark embedding locations. Formation of the required four feature points for such computation is described. Experimental results are shown to demonstrate failure of an existing affine-invariant watermarking method even at very small projective distortion, as well as the success of the proposed method in withstanding distortion due to the same transformation over wide projection angles.