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Linear chirps, a special case of polynomial phase exponentials, have recently been proposed for digital watermarking. In this paper, the authors propose a known-host-state methodology for designing image watermarks that are robust to compression. They use a two-dimensional frequency-modulated chirp as a spreading function in a block-based spatial watermarking scheme. In each block, the chirp is used to embed binary phase information. Chirp parameters allow for spectral shaping of the watermark to match host content. Since host state is known to the embedder, it is possible to tune the chirp for optimum performance, particularly against compression.
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