We present a reversible watermarking algorithm for 2D vector maps using difference expansion and companding technique in this paper. To efficiently compress the location map that records the expanded positions and non-expanded positions, we introduce the companding technique. By this technique, the larger values of differences become to smaller ones. Thus, in the location map, the number of 1s is much more than that of 0s. Then, the location map is compressed into a shorter bit-stream by the arithmetic encoder. Accordingly, the compressed location map consumes less embedding space and the embedding capacity is increased. Experiments show that our algorithm can achieve high embedding capacity with low distortion.
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