Measuring digital watermark strength using error correction coding metrics
View Patent ↗A method of embedding a digital watermark into a halftone image. The method redundantly encodes a multi-bit message, and transforms the encoded message to a multilevel per pixel watermark image. It then derives halftone thresholds from the multilevel per pixel watermark image. These thresholds are then used to convert target images into watermarked halftone images. The multilevel pixels in the target image are used to select corresponding halftone thresholds from the halftone thresholds derived from the watermark image. The selected thresholds are applied to corresponding multilevel pixels in the watermark image to create the watermarked halftone image of the target image. A method of measuring digital watermark strength. In this method, a watermarked signal is processed to extract estimates of error correction encoded bits embedded into the watermarked signal. Then, the error correction encoded bits are decoded to compute a message payload. The message payload is re-encoded to compute error correction encoded bits. A measure of watermark strength is computed from the error correction encoded bits and the estimates of error correction encoded bits. In particular, in one implementation the soft bit estimates decoded from the watermarked signal are multiplied by corresponding re-computed error correction encoded bits and summed to get a measure of the watermark signal strength. This measurement may be compared with a threshold to detect tampering with the watermarked signal, such as compression, scanning and re-printing, photo-copying, etc. For printing applications, the embedded digital watermark may be used to carry printer information that is later decoded by a watermark detector and used to examine a digital scan of a printed object and determine whether the printed object is authentic.
1. A method of measuring digital watermark strength comprising:
processing a watermarked signal to extract estimates of error correction encoded bits embedded into the watermarked signal;
decoding the error correction encoded bits to compute a message payload;
re-encoding the message payload to compute error correction encoded bits;
computing a measure of watermark strength from the error correction encoded bits and the estimates of error correction encoded bits.
2. The method of claim 1 wherein the measure of watermark strength is compared with a strength threshold to detect degradation of the watermark signal.
3. The method of claim 2 wherein the watermarked signal is an image captured of a printed object, and the strength threshold is used to detect whether the printed object is an original or a copy.
4. The method of claim 1 wherein the message payload includes printer information about a printer used to print the watermarked signal; and the printer information is used to determine whether a printed object bearing the watermarked signal is an original or a copy.
5. A computer readable medium on which is stored instructions for executing the method of claim 1 .
6. A method of measuring digital watermark strength comprising:
detecting a watermark signal in a watermarked signal;
decoding a message encoded in the detected watermark signal;
re-creating an original watermark signal from the message; and
comparing the original watermark signal and the detected watermark signal as a measure of strength of the detected watermark signal.
7. The method of claim 6 wherein the comparing includes multiplying the original and the detected watermark signal.
8. The method of claim 6 wherein the decoding includes performing error correction decoding of the message from the detected watermark signal; and wherein the re-creating includes error correction coding the decoded message to form the original watermark signal.
9. The method of claim 6 wherein the measure of watermark strength is compared with a strength threshold to detect degradation of the watermark signal.
10. The method of claim 9 wherein the watermarked signal is an image captured of a printed object, and the strength threshold is used to detect whether the printed object is an original or a copy.
11. The method of claim 6 wherein the message includes printer information about a printer used to print the watermarked signal; and the printer infonnation is used to determine whether a printed object bearing the watermarked signal is an original or a copy.