Digital watermarking method
The invention relates to a method of watermarking a binary stream resulting from the encoding of a set of coefficients comprising at least one coefficient encoded by predictive coding, called predictively coded coefficient. The method comprises the steps of: a) identifying, in the binary stream, a first binary word representing the predictively coded coefficient; b) computing a modified coefficient from the predictively coded coefficient on the basis of at least one watermarking bit; c) encoding the modified coefficient into a second binary word; and d) replacing in the binary stream the first binary word with the second binary word.
1. Method of watermarking a binary stream resulting from the encoding of a set of coefficients representative of a sequence of images comprising at least one DC coefficient of an Intra block encoded by predictive coding, called predictively coded DC coefficient, the method comprises:
a) identifying, in said binary stream, a first binary word representing said predictively coded DC coefficient;
b) computing a modified DC coefficient from said predictively coded DC coefficient on the basis of at least one watermarking bit;
c) encoding said modified DC coefficient into a second binary word; and
d) replacing in said binary stream said first binary word with said second binary word;
wherein said steps a) to d) are applied only on images having a value of quantizing precision being greater than a first predefined threshold.
2. Method according to claim 1 , wherein said modified DC coefficient is computed as the sum of said predictively coded DC coefficient and a marking value derived from said at least one watermarking bit.
3. The method according to claim 1 , wherein the binary stream is an MPEG2 binary stream and wherein the value of quantizing precision is intra_dc_precision.
4. The method according to claim 1 , wherein said steps a) to d) are only applied on parts of the images for which a temporal visibility criteria is lower than a second threshold.
5. The method according to claim 4 , wherein said temporal visibility criteria is the following:
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where:
t is the index of an image in the temporal neighbourhood of said INTRA image;
t l is the index of said INTRA image; and
V i (t,b) is the i th motion vector of a block b in an image of index t.
6. The method according to claim 1 , wherein said steps a) to d) are only applied on images for which a spatial visibility criteria is lower than a third threshold.
7. The method according to claim 1 , wherein said steps a) to d) are only applied on images for which inverse number of colors is lower than a fourth threshold.