I use NSCT or DWT for watermarking DICOM image thus:
coeffs = nsctdec( double(im), nlevels, dfilter, pfilter ); % NSCT Decomposition
level14_1 = coeffs{1,4}{1,1};
block=level14_1(i:i+blocksize-1,j:j+blocksize-1);
watermark=(alpha.*goldcode*watermarkBit); % Create Watermark
stego=(double(block)+ watermark);% Additive Watermarking
The extraction also using Maximum Likehood estimator for additive spread spectrum.
I can extract watermark from the level 4 sub-band correct before inverse NSCT. However, after INSCT and sending the DICOM to the receiving side, when I do NSCT in order to recover the watermark from the NSCT sub-band, the error rate increases to almost 0.5. Which means watermark is lost. There is no attack.
Does this relate to file format or nature of the NSCT or DWT transform? Any ideas?
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