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Watermarking

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					     Watermarking (2)


• University of Palestine
• Eng. Wisam Zaqoot
• May 2010
 Proving ownership using a digital watermark

• Ownership is proved by showing that an image in question
   contains a watermark that depends on owner’s secret key
• If pirate embeds his own watermark, the ownership can be
   resolved by producing the original image or the
   watermarked image (neither contains pirate’s watermark)



Detectable watermark:           Readable watermark:
Pseudo-random sequence          One can recover a short
is either present or not        message, e.g. info about
present (1 bit embedded)        the owner (100 bits)
        Fingerprinting or traitor tracing
Marking copies of one document with a customer signature.

                                  original

                              +
                        …
       W1     W2                             WN




                        …                         N customers

Robust, secure, invisible watermark, resistant with respect
to the collusion attack (averaging copies of documents with
different marks).
      Adding captions to images, additional
              information to videos
Typical application:
• Adding subtitles in multiple languages
• Additional audio tracks to video
• Tracking the use of the data (history file)
• Adding comments, captions to images

Watermark requirements:
• Moderately robust scheme
• Robustness with respect to lossy compression, noise adding,
  and A/D D/A conversion
• Original images (frames) not available for message extraction
• Security requirement not so strong
• Fast detection, watermark embedding can be more time
  consuming
              Watermarking principles
     In spatial domain                        In transform domain
     watermark embedded                       watermark embedded in the
     by directly modifying                    transform space by modifying
     the pixel values                         coefficients


          +         =
                                             DCT              Inverse DCT

                                                     Modify
                                                      DCT
Watermarking for color images
• One or more selected color channels.
• Luminance


Oblivious vs. non-oblivious watermarking
* non-oblivious = original image is needed for extraction
* oblivious = original image is not necessary.
            Watermark detection
Oblivious watermarking:



           ˆ
    where X` is a possibly corrupted watermarked
    image, K` is the extraction key, D represents the
    watermark extraction/detection function, and W ˆ
    is the extracted watermark information

   Note: Oblivious schemes are attractive for many
    applications where it is not feasible to require
    the original image to decode a watermark.
            Watermark detection
Non-oblivious watermarking:
• Subtract the original image from the watermarked (attacked)
  image, and extract the watermark sequence ’ (may be
  corrupted due to image distortion)
• Correlate  with ’
   = original watermark sequence
                                '
               sim( , ' ) 
                                ' '
 sim(, ’) is called similarity
 sim(, ’) > Th => watermark is present
 sim(, ’) < Th => watermark is not present
Watermark detection

				
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posted:10/17/2012
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