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									   Concepts of Multimedia
Processing and Transmission

        IT 481, Lecture #6
     Dennis McCaughey, Ph.D.
         17 October, 2006
                          Project

     Questions to Sheryl Pinto
     spinto@gmu.edu
     Project will be recovering a student specific
      watermark from the bird image.
     MATLAB code will be provided you only
      have to execute it. This is not a
      programming project. e-mail Sheryl she will
      reply with the code.
     Criteria:
       – Recover the watermark
       – Assess the visibility and robustness of the
         watermarking method

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                    Homework #1

     E-mail Sheryl Pinto requesting MATLAB
      code and “bird” picture.
     She will reply with the code.
     Load these on a CD and bring to 10/23
      class




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      High Level BitStream Organization




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                    MPEG-2 Bit Stream Syntax




                                                GOF: Group of Frames
                                    Slide: Courtesy, Hung Nguyen
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      Sequences and Groups of Pictures

     Video Sequence commences with a video
      sequence header which may be followed by
      a group of pictures header and by one or
      more coded frames
     Group of Picture Header is an optional
      header that can be used immediately before
      a coded I-Frame to indicate to indicate to
      the decoder if the first consecutive B-
      pictures immediately following can be
      reconstructed properly in the case of
      random access
     I-Pictures are use to assist random access

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                         Slice

     A Slice is a sequence of consecutive
      macroblocks
     Every Slice must contain at least one
      macroblock and the last one may not be
      skipped
     The first and last macroblocks within slice
      must be in the same horizontal row
     Slices may not overlap
     Slices are not required to cover the entire
      picture as some macroblocks may be
      skipped
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                    MacroBlock

     A macroblock contains a section of the
      luminance component and the spatially
      corresponding chrominance components




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          Examples of Run-Length Codes




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                    Broadcast Environment




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                    Watermarking

     Watermarking is a secret code described by
      a digital signal carrying information about
      the copyright property of the product.
     The watermark is embedded in the digital
      data in such a way that it is not visually
      perceptible.
     The copyright owner should be the only
      person who can show the existence of his
      own watermark and to prove then origin of
      the product.


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                    Watermark Requirements

     Alterations introduced into the image or
      audio should be perceptually invisible.
     A water mark must be undetectable and not
      removable by an attacker.
     A sufficient n umber of watermarks in the
      same image or audio, detectable by their
      own key, can be produced.
     The detection of the watermark should not
      require the original image or audio.
     A watermark should be robust against
      attacks which preserve the desired quality of
      the image or audio.

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           Main Features of Watermarking

     Perceptual Invisibility
     Trustworthy Detection
     Associated Key
     Automated Detection/Search
     Statistical Invisibility
     Multiple Watermarks
     Robustness




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                    Perceptual Invisibility




     Watermark should not degrade the
      perceived image/audio quality
     Differences may become apparent when the
      original and watermarked versions are
      directly compared




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                    Trustworthy Detection



     Watermarks should constitute a sufficient
      and trustworthy part of ownership.
     False alarms should be extremely rare.
     Watermarks signatures/signals should be
      complex.
     An enormous set of watermarks prevents
      recovery by trial-and-error methods.



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                    Associated key

     Watermarks should be associated with an
      identifiable number called the watermark
      key.
     Key used to cast, detect and remove the
      watermark.
     The key should be private and should
      exclusively characterize the legal owner.
     Any signal removed from the image/audio is
      assumed to be valid only if it can be
      associated with the key via a well
      established algorithm
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              Automated Detection/Search




   Watermark should combine with a search
    algorithm.




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                    Statistical Invisibility



     Watermark should not be recoverable using
      statistical methods.
     The possession of a great number of
      watermarked images, embedded with the
      same key should not enable the recovery of
      the watermark through statistical methods.
       – Watermarks should be image/audio
         independent.



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                    Multiple Watermarks




     Multiple watermarks assist in the case
      where someone illicitly watermarks and
      already watermarked image/audio.
     Convenient in transferring copyrighted
      material.



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                       Robustness



     A watermark should survive some
      modifications to the data.
     Common manipulations to image/video
       – Data Compression
       – Filtering
       – Color, quantization , brightness modifications,
         geometric distortions, etc
       – Other trans-coding operations.


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         Robustness, Resilience & Detection


                  Unintentional         Intentional
Applications        Attacks               Attacks        Every     High         Applications
  Domain                                                Decoder   Capacity       Example
                AT1     AT2    AT3       AT4      AT5
                                                                                 Value-added
    A1          Yes     Yes   Maybe      No       No      Yes       Yes
                                                                                  metadata

    A2          Yes     Yes       Yes    Yes      Yes     Yes       No         Copy Protection

    A3          Yes     Yes       Yes    Yes      Yes     No        No       Ownership/fingerprint

    A4          Yes     No        No     No      Some     Yes       No          Authentication

    A5          Yes     Yes       NO     No       Yes     Yes       Yes          Broadcasting
                                                                                  Secret
    A6          Yes     Yes   Maybe     Maybe     Yes     No        Yes
                                                                               Communication




                                                                                                    21
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                    Application Domains

     A1: Carrying value-added metadata
       – Additional information such as hyperlinks, content based
         indexing
       – Malicious and non-malicious attacks
       – Survive MPEG encoding
     A2: Copy protection and conditional access
       – Control Intellectual Property Management and Protection
       – View and copy options
       – Every compliant decoder must be able to trigger protection
         or royalty collection mechanisms at the time of decoding
       – Unauthorized individuals should not be able to defeat the
         watermarks by any means
     A3: Ownership assertion, recipient tracking
       – Establish ownership and determine origin of unauthorized
         duplication.
       – Prosecution of copyright infringement

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               Application Domains Cont’d

   A4: Authentication and verification
    – Allows fragile watermarks; if contents modified watermarks
      should disappear.
    – Helps in identifying areas that wer modified
   A5: Broadcast monitoring
    – Monitor where and when the contents are played
    – Advertisements. Here heavy content degradation is less of an
      issue.
    – Watermark removal, invalidation and forgery can be significant
      concern
    – Counterfeiting should be intractable for the system to be effective
   A6: Secret communication or steganography
    – Data hiding may require higher capacity watermarks than other
      applications
    – Secrecy may be the overriding concern in some applications


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                             Attacks

     AT1: Basic attacks
       – Lossy compression, frame dropping & temporal rescaling
     AT2: Simple attacks
       – Blurring, median filtering, noise addition gamma correction
         and sharpening
     AT3: Normal attacks
       – Translation, cropping and scaling
     AT4: Enhanced attacks
       – Aspect ratio change & random geometric perturbations
         (Stirmark)
     AT5: Advanced Attacks
       – Delete/insert watermarks, single document watermark
         estimation attacks & multiple-document statistical attacks
                                                                               24
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                    Human Perception

     Watermarking schemes take advantage of
      the fact that the human audio and visual
      systems are imperfect detectors.
     Audio & visual signals must have a
      minimum intensity or contrast before they
      are perceptible.
     These minima are spatially, temporally and
      frequency dependent.
     These dependencies are either implicitly or
      explicitly exploited


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      Transform Domain Considerations

     The human eye is more sensitive to noise in
      the lower frequency range than in the higher
      frequency counterparts
     However, energy in most images is
      concentrated in the lower frequency range.
     Quantization used in DCT based
      compression reflects the HVS which is less
      sensitive in the higher frequencies
     A trade is required to balance watermark
      invisibility and survivability resulting in the
      use of the mid-frequency terms.
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          Transform Domain Considerations

                         DCT Spectrum
  1


                                                              An alteration of
0.5                                                            a transform
                                                               coefficient is
                                                               spread across
  0
      0     20      40    60       80    100   120   140
                                                               the entire spatial
                                                               block
                         Time Sequence
  1                                                           A one
                                                               dimensional
                                                               example:
0.5




  0
      0     20      40    60       80    100   120   140


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                Data Embedding Algorithm




                                          Information



                 Signal                   Embedding           Signal
         (image, audio or video)           Algorithm    With embedded data



                                                  Key
                    Perceptual Analysis




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                    Embedded Data Examples

     Multilingual soundtracks within a motion
      picture
     Copyright data
     Distribution permissions
     Data used for accounting and billing and
      royalties
     Etc.




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                    Watermarking Techniques

     Non-Blind: Watermark recovery requires the
      original
     Blind: Watermark recovery does not require
      the original
     Spatial domain or transform domain
      embedding
     Spatial domain:
       – LSB, color pallet, geometric
     Transform Domain:
       – FFT, DCT, Wavelet

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An Algorithm Proposed by Busch, Funk
           and Wolthusen




                              RGB to                      Quantize Y-
                     A/D                   Block DCT
                              YCbCr                          DCT


Camera

                             Inverse
                                          Inverse Block    YCbCr to
                    Embed   Quantize Y-
                                              DCT           RGB
                               DCT

                                                                        Digitized
                                                                        Video to
                                                                        MPEG-2




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                    Algorithm Considerations

     Watermark embedding position determined
      through a pseudo-random number generator
      that determines the order of block
      processing and the coefficient to be
      modified
       – Embed all available blocks
     Key may be public or secret leading to a
      public or secret watermark
     Redundantly embed the watermark to
      achieve survivability to MPEG-2
      compression

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                    Selected Block Embedding

 1.    Block is transformed using the DCT
 2.    Perform edge detection
 3.    Select pair of DCT coefficients from the sub-band
       used using a Pseudo random permutation
 4.    Quantize the selected coefficients using the
       MPEG-2 algorithm
 5.    Determine if the coefficient pair is suitable for
       embedding
       Avoid “Edge” and “Plain” blocks
 6.    Enforce a differential relationship between the
       coefficients in the pair to encode a “1” or a “0”
 7.    Inverse quantize the modified coefficients

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   Busch Watermarking DCT Coefficients

              0   1   2   3     4   5   6   7             0   1   2   3   4   5   6   7
          0                                           0
          1                                           1
          2                                           2
          3                                           3
          4                                           4
          5                                           5
          6                                           6
          7                                           7


                      Level 1                                 Coefficients Used For Edge Detection

                      Level 2

                      Level 3                   If the absolute value of one of the
                      Public                    highlighted coefficients is greater
                                                than 39, the block is classified as
If the quantized value of one                   an “edge” and not used.
of the coefficients in the
selected band is zero the block
is classified as a “plain” block
                                                                                                             34
  IT 481, Fall 2006                                                                                  08/28/2006
                              Visibility

     “Edge” blocks, if modified, are highly visible
      in video and are to be avoided
     “Plain” blocks are not so sensitive, so they
      can be used if care is exercised
       – In one of the bands randomly select a pair of coefficients and
         randomly select one to be the “first” (DCT1) and another to the
         “second” (DCT2)
       – To encode a “one” set DCT1 = (ABS(DCT1)+ ABS(DCT2))/2 + K,
         Preserving the sign(DCT1)
       – To encode a “zero” set DCT1 = (ABS(DCT1)+ ABS(DCT2))/2 - K,
         Preserving the sign(DCT1)
       – K is elected as a compromise between visibility and robustness
         to MPEG-2 compression


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                    Reference




 C. Busch, W. Funk, and S. Wolthusen: “Digital
   Watermarking: From Concepts to Real-Time
   Video Applications”; IEEE Computer
   Graphics and Applications, 1999




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IT 481, Fall 2006                            08/28/2006

								
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