Visualising high-dimensional images with low-dimensional displays

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Sensor Transforms to Improve Metamerism-Based
Watermarking
Mark S. Drew School of Computing
Science Simon Fraser University mark_at_cs.sfu.ca
Raja Bala Xerox Research Center
Webster, Xerox Corp. Raja.Bala_at_xerox.com
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Is it possible to use colour to digitally hide a
watermark?

• i.e., embed information into a document in a way
  that is
• invisible,
• easily seen under a special environment meant
  to reveal it, and
• difficult to remove
• Here, were concentrating on visible
  watermarking, as opposed to purely digital
  watermarking steganography i.e.,
  bit-flipping

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A brief history

• Bala et al. CIC 2007 Substrate Fluorescence
  Bane or Boon?
• use fluorescent property of paper substrate to
  make watermark visible under a portable UV lamp
  

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• more

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brief history Can this be done without UV
using just visible light?

• Bala et al. CIC 2009 Watermark Encoding and
  Detection using Narrowband Illumination
• use narrowband LED lights
• consider a pair of inks, with reflectance
  spectra that are metameric under D50

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Ok, how does this LED-light approach do?
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So, not bad Another example
but could we not do better?
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So far, we used illuminant metamerism, in order
to see watermark.
We could use observer metamerism as well, to
further break apart the colours, by interposing a
cameradisplay system.
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brief history Combating cinema piracy in video
by using observer metamerism to generate a
watermark if a camcorder illegally shoots a film

• Doyen et al. CIC 2009 Description and
  Evaluation of the Variability of Human Color
  Vision in an Anti-Piracy Context
• use 3-primary projector for part of film, and 4
  primaries for watermark camcorder sees the
  difference
• but, observer variability is substantial, and
  would have to be taken into consideration in
  determining a watermark/disturbance signal that
  is invisible in the cine theater to all "normal"
  observers, and yet different enough to show up
  when captured by a camcorder

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For this paper ?printed materials, not video,
since we have a camera and hence digital image
processing, we go on to transform into a new
colour space
so as to optimally disambiguate the foreground
(the watermark) from the background
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We start with 6 inks 3 pairs that are
metameric under D50
Again, these are max-K, min-K metamer pairs
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Again, we see max-differences
Differences between spectral pairs
Start off by using fixed set of LEDs
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These inks are metamers under D50
1 2
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Add a camera
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Ok, lets go to a transformed colour space
matrix with M
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Result of optimization
Progress over optimization
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Ok, for all pairs, optimization improves
discriminability. But, could we not do better by
optimizing on the LED lights as well?
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Let the (float) weights over the set of LEDs be w
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Optimized weights w
no matrixing
optimized matrixing
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But, we may have only binary weights available
result is almost as good -
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Compare objective function
but, using non-perceptual differences
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Conclusions

• matrixing has a dramatic effect
• float weights are best, but optimizing
  for floats and then binarizing is almost as
  effective

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Future work

• choice of inks still metamers, but optimized
  blends along iso-colour loci
• ensure watermark invisible over normal lighting
  and over observers
• optimize on perceptual differences (could just
  use Jacobian of CIELAB transform)
• some metamer pairs dont separate as much ??
  weight some pairs more
• model LEDs better
• could design lights just for this task ACM
  Transactions on Graphics 2009
• include more metamer pairs

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Thanks! Natural Sciences and Engineering
Research Council of Canada
Xerox Corporation