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Sensor Transforms to Sensor Transforms to Improve Metamerism-Based Improve Metamerism-Based
WatermarkingWatermarkingMark S. Drew
School of Computing Science
Simon Fraser [email protected]
Raja Bala Xerox Research Center Webster,
Xerox [email protected]
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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, we’re 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:
under D50 under UV
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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:
max K
min K
So use an LED illuminant to emphasize difference == break metamerism:
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Ok, how does this LED-light approach do?
“Watermark example for the strongest watermark signal. Top shows image photographed under daylight illumination. Bottom shows the same image photographed under the LED illumination”
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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 camera+display 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
3 x 3
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We start with 6 inks == 3 pairsthat are metameric under D50:
Again, these aremax-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
LEDs chosen
500 580 660
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These inks are metamers under D50:
1
2
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illuminant metamerism: D65
illuminant metamerism:
LEDs
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Ok, let’s go to a transformed colour space: matrix with M
-positivity of transformed colours-Maximize sum of squared differences between members of metamer pairs-set a normalization, to constrain M
Requirements of an optimization:
normalize imageto max brightness
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Ok, for all pairs, optimizationimproves discriminability.
But, could we not do better by optimizingon the LED lights as well?
Suppose we have available 31 narrowband LED illuminants
maxima
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Let the (float) weights over the set of LEDs be w
Define 93 x 93 matrix W as
Our optimization now reads:
Also define R = 31 x 6 x 3 set of RGB values under each of the LEDs, split into R1 and R2
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But, we may have only binary weights available: result is almost as good −
4 nonzero contributions from float weights w>0.2
re-optimized matrix M for these LED lights
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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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-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 don’t 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
Future work: