Applying digital halftoning to digital
watermarking and LCD displays
For authentication, security and other
purposes, images often need to be embedded into other images.
In digital watermarking, a watermark image is embedded into
an original image, which can be used for tamper detection, image
fingerprinting and copyright protection. And if the image is
tampered with, the watermark can be used to restore it.
Digital printers use digital
halftoning, a technique that relies on only a handful of colors
to create images that appear to have many colors. This process
operates on pixels in color spaces, such as grayscale or red-green-blue
(RGB). IBM Research has developed a flexible image embedding algorithm
by generalizing digital halftoning to operate on Cartesian products
of image color spaces. More specifically, the embedding algorithm
embeds m images into n images by considering the n+m images as
a single extended image whose pixels lay in the Cartesian product
of n+m color spaces. This extended image is halftoned using pixels
that satisfy constraints relating the pixels between the two groups
of images. These constraints are imposed by the extraction algorithm
that recovers the m images from the n images. The output of the
embedding algorithm will be n images in the corresponding portion
of the halftoned extended image. The constraints and thus the
extraction algorithm are chosen according to the application.
We show three applications where this algorithm can be applied.
First, consider a case where a scrambled version of the image
is embedded into the same image. This image is subsequently tampered
with, as shown in Fig. 1a. The embedded image can be used to detect
and localize the tampering. Furthermore, the embedded image can
be used to reverse the tampering and recover the original image,
as shown in Fig. 1b.
Figure 1: A watermarked
image that has been changed
(b) The change is detected and the original image is recovered.
In another application,
a watermark image is embedded into two halftone images. The extraction
of the embedded watermark is accomplished by superimposing or
combining these two watermarked images. This is useful in cases
where the watermark extraction mechanism is computationally simple.
As shown in Fig. 2, performing the XOR operation pixel by pixel
on each of the color planes on the top two halftone images results
in the bottom third image, which is the watermark image. Another
operation that can be used for extracting the watermark is the
OR operation which corresponds to simply overlaying the two images.
One advantage over previous watermarking algorithms of this type
is that the extracted watermark image can be of high complexity
Figure 2: Performing the
XOR operation on each of the color channels on the top two images
results in the bottom watermark image.
In the third application,
we exploit the fact that a pixel on a twisted-nematic (TN) mode
Liquid-Crystal Display (LCD) (commonly used in laptops, PDAs and
digital cameras) incurs a large change in color and luminance
when the viewing angle is changed. This allows us to create images
which look different when viewed at different angles relative
to the display. An example of this is shown in Fig. 3a and 3b,
where the same image appears completely different on a TN mode
LCD display when viewed at different angles. When the two embedded
images are the same, this technique can be used to enhance the
viewing angle characteristics of TN mode LCD displays.
Figure 3: The same image
appears different on a TN-mode LCD display when viewed at two different
C. W. Wu, “Multimedia
data hiding and authentication via halftoning and coordinate projection,”
EURASIP Journal of Applied Signal Processing, vol. 2002, no. 2,
pp. 143-151, 2002
C. W. Wu, G. Thompson and S. L. Wright,
Images Viewable on Twisted-Nematic Mode Liquid-Crystal Displays,”
IEEE Signal Processing Letters, vol. 10, no. 8, pp.225-227, 2003.
C. W. Wu, G. Thompson and M. Stanich,
Watermarking and Steganography via Overlays of Halftone Images,"
SPIE 49th Annual Meeting, Denver, CO, Proceedings of SPIE, vol.
5561, pp. 152-163, 2004.
S. L. Wright, P. F. Greier, "Low-cost
Method to Improve Viewing-Angle Characteristics of Twisted-Nematic
Mode Liquid-Crystal Displays," SID Symposium Digest of
Technical Papers, vol. 33, no. 1, pp. 717-719, 2002.
What is the most exciting potential
future use for the work you're doing?
As images and other multimedia
content are becoming a more and more dominant form of digital
information being exchanged, some aspects of digital information
such as security and authentication need to be addressed
in images too.
What is the most interesting part
of your research?
The ability to use various techniques from a wide
variety of disciplines to solve different problems. Our
teams consist of people with expertise in dynamical systems
theory, image processing, digital printing and halftoning
and liquid crystal displays.
What inspired you to go into this
I have always been interested in images and the
changes you can make to them. I am a big movie buff and
as a kid wanted to grow up to work in movie special effects.
What is your favorite invention
of all time?
I would have to say the airplane. Every time I
fly, I am still amazed that a ton of steel can so swiftly
ascend and descend in the sky.