High Capacity Color Barcodes Using Dot Orientation and
Color Separability
Orhan Bulan
a
, Vishal Monga
b
, Gaurav Sharma
a
a
University of Rochester, Rochester, NY, USA
b
Xerox Corporation, Webster, NY, USA
ABSTRACT
Barcodes are widely utilized for embedding data in printed format to provide automated identification and
tracking capabilities in a number of applications. In these applications, it is desirable to maximize the number
of bits embedded per unit print area in order to either reduce the area requirements of the barcodes or to
offer an increased payload, which in turn enlarges the class of applications for these barcodes. In this paper,
we present a new high capacity color barcode. Our method operates by embedding independent data in two
different printer colorant channels via halftone-dot orientation modulation. In the print, the dots of the two
colorants occupy the same spatial region. At the detector, however, by using the complementary sensor channels
to estimate the colorant channels we can recover the data in each individual colorant channel. The method
therefore (approximately) doubles the capacity of encoding methods based on a single colorant channel and
provides an embedding rate that is higher than other known barcode alternatives. The effectiveness of the
proposed technique is demonstrated by experiments conducted on Xerographic printers. Data embedded at
a high density by using the two cyan and yellow colorant channels for halftone dot orientation modulation is
successfully recovered by using the red and blue channels for the detection, with an overall symbol error rate
that is quite small.
Keywords: H ardcopy data coding, color barcodes, orientation modulation, color separation
1. INTRODUCTION
Embedding information in printed documents continues to be a problem of considerable interest in security
applications. A large number of legal and official documents such as IDs, passports, and other transactional
data are consumed in the printed format. Hence, systems and algorithms for protecting and interacting with
hardcopy content are a necessity. In this context, techniques for embedding and extracting digital data in
hardcopy documents, which are fundamentally analog, are of particular interest because these methods can
add security and functionality and features that are associated with digital techniques, for example by enabling
cryptographic constructs for authentication, data integrity verification, non-repudiability, etc.
11, 12
Methods for
digitally embedding and extracting data in hardcopy documents can be classified as either data hiding or data
encoding methods depending, respectively, on whether the data is imperceptibility inserted in a cover document
whose primary function is other than the communication of the data or whether a region of the document is
devoted solely to the objective of communicating the embedded data. Methods in both classes are of interest
for a variety of hardcopy security applications and in the literature a number of techniques have been reported
fordatahiding
3–10
and for data encoding.
1, 2, 17
In this paper, we focus on 2-D barcodes, a specific technique
within the latter category.
Two-dimensional (2-D) barcodes are widely utilized for data encoding in hardcopy documents.
1, 2
Besides
being convenient and cheap, they offer greater capacity for data embedding than their one-dimensional counter-
parts and also in comparison with hardcopy data hiding techniques. The increased capacity can often enable
Send correspondence to O. Bulan: E-mail: bulan@ece.rochester.edu, Telephone: 1 585 275-8122, Address: Elec-
trical and Computer Engineering Department, University of Rochester, Rochester, NY, 14627-0126, USA, WWW:
www.ece.rochester.edu/projects/iplab
This work was supported in part by Xerox Corporation and by the a grant from New York State Office of Science,
Technology and Academic Research (NYSTAR) through the Center for Electronic Imaging Systems (CEIS).
Media Forensics and Security, edited by Edward J. Delp III, Jana Dittmann,
Nasir D. Memon, Ping Wah Wong, Proc. of SPIE-IS&T Electronic Imaging, SPIE Vol. 7254,
725417 · © 2009 SPIE-IS&T · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.807742
SPIE-IS&T/ Vol. 7254 725417-1
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additional security applications. For instance, in personal ID’s, driving licenses and passports, this additional
capacity can enable the embedding of a sample speech signal, a picture, or other identifying personal informa-
tion which in turn may be used to establish authenticity of the printed document and/or the identity of the
user. Particular examples proposed in the literature include identification cards based on biometrics
13
and print
signatures for document authentication
14
). Several of these applications favor a push toward higher data rates
for two-dimensional codes, which is the specific aspect that we address in our work.
Many of the existing 2-D barcodes in the literature, are based on monochromatic and single ink printing.
The extension to color is highly desirable for the benefits it carries in terms of embedding rate per unit area
and hence, to meet the requirements of cryptographic protocol and several new applications. To the best of our
knowledge, there are two known color barcode schemes: one recently developed at Microsoft
15, 16
and the other
being DataGlyphs.
17, 18
The former method encodes the data as triangles in one of four colors (black, red, green
and yellow) where the color is chosen based on the data. While this allows for each triangle to carry a 4 − ary
value, as we see subsequently, the flexibility afforded by the spectral difference between the colorants is not fully
exploited. DataGlyphs, on the other hand, in their color instantiation operate by simply using the same glyph
pattern in a dot-on-dot mode that fundamentally offer the same capacity as a single channel.
In this paper, we propose a new high capacity color barcode technique by exploiting halftone-dot orientation
and color separability. In previous work, we developed a hardcopy data hiding technique
10, 19
that uses elliptically
shaped halftone dots and embeds data in the choice of the ellipse orientation. Here we extend that method to
embedding using color halftone dots. In particular, we exploit the spectral (wavelength) characteristics of the
Cyan, Magenta, Yellow colorants commonly used in digital printing, and those of Red, Green and Blue sensors
widely used in desktop scanners. We demonstrate that carefully designed mixtures of two colorants can actually
be well-separated from the scan and hence result in double the capacity of a embedding method that works on
a single colorant channel. We demonstrate experimentally that our method can achieve the highest embedding
rates of all hardcopy barcodes known in the literature.
The rest of this paper is organized as follows. Section 2 gives an overview of data embedding via halftone-dot
orientation modulation. In Section 3 we first introduce color separability and then describe the proposed data
coding method. We present the preliminary results in Section 4. Section 5 concludes the paper by summarizing
the key aspects of our scheme.
2. DATA EMBEDDING VIA HALFTONE-DOT ORIENTATION MODULATION
In recent work, we developed a data hiding method for hardcopy images via halftone dot orientation modulation.
10
This method specifically offers the ability to generate elliptically shaped halftone dots with orientation control.
Data embedding is then carried out in a particular choice of orientation. Figure 1 shows a zoomed version of a
constant graylevel image where halftone dots are oriented along vertical, horizontal and diagonal (+/-45 degree)
directions, i.e. each orientation may be thought of as representing a binary symbol.
(c) Dot orientation along vertical,
horizontal and diagonal directions
(b) Graylevel halftone image
Figure 1. Orientation based data embedding
At the receiver, first the scan is synchronized both for global affine transforms, and local variations.
10
Once
a halftone cell is identified in the (scanned) synchronized image, detection is based on M statistical moments,
each corresponding to an ellipse orientation. For example, the moment along Y − axis is calculated as:
SPIE-IS&T/ Vol. 7254 725417-2
σ
1
=
1
A
x,y∈C
I
s
(x, y)(y − y)
2
(1)
where A =
x,y∈C
I
s
(x, y)andy =
1
A
x,y∈C
I
s
(x, y)y represents the co-ordinate of the center of mass of the
halftone dot. The remaining M − 1momentsσ
i
,i=2, 3 ··· , (M − 1) can be computed similarly.
3. COLOR BARCODES USING DOT ORIENTATION AND COLOR SEPARABILITY
3.1. Color Separation
Color devices exhibit considerable diversity in their color spaces. While color is physically a continuous stimulus,
devices typically use a lower-dimensional (3-D/4-D) representation of color. Printing systems are mostly based
on cyan, magenta, and yellow (CMY) colorants, while displays and capture devices such as scanners, digital
cameras use a color model based on red, green, and blue (RGB) phosphors or sensors to represent color images.
Based on the utilization of RGB or CMY spaces, color devices can be categorized as additive or subtractive.
20
M
YC
RGB
(b)
Wavelength Wavelength
(a)
Figure 2. Spectra of RGB and CMY colorants
Additive color systems such as cameras and scanners represent color through the combination of additive
red, green, and blue primaries. The additive mixing of two primaries, forms cyan, magenta and yellow while
the combination of all three gives white and the absence of all results in black. Subtractive systems, on the
other hand, use CMY space and form color by subtracting unwanted spectral components from the white light.
In particular, cyan absorbs the spectral region corresponding to red sensation, magenta absorbs the region
corresponding to green and yellow eliminates blue. Figure 2 (a) shows the spectral emissions of RGB lights
and Fig. 2 (b) denotes the spectral transmittance of CMY colorants where each absorbs in one region of the
spectrum.
RedCyan
Magenta
Yellow
G
B
R
Black
White Black
White
C
Y
M
Green
Blue
Figure 3. CMY and RGB color cubes
SPIE-IS&T/ Vol. 7254 725417-3
Note that based on their spectral characteristics, colorants in CMY space are complementary to RGB colorants
respectively. Fig. 3 shows idealized CMY and RGB color cubes to illustrate this point.
B
Yellow
Magenta
Cyan
255
255
255
255
255
255
0
0
0
B, G
R
R,B
G
R,G
Figure 4. Complementary colors
3.2. Color Barcodes
We exploit this complementary relation between CMY and RGB colorants for creating color barcodes. The
underlying principle of color separability is illustrated in Fig. 4 for an idealized color printing and scanning
system. In the figure, we generate ramp functions ranging from zero color level to full color level (digital value
255 in typical 8-bit representation) for each of the C, M, and Y colorants. Corresponding to each ramp, we plot
the variation in the RGB channels as observed by an idealized scanner. Note that in this idealized scenario, the
CMY colorants only absorb the light energy in non-overlapping wavelength bands and therefore do not interfere
with bands corresponding to other colorants. The red channel can hence convey information about the Cyan
colorant, the green for Magenta, and blue for Yellow.
Similar plots for two color ramps are shown in Fig. 5. Note in the idealized case, the two complementary
channels can be used to separate the mixture of two colorants. In practice, however as revealed in Fig. 2 the
absorption bands of individual colorants interfere with each other and cause unwanted absorptions in the spectra
of the color print. We therefore, select cyan and yellow channels to embed data since their absorption bands in
the spectra are the most distant from each other as shown in Fig. 2 (b).
0
R
B, G
B
R, G
G
R, B
Magenta+Yellow
Cyan+Magenta
Cyan+Yellow
255
255
255
255
255
255
0
0
Figure 5. Two color mixture
In each selected color channel i.e. cyan and yellow, we then embed data by halftone-dot orientation modulation
described in Sec. 2. The data embedding capacity offered by the orientation modulation technique exhibits a
SPIE-IS&T/ Vol. 7254 725417-4
considerable variation
∗
with the digital level i.e. area coverage for the colorant.
21
We therefore, determine the
input digital level for each colorant at which our modulation technique offers the highest capacity. We then
generate the halftoned barcode bearing the embedded data with the pre-determined color levels. Figure 6(a)
shows the proposed digital barcode with 4 − ary modulation.
(a) Color Barcode via halftone dot orienta-
tion modulation
(b) Scanned Color Barcode
(c) Separated cyan from the scanned bar-
code (Red separation)
(d) Separated yellow from the scanned bar-
code (Blue separation)
Figure 6.
4. EXPERIMENTAL RESULTS
We evaluate the performance of the proposed color barcodes experimentally by utilizing a xerographic printer
and a flatbed scanner. In our experiments, we perform dot-on-dot halftoning and utilize 0/90
◦
orthogonal CY
halftone screens with a frequency of 120 cells per inch (cpi) to generate the halftone barcode. The resulting digital
barcode is printed on a xerographic color printer with an addressability of 2400 dots per inch (dpi). The resulting
print is then scanned on a flatbed scanner and colors are separated from the scanned image. Figure 6 (c) and
(d) illustrates the separated cyan and yellow colorants from the scanned barcode shown in Fig. 6 (b). Following
∗
This variation is identical to the variation in monochrome images, which has been studied in our prior work.
21
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