BACKGROUND AND SUMMARY
The teachings presented herein relate generally to stereoscopic
images. More specifically, the teachings presented herein relate to the creation
of moiré-based auto-stereoscopic watermarks in rendered images.
The principle of stereoscopic vision is well understood.
At the most basic level, each of the viewer's two eyes must perceive the subject
matter to be viewed from a slightly different perspective. That is to say that,
although the differences are generally quite subtle, each eye receives a different
Several methods are commonly used to produce stereoscopic
images. On the one hand, these include the use of direction selective screens onto
which two or more images may be projected simultaneously. Depending on the viewer's
position, a different image may be observed by each eye. Where only two images are
required, it is common practice to use polarizing techniques. Each image is projected
with a characteristic polarization and when viewing through complementary polarizing
viewing spectacles, each eye only sees the picture intended for its reception.
Most existing methods to view printed stereoscopic images
require either special glasses (colored or polarized) or lenticular lenses. The
stereogram may be the one exception which does not need any special viewing aid;
however, many people find that it is very difficult and uncomfortable to see the
hidden stereo image. Lenticular lenses are common but incur some additional expense
and complexity as they require an embossed transparent material for operation.
With most current digital watermark technologies, to retrieve
embedded watermark information from printed documents requires scanning and processing
by a scanner and a computer. It is desirable to provide a simple and quick way to
provide invisible watermarks embedded in documents. It is desirable to provide these
invisible watermarks with means that will quickly make them visible in a given print
item, where an observer is provided only with a simple overlay without the need
for lenticular or other special lenses, and for which this means is readily and
inexpensively generated with common materials using conventional printing apparatus.
Disclosed in embodiments herein is a moiré-based auto-stereoscopic
watermark system. The watermark system comprises a substantially transparent substrate
having a first side. As applied to and placed on the first side of the substantially
transparent substrate is a first side applied marking material of periodic structure
having a first frequency. The watermark system further comprises a second substrate.
As applied to the second substrate is a second side applied marking material which
further comprises a first partition and a second partition, the first partition
having a periodic structure at a second frequency, that second frequency being related
to but some delta away from the first frequency. The second partition has a periodic
structure at a third frequency, that third frequency being related to but some delta
away from the first frequency, such that when the substantially transparent substrate
is placed upon the second substrate in substantial alignment with the second side
applied marking material, a moiré-based auto-stereoscopic watermark image is
evident to an observer.
Further disclosed in embodiments herein is an alternate
moiré-based auto-stereoscopic watermark. The watermark includes a substantially
transparent substrate having a first side applied marking material having a periodic
structure at a first frequency, as applied to and placed on at least one side of
the substantially transparent substrate. The watermark also includes an additional
substrate having a selected side, and a second side applied marking material as
applied to the selected side of the additional substrate. The second side applied
marking material further comprises a first partition and a second partition, the
first partition having a periodic structure at a second frequency, that second frequency
being related to but some delta away from the first frequency, and the second partition
having a periodic structure at the first frequency. The above are arranged such
that when the substantially transparent substrate is placed upon the additional
substrate in substantial alignment with the second side applied marking material
a moiré-based auto-stereoscopic watermark image is made evident to an observer.
In a further embodiment the additional substrate is translucent.
In a further embodiment the substantially transparent substrate is translucent.
In a further embodiment the additional substrate is brought into close proximity
and substantial alignment with the substantially transparent substrate.
In a further embodiment the watermark further comprises a third substrate placed
between the substantially transparent substrate and the additional substrate as
brought into close proximity and substantial alignment with the substantially transparent
substrate and additional substrate.
In a further embodiment the applied marking material is toner-based.
In a further embodiment the applied marking material is ink-based.
In a further embodiment the applied marking material is wax-based.
In a further embodiment the applied marking material is paint.
In a further embodiment the applied marking material is liquid-based.
In a further embodiment the applied marking material is shellac.
In a further embodiment the periodic structure is a line screen halftone.
In a further embodiment the first frequency is 120 LPI, the second frequency is
123 LPI and the third frequency is 117 LPI.
Further disclosed in embodiments herein is a method for
providing a moiré-based auto-stereoscopic watermark imaging system, by applying
marking material with a halftone periodic structure at a first frequency to a substantially
transparent substrate. The method further comprises applying marking material within
a first partition on a selected side of a second substrate with a halftone periodic
structure at a second frequency, that second frequency being related to but some
delta away from the first frequency. The method also comprises applying marking
material within a second partition on the selected side of the second substrate
with a halftone periodic structure at a third frequency, that third frequency being
related to but some delta away from the first frequency. This is followed by placing
the substantially transparent substrate upon the second substrate in substantial
alignment with the selected side applied marking materials such that a moiré-based
auto-stereoscopic watermark image is evident to an observer.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a line halftone with spatial frequency f1
= 16 LPI.
Figure 2 shows line halftones with spatial frequencies
f2 = 17 LPI.
Figure 3 shows an overlapping of the two line halftones
f1 & f2 from Figures 1 & 2, one atop the other.
Figure 4 shows the lateral shifting of the line halftone
f2 from Figure 2.
Figure 5 shows the Moiré shift resultant from the
overlap of f1 upon the lateral shift of f2 from Figure 4.
Figure 6 shows the lateral shifting of the line halftone
f1 from Figure 1.
Figure 7 shows the Moiré shift resultant from the
overlap of f2 upon the lateral shift of f1 from Figure 6.
Figure 8 shows as schematically depicted the eyes of an
observer as viewing a transparency provided with duplex printing thereupon.
Figure 9 shows an exploded-view schematical depiction of
one possible exemplary embodiment.
Figure 10 schematically depicts the superimposition of
a watermark image into an image of interest to yield moiré-based auto-stereoscopic
Figure 11 depicts the manual overlay of a suitably prepared
printed transparency upon the moiré-based auto-stereoscopic printed page of
A methodology is herein taught using duplex printing on
transparencies to create auto-stereoscopic images viewed in a "see-through" manner.
By choosing different halftone structures for each of the two sides of a transparency,
a moiré pattern resulting due to halftone overlapping can be observed. When
the transparency is viewed at different angles (as is inherent with the distance
from the left eye to the right eye of an individual observer), a very small lateral
shift occurs between the front-side and the back-side prints due to the thickness
of the transparency and would not be otherwise normally noticeable. However, the
corresponding resulting moiré shift can be much greater than the above-mentioned
lateral shift and can also be in a different direction. As a result, the moiré
result is apparently visually perceived as located in the space in front of or behind
the transparency. For example, using an ordinary transparency with an approximately
100 micron thickness as is typical for laser printers, the appearance of perceived
depth from the resulting moiré can be as large as hundred times the thickness
of the transparency, or about 10 mm deep. The method provided herein teaches how
to select halftone frequencies and estimate the resulting appearance of depth for
the corresponding resulting moirés. An example embodiment is taught below showing
how moiré images with two different depths are created. There are many possible
applications for this method, such as in security printing, for advertising novelties,
or in the enhancement of graphics content.
A further methodology is herein taught directed to the
creation of moiré-based auto-stereoscopic watermarks in rendered images. By
choosing different halftone structures, which differ by having different spatial
frequencies for each of two delineated partitions in an image, it becomes possible
to embed arbitrary binary patterns into printed documents as digital watermarks.
The invisible watermarks become moiré auto-stereoscopic images when the prints
are viewed through an overlaid transparency "decoder" suitably prepared by virtue
of being rendered with a uniform halftone structure having the correct special frequency
in relationship with the partition frequencies employed in the printed document.
When two different halftones with similar spatial frequencies
overlap each other, a moiré pattern may be observed. For example, Figs. 1 and
2 are two line halftones with spatial frequencies f1 = 16 LPI (lines-per-inch) and
f2 = 17 LPI, respectively. By overlapping the two halftones together, a moiré
pattern, as shown in Fig. 3, can be observed. It is well known by those skilled
in the art, that moiré frequency is equal to the difference or delta of the
two line halftones, i.e., &Dgr;f = f2 - f1.
For the current example as provided in Fig. 3, &Dgr;f = 1 LPI.
If one of the two line halftones is moved laterally with
respect to another, the moiré also moves laterally but in different speed.
To demonstrate the effect of a relative movement of two overlapped halftones, the
line halftone f2 in Fig. 1 is shifted toward left in a step equal to a quarter of
the period of the line halftone, or 0.25x1/17 inches. Fig. 4 shows the shift sequence
of the line halftone f2 after four steps. The total lateral shift provided there
is equal to 1/17 inches.
By overlapping the shifted line halftone f2 to the halftone
f1, one can see that the moiré is also shifted toward left in this case as
depicted in Fig. 5 and the total lateral shift of the moiré after four steps
is exactly equal to the period of the moiré, or one inch. It is also not difficult
to see that the moiré moves in an opposite direction, if the lateral shift
happens to the line halftone f1, when f2 > f1. Figs. 6 and 7 similarly demonstrate
the result with a lateral shift of f1.
For the current analysis, we may assume that the moiré
frequency is much, much less than the halftone special frequency or: &Dgr;f <<
f, where f = (f1 + f2)/2 and &Dgr;f = f2 - f1. Therefore, ignoring small differences
in calculation, we may summarize the two cases of moiré shift in Figs. 5 and
7 as follows: when the two overlapped halftone lines, f1 and f2, have a relative
lateral shift, the moiré always moves in the direction defined by the movement
of f2, the halftone with a higher spatial frequency. The moiré moves M times
faster than the relative movement between f1 and f2, or M may be expressed as given
In Fig. 8 there is schematically depicted the eyes of an
observer as viewing a transparency provided with duplex printing thereupon. When
a transparent substrate 800 as provided with print 810 on both transparency sides
830 & 840 is viewed in a such "see-through" matter, each of the two individual eyes
820 of an observer are seeing slightly different overlapping images as provided
by the two sides of the transparency 830 & 840. Due to the difference of viewing
angles, &thgr;, between each of the two eyes, and the finite thickness of the transparency
860, or "H", as compared to the image seen by the left eye, the right eye sees the
print on the back side of the transparency with a small lateral shift, "S", with
respect to the print on the front side. The shift "S" is approximately equal to:
S = &thgr;H; and gives the appearance of shifting to the right in this example.
Since a normal transparency 800 is only about 100-micron thick and &thgr;, the
difference of viewing angles by two eyes 820, is typically less than 0.5 degrees
at a normal reading distance, the lateral shift "S" is too small to create any stereoscopic
view for most such duplex prints.
However, if the two line halftones depicted in Figs. 1
and 2 as provided with different spatial frequencies, f1 and f2, and are printed
on two sides of a transparency respectively, a moiré will be observed clearly
in a "see-through" viewing configuration. Because of the stereoscopic view of the
two-sided print due to the thickness of the transparency, the resultant moiré
seen by the two eyes 820 of the observer is different. If the line halftone with
a higher spatial frequency f2 is printed on the back side 830 and the halftone with
a lower frequency f1 is on the front side 840, the moiré is moving towards
to the right when the viewing is changed from left to right. As described in above,
the shift of the moiré is much greater than that due to "S" (lateral shift),
the relative lateral shift of the two-side prints. Indeed, it is magnified by a
factor M, as given by Equation 1 above: M = f/&Dgr;f, where f = (f1 + f2)/2 and
&Dgr;f = f2 - f1. Therefore, the moiré appears as if it were printed on the
back side of a much thicker transparency. In other words, the stereoscopic view
of the overlapping of the two line halftones creates a stereo moiré image located
in the space behind the transparency at a distance approximately M times the thickness
of the transparency "H". If the same transparency is viewed with the line halftone
f2 on the front side and f1 on the back side, the moiré will appears as located
in the space in front of the transparency and also at a distance approximately equal
to M x H. With current printing technologies it is not difficult to generate halftone
line structures with a fairly large frequency range, so the magnification M can
be easily varied between zero up to a hundred. Hence, by choosing right combinations
of f1 and f2 for the duplex printing, it is possible to create moiré-based
auto-stereoscopic images with a depth range in the order of a few millimeters.
In one example a transparency shows a stereoscopic moiré
image having two depth levels provided by using the technique described above. On
one side of the transparency is provided a uniform line halftone with a spatial
frequency of 120 LPI as printed. On the other side, the printing consists of two
partitions: what is to be perceived as the background is printed using a line halftone
with a 123 LPI spatial frequency, while a logo image partition is printed using
a line halftone with a 117 LPI frequency. The spatial frequency difference between
the line halftones on two sides is approximately equal to 3 LPI, thus, the corresponding
shift-magnification factor M, as given by Equation 1 above, is about 40. Since the
moiré produced by the two partition print images as visually located appear
in two spatial planes as separated by the transparency, the total depth of this
moiré image is about 80H, where "H" is the thickness of the transparency, and
so is about 100 microns. Thus is yielded a moiré stereoscopic pattern clearly
discernable to the human eye with out aid of lenses or other means.
The effective limitation to this magnification factor M,
as given by Equation 1 above is constrained by two things. First is a given selected
printer provides technology constraints as to the maximum print system frequency
resolution, which at present is typically 300 LPI. This limits the upper moiré
frequency limit. The second constraint is the human visual response to low frequency
moiré as where low frequency beats (i.e. large &Dgr;f), if too low, will
simply be right off the page.
Figure 9 provides an exploded view, schematical depiction
of one exemplary embodiment. Here transparent substrate 800 is provided with a front-side
applied marking material of periodic structure 900, as well as, a back-side applied
marking material of periodic structure 910. In this depiction, for the sake of explanation,
the applied marking materials are shown as planes standing free of the substrate.
However, in at least one embodiment, these applied marking materials of periodic
structure are applied directly upon the transparent substrate's front and back sides,
either by duplex printing or by way of conventional two pass printing. In another
embodiment, one of the two applied planes of marking materials of periodic structure
is alternatively applied to a second substrate instead of the transparent substrate
800 and that second substrate is then brought into close proximity and substantial
alignment with the transparent substrate 800. Additional transparent substrates
may also be placed between the transparent substrate 800 and the second substrate
to effectuate a larger "H" and thus increase the perceived depth for an observer
820 as explained above.
On one side of the transparent substrate 800, the applied
marking material of periodic structure 900 is provided by printing a uniform line
halftone with a selected median spatial frequency. On the other side, for the applied
marking material of periodic structure 910 there is provided by printing, two partitions:
that which is to be perceived as the background partition 920 in this embodiment
is printed using a line halftone with spatial frequency equal to the median
plus some delta or difference in frequency amount (1/2&Dgr;f); while the
desired image partition 930 is printed using a line halftone with a spatial frequency
equal to the median minus the delta frequency amount. The spatial frequency
difference between the line halftones on two sides creates a corresponding shift-magnification
factor M. The moiré produced by the two print partitions 920 and 930 image
as visually located in appearance in two separate spatial planes as separated by
the transparency, with an effective amplified total depth as equal to the shift-magnification
factor M times the thickness of the transparency. Thus is yielded a moiré stereoscopic
pattern for the desired image partition 930 as clearly discernable to the human
eye with out aid of lenses or other means.
As will be evident to those skilled in the art, transparent
substrate 800 may be plastic, glass, Plexiglas, etc. as well as the typical presentation
transparency slide intended for usage by print systems as employed in combination
with overhead projectors. Indeed transparent substrate 800 may be only partially
transparent or translucent, though the effect will be impeded. As should be clear
from the above teachings thicker substrates such as glass will yield a more pronounced
effect. As will also be obvious to those skilled in the art, the applied marking
materials as discussed above may include liquid-based, toner-based, wax-based, inks
or powders or solids, as well as paint or other pigment based materials.
The teachings provided above may also be so employed so
as to apply the two line screens of different frequencies, f1 and f2, to embed watermarks
into halftone rendered prints. Fig. 10 schematically depicts such a superimposition
of a watermark image into an image of interest to yield moiré-based auto-stereoscopic
printed page. In this example, the flat binary desired watermark image pattern 100
shown in Fig. 10 specifies the two partitions for the usage of different frequency
halftone screens: the white partition 102 is for f1, the first line screen, and
the black partition 104, for f2, the second. When the input image 110, as shown
also in Fig. 10, is halftoned, one of the two line screens is chosen at a given
time as based on the binary value of the watermark pattern at the corresponding
location of desired watermark image pattern 100. The rendered halftone output 111
can be printed on a normal paper as is depicted in Fig. 11.
Fig. 11 depicts the manual overlay of a suitably prepared
printed transparency 800 upon the moiré-based auto-stereoscopic printed page
111 of Fig. 10. Separately prepared, a uniform halftone line structure with halftone
frequency f3 is printed on a transparency 800 and is used as a "decoder" overlay
for the purpose of revealing the moiré watermark. To view the embedded watermarks
one places the transparency 800 on the top of the printed image 111, with the printed
side of the transparency facing up, so the two printed halftones, one on the transparency
800 and another one on the paper 111, are separated by at least the thickness of
the transparency. Similar to the duplex printing on a single transparency 800 described
previously above, by properly choosing the line frequencies f1 and f2 for the image
111 and the line frequency f3 for the transparency 800, stereoscopic moirés
images are revealed. This can be observed as is depicted in Fig. 11 within the area
of intersection between transparency 800 and rendered image 111.
Of course, for this example embodiment, the transparency
800 needs to be so placed that the lines on both prints 800 and 111 are aligned
to the same angle to obtain the desired moiré patterns. However, the requirement
is quite relaxed, requiring alignment only to within a few degrees. As may be readily
observed when the example is actually at hand, is the fact that slightly different
angles will yield tilted moirés, but will never-the-less provide the same effective
For the example embodiment shown in Fig. 10 the choice
of f2 = f3 = 117LPI, and f1 = 120LPI, was made. Thus, the noticeable moirés
with a 3-LPI frequency delta due to the difference between f1 and f3 are observed
only in the area specified by the white partition of the binary watermark pattern
shown in Fig. 10. The moirés appear as located a few millimeters behind the
paper surface, as calculated and discussed above, while the logo patterns appear
to be right on the paper surface in this example embodiment. The final result is
that the embedded watermark patterns are retrieved as viewable vivid 3-D logo patterns.
Above choice of those line frequencies is only one example.
Other frequency combinations may also be used for different 3-D appearances. Furthermore,
moirés can be also obtained by overlapping two halftone patterns using two
cluster screens with slightly different frequencies and/or angles, so, as will be
understood by one skilled in the art, the above teachings may also applied to cases
with cluster screens.
Doubtless those skilled in the art will recognize may applications
for the teaching provided herein. In particular is in the authentication of print
items including tickets, coupons, diplomas and certificates. Further application
may be found where the normal course of physical arrangement provides a transparent
layer over a printed layer. For example where a package with print thereupon is
wrapped with a suitably rendered transparent wrap over it. Another example would
be book slip covers further provided with an additional suitably rendered transparent
cover wrap, such that auto-stereoscopic moiré images are incorporated into
the artistic design appearing on the cover.