We present a stochastically seeded, clustered halftoning method with parametric control of dot shapes as well as seed placement adaption to local image structure. While the literature on other randomly structured screens (blue noise, green noise, and stochastic halftoning) is quite extensive, there is very little discussion on optimization of dot shape for robustness or preferred tile appearance. The parametric control presented here can enable an imaging engineer to optimize robustness for a given printing system. The halftoning method that we introduce defines dot centers as seeds that are placed typically in a random, high spatial frequency configuration. Spot functions are defined about these randomly placed seeds, where the spot function allows control of dot cluster growth, touch points, cluster angle, and eccentricity. The spot function can also be applied to regular and irregular polygonal halftone tiling. The adaptability of the seed placement provided by the halftoning method allows for better edge rendition than conventional isotropic methods and enables the implementation of data-embedding techniques. The resulting monochromatic screens can be seamlessly integrated with novel successive filling techniques in order to provide dot-off-dot vector halftoning capabilities.
In this paper, a QR code is presented with a dual resolution structure. It contains a high resolution layer that is coded in luminance and is in consistency with the conventional QR code, and a low resolution layer providing additional error checking information, that is coded in chrominance and is robust to blurring. The proposed QR code is compatible to its underlying conventional black and white barcode as it can be read by their decoders. Its advantage is additional reliability when a color decoder is used. In particular, it enhances the decoding accuracy for devices such as mobile devices for barcodes printed in small sizes.
Hexagons are widely observed as a packing or tiling geometry in nature, yet they appear to have been avoided in conventional halftone tiling. A goal of the present study is to understand the potential barriers that have prevented their use and present new halftone geometry options that overcome the issues while offering several potential benefits. While conventional halftone geometries often include the fourth screen (e.g., yellow) in a suboptimal manner, the hexagonal geometry presented here can include a clustered-dot fourth screen moiré-free. Hexagonal screens can appear to have smoother texture. Due to differences in packing geometry and touch point geometry, hexagons have the potential to possess different tone reproduction characteristics, which may be favorable for some marking processes. We also present a parametrically controlled hexagonal halftone spot function that allows for optimization of dot touch points and provides compact growth. The controllable touch points can prevent a tone reproduction discontinuity, while the compact growth throughout the gray range ensures maximum stability. Additionally, we present a three-colorant dot-off-dot halftone configuration using hexagonal geometry. Examples are provided.
Human visual system has the property of perceiving the object color to remain constant regardless of the prevailing
illumination. However, digital cameras usually lack this capability, and the captured images are digitally corrected to
discount the color of the scene light based on the estimated illuminant. Illumination estimation might be erroneous in
some artificial or chromatic lighting conditions. A method was proposed to correct digital photos captured with a
smartphone camera using the smartphone owner's face as the reference. Taking the advantage of the latest smartphones
with two build-in cameras, we could use the front camera to capture the smartphone owner's face and compare with the
saved reference face image in order to estimate the scene illuminant. After that, we could properly adjust the capture
setting for the main camera in order to take a decent target image; or we could automatically correct the target image
based on the estimated illumination by comparing two face images. The method was implemented on the iOS mobile
platform. Experimental result shows that the adjusted images using the proposed method are generally more favorable
than the pictures taken directly by the default camera application.
We present a stochastically seeded halftoning method with parametric control of dot shapes as well as seed placement
adaption to local image structure. While the literature on other randomly structured screens (blue noise, green noise,
FM) is quite extensive, there is very little discussion on optimization of dot shape for robustness or preferred tile
appearance. The halftoning method that we introduce defines dot centers as seeds that are placed, typically in a random
high spatial frequency configuration. Spot functions are defined about these randomly placed seeds, where the spot
function allows control of dot cluster growth, touch points, cluster angle, and eccentricity. The spot function can also be
applied to regular and irregular polygonal halftone tiling. The seed adaption aspect of the halftoning method allows for
better edge rendition than conventional isotropic methods.
A halftone configuration is presented that utilizes three or four rotated hexagonal screens, or more precisely, screens with
hexagonally tiled clusters, for moiré-free color printing. Halftone designers consider many options to deliver a screen
with desirable characteristics, and often must settle for less than desirable results. The present method presents a new
option with several beneficial properties compared to conventional square-cell-based screens. Hexagonal screens can
appear to have smoother texture. Due to differences in packing geometry and touch point geometry, hexagons have the
potential to possess different tone reproduction characteristics, which may be favorable for some marking processes. A
fourth screen (e.g., yellow) can be included moiré-free, thereby avoiding problems associated with stochastic solutions
for yellow. We also present a corresponding parametrically controlled hexagonal halftone spot function that allows for
optimization of dot touch points and provides compact growth. The optimized touch points can prevent a tone
reproduction bump, while the compact growth throughout the gray range ensures maximum stability. Examples are
A method is provided for embedding a UV fluorescent watermark in a color halftone image printed on paper. The
described method implements two different strategies to halftone a watermark region and a background region. One
strategy uses dot-on-dot halftoning to maximize the usage of black ink and minimize ink dispersion, while the other
strategy uses successive-filling halftoning to maximize ink dispersion. An accurate color look-up-table (LUT) is built to
directly transform the colorant values for one halftoning strategy to the colorant values for the other strategy. With the
color transformation applied on one region, the binary outputs in both watermark and background regions halftoned with
different strategies exhibit similar color appearance under normal lighting condition. However, under UV illumination,
due to the fluorescent effect caused by different paper coverages in two regions, the embedded watermark becomes clearly visible.
In this paper we describe a method using duplex printing on transparencies to create auto-stereoscopic images viewed in
a "see-through" manner. By choosing different halftone structures, differing by having different spatial frequencies for
each of the two sides of a transparency, a moire pattern resulting due to halftone overlapping can be observed. On one
side of the transparency is provided a uniform halftone with a selected median spatial frequency <i>f</i>. On the other side, the
printing consists of two partitions: what is to be perceived as the background is printed using a halftone with spatial
frequency equal to the median frequency <i>f</i> plus some ▵<i>f</i>, while a desired image partition is printed using a halftone with
a spatial frequency equal to the median frequency <i>f</i> minus the same ▵<i>f</i>. The spatial frequency difference between the
halftones on two sides creates a corresponding shift-magnification factor <i>M</i>. The moire produced by the two partition
print images as visually located appear in two separate spatial planes as separated by the transparency, with an amplified
total depth of the shift-magnification factor <i>M</i> times the thickness of the transparency. This yields a moire stereoscopic
pattern for the desired image partition as clearly discernable to the human eye with out aid of lenses or other means.
A monochromatic binary printer model at the cluster level defined by the halftone frequencies is presented in this paper.
The halftone process using a cluster halftone screen creates halftone structures with aligned clusters within halftone cells
tiled to fill the image plane. Each halftone cell, or each tile, can be treated as a "primary color", which can be calibrated,
or macroscopically measured, from a corresponding halftone pattern. The cluster-based printer model estimates the
average color of a halftone pattern containing different halftone cells using a modified Neugebauer spectral
(colorimetric) color mixing model incorporating the Yule-Nielsen effect. A method based on the proposed clusterbased
printer model for quickly and accurately characterizing the complete tone reproduction curve (TRC) for multicenter
halftone screens is also described with the experiment result.
Presented is a unique halftone geometric configuration
for color halftone printing with an arbitrary number of colorants. The
configuration uses a plurality of halftone screens to produce prints
that are moiré free and possess uniform hexagonal periodic rosettes.
Moiré-free N-color halftoning is achieved by selecting halftone
frequencies from harmonics of two rosette fundamental frequencies.
With such screen configurations, the interference
between any halftone frequency components, fundamentals, or
higher-order harmonics of different colorants is a linear integer combination
of the two rosette fundamental frequencies. The linear integer
combination produces a hexagonal grid of frequency components
with no components lower than the rosette frequencies other
than the zero-frequency component. Thus, no visible interference,
or moiré, occurs within the halftone screen combinations. The observed
halftone pattern is a visually pleasing, two-dimensionally uniformly
repeated hexagonal rosette. The uniform-rosette configurations
can be implemented using digital single-cell nonorthogonal
halftone screens. The uniform-rosette method can be used with conventional
cyan-magenta-yellow-black halftone printing or with printing
systems that require a relatively large number of halftone
screens. The additional screens can be used for enhanced printing
applications, such as printing with high-fidelity colorants, light colorants,
or special colorants, such as white, metallic, and
Color printer calibration is always performed in the presence of noise. A major part of this noise is due to the spatial
non-uniformity of the printer. In many cases, the spatial non-uniformity does not repeat the same pattern page by page.
Some calibration techniques use randomizing the locations of patches and averaging a large numbers of measurements to
reduce the noise. Since the non-uniformity is a kind of systematic errors, it should be estimated and corrected in a more
effective way. This presentation describes a method to provide a more accurate color calibration/characterization result
by estimating the spatial non-uniformity presented on the printed target page and applying a correction to the very same
page itself. Instead of defining the noise in the color measurement space, e.g. the CIE Lab space, the new method
specifies the noise in the printer-dependent color space, e.g. the CMY space. After the first-round color calibration, an
inverse transform, from CIE Lab to CMY, is derived from measurements of the entire printed target. The noise dC, dM,
or dY is determined as the difference between the original CMY values and the output of the inverse transform with the
measured Lab values as the transform input. Since color patch locations are stochastically arranged, any "noticeable"
spatial pattern of the noise is most likely due to the printer non-uniformity in the corresponding channel. The nonuniformity
can be estimated by spatially smoothing the noise terms and the result can be subtracted from the original CMY input for a second-round calibration to achieve higher color accuracy.
Substrates found in standard digital color printing applications frequently contain optical brightening agents
(OBAs). These agents fluoresce under near UV light and are predominantly intended to increase the perceived
paper white and thus create a paper look and feel which is preferred by customers. The fluorescence
phenomenon poses a considerable challenge in standard color management applications, however, the
problem description can be inverted and information can be embedded in a printed color image that is perceptually
invisible under normal illumination, but revealed via substrate fluorescence under UV illumination.
From a practical standpoint, the approach works with standard high brightness office-type papers and does
not require any special materials or media, or any modifications to the imaging path inside the machine. This
means that certain security aspects can now produced in an essentially cost-neutral way.
Color printer calibration is the process of deriving correction
functions for device signals (e.g., CMYK), so that the device can
be maintained with a fixed known characteristic color response.
Since the colorimetric response of the printer can be a strong function
of the halftone, the calibration process must be repeated for
every halftone supported by the printer. The effort involved in the
calibration process thus increases linearly with the number of halftoning
methods. In the past few years, it has become common for
high-end digital color printers to be equipped with a large number of
halftones, thus making the calibration process onerous. We propose
a halftone independent method for correcting color (CMY or CMYK)
printer drift. Our corrections are derived by measuring a small number
of halftone independent fundamental binary patterns based on
the 22 binary printer model by Wang et al. Hence, the required
measurements do not increase as more halftoning methods are
added. First, we derive a halftone correction factor (HCF) that exploits
the knowledge of the relationship between the true printer
response and the 22-model predicted response for a given halftoning
scheme. Therefore, the true color drift can be accurately predicted
from halftone-independent measurements and corrected correspondingly.
Further, we develop extensions of our proposed color
correction framework to the case when the measurements of our
fundamental binary patches are acquired by a common desktop
scanner. Finally, we exploit the application of the HCF to correct
color drift across different media (papers) and for halftoneindependent
spatial nonuniformity correction.
By selecting halftone frequencies from high-order harmonics of two common rosette fundamentals for all color
separations, a true moiré-free color halftoning can be achieved. With such screen configurations, the interference
between any two frequency components, fundamentals or high-order harmonics, of different colors will also result in a
linear combination of the two rosette fundamentals. Thereby, no visible interference, or moiré, at all will be shown in
the output. The halftone outputs are two-dimensionally repeated patterns, as visually pleasant uniform rosettes. The
uniform-rosette configurations can be implemented by single-cell non-orthogonal halftone screens for digital halftoning.
Unlike "dot-on-dot" screening, or using one screen for all colors, uniform-rosette halftoning is robust to mis-registration
between color separations. Several designs of uniform-rosette halftone screens have been successfully applied to Xerox
iGen3 color printers for high-quality color reproduction.
Color printer calibration is the process of deriving correction functions for device CMYK signals, so that
the device can be maintained with a fixed known characteristic color response. Since the colorimetric
response of the printer can be a strong function of the halftone, the calibration process must be repeated for
every halftone supported by the printer. The effort involved in the calibration process thus increases
linearly with the number of halftoning methods. In the past few years, it has become common for high-end
digital color printers to be equipped with a large number of halftones thus making the calibration process
onerous . We propose a halftone independent method for correcting color (CMY/CMYK) printer drift. Our
corrections are derived by measuring a small number of halftone independent fundamental binary patterns
based on the 2×2 binary printer model by Wang et. al. Hence, the required measurements do not increase
as more halftoning methods are added. The key novelty in our work is in identifying an invariant halftone
correction factor (HCF) that exploits the knowledge of the relationship between the true printer response
and the 2×2 predicted response for a given halftoning scheme. We evaluate our scheme both quantitatively
and qualitatively against the printer color correction transform derived with the printer in its "default
state". Results indicate that the proposed method is very successful in calibrating a printer across a wide
variety of halftones.
Glossmark technology is a halftone-based digital imaging process to embed visible watermarks into xerographic color prints. The gloss of a xerographic print depends not only on surface roughness of paper and toner, but also on the microscopic structure created by the halftone process. The surface relief of a halftone image can be treated as a two-dimensional phase grating. The shape, or profile, of the surface relief determines the reflected pattern of the illumination. A strong angular differential gloss can be obtained by employing two anisotropic halftone screens in the halftone process. A careful design of these screens enables embedding Glossmark images while maintaining the high quality of the color reproduction. The printing process, that simultaneously creates high quality primary and Glossmark images in a single step, requires neither special equipment nor special paper or toner. Glossmark images, shown in a high contrast of gloss, provide a perfect simulation of the traditional paper watermarks, while their digital implementation makes it easy to embed variable data as digital watermarks into individual documents.
A technique for watermarking duplex printed pages is presented. The
technique produces visible watermark patterns like conventional
watermarks embedded in paper fabric. Watermark information is embedded
in halftones used to print images on either side. The watermark
pattern is imperceptible when images printed on either side are viewed
independently but becomes visible when the sheet of paper is held up
against a light. The technique employs clustered dot halftones and
embeds the watermark pattern as controlled local phase
variations. Illumination with a back-light superimposes the halftone
patterns on the two sides. Regions where the front and back-side
halftones are in phase agreement appear lighter in show-through
viewing, whereas regions over which the front and back side halftones
are in phase disagreement appear darker. The image printed on one side
has a controlled variation of the halftone phase and the one printed
on the other side provides a constant phase reference. The watermark
pattern is revealed when the sheet is viewed in "show-through mode"
superimposing the halftones on the two sides. Threshold arrays for the
halftone screens are designed to allow incorporation of a variety of
halftone patterns while minimizing artifacts in images printed using
A new technique for design of stochastic screens is proposed that produces screens that are robust against mis-registration in multi-pass printing. Conventional stochastic screens are designed through an optimization process that minimizes low-frequency structure in halftone images under the assumption that the placement of pixels is accurate. In inkjet printing, however, a page is often printed in multiple passes to allow for better drying of inks and to minimize appearance of a head signature. Any potential mis-registration between the passes is typically not comprehended in the conventional stochastic screen design process. The mis-registration between the passes can therefore cause significantly increased graininess (low-frequency structure) in printed images produced with stochastic screens even though the corresponding electronic bitmaps are free from low-frequency structure. In this paper, we propose modifications to the stochastic screen design process that take the two pass printing into account and produce halftones that are robust to inter-pass mis-registration errors. This allows reduced tolerances and alignment requirements in manufacturing that translate to lower cost. The proposed technique works by modifying the screen design process to ensure that a majority of the minority pixels are concentrated in a single pass, which provides improved robustness to mis-registration between the passes. Experimental results demonstrate that the proposed design technique performs significantly better than conventional stochastic screens in the presence of mis-registration errors.
In most existing color reproduction systems, color correction is performed in an open-looped fashion. For multiple generation color copying, color fidelity cannot be guaranteed as the errors introduced in color correction may accumulate. In this paper, we propose a method of solving the error accumulation problem by embedding color information as invisible digital watermark in hardcopies. When the hardcopy is scanned, the embedded information can be retrieved to provide real-time calibration. As the method is close-looped in nature, it may reduce error accumulation and improve color fidelity, particularly when copies go through multiple generation reproduction.
In color reproduction, the most troublesome moire pattern is the second-order moire, or the three-color moire, usually produced by mixing of cyan, magenta and black halftone outputs. A classical 3-color zero-moire solution is using three identical cluster halftone screens with different rotations: 15, 45 and 75°, respectively. However, for most digital printing devices, the size and shape of halftone screens are constrained by the "digital grid", which defines the locations of printed dots; and therefore, an exact 15 or 75° rotation of a cluster screen is impossible. Although there are many alternative approaches for moire-free color halftoning, most of them only provide approximate solutions and/or have a tendency to generate additional artifacts associated with halftone outputs. The difficulty to achieve moire-free color halftoning is greatly relieved by using non-orthogonal halftone screens, i.e., screens in general parallelogram shapes. In this paper, a general condition for 3-color zero-moire solutions is derived. A procedure using integer equations to search moire-free solutions for different applications is also described.
Colorimetric data can be readily computed from measured spectral data, however, as illustrated by metameric pairs, the mapping from spectral data to colorimetric values is many-to-one and therefore typically not invertible. In this paper, we investigate inversions of the spectrum-to-colorimetry mapping when the input is constrained to a single color reproduction medium. Under this constraint, accurate recovery of spectral data from colorimetric data is demonstrated for a number of different color reproduction processes. Applications of the spectrum reconstruction process are discussed and demonstrated through examples.
This paper introduces a moire-free color half toning algorithm. Moire patterns might appear in traditional halftone images. This could be caused by interference among halftone screens of different colors, or beating between he input image and the screens. In our algorithm, we adjust the halftone screens of different colors, or beating between the input image and the screens. In our algorithm, we adjust the halftone output in such a way that the low-frequency difference between the input and the appearance of the output is minimized. Due to non-linear color mixing, the appearance of a color print is not a simple combination of CMYK four channels, and is hence not easily predictable. We use the 2 by 2 printer model to estimate the pixel-level CIE-Lab values from the halftone output. Next we calculate the difference between the original CIE-Lab input image and the 2 by 2 estimation and run a low-pass filtering on the result. Then, we feed back the low-frequency difference into the input. The low-frequency-feedback process cancels the potential color moire patterns.
Invisible watermark images can be incorporated in printed halftone images using specially designed halftone screens. The watermark information is embedded into the image by varying the spatial correlation of the halftone texture. The halftone screen with embedded watermarks can be used exactly as a normal halftone screen, so there is no additional image processing required for processing individual images to embed watermarks. Once the binary output image is printed on the paper, the correlation of the binary image is converted into physical spatial correlation between neighboring areas of the printed image. This correlation relation is not visible to the eye but it can be detected by scanning the printed image on a desktop scanner and processing the scanned image. Printer and scanner distortions can interfere with the self-alignment of the scanned image, so localized adjustments are made to detect the embedded spatial correlation information in the watermarked image. The retrieval of this watermark is robust to copying and distortion and it can be detected in reproductions of the halftone image.
Introduction of 2 X 2 centering can significantly reduce the number of dot patterns required in color calibration. As a result, local calibration can be performed using direct measurement and fast table lookup. In this paper, 2 X 2 calibration is applied to error diffusion. A novel concept for error diffusion is introduced. It matches the color increment instead of color. The concept makes integration of error diffusion and 2 X 2 calibration natural and free of causality issue.
A method will be shown to incorporate digital watermarks in printed halftone images using stochastic screens. The watermark is not visible to the eye and introduces no loss in image quality. Although it cannot be seen, the watermark can be extracted at a later time with post processing. Watermarks of high contrast are incorporated in the image by varying the statistics of the stochastic screen. The watermark information can be made visible by comparing the relative changes in spatial correlation in the halftone texture of the image. Watermarking allows a printed image to be tested for the purposes of identifying the owner or the source of the image. Arbitrary customer information can be incorporated into the image, including variable information, such as the data or time of day. The technique is robust to copying of the printed image and the watermark can be detected in reproductions of the halftoned image.
We propose a modification to the process of error-diffused quantization often used for printing computer-generated holograms on bitonal devices. The output of many devices (e.g. laser printers) consists of `dots' printed at locations on a fixed grid. The printed patterns often are modeled as ideal square pixels. However, real dots typically are more round than square, and often overlap adjacent pixels. These variations can affect the quality of the reconstructed hologram significantly.