Note: Descriptions are shown in the official language in which they were submitted.
CA 02218122 1997-10-08
HYBRID IMAGING SYSTEM
The present application is a continuation-in-part of Application Serial No.
08/625,324 filed on April 1, 1996, also assigned to Xerox Corporation.
The present invention relates to a digitized hybrid imaging system as may be
used in black and white or color printing systems (such as in
electrophotographic
printers and copiers), and more particularly, to an apparatus and method for
improving output image quality according to the use of a plurality of imaging
techniques in halftoning black and white and/or color documents.
In the operation of a copier or printer, particularly color machines, it is
highly desirable to have means for processing and enhancing text and image
quality (hereinafter referred to as "image quality" or the like unless
otherwise
noted). Particularly in the case of single or multi-pass color printers, it is
highly
desirable that an image processing system be employed to reduce imaging
problems caused by halftoning systems not suited to a variety of image types.
Likewise, certain image processing systems may be more successfully employed
in
particular printer hardware situations. While the present invention is quite
suitable
for use on the Xerox 4900 family of printers in which aspects of it have been
tested, it may be likewise highly useful with a variety of other xerographic
as well
as non-xerographic printing systems.
In the process of digital electrostatographic printing, an electrostatic
charge
pattern or latent image corresponding to an original or electronic document
may
be produced by a raster output scanner on an insulating medium. A viewable
record is then produced by developing the latent image with particles of
granulated material to form a powder image thereof. Thereafter, the visible
powder image is fused to the insulating medium, or transferred to a suitable
support material and fused thereto. Development of the latent image is
achieved
by bringing a developer mix into contact therewith. Typical developer mixes
generally comprise dyed or colored thermoplastic particles of granulated
material
CA 02218122 1997-10-08
known in the art as toner particles, which are mixed with carrier granules,
such as
ferromagnetic granules. When appropriate, toner particles are mixed with
carrier
granules and the toner particles are charged triboelectrically to the correct
polarity.
As the developer mix is brought into contact with the electrostatic latent
image, the
toner particles adhere thereto. However, as toner particles are depleted from
the
developer mix, additional toner particles must be supplied. Imaging systems
may
be more or less successful in printing high quality images of varying types in
electrostatographic systems which may have output capabilities or efficiencies
unlike those found in ink jet or other systems.
Various systems have been employed to include those set forth in the
following disclosures which may be relevant to various aspects of the hybrid
imaging systems of the present invention:
US-A-5,477,305
Patentee: Parker et al.
Issued: December 19, 1995
US-A-5,341,228
Patentee: Parker et al.
Issued: August 23, 1994
US-A-5,323,247
Patentee: Parker et al.
Issued: June 21, 1994
US-A-5,321,525
Patentee: Hains
Issued: June 14, 1994
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US-A-5,291,296
Patentee: Hains
Issued: March 1, 1994
U S-A-5,111, 310
Patentee: Parker et al.
Issued: May 5, 1992
US-A-4,955,065
Patentee: Ulichney
Issued: September 4, 1990
US-A-4,736,254
Patentee: Kotera et aI.
Issued: April 5, 1988
US-A-4,698,691
Patentee: Suzuki et al.
Issued: October 6, 1987
US-A-4,245,258
Patentee: Holladay
Issued: January 13, 1991.
"Dithering with Blue Noise" by Robert A. Ulichney.
Proceedings of the IEE, Vol. 76, No. 1, January 1988. Pages 56-79.
"Modified Approach to the Construction of a Blue Noise Mask"
by Meng Yao and Kevin J. Parker of the University of Rochester.
Journal of Electronic Imaging, January 1994, Vol. 3(1). Pages 92-97.
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"Digital Halftoning Using a Blue Noise Mask"
by Theophano Mista and Kevin J. Parker of the University of Rochester.
SPIE Vol. 1452 Image Processing Algorithms and Techniques II (1991).
Pages 47-56.
US-A-5,447,305 teaches a method of and system for rendering a halftone
image of a gray scale image by utilizing a pixel-by-pixel comparison of the
gray
scale image against a blue noise mask disclosed in which the gray scale image
is
scanned on a pixel-by-pixel basis and compared on a pixel-by-pixel basis to an
array of corresponding data points contained in a blue noise mask. Multiple
masks
may be used to halftone color images. Modifications can be made by a user to
improve mask performance.
US-A-5,341,228 teaches a method of and system for rendering a halftone
image of a gray scale image by utilizing a pixel-by-pixel comparison of the
gray
scale image against a blue noise mask disclosed in which the gray scale image
is
scanned on a pixel-by-pixel basis and compared on a pixel-by-pixel basis to an
array of corresponding data points contained in a blue noise mask stored in a
PROM or computer memory in order to produce the desired halftoned image.
US-A-5,323,247 also disclosed a method of and system for rendering a
halftone image of a gray scale image by utilizing a pixel-by-pixel comparison
of
the gray scale image against a blue noise mask in which the gray scale image
is
scanned on a pixel-by-pixel basis and compared on a pixel-by-pixel basis to an
array of corresponding data points contained in a blue noise mask stored in a
PROM or computer memory in order to produce the desired halftoned image.
US-A-5,321,525 discloses a method of quantizing pixel values in an image
formed by a plurality of pixels, each pixel representing an optical density of
the
image at a location within the image, and having an original optical density
value
selected from one of a set of 'c' original optical density values that has a
number of
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members larger than a desired output set of 'd' optical density values through
a
process of combined halftoning and cell-to-cell error diffusion.
US-A-5,291,296 discloses a method of halftoning according to a "quad dot"
system, and is also referred to below.
US-A-5,111,310 discloses a method of and system for rendering a halftone
image of a gray scale image by utilizing a pixel-by-pixel comparison of the
gray
scale image against a blue noise mask in which the gray scale image is scanned
on
a pixel-by-pixel basis and compared on a pixel-by-pixel basis to an array of
corresponding data points contained in a blue noise mask stored in a PROM or
computer memory in order to produce the desired halftoned image.
US-A-4,955,065 discloses a digital image processing system for converting
continuous tone pixel values representing an image into halftone or dithered
pixel
values, with the dithered pixel values representing each pixel having fewer
bits
than are used to represent each pixel in the continuous tone image.
US-A-4,736,254 discloses a halftone signal having one of two discrete levels
is generated for each print position along each print line by comparison
between a
gray scale value of an original with a threshold value stored in a memory. The
memory having a matrix array of cells each storing a particular threshold
value
where M, N, a and (3 are integers.
US-A-4,698,691 discloses a halftone image processing method for providing
image information in a bit distribution by specifying a matrix pattern in
response to
tone data which is indicative of a recording density. Several matrix pattern
groups
which are prepared each comprising matrix patterns which are larger in number
than dots which define a dot matrix.
US-A-4,245,258 discloses an electrical screening system for binary displays
or binary graphic recording systems which suppresses false contours. The
suppression is achieved by increasing the number of gray levels that a given m
x n
matrix of pixels can represent.
The article "Dithering with Blue Noise" describes and compares image
processing systems employing blue noise with error diffusion and other
outputs.
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Digital halftoning processes and desirable characteristics are compared and
summarized; optimized blue noise generations are explained and demonstrated.
The article "Modified Approach to the Construction of a Blue Noise Mask"
teaches a modified method of and system for rendering a halftone image of a
gray scale image by utilizing a pixel-by-pixel comparison of the gray scale
image
against a blue noise mask. Steps to produce improved masks are explained.
The article "Digital halftoning Using a blue Noise Mask" likewise teaches
earlier methods by Mista and Parker for rendering a halftone images of a gray
scale utilizing a pixel-by-pixel comparison of the gray image against a blue
noise
mask.
Therefore various aspects of the invention are provided as follows:
A method of constructing a composite screen for halftoning a digitized
color image having a predetermined number of color separations, comprising:
selecting a first halftoning system of a first type having a first set of
pixel
placement characteristics;
selecting a second halftoning system of a second type having a second set
of pixel placement characteristics;
determining a gray scale transition region for transitioning from the first
halftoning system to the second halftoning system;
merging the first halftoning system with the second halftoning system so
as to provide a hybrid halftoning system in the gray scale transition region,
said
hybrid halftoning system including a hybrid set of pixel characteristics of
said
first set of pixel placement characteristics and said second set of pixel
placement
characteristics; and
rendering a first number of color separations of a color image using said
hybrid halftoning system, the first number of color separations being equal to
a
value which is less the predetermined number of color separations.
A method of constructing a composite screen for halftoning a digitized
image, comprising:
selecting a first halftoning system of a first type having a first set of
pixel
placement characteristics;
selecting a second halftoning system of a second type having a_second set
of pixel placement= characteristics;
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determining a gray scale transition region for transitioning from the first
halftoning system to the second halftoning system; and
merging the first halftoning system with the second halftoning system so
as to provide a hybrid halftoning system in the gray scale transition region,
said
hybrid halftoning system including a hybrid set of pixel_characteristics of
said
first set of pixel placement characteristics and said second set of pixel
placement
characteristics;
selecting a third halftoning system of a second type having a third set of
pixel placement characteristics;
determining a second gray scale transition region for transitioning from
the second halftoning system to the third halftoning system; and
merging the second halftoning system with the third halftoning system so
as to provide a hybrid halftoning system in the second gray scale transition
region, said hybrid halftoning system including a second hybrid set of pixel
characteristics of said second set of pixel placement characteristics and said
third
set of pixel placement characteristics.
Other features of the present invention will become apparent as the
following description proceeds and upon reference to the drawings, in which:
Figure 1 is a flowchart showing a hybrid dot screening system of the
present invention;
Figure 2 is a flowchart showing the exemplary use of multiple image
screening techniques in a system of the present invention;
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Figure 3 is a representative gray level rendering option menu screen of the
present invention;
Figure 4 is flow chart of an embodiment of the present invention; and
Figure 5 is a schematic elevational view showing an exemplary color
xerographic printing machine and networked PC incorporating features of the
present invention therein.
While the present invention will hereinafter be described in connection
with preferred embodiments thereof, it will be understood that it is not
intended to
limit the invention to these embodiments. On the contrary, it is intended to
cover
all alternatives, modifications and equivalents, as may be included within the
spirit
and scope of the invention as defined by the appended claims.
For a general understanding of the features of the present invention,
reference is made to the drawings. Figure 5 is a schematic elevational view
showing an exemplary electrophotographic printing/copying machine and a
networked PC which may incorporate features of the present invention therein.
It
will become evident from the following discussion that the system of the
present
invention is equally well suited for use in a wide variety of printing and
copying
systems, and therefore is not limited in application to the particular
system(s)
shown and described herein.
To begin by way of general explanation, Figure 5 is a schematic elevational
view showing an electrophotographic printing machine and networked PC which
may incorporate features of the present invention therein. An image processing
station (IPS), indicated generally by the reference numeral 12, contains data
processing and control electronics which prepare and manage the image data
flow
to a raster output scanner (ROS), indicated generally by the reference numeral
16.
A network of one or more personal computers (PC), indicated generally by the
reference, numeral 5, is shown interfacing/in communication with IPS 12. A
user
interface (UI), indicated generally by the reference numeral 14, is also in
communication with IPS 12.
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UI 14 enables an operator to control and monitor various operator
adjustable functions and maintenance activities. The operator actuates the
appropriate keys of UI 14 to adjust the parameters of the copy. UI 14 may be a
touch screen, or any other suitable control panel, providing an operator
interface
with the system. The output signal from UI 14 is transmitted to IPS 12. UI 14
may
also display electronic documents on a display screen (not shown in Figure
17), as
well as carry out the system of the present invention as described in
association
with Figures 2 through 4 below.
As further shown in Figure 5, a multiple color original document 38 may be
positioned on (optional) raster input scanner (RIS), indicated generally by
the
reference numeral 10. The RIS contains document illumination lamps, optics, a
mechanical scanning drive, and a charge coupled device (CCD array) or full
width
color scanning array. RIS 10 captures the entire image from original document
38
and converts it to a series of raster scan lines and moreover measures a set
of
primary color densities, i.e., red, green and blue densities, at each point of
the
original document. RIS 10 may provide data on the scanned image to IPS 12,
indirectly to PC 5 and/or directly to PC 5.
Digitized electronic documents may be created, screened, modified, stored
and/or otherwise processed by PC 5 prior to transmission/relay to IPS 12 for
printing on printer 18. The display of PC 5 may show electronic documents on a
screen (not shown in Figure 5). IPS 12 may include the processor(s) and
controller(s) (not shown in Figure 5) required to perform the system of the
present
invention.
IPS 12 also may transmits signals corresponding to the desired electronic or
scanned image to ROS 16, which creates the output copy image. ROS 16 includes
a laser with rotating polygon mirror blocks. The ROS illuminates, via mirror
37,
the charged portion of a photoconductive belt 20 of a printer or marking
engine,
indicated generally by the reference numeral 18, at a rate of about 400 pixels
per
inch, to achieve a set of subtractive primary latent images. (Other
implementations
may include other pixel resolutions of varying types 600 X 600 dpi, or even
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asymmetrical resolutions, such as 300 X1200 dpi, both configurations of which
are
employed in versions of the Xerox 4900 printer.) The ROS will expose the
photoconductive belt to record three or four latent images which correspond to
the
signals transmitted from IPS 12. One latent image is developed with cyan
developer material. Another latent image is developed with magenta developer
material and the third latent image is developed with yellow developer
material. A
black latent image may be developed in lieu of or in addition to other
(colored)
latent images. These developed images are transferred to a copy sheet in
superimposed registration with one another to form a multicolored image on the
copy sheet. This multicolored image is then fused to the copy sheet forming a
color copy.
With continued reference to Figure 5, printer or marking engine 18 is an
electrophotographic printing machine. Photoconductive belt 20 of marking
engine
18 is preferably made from a photoconductive material. The photoconductive
belt
moves in the direction of arrow 22 to advance successive portions of the
photoconductive surface sequentially through the various processing stations
disposed about the path of movement thereof. Photoconductive belt 20 is
entrained about rollers 23 and 26, tensioning roller 28, and drive roller 30.
Drive
roller 30 is rotated by a motor 32 coupled thereto by suitable means such as a
belt
drive. As roller 30 rotates, it advances belt 20 in the direction of arrow 22.
Initially, a portion of photoconductive belt 20 passes through a charging
station, indicated generally by the reference numeral 33. At charging station
33, a
corona generating device 34 charges photoconductive belt 20 to a relatively
high,
substantially uniform potential.
Next, the charged photoconductive surface is rotated to an exposure station,
indicated generally by the reference numeral 35. Exposure station 35 receives
a
modulated light beam corresponding to information derived by RIS 10 having
multicolored original document 38 positioned thereat. The modulated light beam
impinges on the surface of photoconductive belt 20. The beam illuminates the
charged portion of the photoconductive belt to form an electrostatic latent
image.
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The photoconductive belt is exposed three or four times to record three or
four
latent images thereon.
After the electrostatic latent images have been recorded on photoconductive
belt 20, the belt advances such latent images to a development station,
indicated
generally by the reference numeral 39. The development station includes four
individual developer units indicated by reference numerals 40, 42, 44 and 46.
The
developer units are of a type generally referred to in the art as "magnetic
brush
development units." Typically, a magnetic brush development system employs a
magnetizable developer material including magnetic carrier granules having
toner
particles adhering triboelectrically thereto. The developer material is
continually
brought through a directional flux field to form a brush of developer
material. The
developer material is constantly moving so as to continually provide the brush
with
fresh developer material. Development is achieved by bringing the brush of
developer material into contact with the photoconductive surface. Developer
units
40, 42, and 44, respectively, apply toner particles of a specific color which
corresponds to the complement of the specific color separated electrostatic
latent
image recorded on the photoconductive surface.
The color of each of the toner particles is adapted to absorb light within a
preselected spectral region of the electromagnetic wave spectrum. For example,
an electrostatic latent image formed by discharging the portions of charge on
the
photoconductive belt corresponding to the green regions of the original
document
will record the red and blue portions as areas of relatively high charge
density on
photoconductive belt 20, while the green areas will be reduced to a voltage
level
ineffective for development. The charged areas are then made visible by having
developer unit 40 apply green absorbing (magenta) toner particles onto the
electrostatic latent image recorded on photoconductive belt 20. Similarly, a
blue
separation is developed by developer unit 42 with blue absorbing (yellow)
toner
particles, while the red separation is developed by developer unit 44 with red
absorbing (cyan) toner particles. Developer unit 46 contains black toner
particles
and may be used to develop the electrostatic latent image formed from a black
and
CA 02218122 1997-10-08
white original document. Each of the developer units is moved into and out of
an
operative position. In the operative position, the magnetic brush is
substantially
adjacent the photoconductive belt, while in the nonoperative position, the
magnetic brush is spaced therefrom. During development of each electrostatic
latent image, only one developer unit is in the operative position, the
remaining
developer units are in the nonoperative position.
After development, the toner image is moved to a transfer station, indicated
generally by the reference numeral 65. Transfer station 65 includes a transfer
zone, generally indicated by reference numeral 64. In transfer zone 64, the
toner
image is transferred to a sheet of support material, such as plain paper
amongst
others. At transfer station 65, a sheet transport apparatus, indicated
generally by
the reference numeral 48, moves the sheet into contact with photoconductive
belt
20. Sheet transport 48 has a pair of spaced belts 54 entrained about a pair of
substantially cylindrical rollers 50 and 53. A sheet gripper (not shown in
Figure 5)
extends between belts 54 and moves in unison therewith. A sheet 25 is advanced
from a stack of sheets 56 disposed on a tray. A friction retard feeder 58
advances
the uppermost sheet from stack 56 onto a pre-transfer transport 60. Transport
60
advances the sheet (not shown in Figure 5) to sheet transport 48. The sheet is
advanced by transport 60 in synchronism with the movement of the sheet
gripper.
The sheet gripper then closes securing the sheet thereto for movement
therewith in
a recirculating path. The leading edge of the sheet (again, not shown in
Figure 5) is
secured releasably by the sheet gripper. As belts 54 move in the direction of
arrow
62, the sheet moves into contact with the photoconductive belt, in synchronism
with the toner image developed thereon. In transfer zone 64, a corona
generating
device 66 sprays ions onto the backside of the sheet so as to charge the sheet
to the
proper magnitude and polarity for attracting the toner image from
photoconductive
belt 20 thereto. The sheet remains secured to the sheet gripper so as to move
in a
recirculating path for three cycles. In this way, three or four different
color toner
images are transferred to the sheet in superimposed registration with one
another.
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One skilled in the art will appreciate that the sheet may move in a
recirculating path for four cycles when under color black removal is used.
Each of
the electrostatic latent images recorded on the photoconductive surface is
developed with the appropriately colored toner and transferred, in
superimposed
registration with one another, to the sheet to form the multicolored copy of
the
colored original document. After the last transfer operation, the sheet
transport
system directs the sheet to a vacuum conveyor 68. Vacuum conveyor 68
transports the sheet, in the direction of arrow 70, to a fusing station,
indicated
generally by the reference numeral 71, where the transferred toner image is
permanently fused to the sheet. Thereafter, the sheet is advanced by a pair of
rolls
76 to a catch tray 78 for subsequent removal therefrom by the machine
operator.
The final processing station in the direction of movement of belt 20, as
indicated by arrow 22, is a photoreceptor cleaning apparatus, indicated
generally
by the reference numeral 73. A rotatably mounted fibrous brush 72 may be
positioned in the cleaning station and maintained in contact with
photoconductive
belt 20 to remove residual toner particles remaining after the transfer
operation.
Thereafter, lamp 82 illuminates photoconductive belt 20 to remove any residual
charge remaining thereon prior to the start of the next successive cycle. As
mentioned above, other xerographic and non-xerographic printer hardware
implementations may be used with the hybrid imaging systems of the present
invention, such as in the case of versions of the Xerox 4900 printer (which
employs
an intermediate transfer system) in which certain aspects of the system as
outlined
below have been tested.
Figure 1 shows a system for halftoning gray scale black and white or color
images which utilizes pixel-by-pixel comparison of the image against a ordered
hybrid dot screen. The system includes the use of a ordered dot matrix look-up
table or thresholding system (such as a 2 X 2 ordered dot matrix), wherein
each
quadrant of the matrix is always filled in a particular order. For example, in
an up
to a 25% "fill" of a gray scale area printed output, the first designated
quadrant of
the matrix in a continuous halftone area will always be used or filled before
the
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second ordered quadrant is utilized. Within each of the gray scale quadrant
halftoning ranges (0-25%, 26-50%; 51- 75%; and 76-100%), halftoning may be
accomplished using a variety of stochastic screening, thresholding, dithering,
randomized dot systems (such as blue noise-emulating functions) or other
compartmentally useful imaging techniques. While the deterministic nature of
such an ordered dot system may not work well on some image types or with some
imaging situations or hardware implementations (such as by resulting in
"checkerboard" effects at the transition regions), such a system may be
employed
in many situations with good to excellent results.
Figure 1 shows the hybrid "matrix" dot screen system in which a stochastic
screen function of the dimensions M x N for use in selected instances (such as
128X128). Thereafter a list of x, y coordinates sorted in the matrix in the
order in
which they turn on is made. The scale of the matrix P x Q may preferably be
linearly translated into the new coordinates. For a 2x2 matrix, the function
is
scaled according to the dimension shown in Figure 1 of M' and N' and the
coordinates of x' and Y. Thereafter a list for x" and y" is created for all
x', y'
coordinates; this operation is repeated P*Q (four times in the illustrated
example)
to create a matrix of the desired matrix dot size. Thereafter, the matrix is
projected
into a new array. At the same time, before or after the stochastic screen
generation, the matrix dot scaling steps occur. Preferably a list of x, y
coordinates
for sorting thresholds at the coordinate 0,0; 1,1; 0,1; 1,0 are created. A
counter
sorted list of x, y coordinates for the matrix is thereafter generated. Again,
these
operations may preferably be performed linearly so as to create a more
efficient
system for generating the hybrid dot. Thereafter, an ordered list of the size
(P x
Q).(M x N) that is (2x2).(128x128) is generated resulting in 65,536 address
list
lookup table. Pixel-by-pixel comparison may thereby be performed on this
hybrid
dot screen or listed lookup table. The stochastic screen function may be of a
nature to emulate blue noise or many other systems of generating random
screens
to fill in dot quadrants may be used, as described above and below.
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The absence of low frequency components in the frequency domain
corresponds to the absence of "disturbing artifacts" in the spatial domain
(meaning
the actual appearance of the dot profiles when printed). While the hybrid dot
system of the present invention will result in ordered matrix dot filling,
desirable
outputs are obtained using the hybrid dot system. The cutoff frequency fg,
which is
termed the Principal Frequency, depends as follows on the gray level g:
~g- /Rforg<-2
fg 1
41-g/Rforg > 2
where R, as before, is the distance between addressable points on the display
and
the gray level g is normalized between 0 and 1. According to this formula, fg
achieves its maximum value where g=1/2 (50%), since at that level the
populations
of black and white dots are equal and thus very high frequency components
appear in the binary image. It is at this gray level that would appear the
most
difficult location to attain dot profiles without disturbing artifacts.
In one example, a stochastic screen function may be generated according to
a number of steps proposed in the Article "Modified approach to the
construction
of a blue noise mask":
1. Set the number M of pairs of 1's and 0's to be swapped in
each iteration.
2. Rotate the 1-D filter with anisotropy to make the 2-D
filter.
3. Create the initial binary pattern for level gl+Og by
converting randomly KO's to l's in the binary pattern for
gi (where K=W x w/L, W x W is the size of the BNM and
L is the total number of levels).
4. Take the FFT (fourier transform) of the binary pattern for
level g,+Ag.
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5. Filter the current binary pattern with the 2-D filter
appropriate for level g,+Og.
6. Take the IFFT (inverse fourier transform) of the filtered
pattern.
7. Form an error array by computing the difference between
the filtered pattern and g,+Ag.
8. Sort the errors into two cases:
For the K1's that are in the binary pattern for
level gj+Og but not in the binary pattern for g,,
sort the positive errors.
For the 0's in the binary pattern, sort the negative
errors.
9. Swap the M pairs of 1's and 0's that have the highest
positive errors and negative errors.
10. Compute the MSE (mean square error) of the filtered
pattern with respect to the gray level gl+Ag. If the MSE
drops, go to step 5 and proceed to the next iteration. If
the MSE increases but M*1, reduce M by half, go to step
5. Otherwise, go to step 12.
11. Update the mask:
,
mii,j] = mi,A+ 6P[i, 1, 5. +Og
where the bar is the NOT operation.
12. If g,+Og<255, let g,=g,+Og reset M, and go to step 2.
This further modified approach for the generation of a blue noise-emulating
function can be enhanced by performing additional steps. For example, the
dynamic range does not work well in some printer hardware system
implementations. The method was not designed to be used as part of a hybrid
dot
CA 02218122 1997-10-08
as required in the present invention. By way of further example, the
aforementioned method does not relate the amount of residual low frequency
power in executing the error decision when determining the swaps made at each
level. Lastly, the algorithms may not be readily adaptable for automatic
execution
on a computer.
The hybrid dot system preferably includes a modified iterative stochastic
screen function generated according to the following steps:
A. Generate a stochastic function with the steps proposed above.
(Equal numbers of pixels are turned on in each step.)
B. Take the L* (luminance or "lightness") measurement of the
resultant stochastic function.
C. Invert the measurement curve so that the output L* curve is linear
with respect to digital count.
D. Use the inverted curve to determine the number of pixels to turn
on at each level.
E. Generate the first level bitmap as "seed".
F. Starting at level above, calculate the number of pixels to turn on at
the current level according to the inverted L* curve.
G. Use the numbered steps outlined above to identify the locations
with highest DC level and pixel value of 0.
H. For the number of pixels to be turned on at the current level, turn
on pixels at locations with highest DC level in a descending order.
1. Sort pixels that is currently off (0) in a descending order of DC
level. Repeat the same procedure on pixels that are turned on (1)
at this level.
J. Swap N pixels on each list
K. If the resultant DC level of the bitmap decreases, repeat step G.
Otherwise, divide N by 2 and repeat step G. If N=1 restart loop
with half of the pixels to begin with. If the starting value of N=2,
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repeat step G until the DC level of the bitmap reaches a steady
state.
L. Take the FFT; look for maximum DC levels within transform
range. Look up number of pixels to be added to the next
level from the step D inverted curve.
M. Add pixels to positions of highest DC value in descending order.
Go to step G and repeat.
In this manner, an improved stochastic function can be generated for
use in the hybrid dot of the present invention. Several important aspects of
the improved methods outlined above enable optimization of the function to be
used in the hybrid dot. First by actually measuring luminance on a sensing
device as set forth in step B, the outputs of the screen can be known and its
performance optimized according to the printing hardware (such as the 4900)
that the screen will be used on. Further, steps B-F involve a summation
operation that by using the inverted curve permits the creation of a more
linear (consistent) output. Additionally, the DC levels (step K) are placed in
a
buffer, at which time the repeatability of the results can be established,
such
that "steady state" conditions may be identified and checked. Finally, the
system employs a repeating loop logic that permits the levels of the improved
screen to be built automatically.
Figure 2 shows a halftoning menu system which in itself can employ an
imaging system including a further "hybrid" imaging system including multiple
types of screening or imaging techniques in generating or rendering black and
white or color images. For example, black and white image halftoning might
be performed such that at level 1, that is, 0 to x' gray levels hybrid dot
screening according to the system outlined above with regard to Figure 1 may
be performed. With regard to level 2, that is, x' up to 256 gray levels quad
dot
screening according to U.S. Patent No. 5,291,296 may be used. By way of
further example according to the system set forth in Figure 2, if only 3 color
print imaging is employed, certain colors may be halftoned according to
designated gray levels while other colors are halftoned according to a
different
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gray level screening technique. Figure 2 also shows an embodiment in which
cyan
and magenta halftoning is performed at two different levels by two different
halftoning systems. At level 1, that is, 0 to y', gray level hybrid dot
screening may
be performed, whereas at gray level 2, that is, y' to 256 gray level, an error
diffusion system is used to halftone these gray levels. As further shown in
the
example of Figure 2, for yellow images, halftoning is completed by a single
method
(hybrid dot screening) for all gray levels. When undercolor removal is used
(that is,
black toner is used to darken the output image to the correct level so as to
lower
colored toner use levels), the entire Figure 2 system may be used. The Figure
2
color image halftoning scenarios can be modified in a variety of situations in
accordance with the spirit of the present invention. In the color imaging
breakdown portion suggested, the Figure 2 example highlights such concepts as,
for example, that for a lighter color such as yellow, the imaging system may
be less
critical to the output of the final halftoned image. For certain (such as
darker)
colors, the halftoning system used may be more critical and have a greater
influence over the quality of the image generated at particular gray levels.
In accordance with the system described in association with Figure 2, a
variety of modifications are envisioned such that the quality of the final
rendered
composite (1-4 color) halftoned image is maximized. Modifications on the
hybrid
imaging system of the present invention may be used to reduce the occurrence
of
undesirable image artifacts such as contouring in the highlight regions,
noisiness of
halftone images through all gray levels and other undesirable effects may be
employed. The hybridized use of multiple screening techniques capitalizes on
the
fact that certain gray level ranges can mean more desirably halftoning with a
particular screening technique. Finally, in some instances, when a particular
color
or image type is being gray scaled (such as yellow), the system recognizes
that the
most efficient (and simplified) gray scale imaging technique can be used
without
detriment to the image quality of the final composite image.
The hybrid dot thresholding system of Figure 1 has been shown to be
particularly useful as an imaging option in four color printers, such as in
the Xerox
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4900 family of printers. The 4900 or other networkable or as a stand-alone
printers
or copiers may permit users to select between several halftoning options. In
one
embodiment, a 128 x 128 (N X M) size stochastic or blue noise emulating
function
may be used in conjunction with the 2 X 2 ordered dot matrix to achieve a
desired
output of 256 gray levels in both black and white or color implementations. A
linearized scaled function permits projection into the hybrid dot array
according to
a sorted list of matrix dot coordinates. The resultant method can yield
quality dot
patterns across the gray scale. A single hybrid dot halftoning system may be
used
for each color, or different thresholding (hybrid dot or other) systems may be
used.
While the ordered filling of this ordered dot system can result "checkerboard"
effects (particularly at or near the 25%, 50% or 75% dot fill areas), this
effect can
be quite desirable in many imaging scenarios, (or other halftoning systems may
be
used to prevent particularly undesirable outputs that might be generated as a
result
of this system as shown in Figure 2).
In traditional halftone technology, there may often be a tradeoff between
the use of the standard number of gray levels (256) in a dot profile and the
spatial
frequency of the screen. If the resolution of the dot was high, then it
sacrificed the
number of gray levels (i.e., the number of micro dots in a halftone cell). The
most
common dot growth pattern is known as a clustered dot. This type of screen
grows
out from the center of the dot as the gray levels increase. Several
alternatives to
this tradeoff have been employed with varying success. For instance, a matrix
dot
that is several times larger than a traditional cluster dot, but has multiple
centers
can be used to achieve a larger number of levels without sacrificing spatial
resolution. The Xerox quad dot (U.S. Pat. No. 5,291,296) discussed above is an
example of a multi centered dot with 4 centers and the ability to have 4 times
as
many levels. This dot can significantly improve image quality. There is a
limit to
the number of centers one can design into this dot because low frequency
artifacts
become more apparent as the dot grows. Another approach to halftoning is error
diffusion. Here, a gray level is thresholded and the error between the
threshold
and the gray level is distributed downstream to neighboring pixels. This
technique
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is very good at rendering pictorial images, although it exhibits "worm" like
structures and is computationally intensive to implement. Typically, extra
hardware is necessary to perform error diffusion in a timely manner.
The Figure 1 system provides desirable halftoning results particularly for
pictorial images is stochastic screening. The halftone cell is relatively
large and
affords many gray levels, and avoids problems that can occur with a large dot
by
minimizing visual structure within the dot itself. In some scenarios,
stochastic
screens are used wherein the constraint to minimize visual structure is to
generate
high frequency noise with the principle frequency in a relatively insensitive
region
of the human visual contrast sensitivity function. Such masks typically appear
to
be visually noisier than structured dots, although the increased latitude in
number
of levels makes them valuable. The selective combination of several
thresholding
techniques can potentially solve the problems of low frequency structure of
multi
centered dot and the perceived noisiness of stochastic by utilizing several
halftoning system on a selective basis, depending on gray level thresholds or
ranges. For example, a quad dot system may be merged in such a way with
another imaging technique that the advantageous properties of both approach
may
be used to create a hybrid significantly better than one system or another.
The
present invention allows different screening or thresholding systems to be
selectively used to render image areas according to gray level. Further,
different
object types may also be rendered differently. (The term "object" as used
herein
generally refers to image types including text, graphics and bitmaps, although
other
classes or types of objects may differentiated in other systems.) For example,
graphic objects often desirably may be output with vivid, bold colors than
other
types of images, while photos generally need to have less dramatic, more
natural
tones applied to produce a more desirable output. A particular desired
colorization
scheme may be related to the halftoning system used. The limitations and/or
characteristics of particular print engines can also dictate which halftoning,
screening or thresholding systems are most useful in rendering a multi-object
document or image.
CA 02218122 1997-10-08
Figure 3 shows an exemplary user interface hybrid halftoning selection
system, in which a "Hybrid Imaging System Preset Menu" is used to display
select
the various available gray level rendering options A user can proceed to
display a
graphical representation of one or more presets on screen display 100, and
then
select for implementation a preloaded gray level rendering preset from the
available menu selections. Gray Scale Level Rendering Preset display portions
(shown as "Preset" #1, 2 and 3 in Fig. 3) can be used to graphically represent
the
use of two or more in which one imaging technique transitions to (or merges
with)
another, as described in greater detail below. Some of the types of rendering
techniques that may be selected according to the Figure 3 menu are as follows:
Matrix Dot: Stochastic or random noise dot in an ordered 2x2 matrix (1,3,2
4 order), often useful in a variety of higher density color images.
Cluster Dot: Larger cluster of dots providing increased gray levels often
useful in a variety of higher density color images.
Random or Stochastic Dot: Randomly generated noise dot (without ordered
matrix); useful certain applications such as pictorials and graphics.
Quad Dot: Versatile halftoning method; often best for medium to high
density images.
White Noise: Generally useful for lower density applications.
Blue Noise: Also generally useful for lower density applications.
Error Diffusion: Versatile halftoning method; often used for non-text images.
Combinations and variations on these techniques may also be used, such as are
included in selection #1 shown on the of the representative menu screen of
Figure
3. The descriptions provided above are intended to be exemplary of the type of
information to be provided to a user on the display, and while being useful in
some situations, may not hold true in many imaging applications.
In some imaging applications, a single size cluster halftone screen simply
does not provide enough simulated gray levels. Larger sized dot cluster
systems
may result in more pleasing/higher quality rendered images. The present
invention
includes a system in which one imaging system is transitioned to another
imaging
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system, whereby a rendering system (such as clustered quad dot) is
effectively merged with a second imaging system (such as white noise) to
provide relatively contour-free transitioning from one imaging system to
another.
In the selection #1 example of Figure 3, a Cluster Quad Dot rendering
system is merged with a White Noise rendering system by of the present
invention. For example, beginning (in a descending manner) at gray level 65,
pixels are removed from a cluster dot rendering system according to a white
noise pattern. In this manner, additional gray levels are added to the cluster
dot rendering system through this merging of one rendering system with
another. In this manner, in the transition zone, the two imaging techniques
are related related to each other; that is, the transition zone join the two
imaging techniques so as to include aspects or traits of both. By way of
example, as pixels are removed from a cluster dot rendering system at gray
level 65 according to a white noise pattern (Preset #1 of Fig. 3) just below
gray level 65, as only a few dots in the clusters are removed, the imaging
technique is a closer relative of the cluster dot rendering system than the
white noise rendering system it gradually transitions to. Distinctive and
undesirable contouring or" stepping" can be thus be avoided or minimized.
U.S. Patent No. 5,740,279 entitled "Cluster Dot Halftoning System" also
assigned to Xerox, describes in relevant part an optimization technique that
can be used to in the transition region described above. This implementation
of the system of the present invention provides that pixels are turned-on in
clusters but the clusters themselves are "selected" based on a optimized
stochastic or randomized screening system. In traditional stochastic screen
design, it is the turn-on order of each pixel that is stochastic instead of a
cluster of pixels. In particular, the screen of the present invention is
useful to
eliminate moire patterns as well as to provide for a more stable imaging
platform (that is, a process that will be less prone to resulting in
undesirable
image outputs such as may occur in xerographic or other types of print
engines). The present invention thus permits the combination of two or more
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imaging systems so as to capitalize on the advantages and/or minimize the
weaknesses of each imaging technique. A variety of halftone screens may be
used
in conjunction with the present invention, to include the examples described
or
incorporated herein by reference above.
The Figure 4 flow chart provides a general description of how a hybrid
imaging system is assembled. composite screen system 100 may be assembled
according to one embodiment of the present invention (as will later be
described
in additional detail.) A composite screen for halftoning a digitized image is
constructed according to Figure 4. In a first or preliminary step, the
multiple
halftoning systems to be used to create a hybrid or composite screen (such as
the
"Gray Scale Level Rendering Presets" 1, 2 and 3 shown in Fig. 3). Figure 4
shows
this step as the block "Select a first halftoning system and a second
halftoning
system". Next, the gray scale transition region or start point for
transitioning from
the first halftoning system to the second halftoning system must be
established.
This step is shown as the "Define the gray scale transition region" block in
Figure
4. In the next step, the first halftoning system is merged with the second
halftoning
system so as to provide a hybrid halftoning system in the gray scale
transition
region. In order to insure a smooth transition of the first halftoning system
having
what may be called a first set of pixel placement characteristics to the
second
halftoning system having a second set of pixel placement characteristics. This
step
is shown as the "Merge the first and second halftoning systems in the
transition
region" block of Figure 4. The transition region must, as described in the
examples
above, include characteristics or traits that are a hybrid or combination of
these
two systems. Thereafter, the image is rendered with the composite or hybrid
image
system. according to the "Render Image with Hybrid Image System" block of
Figure 4.
While present invention has been described in conjunction with various
embodiments, it is evident that many alternatives, modifications, and
variations
will be apparent to those skilled in the art. Accordingly, it is intended to
embrace
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all such alternatives, modifications, and variations as fall within the spirit
and
broad scope of the appended claims.
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