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BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to encoding pictorial imagery for
reproduction on binary display and/or printing systems, and more
particularly to increasing the number of discernible gray levels in
halftone reproduction.
2. Background Art
Representation of the intensity, i.e., the gray level, of a color by binary
displays and printers has been the object of a variety of algorithms.
Binary displays and printers are capable of making a mark, usually in the
form of a dot, of a given, uniform size and at a specified resolution in
marks per unit length, typically dots per inch. It has been common to
place the marks according to a variety of geometrical patterns such that a
group of marks when seen by the eye gives a rendition of an intermediate
color tone between the color of the background (usually white paper stock)
and total coverage, or solid density.
Continuous tone images contain an apparent continuum of gray levels. Some
scenes, when viewed by humans, may require more than 256 discrete gray
levels to give the appearance of a continuum of gray levels from one shade
to another.
As an approximation to continuous tone images, pictorial imagery is
represented via halftone technologies. In order to record or display a
halftone image with a scanning system, one picture element on the
recording or display surface consists of a j.times.k matrix of
sub-elements where j and k are positive integers. A halftone image is
reproduced by printing the respective sub-elements or leaving them blank.
That is, by suitably distributing the printed marks.
Halftone image processing algorithms are evaluated in part, by their
capability of delivering a complete gray scale at normal viewing
distances. The capability of a particular process to reproduce high
frequency renditions (fine detail) with high contrast modulation makes
that procedure superior to one which reproduces such fine detail with
lesser or no output contrast.
Another measure of image processing algorithm merit is the tendency to
produce visual details in the output image that are not part of the
original image, but are the result of the image processing algorithm. Such
details are called artifacts, and include moire patterns, false contours,
and false textures. Moire patterns are false details created most often by
the beating between two relatively high frequency processes resulting in a
signal whose spacial frequency is low enough to be seen by the viewer.
False contours are the result of gray scale quantization steps which are
sufficiently large to create a visible contour when the input image is
truly a smooth, gradual variation from one to the other. False textures
are artificial changes in the image texture which occur when input gray
levels vary slowly and smoothly and the output generates an artificial
boundary between the textural patterns for one gray level and the textural
patterns for the next gray level.
Briefly, several of the commonly used processing algorithms include fixed
level thresholding, adaptive thresholding, orthographic tone scale fonts,
and electronic screening. The present invention is concerned with the
latter, electronic screening.
FIG. 1 shows a schematic view of the electronic screening process. Signal
X.sub.i represents the lightness or gray level information at a sampling
point i of an image. Input signal X.sub.i of sample image picture elements
is compared with a series of threshold values C.sub.i selected in
sequential order from a two-dimensional matrix defined to be the halftone
cell threshold set, and a print/no-print decision is made. The series of
threshold values and their arrangement within the threshold set determine
the gray scale range, the frequency, angle, and other properties of the
halftone pictorial image. Each threshold level C.sub.i is determined by a
comparison j.times.k matrix. When the input signal X.sub.i exceeds the
threshold level C.sub.i, the corresponding sub-element is determined to
have a print level or logic level "ONE". By comparing the input signal
X.sub.i with the threshold levels, j.times.k output signals O.sub.i are
produced. A density pattern consisting of a combination of j.times.k
sub-elements is obtained by dividing each picture element into j.times.k
sub-elements and systematically printing them or leaving them blank.
FIG. 2 shows a typical two dimensional matrix halftone cell for electronic
halftoning with 18 possible gray levels, used as a 45.degree. angular
screen. When the cell is repeated horizontally and vertically, it creates
the entire screen function. FIG. 3 shows the possible "nonwhite" halftone
sub-elements which may be generated by the screen function of FIG. 2.
A problem exists with the number of density levels attainable with a
limited resolution and acceptable screen frequency. A 94.5 lines per inch,
45.degree. screen using a 400 dpi system results in nineteen level
halftoning, including white. Nineteen levels is not generally sufficient;
more gradations being preferred. One way to get more gray levels is to
reduce the number of lines per inch, but this decreases the screen
frequency to a visible level.
Various screen functions have been proposed for electronic screening to
minimize the number of gray levels required to manifest acceptable
pictorial imagery. These existing types of screen functions, also referred
to as "threshold value matrices," are roughly divided into the following
two groups: (1) those in which sub-elements grow around the center core
and (2) those in which the spatial frequency of the sub-elements is made
to be as high as possible.
Group-1 screen functions are generally known as "fattening" or "dot
concentration" type functions. FIG. 4 is a 4.times.4 group-1 matrix. As
shown in FIG. 5, sixteen gray levels (plus all white) are obtained by
sequentially increasing the number of sub-elements which are printed in
black.
Group-2 screen functions are known generally as "dot dispersion" type. The
best known dot dispersion type screen functions were developed by Bayer,
Lippel, and Jarvis. FIG. 6 is typical a 4.times.4 group-2 matrix. As shown
in FIG. 7, sixteen gray levels (plus all white) are obtained by
sequentially increasing the number of sub-elements which are printed.
DISCLOSURE OF INVENTION
It has been found that by arranging the sub-elements differently in a cell,
different apparent densities are attainable; even though the same number
of sub-elements are present in each pattern. For example, a six-dot
group-1 pattern in a 4.times.4 cell will have a different apparent density
than a six-dot group-2 pattern in a similar cell.
This is caused by the fact that in a group-2 matrix, the sub-elements are
dispersed in a manner to create a high spacial frequency so that the
overlap of the sub-elements are minimal, whereas in a group-1 matrix, the
sub-elements are grouped together so that there is greater overlap.
Therefore the sub-elements of a group-1 matrix will cover less area than
an equal number of sub-elements in a group-2 matrix.
Employing this characteristic, it is an object of the present invention to
provide image processing for rendering halftone images with an increased
number of gray level steps without increasing the number of sub-elements
in the halftone cells.
According to the present invention, a document copying apparatus includes:
means for comparing input image signals representing low optical density
picture elements of the original document to a series of threshold values
selected in sequential order from a first two dimensional matrix halftone
cell in which an output signal density pattern grows in a low spacial
frequency manner; means for comparing input image signals representing
high optical density picture elements of the original document to a series
of threshold values selected in sequential order from a second two
dimensional matrix halftone cell in which an output signal density pattern
grows in a high spacial frequency manner; and means for comparing input
image signals representing mid-optical density picture elements of the
original document to a series of threshold values selected in sequential
order from the first or the second two dimensional matrix halftone cells
according to the optical density of the picture element.
According to another feature of the present invention, document copying
apparatus includes: means for comparing input image signals representing
picture elements in low optical density areas of the original document to
a series of threshold values selected in sequential order from a first two
dimensional matrix halftone cell in which an output signal density pattern
grows in a low spacial frequency manner; means for comparing input image
signals representing picture elements in high optical density areas of the
original document to a series of threshold values selected in sequential
order from a second two dimensional matrix halftone cell in which an
output signal density pattern grows in a high spacial frequency manner;
and means for comparing input image signals representing picture elements
in mid-optical density areas of the original document to a series of
threshold values selected in sequential order from the first or the second
two dimensional matrix halftone cells according to the optical density of
the area of the picture element.
According to yet another feature of the present invention, apparatus for
reproducing a gray level image in a bi-tonal medium includes means for
comparing a picture element value with a threshold value to make a print
or no-print decision characterized by a corresponding one of two print
values; a first dot concentration type two dimensional matrix halftone
cell set; a second dot dispersion type two dimensional matrix halftone
cell set; and means for selecting the threshold value from the first set
when the picture element is of low density level; for selecting the
threshold value from the second set when the picture element is of high
density level; and for selecting the threshold value from the first or the
second sets when the picture element is of mid-density level.
The invention, and its objects and advantages, will become more apparent in
the detailed description of the preferred embodiments presented below.
BRIEF DESCRIPTION OF THE DRAWINGS
In the detailed description of the preferred embodiments of the invention
presented below, reference is made to the accompanying drawings, in which:
FIG. 1 is a schematic view of the electronic screening process known in the
prior art;
FIG. 2 is a typical electronic halftone function unit cell known in the
prior art;
FIG. 3 is a view of the possible halftone patterns of the unit cell of FIG.
2;
FIG. 4 is a 4.times.4 electronic halftone dot concentration function unit
cell known in the prior art;
FIG. 5 is a view of the possible halftone patterns of the unit cell of FIG.
4;
FIG. 6 is a 4.times.4 electronic halftone dot dispersion function unit cell
known in the prior art;
FIG. 7 is a view of the possible halftone patterns of the unit cell of FIG.
6;
FIG. 8 is a comparison of the apparent density produced by the unit cells
of FIG. 4 and FIG. 6;
FIG. 9 is a schematic view of the electronic screening process according to
a preferred embodiment of the present invention;
FIG. 10 is a schematic view of the timing of signals generated in the
screening process of FIG. 9;
FIG. 11 is a schematic view of the electronic screening process according
to another preferred embodiment of the present invention; and
FIG. 12 is a comparison of the apparent density produced by the screening
process of FIG. 11.
BEST MODE FOR CARRYING OUT THE INVENTION
The present description will be directed in particular to elements forming
part of, or cooperating more directly with, apparatus in accordance with
the present invention. It is to be understood that elements not
specifically shown or described may take various forms well known to those
skilled in the art.
It has been recognized that the apparent density produced by a group-1
matrix as a function of the number of printed sub-elements differs from
the apparent density produced by a group-2 matrix for the same number of
printed sub-elements. Further, the difference in apparent density between
the group-1 and group-2 matrices changes with the density range.
FIG. 8 is a comparison of the apparent density produced by group-1 and
group-2 matrices as a function of the number of sub-elements which are
printed. As can be seen from the figure, the slope of the group-1 matrix
curve is greater in the high density portion of the curve, and the slope
of the group-2 matrix curve is greater in the low density portion of the
curve.
Since their density slopes are less in low density regions, group-1
matrices exhibit more observable density levels, and therefore less
density contouring, at low densities than group-2 matrices because the
group-2 matrix tone scale is compressed, resulting in rapid low density
rise (high contrast). Accordingly, group-1 matrices would be preferred at
low densities. On the otherhand, group-2 matrices exhibit more observable
density levels, and therefore less density contouring, at high densities
than group-1 matrices because the type 1 matrix tone scale is compressed
at these densities, resulting in rapid high density rise (high contrast).
Accordingly, group-2 matrices would be preferred at high densities.
Accordingly, the present invention provides for mixing group-1 and group-2
matrices so that a group-1 matrix is used in low density areas of the
image and a group-2 matrix is used in high density areas of the image.
Good results have been observed when the cut between group-1 and group-2
matrices have been made at density levels of between 60 and 80 on a 255
gray level scale.
FIG. 9 is a schematic view of the electronic screening process for
switching between group-1 and group-2 matrices according to the average
density of an area. As a scan line, designated (J-1), of the input signal
X.sub.i is scanned, the electrical signals representing the density for
each successive picture element along the scan line are stored in an
analog shift register 12. As the next scan line, designated J, is scanned,
the electrical signals from that line replace those of the preceding scan
line in register 12, and the replaced signals are shifted into a second
shift register 13.
Now, as the next line, designated (J+1), is scanned, its electrical signals
replace those from line J in register 12 and are simultaneously applied
along a conductor 14 to an average 15. The signal outputs from register 12
replace those from the (J-1) line in register 13 and are simultaneously
applied along conductor 16 to averager 15. Simultaneously, the signals
from scan line (J-1) shift from register 13 along a conductor 17 to
averager 15.
It is now seen that averager 15 of FIG. 9 serially receives the electrical
signals from scan lines (J+1), J, and (J-1), with all three inputs
synchronized such that the corresponding samples from each scan line
arrive simultaneously.
Referring for the moment to FIG. 10, averager 15 of FIG. 9 compares the
amplitude of the nine adjacent samples in a two-dimensional array of
sampled information. FIG. 10 depicts samples (J+1,K+1), (J+1,K), and
(J+1,K-1) from scan line (J+1); samples (J,K+1), (J,K), and (J,K-1) from
scan line J; and (J-1,K+1), (J-1,K), and (J-1K-1) from scan line (J-1).
Referring back to FIG. 9, a comparator 20 compares the averaged density
signal from averager 15 with a reference threshold level determined on the
basis of the density level selected for switching between group-1 and
group-2 matrices. At low average density levels, the output of comparator
20 is a logical zero which, when inverted and applied to an AND gate 22,
causes the threshold level C.sub.i to be selected in sequential order from
a two-dimensional group-1 matrix 24. At high average density levels, the
output of comparator 20 is a logical "ONE" which when applied to AND gate
22 causes the threshold level C.sub.i to be selected in sequential order
from a two-dimensional group-2 matrix 28.
FIG. 8, which is not to scale, shows this graphically by plotting the
apparent density verses the number of printed sub-elements for group-1 and
group-2 matrices. For group-2 matrices, the density rises quickly at low
densities and has a lower slope at maximum densities. In the case of
group-1 matrices, the density rises more slowly at low densities and has a
greater slope at maximum densities.
It is apparent that in the mid-density range, a given number of printed
sub-elements will have different apparent densities for different matrix
types. In accordance with another embodiment of the present invention
illustrated in FIG. 11, this characteristic is used to attain an even
greater number of gray scale levels by mixing the matrices in the
mid-density range.
For example, there are illustrated in FIG. 12, six different vertical lines
representing different number of printed sub-elements. The intersections
of the vertical lines with the density curves for group-1 and group-2
matrices are brought to the density scale where, going from bottom to top,
eleven different densities are attainable by switching between matrices in
the sequence 1,2,1,2,1,1 or 2,2,1,2,1,1. It should be noted that as many
as twelve different densities are theoretically possible, but it is at
least possible that there will be overlap between a dot pattern of one
matrix and a dot pattern of the other matrix, as is seen between the third
vertical line of the group-2 matrix and the forth vertical line of the
group-1 matrix.
While it would be possible to use the mixed-dot process for the entire
range, selecting between matrices for each density; the fact that there
are more steps in the low density range for a group-1 matrix and more
steps in the high density range for a group-2 matrix can be exploited by
using concentrated dot in the low density range and dispersion dot in the
high density range. In the mid-region, both groups are mixed in accordance
with a look up table 30 of FIG. 11. The input to the table is the average
gray level value of a sample image picture elements. The look up table
will provide an output indicating the number of sub-elements to be printed
and the matrix to be used for that gray level value.
The invention has been described in detail with particular reference to
preferred embodiments thereof, but it will be understood that variations
and modifications can be effected within the spirit and scope of the
invention.
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