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BACKGROUND OF THE INVENTION
The invention relates to image uniformity and compatibility, and more
particularly, to image uniformity and compatibility by the use of digital
darkness control or pixel stretch techniques in electronic imaging devices
regardless of differences in development systems.
Resolution conversion is well known in the prior art. For example, Sharp
U.S. Pat. No. 3,573,789 shows a resolution conversion technique by
shifting each pixel with surrounding pixels into a resolution expander
that automatically produces a multiple number of pixels corresponding to
the center pixel. Image enhancement techniques to improve the quality of
the image are also well known. For example, Coviello U.S. Pat. No.
4,450,483 statistically analyses a pixel with its surrounding pixels to
make a determination whether or not the center pixel should remain as a
black or white pixel or be changed to either a black or white pixel to
improve the quality of the overall image. Walsh U.S. Pat. No. 4,437,122
does image enhancement of a digital image by taking each pixel of the
digital image and a neighborhood surrounding the pixel and comparing this
pattern to a set of reference patterns. Depending upon the match between
the pixel and its neighborhood with a particular pattern, the center pixel
is expanded into a plurality of predetermined pixels enhancing the overall
quality of the image.
An ongoing difficulty in the art of recreating and reproducing images on a
medium is the desirability of recreating the original image as close as
possible regardless of the particular system used in recreating the image.
In other words, it is important to match the images or have the images
look alike regardless of the particular system in a given machine that is
used in developing the image. Because of variances in development systems
such as magnetic brush development, cascade development, and liquid
development, the reproduced image will necessarily take on different
characteristics in the reproduction of portions of the image in such
characteristics as line width and solid area development. In addition,
within a particular development system itself, the reproduction of a
likeness of an original can be further altered by settings such as various
degrees of copy quality such as normal, copy light, or copy dark. It is
also a challenge in the prior art with multiple font types to be able to
closely approximate electronically stored fonts in the reproduced image or
text. This often involves painstaking trial and error of electronic
representations of fonts and the analysis of the reproduced image. This
process is, of course, further complicated, as mentioned above by
different development processes that effect the reproduced image.
It would therefore be desirable to provide an electronic adjustment to an
original image in order to compensate for various development systems in
the prior art as well as to compensate for quality settings within a given
development system. It would also be desirable to compensate for the use
of different fonts and to be able-to match fonts within a reproduction
system regardless of the font used and also taking into account the
degrees of difference in development systems. It would also be desirable
to compensate for variations in prior art systems by the use of an
electronic adjustment to an original image that is two dimensional, that
is, it can be made in the direction of a scanning beam or in the direction
of movement of a medium in relation to the beam.
It is an object of the present invention, therefore, to provide a new an
improved technique for improving reproduced images regardless of the
development system and regardless of the font. It is another object of the
present invention to improve the line delineation of reproduced images by
adding pixels or partial pixels in both the X and Y- direction regardless
of development characteristics. It is still another object of the present
invention to be able to change the size of partial pixels in recreating an
image in order to compensate for degrees of settings of the development
system. Other advantages of the present invention will become apparent as
the following description proceeds, and the features characterizing the
invention will be pointed out with particularity in the claims annexed to
and forming a part of this specification.
SUMMARY OF THE INVENTION
The present invention is concerned with a printing machine having an
imaging surface, a scanning system for modulating a beam and scanning an
image onto the imaging surface, a device to designate a relative darkness
factor, a store for holding a partial array of the image to be reproduced,
a comparator for relating the partial array of the image to be reproduced
with standard reference formats to produce correlation signals, modulating
logic responsive to the correlation signals and the relative darkness
factor to provide timing adjustments, and a modulator for modulating the
beam and scanning the image onto the imaging surface in response to the
timing adjustments.
For a better understanding of the present invention, reference may be had
to the accompanying drawings wherein the same reference numerals have been
applied to like parts and wherein:
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a raster output scanner incorporating the
present invention;
FIG. 2 illustrates digital darkness control in both the fast scan and slow
scan directions in accordance with the present invention;
FIG. 3 illustrates the digital darkness control technique to enhance image
quality using a 2.times.2 pixel array in accordance with the present
invention;
FIG. 4 illustrates a real time hardware implementation of the digital
darkness control technique;
FIG. 5 illustrates a 3.times.3 pixel array corresponding to a set of
decoding rules in accordance with the present invention;
FIGS. 6a-6d illustrate a typical darkness adjustment for a given pixel
matrix;
FIGS. 7a-7d illustrate another typical darkness adjustment for a given
pixel matrix; and
FIG. 8 is a flow chart illustrating digital darkness control in accordance
with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is applicable to a wide variety of electronic imaging
or printing systems such as typical laser based printing systems. Such
systems may often be suitably divided into a scanner section, a controller
section, and a printer section. While a specific printing system may be
described, the present invention may be used with other types of printing
systems such as ink jet, ionographic, etc.
The printer section typically often comprises a laser type printer
separated into a Raster Output Scanner (ROS) section, Print Module
Section, Paper Supply section, and Finisher. With reference to FIG. 1, the
ROS includes a laser 8 with beam 10 shaped by optics 12 and split into two
beams 8a and 8b by beam splitter 14. Each beam 8a, 8b is modulated at 16a,
16b in accordance with the content of an image signal input by an
acousto-optic modulator to provide dual imaging beams scanned across a
moving photoreceptor 18 by the mirrored facets 20 of a rotating polygon 22
to expose two image lines on the photoreceptor with each scan. This
creates the latent electrostatic images represented by the image signal
input to a modulator. Photoreceptor 18 is uniformly charged by a corotron
at a charging station preparatory to exposure the imaging beams. The
latent electrostatic images are developed and transferred to print media
delivered by a suitable paper supply section.
The print media, may comprise any of a variety of sheet sizes, types, and
colors. For transfer, the print media is brought forward in timed
registration with the developed image on photoreceptor from suitable paper
trays. The developed image transferred to the print media is permanently
fixed or fused by a fuser and the resulting prints discharged to either an
output tray or to a finisher. Again, it should be understood that it is
within the scope of the present invention to be applicable to any suitable
projecting or imaging receiving system such as ionographic or ink jet.
A typical controller for such a machine is divided into an image input
controller, User interface (UI)-controller, main memory, image
manipulation section and image output controller. Scanned image data is
compressed by an image compressor, segmented into slices N scanlines wide,
each slice having a slice pointer. The compressed image data together with
slice pointers and any related image descriptors providing image specific
information (such as height and width of the document in pixels, the
compression method used, pointers to the compressed image data, and
pointers to the image slice pointers) are placed in an image file. The
image files, which represent different print jobs, are temporarily stored
in a system memory which comprises a Random Access Memory or RAM pending
transfer to the main memory where the data is held pending use.
The User Interface often includes a combined operator controller/CRT
display consisting of an interactive touchscreen, keyboard, and mouse, and
interfaces the operator with the printing system enabling the operator to
program print jobs and other instructions, to obtain system operating
information, instructions, programming information, diagnostic
information, etc. Items displayed on the touchscreen such as files and
icons are actuated by either touching the displayed item on the screen
with a finger or by using a mouse to point a cursor to the item selected
and keying the mouse.
When the compressed image data in main memory requires further processing,
or is required for display on the touchscreen or is required by printer
section, the data is accessed in main memory. Where further processing is
required, the data is transferred to the image manipulation section where
the additional processing steps such as collation, make ready,
decomposition, etc. are carried out. Following processing, the data may be
returned to main memory or sent to the image output controller.
Image data output to image output controller is decompressed and readied
for printing by image generating processors. Following this, the data is
output by suitable dispatch processors to the printer section. For
additional detail, reference is made to U.S. Pat. No. 5,081,494 and
4,686,542 incorporated herein.
With respect to FIG. 2 in accordance with the present invention, there is
illustrated a portion of a scanned image, each square representing either
a black B or white W pixel. As shown in FIG. 2, there is a 4.times.4 black
pixel area surrounded by a white pixel area. The process of reproducing an
image that is a faithful reproduction of the original image is difficult.
For example, the original image is converted to light and dark spots that
are used to modulate a laser beam scanning an image receiver. The image on
the receiver is then developed with toner and transferred to a copy sheet
for fusing. Within this operation, there are inherent difficulties in
accurately reproducing a true replica of the original image. For example,
often times black lines or black areas can be too constricted or too wide.
For example, assume that FIG. 2 represents a correct reproduction of the
image as digitally recorded and delivered to the modulating circuitry.
However, often times, the finished result is not the best reproduction in
either the X-direction, considered to be the fast-scan or beam-scan
direction or in the Y-direction, the slow-scan or movement of the receiver
belt with respect to the laser beam direction.
Assume that the black line width in the X-direction is too narrow and it
would be desirable to increase the width of the black area in the
X-direction. By suitable modulating of the laser scanner, as the laser
scans in the X-direction, the width of the black line can be increased in
the X-direction. This is accomplished by extending the modulation of the
beam for a black dot for a portion of time-the beam is sweeping an area
that should be a white dot. For example, assume that the time length or
period for scan of a pixel in the X-direction is 22 nanoseconds. Then, by
extending the black pixel for a period of time, for example, 10
nanoseconds, into a white pixel area, it is possible to extend the black
area and in effect widen the black line in the X-direction. This is
illustrated in FIG. 2 with respect to the partial black dot areas PBI as
shown in the normally white pixel areas. It should be noted that the same
effect could be achieved by initiating the black pixel area in the white
pixel area preceding the black area. It should also be noted that the same
principle applies to decreasing the width of a black line in the
X-direction by merely decreasing the time period that the scanning beam is
modulated for a black pixel.
In a similar manner, there can be a change in modulation of the laser beam
in the Y-direction or the slow-scan direction as illustrated. In this
case, the effect of an increase in the width of the black line in the
Y-direction is shown as being achieved by scanning partial black dots PBZ
as illustrated. That is, partial black pixel is in each next adjoining
pixel area to the black pixels in the Y-direction are provided. Typically,
a 10 nanosecond period for the black pixel in the X-direction is provided
for the entire area of the pixel area in the Y-direction. Although this
may appear to be disjointed, the overall effect on the human eye on a
developed image is to extend or widen the perceived black line in the
Y-direction.
Simple logic can be used to process the pixels. For example, with reference
to FIG. 3, there is illustrated two laser scan lines showing four pixel
elements A, B, C and D. A scan line buffer of the previous scan line, in
this case, line 1 as shown, can be provided as well as a bit buffer of the
previously printed bit, in this case the bit buffer holding pixel C from
line 2. This allows the pixels to be stretched or constricted in two
dimensions as illustrated with respect to FIG. 2.
Thus, in the 2.times.2 illustration of FIG. 3, in the X or fast-scan
direction in order to increase or partially stretch the black pixel, the
following logic can be used. If D is white and C is black, then extend D
fractionally beyond C. This can be considered an X-adjust. If D is white
and C is white and A or B is black, then make D a fractional black pulse
(Y adjust). In all other situations D=D. These X and Y fractional pulses
could be different or they could be the same size and adjusted with one
control. The 2.times.2 pixel array could be expanded to a 3.times.3 array
or larger. This would require additional buffering of scan lines and
preferably a table look-up.
In accordance with the present invention, FIG. 4 illustrates a typical
hardware implementation of the pixel alteration scheme. To provide a
3.times.3 pixel array, it would be necessary to buffer 3 lines of data as
illustrated in FIG. 5. The 3.times.3 array illustrated in FIG. 5 would be
compared with look-up table 140 and the results being a two-bit output, 00
being a white pixel, 11 being a black pixel, 10 being a delayed pixel in
the X-direction and 01 being a delayed pixel in the Y-direction. The
results of the look-up table would be buffered as shown in 142 and decoded
and modulated as illustrated at 144 to provide the suitable video output
signal through gate 146.
The following rules have provided excellent results in generating images in
accordance with the above-described scheme for a 3.times.3 matrix as shown
in FIG. 5.
If pixel E is black, then make the output 11.
The output is 10 or an X-adjustment, if D is black and E, I, F, C, or H are
white, or E, I, F, C, and B are white. A 01 or Y-adjustment is made if A
is black and E, I,C,F,H,G, and D are white.
Or if D, B and C are black and E, F, G, H, and I are white. Or D, H and I
are black and E, F, A, B, and C are white.
Or if B is black and E, I, G, H, and D are white.
Otherwise the output is white or 00
The modulation of the video beam is also responsive to a relative darkness
factor logic as shown at 145. It is well known in the art to compensate
for high or low density originals, such as a light original. In the prior
art, compensation was achieved by changing the electrical bias in the
development system. For example, in a typical magnetic brush development
system, to allow for a high density original, the developing bias on the
magnetic brushes would be set higher than normal. For low density images,
the development bias would be less than normal. By analogy, these same
type of adjustments can be done by modulating a laser beam to provide
partial pixels in the reproduced image. A density level or darkness factor
is converted to modulation timing to provide these partial pixels in both
x and y-directions. Thus, the darkness factor logic 145 adjusts X delay
circuitry 152 and Y delay circuitry 154 as appropriate.
In accordance with the present invention, for each pixel of the original
image, a series of decisions must be made. The first decision is whether
or not a change is to be made to that particular pixel. If a change is
required, the next decision is whether or not to make the change in the X
or the Y-direction. Finally, a decision is made on the amount or size of
change in either the X or the Y-direction. A component in the amount of
change is the relative darkness factor. To make this determination, each
pixel and a neighborhood of pixels surrounding the target pixel are
examined. For purposes of explanation, it will be assumed that a 3.times.3
matrix of pixels is examined with the target pixel or the pixel to be
changed being the center pixel. It should be understood that any matrix or
area of pixels is contemplated within the scope of this invention to be
analyzed such as a 2.times.2 matrix a 4.times.4 matrix or any other
appropriate number of pixels surrounding a target pixel.
With reference to FIG. 6a, there is disclosed a 3.times.3 matrix of pixels
of original image, in this case the three pixels in the top row being
black as designated in FIG. 6b B.sub.1, B.sub.2 and B.sub.3, the pixels in
the middle row being white as designated by W.sub.1, W.sub.2, and W.sub.3,
and the three pixels in the bottom row of the matrix also being white as
designated by W4, W5 and W6. As is well known in the prior art, the pixels
to be analyzed are stored in a suitable buffer registers with the black
pixels B.sub.1, B.sub.2 and B.sub.3 representing the appropriate pixels or
the image to be scanned in the line immediately above the target pixel,
the pixels W.sub.1, W.sub.2, and W.sub.3 representing the three pixels in
the line of the target pixel W.sub.2 immediately below the pixels of line
1, and the pixels W4, W5 and W6 representing the appropriate pixels from
the line immediately below the line containing the target pixel.
In accordance with the present invention, the adjustment to each center
pixel, if any, will be determined by the nature of the pixels surrounding
the center or target pixel. This is done in accordance predetermined rules
or logic for each 3.times.3 matrix situation. The configuration as
illustrated in FIG. 6b dictates that the center or target pixel W2 be
partially changed to black in the Y-direction, as illustrated in FIG. 6c.
As mentioned above, there is another decision as to the size or degree of
change of the target pixel. Whereas the decision to change in either the X
or Y-direction is a function of the target pixel and the neighboring
pixels, the size of the change is dependent upon the development device
within the particular reproduction system. This can be a function of two
variables, one being the particular development system such as a magnetic
development system or cascade development system and is predetermined. The
other variable is the degree of setting such as copy light or copy dark
within a particular development system. FIG. 6c illustrates the degree of
change for a particular development system with a degree of darkness
setting that is relatively high, and FIG. 6d illustrates the amount of
change of the target pixel for a degree of darkness setting that is
relatively low. Thus, FIGS. 6c and 6d illustrate the degree of difference
of a partial pixel depending upon the degree of setting for a particular
development system.
In a similar manner changes can be made in the X-direction with the amount
of change being primarily a function of a relative darkness setting. With
reference to FIGS. 7a thru 7d there is illustrated a typical scenario for
a matrix of pixels. In this particular case, the top row of pixels are B1,
W1 and W2, the second row are B2, W3 and W4, and the bottom row B3, W5 and
W6. As shown in FIG. 7b this particular configuration dictates the
direction of change to be in the X-direction. FIGS. 7c and 7d illustrate
the degree of change or the different darkness settings, in particular a
relatively dark setting in FIG. 7d and a relatively light setting in FIG.
7c.
FIG. 8 is a flow chart showing digital darkness control. A matrix of pixels
is evaluated as illustrated in block 101. A determination of whether or
not there is an x-direction change is made at block 103. If there is an
x-direction then, there is a determination of the degree of modulation
adjustment in the x-direction at 105 applied to modulator 107. In a
similar manner there is a determination of whether there is a y-direction
change at block 109, if so, the proper modulation adjustment is determined
at 111 and applied to the modulator at 107. The X-modulation adjust 105
and Y-modulation adjust 111 also receive a copy density or relative
darkness adjustment shown at 113. After each pass for a target pixel, the
matrix is indexed to analyze the next target pixel and its neighbors shown
at 119.
While there has been illustrated and described what is at present
considered to be a preferred embodiment of the present invention, it will
be appreciated that numerous changes and modifications are likely to occur
to those skilled in the art, and it is intended to cover in the appended
claims all those changes and modifications which fall within the true
spirit and scope of the present invention.
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