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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a color image processing apparatus and,
more particularly, to a color image processing apparatus for automatically
feeding one at a time a stack of a plurality of documents to process
images printed thereon.
Color copiers available today include one of the type having a single image
reproducing unit and reproducing a full-color image by repeating an
iterative copying process a plurality of times. Specifically, this type of
copier sequentially executes a copying process in each of four fundamental
colors, i.e., cyan (C), magenta (M), yellow (Y), and black (BK) (or only
C, M and Y), the resultant color components being transferred one above
another to a single recording sheet. In a monochromatic mode, the copier
reproduces a single image by a single copying process. The operator,
therefore, has to attend to the switchover of the color mode because the
required copying time greatly differs from the full-color mode to the
monocolor mode, i.e., the operator has to switch over the color mode
depending on the kind of a copy.
To free the operator from such troublesome mode switching operations, the
operation mode of a copier may be switched over automatically by
automatically determining whether a document image is color or
black-and-white, as disclosed in Japanese Patent Laid-Open Publication
(Kokai) No. 63-107274. The apparatus taught by this Laid-Open Publication
compares the densities of Y, M and C of an image signal from which an
achromatic component (black) has been removed (output of an undercolor
removing circuit) with a predetermined threshold value, and determines
that the image is achromatic or black-and-white if all the densities of Y,
M and C are extremely low. Alternatively, the discrimination of an image
as to color/black-and-white may be effected by extracting, among color
components C, M and Y of an image signal, the maximum and minimum values
and producing a difference therebetween, as shown and described in
Japanese Patent Laid-Open Publication (Kokai) No. 62-101179.
A problem with the prior art apparatuses discussed above is that the
accuracy of discrimination of an image as to color/black-and-white
(achromatic color) is too low to eliminate discrimination errors. Should
the threshold value with which image data should be compared be increased
to reduce discrimination errors, all the thin image components would be
determined to be achromatic.
The present invention contemplates to automatically discriminate an image
as to color/achromatic with high accuracy and thereby executes optimal
image processing matching the kind of an image without resorting to
operator's manipulations. To this end, the present invention provides a
color image processing apparatus comprising image reading means having
spectral filter means for reading a visible image printed on a document
sheet in a predetermined image reading area by scanning the image, color
discriminating means for automatically determining whether or not the
image is substantially monochromatic by processing image data produced by
the image reading means and separated into a plurality of fundamental
colors, edge detecting means for detecting edges of the image by
processing the image data, discrimination inhibiting means for inhibiting
the color discriminating means from operating when the edge detecting
means has detected an edge of the image, and control means for
automatically selecting and executing either one of color processing and
monochrome processing in response to a result of discrimination by the
color discriminating means.
The present invention determines whether or not a document image is
monochromatic (especially achromatic) by processing image data derived
from the document. In this type of apparatus, image reading unavoidably
suffers from the deterioration of MTF (Modulation Transfer Function) due
to the influence of a lens included in optics, for example. Specifically,
edge portions of a document image where the tone and/or color sharply
changes are blurred in a reproduction because the tone and/or color of the
edge portions changes slowly in image data. Further, this kind of
degradation of image quality appears with a different characteristic in
each of the color-separated image components (red (R), green (G) and blue
(B)). Moreover, sensors each being assigned to respective one of
color-separated image light (R, G and B) are not free from positional
deviations, so that the color components R, G and B in image data involve
phase deviations corresponding to the positional deviations of the
sensors.
The difference in MTF characteristic and the phase deviation of the color
components stated above directly translate into color deviations in image
data. For example, despite that only black (achromatic color) produced by
combining R, G and B exists on a document image, image data representative
of the document image is partly short of the color component R, G or B due
to the deviation of R, G or B with respect to position (or time). Then,
such a part will turn out to be chromatic. Thus, discriminating an image
as to chromatic/achromatic by processing image data would render colors
different from the colors of a document image. Nevertheless, it is only in
the edge portions of an image where the tone and/or color sharply changes
that erroneous discrimination is apt to occur. In accordance with the
present invention, while image data representative of a document image is
processed, the color discrimination is inhibited when an edge of the
document image is detected. This is successful in minimizing erroneous
discrimination and thereby allowing an image to be discriminated as to
color/monochrome with high accuracy.
By using a signal produced by removing an achromatic (BK: black) component
from image components Y, M and C, it is possible to promote easy
discrimination of an image as to color/monochrome. Usually, this kind of
signal is generated by an undercolor removing (UCR) circuit incorporated
in an image processing unit. Therefore, the circuit arrangement will be
simplified if the existing circuit is also usable for color/monochrome
discrimination.
To simplify the circuit arrangement for the discrimination, the present
invention provides a color image processing apparatus comprising image
reading means having spectral filter means for reading a visible image
printed on a document sheet in a predetermined image reading area by
scanning the image, image processing means interconnected to an output of
the image reading means, color image recording means interconnected to an
output of the image processing means and comprising a plurality of
chromatic recording systems which are different in hue from each other and
an achromatic recording system, undercolor removing means included in the
image processing means for processing input image data to detect an
achromatic color component contained in the image, extracting as
achromatic color data a value of a predetermined ratio associated with the
detected achromatic color component, and subtracting a value corresponding
to the extracted achromatic color data from the input image data to output
chromatic color data, color discriminating means for determining whether
or not the image is substantially monochromatic by processing the
chromatic color data outputted by said undercolor removing means, edge
detecting means for detecting edges of the image by processing the image
data, discrimination inhibiting means for inhibiting the color
discriminating means from operating when the edge detecting means has
detected an edge of the image, and control means for causing the image
reading means to read a single document image a plurality of times, the
control means fixing, during a first image reading operation, the ratio of
the undercolor removing means substantially to 100% and inhibiting the
color image recording means from operating, the control means
automatically and selectively conditioning the color image recording means
to either one of a color processing mode and a monochromatic processing
mode in response to a result of discrimination which the color
discriminating means produces with image data resulted from the first
image reading operation.
In the above construction, each document image is simply read and not
recorded at first (prescanning) and, at this instant, the color
discriminating means determines whether or not the image is monochromatic.
During the second and successive image readings, recording is executed in
color or monochrome. Since the removing ratio of the undercolor removing
means is set to 100% during the prescanning, chromatic data outputted by
the undercolor removing means substantially does not include an achromatic
component. When such data is compared with a particular threshold value,
whether or not the image is monochromatic will be readily determined.
In the event of actual image recording, 100% UCR processing would introduce
a black component over the entire recorded image and thereby make it
difficult to match C, M and Y ink and black ink. Nevertheless, the above
construction of the present invention needs 100% UCR processing only
during prescanning and sets an adequate ratio in the undercolor removing
circuit during the second and successive image processing, promoting easy
matching of C, M and Y ink and black ink in a recorded image with respect
to tone. Hence, the undercolor removing circuit can be shared by the
color/monochrome discrimination and the generation of a black component
for image recording, whereby the circuit arrangement is simplified.
When prescanning is effected as stated above, even monochromatic image
recording needs two times of scanning and, therefore, a substantial
processing time. With a popular inexpensive image processing apparatus
which lacks a frame memory, it is difficult to eliminate prescanning
because such an apparatus is substantially not capable of executing image
reading and recording at the same time.
To eliminate prescanning and thereby reduce required processing time, the
present invention provides a color image processing apparatus comprising
image reading means having spectral filter means for reading a visible
image printed on a document sheet in a predetermined image reading area by
scanning the image, image processing means interconnected to an output of
the image reading means, color image recording means interconnected to an
output of the image processing means and comprising a plurality of
chromatic recording systems which are different in hue from each other and
an achromatic recording system, background removing means included in the
image processing means for correcting tone of input image data and
removing a set background level from the image data, undercolor removing
means included in the image processing means for processing image data
outputted by the background removing means to detect an achromatic color
component contained in the image, extracting as achromatic color data a
value of a predetermined ratio associated with the detected achromatic
color component, and subtracting a value corresponding to the extracted
achromatic color data from the input image data to output chromatic color
data, color discriminating means for determining whether or not the image
is substantially monochromatic by processing the chromatic color data
outputted by the undercolor removing means, edge detecting means for
detecting edges of the image by processing the image data, discrimination
inhibiting means for inhibiting the color discriminating means from
operating when the edge detecting means has detected an edge of the image,
and control means for setting, when the image reading means reads a single
document image for the first time, a first background level in the
background removing means, fixing the ratio of the undercolor removing
means at substantially 100%, and in this condition causing the color image
recording means to record an achromatic color component of the image, the
control means setting, if a result of discrimination by the color
discriminating means resulted from the first image reading operation is
not monochromatic, a second background level in the background removing
means, setting the ratio of the undercolor removing means such that the
ratio is zero if tone is lower than a tone substantially the same as the
first background level and has a predetermined value if otherwise, and in
this condition executing a second and successive reading operations while
conditioning the color image recording means for a recording mode for
recording chromatic color components.
The above construction has the following device in order to eliminate
prescanning. To begin with, during the first scanning, an achromatic
component (black) is recorded at the same time as image reading. The
undercolor removal (UCR) ratio is set to 100%. The background removing
means removes an image component whose density is lower than the first
background level in order to prevent an image area lower in density than
the first background level from being recorded. Usually, when the 100% UCR
ratio is selected, black ink will be recorded over the entire image area
and make it difficult to match C, M and Y ink and black ink. Nevertheless,
the above construction applies undercolor removal to data from which the
background component of an image has been removed, so that black ink is
recorded only in the portions where the density is higher than the
background level, as in recording in black implemented by a skeleton black
recording method known in the art. This allows black ink and C, M and Y
ink to be readily matched to each other. Further, the resultant chromatic
data has undergone 100% UCR processing and facilitates the discrimination
by the color discriminating means.
When the image is determined to be monochromatic by the first scanning, the
recording operation can be ended immediately because the recording in
black has already been completed. In this case, density components lower
than the first background level will not appear in a recorded image. This
is rather favorable considering the fact that a monochromatic image often
contains only characters and, therefore, has noticeable changes in
density, i.e., removing the background erases noise image and leaves only
necessary data. When the image is not monochromatic as determined by the
first scanning, the second and successive scanning are executed while
image data are recorded in chromatic ink of different colors (C, M and Y).
For the second and successive scanning, the background level is changed to
zero to stop the removal of background, while the undercolor removing
means removes the undercolor at a predetermined ratio. Concerning this
ratio, zero is selected for densities lower than the first background
level while an adequate ratio other than zero is selected for higher
densities. More specifically, the background removing level for the first
scanning (first background level) and the boundary level for the
switchover of the undercolor removal for the second and successive
scannings are selected to be the same as each other. Hence, an image
component lower in density than the first background level and, therefore,
not recorded in black ink during the first scanning is recorded to 100% in
chromatic ink during any of the second and successive scanning.
With the apparatus disclosed in the previously stated Laid-Open Patent
Publication No. 63-107274, the operator has to position the individual
documents in a document reading area every time copying processing is to
be executed. An automatic document feeder (ADF) is available for
automatically and continuously processing images which are printed on a
number of document sheets. An ADF feeds one at a time a stack of document
sheets loaded on a tray so as to locate them in an image reading area, the
uppermost document sheet being first. This frees the operator from the
manual replacement of documents and implements continuous automatic
copying. In this kind of conventional ADF, a document feed control signal
is generated in order to replace a document every time the document
reading operation is repeated n consecutive times which corresponds to the
desired number of copies of that document.
However, when the stack of documents on the tray include both of
monochromatic (e.g. black-and-white) documents and color documents, they
cannot be reproduced by fully automatic processing despite the use of an
ADF. Specifically, unless the operator sets up either one of the color and
monochromatic modes every time the kind of a document image
(black-and-white or color) changes, all the documents will be dealt with
in the color mode or the monochromatic mode. While the operator may
separate a desired stack of documents into color documents and
monochromatic documents beforehand in order to minimize the number of
times of the switchover, such an operation is time- and labor-consuming.
The present invention allows, even when a document stack has both of color
and monochromatic document, the documents to be processed continuously
without resorting to the operator's manipulations while minimizing the
processing time. For this purpose, a color image processing apparatus of
the present invention comprises image reading means having spectral filter
means for reading a visible image printed on a document in a predetermined
image reading area by scanning the image, color discriminating means for
automatically determining whether or not the image is substantially
monochromatic by processing image data outputted by the image reading
means and separated into a plurality of fundamental colors, edge detecting
means for detecting edges of the image by processing the image data,
discrimination inhibiting means for inhibiting the color discriminating
means from operating when the edge detecting means has detected an edge of
the image, automatic document feeding means loaded with a plurality of
document sheets each carrying a visible image thereon for feeding the
document sheets one by one to the predetermined image reading area of the
image reading means, and control means for switching over, on the basis of
a result of discrimination by the color discriminating means,
correspondence between the number of times that the image reading means
scans a document sheet and the number of times that the automatic document
feeding means feeds a document sheet.
In the above construction, the color discriminating means detects the color
of an image printed on a document and lying in the image reading area and,
at the same time, automatically determines whether or not the color is
substantially monochromatic. Based on the result of discrimination, the
control means switches over the correspondence between the number of times
that the image reading means scans a document and the number of times that
the automatic document feeding means feeds a document. For example, in a
color copier selectively operable in a full color mode which produces a
single copy by repeating a copying process four consecutive times and a
black-and-white mode which produces a single copy by a single copying
process, every time a new document is fed, whether or not an image printed
thereon is color or black-and-white is discriminated automatically. If the
image is a color image, it is scanned 4.times.n (desired number of copies)
times, and then the next document is fed. If the image is a
black-and-white image, it is scanned n times, and then the next document
is fed. Hence, even when color documents and black-and-white documents are
stacked together, the apparatus automatically discriminates them as to
color and feeds them at adequate timings. This allows the documents to be
continuously processed within a minimum necessary period of time while
freeing the operator from troublesome manipulations. When an image is
printed on a document in ink of single hue such as C, M or Y, it can be
processed by single scanning and, therefore, handled in the same manner as
a black-and-white image.
SUMMARY OF THE INVENTION
It is an object of the present invention to automatically discriminate a
color image and an achromatic image with accuracy so that optimal image
processing matching the kind of an image may be executed without resorting
to operator's manipulations.
It is another object of the present invention to simplify the circuit
construction for color/monochrome discrimination.
It is another object of the present invention to reduce the required
processing time by eliminating prescanning.
It is another object of the present invention to process color documents
and monochromatic documents stacked together continuously without
resorting to operator's manipulations, while minimizing the required
processing time.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will become more apparent from the following detailed
description taken with the accompanying drawings in which:
FIG. 1 is a front view of a digital color copier to which the present
invention is applicable;
FIG. 2 is a schematic block diagram showing an electrical arrangement
associated with the copier of FIG. 1;
FIGS. 3A and 3B are timing charts demonstrating the operation timings of
various sections included in the copier of FIG. 1;
FIG. 4 is a flowchart outlining the operation of a system controller shown
in FIG. 2;
FIG. 5 is a block diagram schematically showing an edge extracting circuit
of FIG. 2;
FIG. 6 is a schematic block diagram of an UCR black generating circuit
shown in FIG. 2;
FIGS. 7A and 7B are graphs indicative of input-output characteristics of a
gamma correcting circuit;
FIGS. 8A to 8C are timing charts each showing the variation of an input
image and the state of an output image signal under a particular
condition;
FIG. 9 is a timing chart showing a specific variation of a signal in the
edge extractin circuit;
FIGS. 10A to 10C are graphs representative of the densities of individual
color components included in an image signal; and
FIG. 11 is a flowchart showing the operation of a control device
representative of a modified embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 of the drawings, a digital color copier to which the
present invention is applicable is shown and generally made up of a laser
printer 100, an ADF 200, an operation board 300, and an image scanner 400.
The image scanner 400 has an image reading section located below a glass
platen 401. The image reading section is mechanically driven in the
left-and-right direction as viewed in the figure, i.e., in the subscanning
direction. Light issuing from a lamp 402 is incident to a document which
is laid on the glass platen 401, so that the resultant reflection from the
document is representative of a density distribution of the latter. The
reflection, or image light, is incident to a dichroic prism 410 by way of
a number of mirrors and a lens. The dichroic prism 410 separates the
incident light on the basis of wavelength into three different colors,
i.e., red (R), green (G), and blue (B). The three color components each is
incident to respective one of three one-dimensional image CCD (Charge
Coupled Device) image sensors which are incorporated in the image scanner
400. The image scanner 400, therefore, senses the color components R, G
and B existing on one main scanning line of the document image at the same
time. Eventually, a two-dimensional image of the document is read with the
image reading section being driven in the subscanning direction as
mentioned previously.
The ADF 200 is located above the scanner 400. A document table 210 may be
loaded with a stack of any desired number of documents. In the event of
document feed, a pick-up roller 212 is brought into contact with the top
of the document stack and driven in a rotary motion. A separation roller
213 serves to prevent two or more documents from being fed together. The
document paid out to a predetermined position is further transported by a
pull-out roller 217 and a belt 216 along the glass platen 401 to a
predetermined reading position. As soon as the document reaches the
reading position, i.e., when its leading edge reaches the leftmost
position of the glass platen 401, it is brought to a stop. After the
document has been read, it is again transported by the belt 216 and
thereby driven out of the glass platen 401. At the same time, the next
document is driven by the belt 216 to the reading position. An optical
sensor 211 is located just ahead of the pick-up roller 212 in order to
determine whether or not documents exist on the document table 210.
Another optical sensor 214 is interposed between the separation roller 213
and the pull-out roller 217 for sensing the leading edge and the size of a
document. Specifically, the sensor 214 is constituted by a plurality of
sensors which are located at spaced positions in the main scanning
direction, so that the size or width of a document as measured in the main
scanning direction may be determined on the basis of the combination of
their outputs. A pulse generator is associated with a drive motor, not
shown, to generate pulses the number of which is associated with the
amount of rotation of the motor. A controller installed in the ADF 200
determines the size or length of a document as measured in the subscanning
direction by counting the output pulses of the pulse generator which
successively appear until the document moves away from the sensor 214. The
pick-up roller 212 and separation roller 213 are driven by a document feed
motor, while the pull-out roller 217 and belt 216 are driven by a
transport motor. A register sensor 215 is located downstream of the
pull-out roller 217 with respect to the direction of document feed.
The laser printer 100 has a photoconductive drum 1 for reproducing an
image. Arranged around the drum 1 are a charger 5, writing unit 3, a
developing unit 4, a transfer drum 2, a cleaning unit 6, etc. The charger
5 uniformly charges the surface of the drum 1 to a high potential by a
corona current. As a laser beam issuing from the writing unit 3
illuminates the charged surface of the drum 1, the charge potential is
varied depending on the intensity of the light. As a result, a particular
potential distribution is developed on the drum 1 in association with the
light intensity. Specifically, the writing unit 3 has a laser diode, not
shown. A laser beam from the laser diode is steered by a polygonal mirror
3b, a lens 3c, a mirror 3d and a mirror 3e to reach the surface of the
drum 1. The polygonal mirror 3b is driven in a high-speed rotary motion by
a motor 3a. A controller applies to the laser diode a pixel-by-pixel
two-level signal (record/non-record) associated with a desired image, such
that the individual pixel positions are synchronous to the angular
positions of the polygonal mirror 3b. More specifically, the laser beam is
controlled on and off at each scanning position of an image in response to
the density (record/non-record) of a pixel located there.
Therefore, the potential distribution developed on the drum 1 forms an
electrostatic latent image which corresponds to the document image with
respect to the density. The developing unit 4 is located downstream of the
writing unit 3 with respect to the direction of rotation of the drum 1 and
develops the latent image by a toner. In the illustrative embodiment, the
developing unit 4 has four developing subunits 4M, 4C, 4Y and 4BK which
are loaded with a magenta (M) toner, a cyan (C) toner, a yellow (Y) toner,
and a black (BK) toner, respectively. In this specific printer
configuration, the four developing subunits are energized one at a time
and, hence, the latent image is developed by one of the toners of
different colors M, C, Y and BK.
A cassette 11 is loaded with a stack of paper sheets. A paper sheet paid
out from the cassette 11 by a feed roller 12 is transported toward the
transfer drum 2 by way of a register roller 13. The paper sheet is moved
by the transfer drum 2 while lying thereon. A transfer charger 7 is
energized to transfer the toner image from the drum 1 to the paper sheet
being moved in close proximity of the surface of the drum 1. In a
monocolor copy mode, the paper sheet carrying the toner image thereon is
separated from the transfer drum 2, fixed by a fixing unit 9, and then
driven out to a tray 10. In a full-color mode, it is necessary to
superpose the four different colors BK, M, C and Y on a single paper
sheet. This is implemented by forming a toner image BK on the drum 1,
transferring the toner image BK to a paper sheet, forming a toner image M
on the drum 1 without the paper sheet being separated from the transfer
drum 2, and then transferring the toner image M to the paper sheet. The
other toner images C and Y are sequentially transferred to the same paper
sheet in the same manner. A single color image is, therefore, reproduced
on a paper sheet by the toner image forming and transferring process which
is repeated four times in total. After all the toner images have been
transferred to the paper sheet, the paper sheet is separated from the
transfer drum 2, transported to the fixing unit 9 for fixing the toner
image, and then discharged to the tray 10.
Referring to FIG. 2, an electrical arrangement incorporated in the color
copier of FIG. 1 is shown. As shown, the arrangement includes a system
controller 50 which supervises the operations of the entire copier and is
implemented as a microcomputer, for example. A synchronization control
circuit 60 generates a clock pulse which is the reference for control
timings. Further, the control circuit 60 receives and delivers various
signals in order to synchronize signals which are interchanged among
various control units. In this specific circuit arrangement, a main scan
sync signal which is the source of scanning timings appears in synchronism
with the scanning position of the polygonal mirror 3b of the laser printer
100. The image scanner 400 transforms the read image signals R, G and B
into digital signals and delivers them as color image data having eight
bits each. The image data are individually subjected to various kinds of
processing at a image processing unit, which will be described, and then
applied to the laser printer 100.
The image processing unit has a gamma correcting circuit 71, a
complementary color generating circuit 72, an UCR black generating circuit
73, a selector 74, and a tone processing circuit 75. The complementary
color generating circuit 72 transforms the color data R, G and B into
complementary color data Y, M and C, respectively. The UCR black
generating circuit 73 separates a black component included in a color
which is the mixture of the input colors Y, M and C, and outputs it as
color data BK. At the same time, this circuit 73 removes the black
components of the other signals. Details of the UCR black generating
circuit 73 will be described in more detail later. The selector 74 selects
one of the outputs Y, M, C and BK of the UCR black generating circuit 73
at a time in response to an instruction from the system controller 50. The
selected signal Y, M, C or BK is fed to the tone processing circuit 75.
While the circuit 75 binarizes the input 8-bit density data, it executes
dither processing in order to implement the output of halftone. The
resultant two-level image data is fed to the laser printer 100.
The outputs of the UCR black generating circuit 73 are connected to a color
discriminating circuit 80 which determines whether or not a document image
contains any chromatic color. The UCR black generating circuit 73
separates black, i.e., an achromatic component BK from the input image
data, as stated earlier. Hence, the output data Y, M and C of the circuit
73 are chromatic components. It follows that whether or not a document
image contains a chromatic color or colors can be determined on the basis
of the outputs Y, M and C of the circuit 73. However, colors detected by
an image reading system are somewhat different from actual colors and,
moreover, a document image may have been smeared in chromatic colors. In
the light of this, the illustrative embodiment ignores chromatic
components of relatively low density and, only when a chromatic color
extends over a predetermined pixel area, produces a chromatic color signal
(SG1: "H").
Only the most significant bit (MSB) of each of the data Y, M and C is fed
from the UCR black generating circuit 73 to the color discriminating
circuit 80. Specifically, only when any one of the chromatic color signals
Y, M and C has a density equal to or higher than 128, or 50%, the circuit
80 determines it valid. When such a valid signal appears, a counter 84
starts counting pixels over which the color of interest extends. The
counter 84 is operated document by document, i.e., it is cleared by a
clear signal CLR at the beginning of document reading and thereby allowed
to count. A signal SIZE is determined on the basis of the detected size of
a document; it remains in a high level or "H" while the range where a
document actually exists is scanned and goes low or "L" while the other
range is scanned. A signal CLK is a clock appearing in synchronism with
the individual pixels.
At first, the counter 84 is cleared by the clear signal CLR to in turn
maintain the output of an inverter 85 in "H". While an area where a
document exists is read, clock pulses appear on the output of an AND gate
82 pixel by pixel. When a chromatic color signal whose density is higher
than 50% appears, the output of an OR gate 81 turns to "H" with the result
that the clock pulses are fed to the count input terminal of the counter
84. More specifically, the counter 84 continuously counts the clock
pulses, or pixels, so long as such a chromatic color signal is present.
The counter 84 is a binary counter. The output terminal of the color
discriminating circuit 80 is interconnected to a bit 8 of the binary
counter 84. In this configuration, when the counter 84 reaches "512", a
discrimination signal SG1 turns from "L" to "H". Then, the output of the
inverter 85 goes low to prevent the clock pulses from being fed to the
counter 84 any further, so that the discrimination signal SG1 remains in
the same status until the next clear signal CLR arrives.
In the illustrative embodiment, the color discriminating circuit 80
includes an edge extracting section 90 for determining whether or not an
edge of an image as represented by a great change in tone has appeared.
The edge extracting section 90 is interconnected to the AND gate 83. When
this section 90 extracts an edge, the clock pulses are inhibited from
reaching the counter 84 even if a chromatic color signal whose density is
higher than 50% is present. In this embodiment, therefore, edges of an
image do not effect the chromatic/achromatic color discrimination at all.
Why this kind of circuit configuration is adopted is as follows.
Granting that an image reading device has ideal characteristics, MTF
(Modulation Transfer Function) is unavoidably lowered from an original
image to an image signal representative of the original image.
Specifically, as shown in FIG. 8A, even when the tone of an input image
varies stepwise, the output of a sensor builds up and falls slowly
resulting in a blurred reproduction. In the condition shown in FIG. 8A,
chromatic color components Y, M and C do not appear at all. In practice,
however, such ideal characteristics are not achievable. Specifically, as
shown in FIG. 8B, despite that an achromatic color is obtained by
combining R, G and B of substantially the same tone, R, G and B in the
sensor output differ from one another with respect to the gradient of
buildup and fall because the degree of deterioration of MTF depends on the
wavelength. This causes chromatic (Y, M and C) components to appear in the
output at the edge portions of the image, despite that the input is an
achromatic color. Further, when three different sensors are each assigned
to respective one of color components R, G and B, they cannot be free from
some deviation in position from one another. Then, as shown in FIG. 8C,
the image will be deviated in phase in the sensor output and cause
chromatic compone | | |
