Image recording apparatus for representing a halftone by use of a plurality of recorded dots

An image recording apparatus comprises an unequal interval quatization circuit for quantizing at an unequal interval an input multilevel image signal to prevent at least one signal having a tone level at which a recording density is unstable from being outputted, a circuit for calculating a difference between an unequal-interval-quantized signal and the input image signal, a buffer memory for temporarily storing the difference, a multiplexer for multiplying a recorded signal with a weight coefficient, an adder for adding a resultant signal from the multiplier to the input image signal, and a printer for recording an image, using the quantized signal as a record signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image recording apparatus used in a 
copy machine or printer. 
2. Description of the Related Art 
An error diffusion method or error distribution method is known as a method 
of processing an input image signal to record a halftone image. The error 
diffusion method is a technique for performing binary coding process of an 
image signal so as to minimize a density difference between an input image 
signal and an output image signal. This technique is very effective to 
output both a halftone image and a high-definition image in a printer 
having only binary representation. A method of inputting a signal 
processed by this error diffusion method to a printer and recording the 
processed signal at the printer is called error diffusion recording. This 
error diffusion recording has a disadvantage in that a texture inherent to 
a smooth halftone image becomes conspicuous. 
To compensate for the drawback of this error diffusion method, a multilevel 
error diffusion method and a multilevel correlative density assignment of 
adjacent pixels ("multilevel algorithm for high quality pseudo 
halftoning", Proceedings of Television, Vol. 44, No. 5, pp. 608-614 
(1990)) which is similar to the multilevel error diffusion method are 
known. In these techniques, however, a tone gradation tends to be 
conspicuous in a highlighted or high-density portion whose recording is 
unstable. A pseudo contour is formed in such a portion to result in a poor 
image. 
To solve this problem, a method of adding random noise to an image signal 
(e.g., "Smoothing Effect in Gray Scale Characteristics Using a 
Pseudo-Noise Method", Proceedings of Television, Vol. 40, No. 11, p. 1113 
(1986)). According to this method, however, the texture of a multilevel 
recording portion becomes different from that of the portion added with 
random noise, resulting in a poor image. 
As a method of correcting a tone gradation, a technique for correcting the 
levels of a signal to smooth levels in advance, using a ROM table is 
known. In this tone correction, however, when a change in density occurs 
in a printer deterioration over time, the pseudo contour tends to be 
formed in the highlighted or high-density portion. 
In error diffusion recording, when an input image signal is subjected to 
undercolor removal, snow noise tends to be conspicuous. Published 
Unexamined Japanese Patent Application No. 3-204273 describes a technique 
for determining a condition in selecting each color to prevent colors from 
generating at random, thereby preventing snow noise. In a practical color 
printer, however, color misregistration between colors to be printed 
occurs to result in a random color combination in stacking the colors to 
actually print an image under a specific condition for a color selection. 
Snow noise is not always reduced. When four-color printing is performed 
after processing such as undercolor removal, a low-density recording 
signal is frequently generated to form a number of non-recorded dots in a 
system in which low-density recording is unstable. As a result, color 
reproduction is often degraded. 
In a color copy machine or color printer, a positional error occurs between 
ink colors in a color printing unit. Black characters are blurred, and the 
color of a halftone portion such as a gray or skin-colored portion changes 
depending on different positions within the printed paper sheet or 
different paper sheets. To prevent this, a technique for using a screen 
angle in color halftone reproduction to obtain different screen angles for 
all colors, locally causing an average positional error any time, and 
stabilizing color reproduction is known. According to this technique, 
image degradation in a character portion greatly occurs, and moire noise 
is undesirably generated. 
To improve the quality of a character portion, Published Unexamined 
Japanese Patent Application No. 63-240175 discloses a technique for 
extracting a character and black regions of a color original, performing 
100% undercolor removal of only a black character region, and performing 
undercolor removal of less than 100% of the remaining region. In practice, 
however, it is difficult to accurately extract the character region in 
100%. When the character region is erroneously identified, e.g., when a 
halftone image region is erroneously identified as the character region, 
100% undercolor removal causes a change in color or an increase in snow 
noise. It is difficult to set an extreme undercolor removal ratio (i.e., 
blackening ratio). A color variation caused by a relative positional error 
in an ink color of the printer occurs in the halftone image region, and 
moire noise cannot be reduced. When undercolor removal of a region (i.e., 
a halftone image region occupying an almost print area) except for the 
character region is performed at a low ratio, an ink consumption amount 
cannot be reduced. In addition, every time the undercolor removal ratio 
changes, the color changes in strict color reproduction. It is, therefore, 
difficult to variably set the undercolor removal ratio. 
As described above, in the conventional error diffusion recording 
technique, texture noise inherent to binary error diffusion recording is 
generated, and pseudo contour noise at discontinuous points in multilevel 
recording is generated. Even if processing for correcting printer tone 
characteristics is performed, the pseudo contour caused by variations in 
tone characteristics due to the deterioration over time is undesirably 
formed. 
In the conventional color image recording apparatus, when four-color 
printing is performed in error diffusion recording, snow noise caused by 
color noise generated at random is generated. In addition, it is difficult 
to faithfully reproduce the colors in a printer having unstable tone 
characteristics, and color reproduction by undercolor removal processing 
is degraded. 
In the conventional color image recording apparatus, a character portion 
tends to be blurred. When the technique having different undercolor 
removal ratios in the character and halftone image regions is used, a 
change in color in the character region occurs and snow noise increases 
due to erroneous identification of the region. In addition, the ink 
consumption amount cannot be reduced. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an image recording 
apparatus capable of performing image recording almost free from pseudo 
contour noise and a change in texture, while using error diffusion 
recording. 
It is another object of the present invention to provide an image recording 
apparatus capable of performing faithful color reproduction even in use of 
error diffusion recording, eliminating instability of printer tone 
characteristics, and performing four-color image recording almost free 
from snow noise. 
It is another object of the present invention to provide an image recording 
apparatus capable of preventing color blurring and snow noise in a 
character region and performing color image recording with a small ink 
consumption amount. 
According to a first aspect of the present invention, there is provided an 
image recording apparatus, which includes an unequal interval quantization 
circuit (a nonlinear quantization circuit) in an error diffusion 
processing section, for quantizing an image signal at an unequal interval 
to eliminate an unstable recording level. 
According to a second aspect of the present invention, there is provided an 
image recording apparatus, wherein multilevel error diffusion processing 
for each color of a four-color signal obtained upon correction including 
undercolor removal of a three-color signal in color recording or a read 
three-color signal so as to reduce snow noise of colors in error diffusion 
recording and to reduce an ink consumption amount even if positional 
errors of all the colors occur in a printing unit, thereby obtaining a 
color recording signal. 
To achieve faithful color reproduction and perform undercolor removal 
having a large degree of freedom, a three-color signal is converted into a 
four-color signal, a table representing a relationship with the four-color 
signal as the recording signal is prepared from a signal measured by a 
read system upon actual printing at a printing unit or a signal estimated 
to be obtained from the system, and the three-color signal is converted 
into the four-color signal from this table. 
To solve the problem caused by an increase in snow noise because 
low-density level recording is frequently performed due to undercolor 
removal or blackening processing, when a blackening amount is small, 
three-color printing is performed, when the blackening amount increases, 
four-color printing is performed. 
The multilevel error diffusion processing may be multilevel error diffusion 
processing from which assignment to the low-density level corresponding to 
the unstable recording state is canceled, or multilevel error diffusion 
recording processing in which a weighting coefficient is caused to 
correspond to a recording level in a feedback loop of the low-density 
level corresponding to the unstable recording state. 
According to a third aspect of the present invention, there is provided an 
image recording apparatus comprising a correction unit for performing 
correction processing of a three-color input image signal including black 
on the basis of a blackening amount and a discrimination result of a 
character/line image region and a halftone image region to obtain a 
four-color image signal, an error diffusion processing unit for performing 
binary or multilevel error diffusion processing of the four-color image 
signal obtained from the correction unit, and a color recording unit for 
recording a color image using, as a recording signal, the color image 
signal processed by the error diffusion processing unit, wherein the 
correction unit sets a higher ratio of substitution to a black signal for 
the character/line image region than that for the halftone image region, 
the ratio of substitution to the black signal for the halftone image 
region being lowered in accordance with a decrease in blackening amount. 
In noise such as pseudo contour noise caused by the recording level 
corresponding to the unstable recording state, the tone characteristics 
immediately change upon tone correction, a tone degradation occurs in the 
tone characteristics, a noise reduction effect by tone correction cannot 
be obtained, and noise reduction is difficult. To the contrary, according 
to the first aspect of the present invention, when the recording control 
level is quantized by threshold processing using the multilevel in error 
diffusion recording, the recording signal is generated using the unequal 
interval (nonlinear quantization) characteristics. The tone can be 
expressed without using the recording level region (density region) 
corresponding to the unstable recording state. Therefore, stable recording 
can be performed. 
The conversion table representing the relationship between the three- and 
four-color signals is prepared for color correction in color reproduction, 
and the three-color signal is converted into the four-color signal. No 
change in color before and after processing such as undercolor removal is 
generally required. The color change component is corrected using this 
conversion table, and any undercolor removal can be performed. In 
addition, a printer is not generally good at low-level printing, and snow 
noise is increased. If a possibility of performing low-level recording is 
present, i.e., when a blackening amount is small, blackening recording is 
not performed, but three-color recording is performed to prevent low-level 
recording. Quantization using the nonlinear quantization characteristics 
is performed in multilevel error diffusion recording processing to 
suppress generation of a low-level signal, thereby reducing snow noise. 
According to the third aspect, the amount of ink actually used is 
determined by the image region identification signal and the blackening 
amount. For this reason, 100% undercolor removal is performed in the 
character region, and a clear image free from color blurring can be 
obtained. When the blackening amount is large, almost 100% undercolor 
removal is performed in the halftone image region to contribute to a 
decrease in ink consumption amount. When the blackening amount is small, 
almost only three-color printing is performed to suppress snow noise. The 
weight coefficient matrix is changed in error diffusion recording of the 
character region requiring high-definition recording and a recording color 
requiring high-definition recording. Therefore, both high-definition 
recording and smoothness in the halftone image region can be satisfied. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will be described below with reference 
to the accompanying drawings. 
FIG. 1 is a block diagram showing an image recording apparatus according to 
the first embodiment of the present invention. When this image recording 
apparatus is used as a copy machine, an image signal obtained by reading 
an original by a scanner 101 is used as an input image signal 102. When 
the image recording apparatus is used as a printer apparatus, an image 
signal input from an input terminal 100 is used as the input image signal 
102. The input image signal 102 is input to an adder 103, and an error 
between a signal to be recorded (to be described later) and an output 
image signal is added to the input image signal 102. An output signal from 
the adder 103 is input to an unequal interval quantization circuit (a 
nonlinear quantization circuit) 104. 
A laser printer generally exhibits tone characteristics shown in FIG. 2. In 
FIG. 2, the ordinate represents 1-R (R: reflectance), and the abscissa 
represents a control signal (recording signal) to be supplied to a 
printer. At this time, a case wherein the control signal changed from 0 to 
F in hexadecimal notation will be described below. The control signal is 
considerably changed at a point of control amount "3" (immediately after a 
density level region corresponding to a highlighted portion a) or a point 
of control amount A (immediately before a density level region 
corresponding to a solid density region b) depending on atmospheric 
conditions, latent image conditions (variations in laser power) of 
electrophotography, and development conditions. For this reason, even when 
these points are output as a recording signal 105, these points are not 
actually recorded by a printer 106, or these points are undesirably 
connected to each other. Therefore, recording is often performed at a 
density considerably different from that of the recording signal 105. 
The nonlinear quantization circuit 104 is constituted by, e.g., an ROM 
table, and generates a control signal in a density level region except for 
control amounts "3" and "A". For example, control signals indicated by 
.largecircle. in FIG. 2, i.e., only signals having 8 types of control 
amounts 0, 4, 5, 6, 7, 8, 9, and F are generated. More specifically, upper 
4 bits of 8 bits of an output of the adder 103 are input to a ROM (16 
addresses), and the ROM outputs control signals having control amounts 0, 
0, 0, 0, 4, 5, 6, 7, 8, 9, F, F, F, F, F, and F. 
When these control signals are input to the printer 106 to perform 
recording, a density shown in FIG. 2 is obtained as an actual density. For 
this reason, an error between the density of the output signal from the 
adder 103 and the density of actual printing must be calculated by a 
density corrected by the tone characteristics (FIG. 2) of the printer 106. 
The output signal from the adder 103 is converted into a density signal of 
actual printing by using the tone characteristics in FIG. 2 as a density 
conversion table 107. The density conversion table 107 has a 4-bit (16 
addresses) input and an 8-bit output. 
An error between the output signal from the adder 103 and the signal 
converted into the signal corresponding to an actual printing density and 
output from the density conversion table 107 is calculated by a difference 
processing circuit 108, and the resultant value is input to an error 
buffer 109 capable of storing the data of a 2-line error signal. A product 
of the error signal stored in the error buffer 109 and a weight 
coefficient matrix, output from a coefficient memory 111, for error 
diffusion processing is calculated by a multiplier 110, and the error is 
input to the adder 103. 
The weight coefficient matrix is constituted by four pixels as shown in 
FIG. 3, and coefficients A, B, C, and D are 1/16, 5/16, 3/16, and 7/16, 
respectively. An error Em-k,n-p having occurred ahead of an input signal 
fmn by a pixel (k,p) is weighted with the weight coefficient matrix of a 
coefficient .alpha.kp, and the weighted error is added to input image 
signal fmn. The resultant value is defined as a correction value f'mn of 
the next pixel, and the correction value f'mn is quantized at an unequal 
interval by the unequal interval quantization circuit, i.e. nonlinear 
quantization circuit 104. At this time, the correction value f'mn is 
expressed by equation (1). An error Emn is a difference between the 
correction value f'mn and an output density signal, i.e., an output signal 
Gmn from the density conversion table 107, and is expressed by equation 
(2). 
##EQU1## 
In this manner, density level regions a and b shown in FIG. 2 and 
representing recording characteristics having an unstable tone are not 
used in the nonlinear quantization circuit 104, and the printer 106 can 
have recording characteristics having a smooth tone by the error diffusion 
recording. If the error diffusion processing is not performed, noise such 
as pseudo contour noise is generated due to a density difference caused by 
a tone degradation in one pixel of the nonlinear quantization circuit 104, 
thereby resulting in a very poor image. When the error diffusion 
processing of equations (1) and (2) is performed, an error generated at 
one dot is diffused and distributed through several pixels, and the 
density error of each pixel can be corrected. For this reason, a pseudo 
contour is rarely formed. 
Intermediate-level signals often occupy a large area in an halftone image. 
However, a contrast of a tone difference is small because a control signal 
is distributed to a large number of levels in each period of intermediate 
levels. Therefore, texture noise unique to error diffusion recording is 
rarely conspicuous. 
Although tone differences locally occur because tones are not used in a 
low-density level region and a high-density level region which are 
unstable density level regions of the printer 106, a contrast of the error 
diffusion recording is smaller than that of binary error diffusion 
recording, and texture noise is rarely conspicuous in the error diffusion 
recording. 
In electrophotographic recording, tone characteristics vary depending on 
exposure conditions or development conditions. In this case, a pseudo 
contour is rarely formed because the most unstable density level region 
which easily varies is not used. Although an entire density varies due to 
a variation in tone characteristics of the printer 106, the variation in 
density can be neglected while a large variation in tone characteristics 
does not occur because the variation in density entirely occurs. 
In this embodiment, a tone is corrected by the density conversion table 107 
while the printer 106 has nonlinear tone characteristics inherent to the 
printer 106. With the above arrangement, a tone control circuit 
incorporated in the printer 106 can easily be realized because highly 
accurate tone correction can be performed by using a coarse control signal 
in the printer 106. For example, when multilevel control is realized by 
controlling the pulse width of a laser beam emitted from a laser, the tone 
control circuit is not operated at a very high speed, and a clock 
frequency need not be increased to an unnecessary level. Therefore, the 
tone control circuit can be realized by a low-cost element. 
As a modification of the first embodiment, an output signal from the 
nonlinear quantization circuit 104 is directly input to the difference 
processing circuit 108 without passing through the density conversion 
table 107 to calculate an error, and the output signal from the density 
conversion table 107 may be corrected such that a reverse table of the 
density conversion table 107, i.e., a table for correcting the tone 
characteristics in FIG. 2, more specifically, a correction table for 
dividing a (1-R) signal of the ordinate at equal intervals to obtain 
control signals corresponding to the divided signals, is inserted before 
the printer 106. In this arrangement, tone correction can also be 
performed. 
However, in order to achieve satisfactory tone correction in the above 
arrangement, the number of bits of a control signal from the printer 106 
must be larger than the number of output bits from the nonlinear 
quantization circuit 104. In addition, the modification is susceptible to 
a variation in tone characteristics compared with the first embodiment. In 
other words, when quantization is performed by a (1-R) signal of the 
ordinate, a density level region close to a density level region having an 
unstable recording state may be selected. An advantage of the above 
modification is that versatility for various printers is improved because 
a correction table is only inserted between the input portion of the 
recording signal 105 and the printer 106. 
FIG. 4 is a block diagram showing the second embodiment of the present 
invention. Tone characteristics shown in FIG. 2 are tone characteristics 
at one dot of a printer 106. When recording is actually performed by the 
printer 106, more specifically, a density level is changed depending on a 
recording mode of an adjacent dot. In other words, in a case wherein a dot 
(indicated by .times.) to be currently printed is surrounded by solid 
density pixels (indicated by ) or quasi-solid pixels (indicated by ) as 
shown in FIG. 5A, in a case wherein the dot is surrounded by intermediate 
density pixels (indicated by .largecircle.) or pixels (indicated by 
.DELTA.) as shown in FIG. 5B, and in a case wherein the dot is surrounded 
by one low density pixel (indicated by .DELTA.) or is not surrounded as 
shown in FIG. 5C, tone characteristics are slightly different from the 
characteristics shown in FIG. 2. 
In this embodiment, a recording signal 105 from a nonlinear quantization 
circuit 104 is input to an output buffer 112, and an output signal is read 
out from the output buffer 112 in a combination of 2.times.2 pixels. One 
pixel of the recording signal 105 is expressed by four bits, and the 
2.times.2 pixels have 16 bits. Therefore, a density conversion table 113 
has a 16-bit address unlike the first embodiment. 
The density conversion table 113 stores all combinations of density levels 
in combinations of 2.times.2 pixels, and an output signal from the output 
buffer 112 is converted into a signal corresponding to an actual printing 
density. As in the first embodiment shown in FIG. 1, a difference between 
the converted signal and an output signal from an adder 103 is calculated 
by an difference processing circuit 108. The arrangement of other parts of 
the second embodiment is the same as that of the first embodiment. In this 
manner, a variation in density caused by the influence of adjacent pixels 
is corrected, and stable and accurate tone characteristics can be 
obtained. 
The third embodiment applied to a color printer will be described below 
with reference to FIG. 6. The color of a color image is expressed by 
stacking color inks in a color printer 205. When tone characteristics as 
shown in FIG. 2 are observed, the characteristics often vary depending on 
the density of an ink which is recorded for the first time. That is, the 
tone characteristics vary depending on a manner of stacking color inks. 
According to the third embodiment, tones having different tone 
characteristics are corrected by the combination of the colors of the 
color inks, and combinations which reproduce unstable colors having 
increased snow noise are removed, thereby realizing stable and smooth 
color reproduction having reduced noise. 
An input color image signal 202 input from the color scanner 201 or an 
input terminal 200 is input to an adder 203. The input color image signal 
202 is constituted by, e.g., R, G, and B signals (three primary color 
signals), and each of the R, G, and B signals has 8 bits. Note that the 
input color image signal 202 need not be constituted by the R, G, and B 
signals, and the input color image signal 202 may be constituted by, e.g., 
L*, a*, and b* signals. 
An output signal from the adder 203 is input to a nonlinear quantization 
circuit 204. The nonlinear quantization circuit 204, as in the above 
embodiments, prevents a signal of a level easily set in an unstable 
recording state from being generated from the output of the nonlinear 
quantization circuit 204. At this time, combinations of colors which 
generate large snow noise caused by not only one ink but also combinations 
of inks are removed. In a color printer 205, combinations in which 4-bit 
tone control is performed for each color are used. Printing is performed 
by all color combinations, and a print sample is read by the color scanner 
201 or a color sensor having characteristics equivalent to those of the 
color scanner 201. However, recording characteristics in a density level 
region having the unstable tone characteristics of one color contain large 
noise even when colors are stacked. For this reason, a print sample need 
not be formed from the beginning. In this manner, the number of operations 
is considerably reduced. In addition, when a print model is estimated, 
combinations of all colors need not be actually printed, a specific 
recording sample may be recorded, and other data may be estimated in 
accordance with the print model. 
A signal required for recording of the print sample and a combination of 
the R, G, and B signals thereof are obtained. A color conversion table 206 
is prepared on the basis of the signal and the combination. This color 
conversion table 206 may receive 4-bit data for each color and output 
8-bit data for each color, and the color conversion table 206 can easily 
be prepared. 
The nonlinear quantization circuit 204 is formed as a table such that 
combinations having unstable printing and large snow noise are removed 
from the color conversion table 206, and the content of the resultant 
table is reversed to the content of the color conversion table 206. The 
color printer 205 may be subjected to 4-bit control, and an input address 
of the nonlinear quantization circuit 204 may have 4 bits. In other words, 
the input address of the nonlinear quantization circuit 204 may have about 
4 bits for each color, or 5 bits for each color in consideration of the 
nonlinearity of color conversion. In this manner, the capacity of a table 
(ROM) constituting the nonlinear quantization circuit 204 can be 
decreased. 
An 8-bit output signal for each color output from the color conversion 
table 206 is input to a subtracter 207, and a difference between an 8-bit 
output signal and an output signal from the adder 203 is calculated. This 
difference is input to an error buffer 208, and a product of the 
difference and a weight coefficient matrix from a weight coefficient 
memory 209 is calculated by a multiplier 210 as in the first embodiment, 
and the product is input to the adder 203, thereby performing error 
diffusion. 
In this embodiment, even in a recording system in which a noise level is 
considerably changed by stacking colors as in thermal transfer recording, 
when printing is performed while avoiding a density level which easily 
generates noise, recording can be performed with noise being suppressed. 
In addition, when correction is performed in consideration of the 
influence of adjacent pixels as in the second embodiment, more highly 
accurate color reproduction can be performed. 
FIG. 7 is a view showing the fourth embodiment of the present invention. An 
input color image signal 302 input from a color scanner 301 or an input 
terminal 300 is constituted by R, G, and B signals generally serving as 
read color-separated signals, and the R, G, and B signals are converted 
into Y, M, C, and K (yellow, magenta, cyan, and black) signals serving as 
ink-color signals by a color conversion table 303. 
If a method of stacking colors is determined, in general, the conversion 
from the R, G, and B signals to the Y, M, and C signals is determined. In 
contrast to this, a method of converting the R, G, and B signals into the 
Y, M, C, and K signals has a degree of freedom. A ratio (ratio of a black 
component constituted by Y, M, and C to a black component expressed by K) 
of the signal K, i.e., a black ink has a degree of freedom. The black 
component constituted by Y, M, and C cannot be simply replaced with the 
black K due to the influence of color stacking or a difference between the 
color tone of the black component (=min(Y, M, C)) constituted by Y, M, and 
C and the color tone of the black K. For this reason, the conversion of 
the R, G, and B signals into the Y, M, C, and K signals is complicated, 
and highly accurate color reproduction cannot be obtained. 
According to this embodiment, as shown in a principle of FIG. 8, Y, M, and 
C ink signals are generated (step 21), and four Y, M, C, and K signals are 
generated in a predetermined algorithm (step 22). At this time, when color 
reproduction by the Y, M, and C ink signals is related to color 
reproduction determined by the four Y, M, C, and K signals generated in 
the predetermined algorithm, these colors need not be strictly equal to 
each other. In other words, a degree of freedom is allowed to some extent. 
For example, assume a method in which color inks (Y, M, and C) are rarely 
used in consideration of economy so as to extend the reproduction range of 
colors to obtain a high-quality image. In this case, the Y, M, and C 
signals are converted into the Y, M, C, and K signals so as to minimize 
amounts of inks (a total amount of Y, M, C, and K inks) to be used. More 
specifically, it becomes, for example, K=min(Y,M,C) signal. Recording 
signal processing (step 23) for actually performing recording in 
accordance with these signals, in this case error diffusion processing, is 
performed to record the signal in a printer (step 24). Recorded color 
patches are read, R, G, and B read signals are obtained by a color 
separation system. In order to obtain four color signals from the R, G, 
and B signals, a table is prepared by the obtained R, G, and B signals and 
the Y, M, C, and K signals. That is, a relationship indicated by the arrow 
in FIG. 8 is obtained. At this time, when a table is prepared by 
performing interpolation processing from a patch having a small number of 
colors, the number of operations may be decreased. In addition, the 
difference processing circuit 303 is constituted as shown in FIG. 9, and 
interpolation processing is performed by an interpolation processing 
circuit 33. When an output from the interpolation processing circuit 33 is 
synthesized with an output from an YMCK table 32 by an adder 34 to obtain 
Y, M, C, and K signals, smooth conversion can be performed even when the 
YMCK table 32 does not have a very large capacity. 
A case wherein a color printer of a type in which a color printer 304 
simultaneously outputs four colors, i.e., a color-sequential-output type 
color printer is used will be described below. In this case, the color 
conversion table 303 converts a color image signal into an ink color 
signal with reference to one color signal, of the Y, M, C, and K signals 
serving as ink signals, output from the color printer 304, and the color 
image signal is sequentially converted into the four ink color signals in 
correspondence with an output color of the color printer 304. The adder 
305 adds correction signals to the ink color signals converted as 
described above. An output signal from the adder 305 is quantized into an 
unequal interval signal by a nonlinear quantization circuit 306. 
In many types of laser printers or thermal transfer printers, recording can 
be performed at several levels or more by controlling a pulse width. 
However, it is difficult to control a large number of densities at equal 
intervals, and recording at a highlighted portion, i.e., a low-density 
level region, becomes easily unstable. In FIG. 10A, control amounts of a 
printer are plotted along the abscissa, and 1-R (R: reflectance) is 
plotted along the ordinate. In a density level region a of the highlighted 
portion, (1-R) is set to be 0 or b by a small change in atmosphere. The 
nonlinear quantization circuit 306 performs quantization without using 
recording levels in the low-density level region a. In other words, only 
recording levels at points indicated by .largecircle. are used, and the 
nonlinear quantization circuit 306 is used as a table such that control 
signals at points indicated by .times. are not generated. In other words, 
the nonlinear quantization circuit 306 having input/output characteristics 
shown in FIG. 10B is formed as a table. A signal quantized at an unequal 
interval by the nonlinear quantization circuit 306 is used as an output 
signal 307, and the output signal 307 is supplied as a recording control 
signal to the color printer 304. 
The signal quantized at an unequal interval by the nonlinear quantization 
circuit 306 is also input to a density conversion table 308, and the 
signal is converted by a density characteristic curve of the color printer 
304 actually output as shown in FIGS. 10A and 10B, and a difference 
between the converted signal and a signal which is not quantized at an 
unequal interval is calculated by the difference processing circuit 309. 
When an output signal from the nonlinear quantization circuit 306 is input 
to the density conversion table 308 and subjected to tone correction of 
the color printer 304, the signal quantized at a small number of 
multilevels by the nonlinear quantization circuit 306 is reproduced into a 
faithful tone by the control of a multilevel printer. Therefore, the tone 
reproduction can easily be realized because the multilevel control circuit 
in the printer 304 can be controlled by a small number of levels. For 
example, multilevel control is controlled by a pulse width, the multilevel 
control circuit need not be operated at a very high speed. 
The error signal obtained by the difference processing circuit 309 is input 
to an error buffer 310. A product of an error signal from the error buffer 
310 and a weight coefficient matrix from a weight coefficient memory 311 
is calculated by a multiplier 312. As shown in FIG. 11, this calculation 
causes the error signal to be diffused to four points A, B, C, and D. 
Weight coefficients are A=1/16, B=5/16, C=3/16, and D=7/16. The diffused 
error signals are added to each other by the adder 305, and are entered in 
an error diffusion loop. When the error signals are entered in the error 
diffusion loop, the loop is operated such that an output whose density is 
converted, i.e., a density output from the color printer 304 coincides 
with an output signal from the color conversion table. Although a control 
signal level (indicated by .times. in FIG. 10A) at a point at which the 
color printer 304 unstably outputs a signal is not output from the 
nonlinear quantization circuit 306, an average density becomes equal to 
that of the color conversion table 303. 
Even when tone characteristics are the nonlinear recording characteristics 
shown in FIG. 10A, an output having average tone characteristics is 
continuously obtained from the color printer 304. 
AS described above, the color printer 304 can record one color at a time. 
For this reason, the color scanner 301 scans an original four times. The 
color printer 304 prints an yellow ink in the first scanning, and 
sequentially prints magenta and cyan inks, and finally a black ink. The 
scanner 301 and the color printer 304 perform scanning and printing in 
synchronization with each other. A positional error recorded by each ink 
occurs due to the elongation of paper or a limit of mechanical accuracy. 
In addition, the direction of the positional error is not constant, and 
the direction is changed every time a copying operation is performed, when 
the positions of colors are shifted as described above, a reproduced color 
is slightly changed. 
In a conventional technique, a screen angle is used, and recording is 
performed while changing the screen angle of each color. In this manner, 
average shift between ink colors at a position on a paper surface is 
eliminated every time a copying operation is performed, thereby obtaining 
constant color reproduction. This technique is known well. However, when a 
screen angle is used, the resolution may be disadvantageously decreased, 
moire noise may be disadvantageously generated. The technique is not used 
in fields other than the field of printing capable of performing 
sufficient high-definition recording. In contrast to this, in this 
embodiment, error diffusion processing is independently performed for each 
color, and a signal itself varies at random. For this reason, a degree of 
random is not changed with a positional error of several pixels or less at 
any position on a paper surface every time a copying operation is 
performed, and color reproduction does not vary, thereby obtaining stable 
color reproduction. 
According to this embodiment, faithful color reproduction corrected by the 
difference processing circuit 309 can be obtained. However, as shown in 
FIG. 10A, a tone degradation occurs at a highlighted portion, and texture 
noise can be slightly detected due to the tone degradation at the 
highlighted portion. In addition, snow noise is disadvantageously 
increased at the highlighted portion due to color stacking. When a color 
conversion table is prepared by a maximum blackening ratio, although an 
amount of ink to be used is advantageously minimized, a print signal is 
frequently used at the highlighted portion by the operation of undercolor 
removal. That is, when blackening is performed, the density of the color 
ink is decreased by an amount of ink used for the blackening. Therefore, 
an amount of color ink is originally small at a low-density portion. For 
this reason, when the color ink at a chromatic portion is replaced with a 
blackening ink, an amount of color ink is further decreased, and the 
density of the low-density portion is decreased. In this manner, when 
blackening is performed at the low-density portion, recording of a further 
unstable level is performed in the printer. 
For this reason, recording is performed by only three color signals without 
performed blackening and undercolor removal at a low-density portion, and 
four-color printing is performed at a level at which a large amount of 
black ink is used. In this case, a print signal is not frequently used in 
an unstable portion. with the above arrangement, although an ink 
consumption amount is increased, the ink consumption amount is not 
actually increased because the amount of color ink replaced with a black 
ink is originally small at the low-density portion. 
A method of forming a color conversion table for changing a blackening 
amount will be described below. FIG. 12 corresponds to the four-color 
signal generation processing 22 described in FIG. 8. A YMC ink signal 41 
is obtained by the three-color signal generation processing 21 in FIG. 8, 
a blackening plate signal K is defined such that the minimum value of the 
Y', M', and C' inks constitutes a black ink plate, i.e., a blackening 
plate K'(42)=min(Y',M',C'). A blackening ratio P is controlled in 
accordance with a blackening plate signal K' as indicated by the broken 
line in FIG. 13, P=0 is kept until the blackening plate signal reaches a 
predetermined value a, and printing is performed by only three color inks. 
The blackening ratio P is increased at a constant rate until the 
blackening plate signal reaches a point b to gradually increase four-color 
printing. P=100% is kept when the blackening plate signal reaches the 
point b. In this manner, the blackening ratio P (43) is determined, a 
product of the blackening plate signal K' and the blackening ratio P is 
calculated by a multiplier 44, and the product is defined as the 
blackening plate signal K which is to be actually used. A difference 
between the blackening plate signal K and the YMC ink signal 41 subjected 
to undercolor removal is calculated by a subtracter 45, and the YMC ink 
signal 41 is converted into Y', -K, M', -K, C', and -K signals. As 
described above, the YMC ink signal 41 is converted into a four-color 
signal 46. 
In this case, although color reproduction by the three-color signal may be 
different from that by the four-color signal due to the nature of inks, 
when a reverse table is prepared as shown in FIG. 8, faithful color 
reproduction can be obtained. In addition, if a curve indicated by the 
solid line in FIG. 13 is used as a curve of switching three-color printing 
to four-color printing, the three-color printing can be more smoothly 
switched to the four-color printing. Therefore, the three-color printing 
can be performed in using a small blackening plate, and a blackening ratio 
can be set to be 100% in using a large blackening plate. 
When inks are stacked at random, and a method of only removing the 
blackening plate K from a chromatic portion of Y, M, and C inks is 
performed as undercolor removal processing, a color tone is degraded in a 
high-density portion. When a color correction table is prepared by a 
reverse table scheme described in FIG. 8, color reproduction can be 
assured. However, in order to form a table for assuring color reproduction 
at a low-illuminance portion to some extent, the table must be prepared 
such that a large number of color patches must be formed in a large number 
of density levels and are loaded. In this case, the number of operations 
is increased. Therefore, when the following method is used, the table can 
be prepared without loading a large number of color patches. 
This equivalently corresponds to that degradation of a color tone is 
suppressed by blackening. As a method of suppressing degradation of a 
color tone, a GCR (Gray Component Replacement) method is known. According 
to the GCR method, assume that a chromatic signal in actual 4-color 
printing and a chromatic signal in actual 3-color printing are represented 
by I and I', respectively, and printing is performed such that the signal 
I satisfies a relationship of (I'-K)/(1-K). That is, when the blackening 
plate signal K is close to 1, the chromatic signal is close to 1. At this 
time, when the color balance of Y, M, and C is slightly degraded, the 
colors are considerably shifted, and a proper color conversion table 
cannot easily be prepared. In order to correct this, undercolor removal is 
preferably performed by equation (3). 
EQU I=(1+K)(I'-K) (3) 
FIG. 14A is a graph showing a relationship between a signal I' before the 
undercolor removal and a signal I after the undercolor removal by using 
the blackening plate signal K as a parameter. According to FIG. 14A, 
although degradation of a color tone is corrected in setting the 
blackening plate signal K at low level, a function of correcting the 
degradation of the color tone is weakened in setting the blackening plate 
signal K at high level. Even when K=1, a change rate of the signal I after 
the undercolor removal is performed is 2, and the signal I is stable 
against a variation in color balance. Degradation of a color tone and 
stability of a color balance cannot be adjusted in equation (3), but can 
be adjusted in equation (4). 
##EQU2## 
When n is decreased in equation (4), degradation of the color tone of a 
low-density color can be prevented. When p is decreased, degradation of 
the color tone of a high-density color can be prevented, but stability is 
easily degraded by a variation in color balance. For example, when n=0.03 
and P=0.9, a preferable result shown in FIG. 14B is obtained. According to 
equations (3) and (4), the degradation of a color tone can be prevented, 
and stability against a variation in color balance can be improved. 
As described above, according to the fourth embodiment of the present 
invention, a YMCK four-color signal is generated from a RGB three-color 
signal by using a color conversion table or the like, and halftone 
processing is independently performed for each color by a multilevel error 
diffusion method. At this time, uniform and stable color reproduction free 
from texture noise can be obtained. In addition, when nonlinear 
quantization or blackening ratio variation recording is combined to the 
error diffusion method, even when unstable recording is performed, 
preferable color reproduction free from a pseudo contour can be obtained. 
When the functions of undercolor removal are preferably changed, 
degradation of a color tone can be advantageously prevented, and stable 
color reproduction at a high-density portion can be advantageously 
obtained. 
In the fourth embodiment shown in FIG. 7, error processing is performed by 
the difference processing circuit 309 on the basis of data obtained by 
density-converting the output signal 307 from the nonlinear quantization 
circuit 306 by a tone correction table 313 in accordance with the color 
printer 304. 
In the fifth embodiment, as shown in FIG. 15, an difference processing 
circuit 309 uses an output signal 307 from a nonlinear quantization 
circuit 306. In this case, when a color printer 304 has nonlinear tone 
characteristics, color reproduction is shifted. For this reason, according 
to this embodiment, a tone correction table 313 for causing a density 
given by the output signal 307 from the nonlinear quantization circuit 306 
to coincide with the density of the color printer 304 is provided. In this 
manner, preferable color reproduction can be obtained. 
The versatility of this embodiment is improved because the output signal 
307 from the nonlinear quantization circuit 306 includes tone 
characteristics inherent to the color printer 304. However, in order to 
effectively use the ability of the color printer 304, the number of 
multilevel control values of the color printer 304 must be increased. 
According to the fourth and fifth embodiments, in order to obtain faithful 
color reproduction, a large-capacity memory (ROM) must be used in a color 
conversion table 303. In order to cope with this, when the interpolation 
processing circuit 33 is used as shown in FIG. 9, an increase in capacity 
of the color conversion table 303 can be prevented. However, a method of 
forming the color conversion table 303 must be modified for better 
effects. In addition, the interpolation processing circuit 33 inevitably 
has a large circuit scale. 
In order to solve the above drawback, according to the sixth embodiment, as 
shown in FIG. 16, a nonlinear quantization table 402 is combined to a 
color conversion table 404, and the number of quantization levels of the 
color conversion table 404 is reduced to the number of quantization levels 
of a color printer 403. In FIG. 16, a printer of a type of simultaneously 
receiving four colors (Y, M, C, and K), e.g., an ink-jet printer, or an 
electrostatic recording (electrophotographic scheme) printer using a 
four-drum scheme is used as the color printer 403. At this time, the 
printer for simultaneously recording colors is not used, but a printer for 
sequentially recording colors may be used as in the previous embodiments. 
In this case, a color scanner and a printer are interlocked to each other 
every time printing for each color is performed, and an ink signal 
simultaneously performs calculations of four colors. An ink amount is 
calculated four times, and the ink signal is preferably switched to be 
supplied to the printer. 
An RGB signal serving as an input image signal 302 input from a color 
scanner 301 or an input terminal 300 is input to an adder 401. The adder 
401, as in the above embodiments, adds errors having being output to the 
RGB signal to diffuse the errors. An output signal from the adder 401 is 
quantized at an unequal interval by the nonlinear quantization table 402 
having a color conversion function. The number of input addresses of the 
nonlinear quantization table 402 is almost equal to the tone quantization 
number of the color printer 403, or is slightly larger than the tone 
quantization number. In this case, in consideration of nonlinearity in 
color conversion, the nonlinear quantization table 402 is prepared such 
that the number of input address bits is set to be 5 for each of R, G and 
B colors because the color printer 403 has 16 tones for each color and 
four bits for each color. In addition, according to this embodiment, an 
8-bit RGB signal is converted into a 5-bit RGB signal by only inputting 
the upper five bits to the nonlinear quantization table 402. Note that the 
nonlinear quantization table may be used to convert the 8-bit signal to 
the 5-bit signal as needed. The number of output bits of the nonlinear 
quantization table 402 is four for each of Y, M, C, and K colors. 
An output signal from the nonlinear quantization table 402 is converted 
into an RBG signal by the color conversion table 404. The color conversion 
table 404 is obtained by measuring color patches printed by 4-bit data of 
each color by an RGB sensor. At this time, as in the previous embodiment, 
the color patches are formed by a color signal subjected to undercolor 
removal processing by equations (3) and (4). In this manner, a decrease in 
ink amount to be used and prevention of degradation of a color tone caused 
by the positional error of the printer can be achieved. In addition, as 
shown in FIG. 13, color reproduction can be stabilized by using a variable 
blackening ratio P as in the previous embodiments. When a print model of 
the color printer 403 is estimated to some extent even when color patches 
are not actually formed, it is more effective that the color conversion 
table 404 is formed by the estimated value. 
The nonlinear quantization table 402 is used as a reverse conversion table 
of the color conversion table 404. At this time, the signal of a color 
patch which causes unstable recording characteristics, more specifically, 
a color patch having a large amount of snow noise, or the signal of a 
color patch rarely recorded is preferably removed as in the previous 
embodiments so as not to be accessed. In this manner, stable recording 
having reduced snow noise can be performed. 
In this embodiment, as a color which causes unstable recording need not be 
used in printing depending on combinations of colors, not only unstable 
recording caused by nonlinearity of one color can be removed, but also the 
signal of a color patch causing unstable recording in which unstable 
recording is performed by stacking colors can be removed. However, a color 
removed from the nonlinear quantization table 402 is not perfectly 
reproduced (the color is not expressed in one pixel). The color is 
averagely reproduced on the basis of the principle of error diffusion 
recording, and faithful color reproduction can be obtained with snow noise 
being reduced. 
An error between an output signal which is output from the color conversion 
table 404 and obtained by estimating an actual signal used in the above 
printing and an output signal output from the adder 401 and serving as a 
target value to be printed is calculated and input to an error buffer 406. 
A product of the error and a weight coefficient matrix from the weight 
coefficient memory 408 is calculated by the multiplier 407 and input to 
the adder 401, thereby feeding back a color shift. Although the same 
weight coefficient matrix is used for each color, different weight 
coefficient matrices may be used for colors. 
As described above, according to this embodiment, a high-quality image can 
be obtained by using a small-capacity table (the nonlinear quantization 
table 402) for converting an RGB signal to an YMCK signal. According to 
this scheme, however, the error buffer 406 or the like requires three 
channels, and weight coefficients must be calculated at a high speed. 
According to this embodiment, although a signal other than the output 
signal from the nonlinear quantization table 402 is used as an RGB signal, 
the signal need not be used as the RGB signal, but is used as an YMC 
signal or an L*a*b* signal. More specifically, when the color printer 403 
is used, the YMC signal or the L*a*b* signals is effectively used. When 
the input color image signal 302 is used as an YMC signal, the number of 
bits of data input to the nonlinear quantization table 402 may be almost 
equal to the number of quantization levels of the color printer 403. 
When the input color image signal 302 is used as the L*a*b* signal, it is 
more effective that weight coefficients multiplied with the output signal 
from the error buffer 406 are different from each other with respect to 
the L*a*b* signals. For example, the L* signal can use the weight 
coefficients shown in FIG. 11 as in the previous embodiments. However, the 
a* and b* signals may have weight coefficients, e.g., A=0, B=0, C=0, D=1. 
In this manner, the circuits of the error buffer 406 and the multiplier 
407 are simplified. 
FIG. 17 is a view showing the seventh embodiment of the present invention. 
An RGB signal serving as an input image signal 502 input from a color 
scanner 501 or an input terminal 500 is input to an adder 503. The adder 
503 adds errors having being output to each other to diffuse an error 
between a target value to be output and an output signal. 
An output signal from the adder 503 is converted into an YMC ink amount 
signal by an YMC color conversion table 504. A color printer 505 is a 
multilevel color printer capable of expressing, e.g., 16 tones. For this 
reason, the converted ink amount signal is quantized in 16 levels to be 
input to the color printer 505. The number of address bits of data input 
to the YMC color conversion table 504 may be equal to or slightly larger 
than the number tone quantization levels of the color printer 505. In this 
case, the table is prepared such that the number of address bits of data 
input to the YMC color conversion table 504 is set to be 5 for each of R, 
G, and B colors because data output from the color printer 505 has 16 
tones and 4 bits for each color. The number of bits of data output from 
the YMC color conversion table 504 is set to be 4 for each color. 
A blackening plate signal K' which is not corrected is generated from the 
output of the YMC color conversion table 504. The blackening plate signal 
K' has a minimum value of three Y, M, and C colors, i.e., 
K'=min(Y',M',C'), and is calculated by a minimum value circuit (MIN) 506. 
A blackening ratio P is determined in accordance with table (P) 507, a 
product of the blackening plate signal K' and the blackening ratio P is 
calculated by a multiplier 508, and the product is defined as an actual 
blackening plate signal K. 
As shown in FIG. 18, the blackening ratio P can be changed by a blackening 
plate signal which is not corrected. In other words, in a curve a, the 
blackening ratio P is set to be 0 (corresponding to three-color printing) 
when K' is small, and the blackening ratio P is set to be 1 (corresponding 
100% undercolor removal) when K' is increased. On the other hand, in a 
curve c, the blackening ratio P is set to be 1 (corresponding 100% 
undercolor removal). A curve b is located between the curve a and the 
curve b. 
The table 507 is constituted by a ROM which stores data of the curves a, b, 
and c in advance, and data of any one of the curves is selected in 
accordance with a result of image region identification (to be described 
later). In other words, the curve c is selected when data reliably 
represents a character region, and the curve a is selected when the data 
represents an halftone image region. In addition, a curve b is selected 
when the data is seemed to represent the character region. In this manner, 
the blackening plate signal K to be actually printed is determined. Note 
that the multiplier 508 may be constituted by a small-capacity ROM because 
the multiplier 508 may output 4-bit data. 
The blackening plate signal K is input to an undercolor removal circuit 509 
and is subtracted from each of the output signals (Y', M', and C') from 
the YMC color conversion table 504. In other words, actual print signals 
are represented by Y, M, and C, and these signals satisfy conditions of 
Y=Y'-K, M=M'-K, and C=C'-K. The signals Y, M, C, and K are input to the 
color printer 505. 
The signals Y, M, C, K are also input to an RGB conversion table 510. The 
RGB conversion table 510 converts the signals Y, M, C, and K into R, G, 
and B signals obtained when the signals Y, M, C, and K are actually 
printed by the color printer 505 and read by the color scanner 501. 
Therefore, the RGB conversion table 510 can be realized by a ROM in which 
print data obtained by actual printing is used as an address and the R, G, 
and B signals read by the color scanner 501 from the printed output are 
present. The address of data input to the RGB conversion table 510 has 
four bits for each of Y, M, C, and K, thereby obtaining total 16 bits. 
8-bit data for each of R, G, and B is output from the RGB conversion table 
510. 
An error between the signal converted by the RGB conversion table 510 and 
an output signal from the adder 503 is calculated by a subtracter 511, and 
the error is input to an error buffer 512. The content of the error buffer 
512 is input to the multiplier 513, and a product of the content of the 
error buffer 512 and the content of a weight coefficient memory 514 for 
error diffusion is calculated to be fed back to the adder 503, and the 
resultant value is input to the input image signal 502 as a correction 
value of the error. 
Weight coefficients of error diffusion are diffused to four points A, B, C, 
and D shown in FIG. 19 when an image is discriminated as a halftone image 
by image area discrimination. A mark .times. in FIG. 19 represents the 
position of a dot to be printed. At this time, the coefficients A, B, C, 
and D are set to be 1/16, 5/16, 3/16, and 7/16. In this case, errors are 
independently diffused with respect to R, G, and B color components. An 
error Em-k,n-p having independently occurred for each color component 
ahead of an input signal fmn by a pixel (k,p) is weighted with a 
coefficient .alpha.kl of the weight coefficient memory 514, and the 
weighted error is added to input image signal fmn. The resultant value is 
defined as a correction value f'mn of the next pixel, and the correction 
value f'mn is quantized at an unequal interval by the nonlinear 
quantization circuit 104. At this time, the correction value f'mn is 
expressed by equation (1). 
As expressed by equation (2), the error Em-k is represented by a difference 
between the correction value f'mn and a signal of a color component 
developed by the color printer 505, i.e., an output signal Gmn from the 
RGB conversion table 510. 
With the above arrangement, the blackening ratio P is changed depending on 
the value of the blackening amount K' or a result of image region 
discrimination. Even when the effective blackening amount K and an 
undercolor removal ratio or an YMC ink amount are changed, stable color 
reproduction can be obtained because the difference between the correction 
value and the output signal Gmn from the RGB conversion table 510 is added 
to the input image signal fmn to be diffused. 
When an image is discriminated as a character image by image region 
discrimination, all the coefficients A, B, C, and D are set to be 0, 
errors are not diffused, and the resolution is preferentially increased. 
When an image is discriminated as an image which does not reliably 
represent a character region, but the image is discriminated as an image 
which is in a region where an edge is preferably emphasized, only the 
coefficient D is set to be 1, and the remaining coefficients A, B, and C 
are set to be 0 to narrow a diffusion region, thereby preventing image 
blurring. 
Image region discrimination will be described below. The input image signal 
502 is input to a luminance extraction circuit 515. In the luminance 
extraction circuit 515, a luminance is calculated by (R+G+B)/3, and the 
resultant value is input to the image region discrimination circuit 516. 
The details of the image region discrimination circuit 516 are shown in 
FIG. 20. 
In FIG. 20, a luminance signal 601 is input to a high-frequency component 
extraction unit 602 to extract a high-frequency component. In other words, 
a luminance signal obtained by causing the luminance signal 601 to pass 
through a line memory 603 is added to the luminance signal 601 by the 
adder 604, thereby extracting the low-frequency component of the luminance 
signal 601. A difference between the low-frequency component and the 
luminance signal 601 is calculated by the subtracter 605 so as to extract 
the high-frequency component of the luminance signal 601. The 
high-frequency component is input to the pattern matching unit 606, and 
binarized by a binary coding circuit 607, and sequentially input to line 
memories 608 and 609. Three pixels are read out at a time from the binary 
coding circuit 607 and the line memories 608 and 609, and the pixels are 
input, as a 3.times.3 pixel pattern, to a pattern table 610 (9-bit input 
address) constituted by a ROM. 
The pattern table 610 classifies the 3.times.3 pixel patterns of the 
high-frequency components of test images such as a halftone image and a 
character image including a dot image in advance. The 3.times.3 pixel 
patterns are classified into three patterns, i.e., a pattern of a 
character image region, a pattern of a halftone image, and a pattern which 
is located near a character and the edge of which is to be emphasized, and 
the these patterns are stored. 
FIGS. 21A to 21C illustrate some of the above patterns. FIG. 21A shows the 
pattern of a character portion, FIG. 21B shows a pattern which is not 
defined as a character and a line but is to be subjected to edge emphasis, 
and FIG. 21C shows a pattern of a halftone image including a dot image or 
the like, a halftone image of which is to be reproduced at a high 
accuracy. The pattern table 610 causes these patterns to correspond to 
memory addresses, respectively, and the pattern table 610 may be 
constituted by a ROM using an output (in this case, a two-bit output is 
used because the patterns are classified into three types) from the 
pattern table 610 as a content. In this manner, an image region 
discrimination signal 611 is obtained from the luminance signal 601. The 
details of the discrimination method is described in Published Unexamined 
Japanese Patent Application No. 60-204177. 
As described above, the blackening ratio P is switched in accordance with 
the image region discrimination signal 611 obtained as described above. In 
this case, the blackening ratio is set to be high (P=1) in a character 
portion, and the blackening ratio P at a low-density portion is set to be 
low in a halftone image portion by changing the blackening amount K', 
thereby reducing snow noise of an image caused by blackening. Even when 
blackening of 100% is performed in a portion having a large blackening 
amount, the image rarely becomes coarse. For this reason, sufficient 
undercolor removal is performed by setting the blackening ratio P to be 1. 
When the extent of error diffusion is changed by the image region 
discrimination signal 611, image blurring caused by the error diffusion 
can be prevented. In other words, error diffusion is not performed in a 
character portion, and the resolution is preferentially set. In halftone 
images, errors are diffused to four pixels. In the halftone images, errors 
are transmitted to only an adjacent pixel to prevent image blurring. 
In this manner, a sharp image free from color blurring can be obtained in a 
character portion, and smooth color reproduction can be obtained in a 
halftone image portion. In addition, sufficient undercolor removal is 
performed in a portion having a large blackening amount. For this reason, 
an amount of effectively used ink is almost equal to that obtained when 
100% blackening is performed to all images, and an ink consumption amount 
is reduced by 50% compared with 3-color printing, thereby performing 
economical recording. 
The eighth embodiment in which an under color removal method is switched by 
an image area discrimination signal to obtain a high-quality image will be 
described below. The arrangement of the eighth embodiment is the same as 
that of the seventh embodiment except that the under color removal method 
is switched to obtain a high-quality image. In the seventh embodiment, the 
undercolor removal processing is performed such that an effective 
blackening signal K is subtracted from an original ink amount signal. In 
this processing, a problem is not posed in a character image. However, in 
the halftone image, a color tone is degraded by stacking blackening plates 
because color inks are stacked at random. In this case, according to the 
seventh embodiment, although the error is corrected by a loop of error 
diffusion, noise is diffused by the error, and the error influences 
degradation of resolution and an increase in noise. 
As described above, when the color inks are stacked at random, and a 
blackening plate K is only removed from a chromatic ink as undercolor 
removal processing, degradation of a color tone at a high density is 
increased. As a method of suppressing the degradation of a color tone, the 
GCR method is known as described above. In addition, as a method of 
solving the drawback of the GCR method, when equations (3) and (4) are 
used, prevention of degradation of a color tone, stability against a 
variation in color balance, high-quality images of a black character and 
the like can be obtained. 
Referring to FIG. 22, the above undercolor removal processing is 
constituted as an undercolor removal processing table 701, ink amount 
signals Y', M', and C' before correction are set as inputs 702, 703, and 
704 of the undercolor removal processing table 701. In addition, the 
blackening plate signal K is set as an input 705, and an image region 
discrimination signal is set as an input 706. When a character image is 
used, these signals are converted into signals except for the blackening 
plate signal K. When an image other than the character image is used, a 
table for undercolor removal processing expressed by equations (3) and (4) 
is selected, and conversion is performed. When the undercolor removal 
processing is performed as described above, a sharp character image which 
is rarely blurred can be reproduced, and a halftone portion is rarely 
darkened, thereby obtaining a high-quality image. 
The ninth embodiment in which a blackening plate generation signal is 
switched by an image discrimination signal will be described below with 
reference to FIG. 23. In the seventh embodiment, as shown in FIG. 17, a 
minimum value of YMC serving as a blackening plate signal K' is obtained 
by the minimum value circuit 506. When the blackening signal K' is 
determined as in the seventh embodiment, an excessive blackening signal is 
present in a portion corresponding to the chromatic portion of a natural 
image, and the chromatic portion may be darkened. In this case, when a 
blackening plate signal K' is not determined by the minimum value of YMC, 
but is determined by a product of ratios of Y, M, and C areas, the 
chromatic portion is rarely darkened. However, in this case, a blackening 
amount is decreased in a character portion or the like, character image 
quality is degraded. For this reason, it is effective that a blackening 
plate generation scheme is switched by an image region discrimination 
signal as in this embodiment. 
FIG. 23 is a block diagram showing a blackening plate generation system in 
this embodiment. In a character region, a blackening plate generation 
circuit 801 outputs a minimum value of Y, M, and C inks in response to an 
image region discrimination signal 802 as in the seventh embodiment. In a 
halftone region, a product 803 of Y, M, and C ink amounts is output. As a 
detailed arrangement of the blackening plate generation circuit 801, 
values which are calculated in advance may be stored as a ROM table. Other 
arrangements are the same as those of each of the previous embodiments. In 
this manner, a solid black character can be reproduced in a character 
portion, and a halftone portion is not darkened, thereby obtaining natural 
color reproduction. 
The tenth embodiment in which non-color information is subjected to error 
diffusion recording after YMCK color conversion is performed will be 
described below with reference to FIG. 24. In FIG. 24, an input image 
signal 502 is input to a YMCK conversion table 521, and converted into an 
ink amount signal. This ink amount signal has not yet been subjected to 
undercolor removal. As in the seventh embodiment, a blackening ratio (P) 
507 is determined by an image region discrimination signal and a 
blackening amount K' from the non-corrected YMCK color conversion table 
521 so as to calculate an effective blackening amount K, and an ink amount 
signal is processed by an undercolor removal circuit 509 on the basis of 
the effective blackening amount K. 
Printed color signals of an output from the undercolor removal circuit 509 
are input to a next adder 503 in correspondence with a color printer 505 
(color printer in which Y, M, C, and K are sequentially printed in an 
image screen) in which a plurality of colors are not simultaneously 
recorded. An output signal from the adder 503 is input to a nonlinear 
quantization table 402, and is quantized at an unequal interval in 
accordance dance with the multilevel of the adder 503. Subsequently, as in 
the seventh embodiment, the signal is subjected to error diffusion 
recording processing. However, unlike the seventh embodiment, an error is 
calculated in one channel. For this reason, the circuits of an error 
buffer 512, a multiplier 513, and a weight coefficient memory 514 are 
simplified. However, in order to realize high-quality color reproduction, 
the YMCK conversion table 521 requires a large-capacity memory. Therefore, 
as described in FIG. 9, interpolation processing of the table is 
preferably performed. 
In the seventh embodiment, weight coefficients of R, G, and B are commonly 
switched in response to a discrimination signal. In the tenth embodiment, 
the weight coefficients used in the previous embodiments are used in 
printing of Y, M, and C ink colors. However, only when a blackening plate 
is printed, unlike the weight coefficients of the printing of the Y, M, 
and C color inks, all the matrix coefficients A, B, C, and D are set to be 
0 in a character region to eliminate error diffusion so as to obtain 
high-definition image because resolution is most important. In other 
regions, only the coefficient D is set to be 1 to decrease an error 
diffusion area, thereby preferentially setting the resolution. In this 
operation, the quality of a black character is improved. 
In addition, as a modification of the tenth embodiment, although not shown, 
the nonlinear quantization table 402 is controlled in response to an 
output signal from an image region discrimination circuit 516, binary 
quantization or quantization of several levels is performed in a character 
region, and quantization of four or more levels is performed in a halftone 
region, thereby outputting a signal form the nonlinear quantization table 
402. In this manner, image quality is effectively improved in a 
high-definition printer in which unstable recording is easily performed 
(phenomenon such as an increase in snow noise occurs) when the number of 
levels is increased. 
The eleventh embodiment on the basis of a multilevel texture dither method 
will be described below with reference to FIG. 25. According to this 
embodiment, an image signal from an input terminal 100 or a scanner 101 is 
input to a nonlinear multilevel dither table 901. In the nonlinear 
multilevel dither table 901, its content is changed in accordance with X 
and Y on an image by an address circuit 902 to generate a multilevel 
dither output. This multilevel dither output is input to a printer 106, 
and is reproduced as an image. In FIG. 26, a mark .largecircle. shows a 
recording multilevel of the printer 106 in this case. In FIG. 26, unstable 
recording characteristics in the first-level tone are shown. Recording at 
second, third, and fourth levels has stable recording characteristics. 
Accordingly, in order to perform a stable recording, the dither circuit 
does not use the first level, but second, third and fourth levels. In 
other words, the tone shown by marks .largecircle., white recording, and 
solid black recording are used. A tone is expressed in 8 levels (9 levels 
including a density of 0). 
FIG. 27 shows a multilevel dither output at this time. FIG. 28 shows the 
detailed content of nonlinear multilevel dither table 901 capable of 
outputting the multilevel dither output in FIG. 27. As shown in FIG. 28, 
0-level to solid, i.e., 8-level image signals are input to the table 901. 
The XY address circuit 902, recognizes whether the image signal represents 
odd-numbered (2n-1) or even-numbered (2n) image data and odd-numbered 
(2m-1) or even-numbered (2m) image data because the dither matrix is set 
to be a 2.times.2 matrix by a current recording position. In other words, 
the nonlinear dither table 901 is accessed by the image signal and an 
address determined by a position to be recorded, and a content 
corresponding to the address is output. For example, when an input image 
signal can be set at three levels, the position of an image is an 
odd-numbered image and is in an even-numbered line, a signal at level 2 is 
output. The tone characteristics of the image output as described above 
exhibit a tone increasing every two levels. Therefore, the tones indicated 
by .DELTA. and .largecircle. in FIG. 26 can be expressed as recording 
outputs. 
According to the first aspect of the present invention, multilevel 
diffusion recording is performed without using unstable recording levels 
of a recording apparatus, thereby performing stable recording. A tone 
recording level corresponding to an unstable recording level is expressed 
by a set of dots of stable tone levels, a large error is not generated 
unlike binary error diffusion recording, and the contrast of texture noise 
is kept low, thereby obtaining a high-quality image free from snow noise. 
In addition, stability of a recording system can be equivalently improved, 
the linearity of an input image signal can be improved, and a dynamic 
range can be widened. For this reason, even when a variation in input 
image signal or a variation in output recording system occurs, stable 
recording can be performed. 
According to the second aspect of the present invention, undercolor removal 
of an input color image signal is performed, and multilevel error 
diffusion processing of each of colors is performed in response to a 
blackening signal. For this reason, even when positional errors occur in 
an output printer, stable color reproduction can be obtained, and a 
high-quality image free from moire noise or texture noise can be obtained. 
In addition, even when a printer having printer characteristics in which 
color reproduction is degraded by tone characteristics or color stacking 
is used, a signal of only a stable portion is used as a control signal to 
obtain an extremely high-quality image. Even when a variation in color 
balance occurs, stable color reproduction can be advantageously obtained. 
According to the third aspect of the present invention, the amount of ink 
actually used is determined by the image region discrimination signal and 
the blackening amount. For this reason, 100% undercolor removal is 
performed in the character region, and a clear image free from color 
blurring can be obtained. When the blackening amount is large, almost 100% 
undercolor removal is performed in the halftone image region to contribute 
to a decrease in ink consumption amount. When the blackening amount is 
small, almost only threecolor printing is performed to suppress snow 
noise. The weighting coefficient matrix is changed in error diffusion 
recording of the character region requiring high-definition recording and 
a recording color requiring high-definition recording. Therefore, both 
high-definition recording and smoothness in the halftone image region can 
be satisfied. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, and representative devices shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents.