Image forming apparatus

The characteristics such as hue, saturation, edge portion or flat portion of image data are decided for each pixel based on read data of three primary colors. The decision is performed on the data which have been subjected to spatial filtering, and the processing of the image data such as smoothing or edge emphasis are changed according to the detected characteristics. For example, the masking coefficient is selected according to the detected hue. The color data is decreased and the black data is generated, according to the detected saturation. The edge is decided only when the sign of the gradients of the three color data agree with each other. Further, edge detection quantity is detected in the edge decision and the edge emphasis or the smoothing is performed by comparing the edge detection quantity with threshold values only when the edge is detected.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus such as a 
copying machine or a printer which forms a multi-color image. 
2. Description of Related Art 
In a printer or the like for reproducing a full-color image, digital image 
data R, G, B of red, green and blue of primary colors obtained by reading 
a document are transformed to data C, M, Y of the complementary colors of 
cyan, magenta and yellow used for image reproduction. 
Therefore, data processing is performed in order to transform the digital 
data of the three colors, red, green and blue, to data for the three 
colors for image reproduction. 
As to the data processing for a full-color image, following three points 
have to be taken into consideration: (a) the compatibility of the 
visibility of black and the saturation of colors, (b) the improvement of 
color reproducibility, and (c) the compatibility of resolution and 
smoothness. 
As to the first point (a) of the reproduction of black in a full-color 
image, pure black is hardly reproduced by overlapping cyan, magenta and 
yellow toners due to the spectral characteristics of the toners. Then, the 
reproducibility of black is improved by using subtractive mixture of the 
reproduction colors, cyan, magenta and yellow, and by using black paint. 
However, in this method, the visibility of black is improved by increasing 
the degree of black paint, while the saturations of chromatic colors are 
lowered. For a full-color image, the improvement of the pureness of 
achromatic colors has to be compatible with the improvement of the 
saturation of chromatic colors. 
Then, the black paint may be proposed to be performed according to the 
decision of achromatic color or chromatic color. However, the decision 
with use of the read colors of red, green and blue is liable to err due 
for example to Moire patterns for a dot document and to errors and noises 
in a document of flat density and in portions where the hue and the 
brightness change gradually. Especially, the decision at an edge portion 
in a image or at a portion where the brightness varies much is liable to 
be erroneous due to the color, shift of print location or the like, so 
that the black paint may deteriorate the image adversely. Further, in an 
image of low saturation, the decision of achromatic color and that of 
chromatic color may alternate rapidly. Then, because the black paint is 
performed only for pixels decided to be achromatic colors, the resultant 
image seems to include random noises. 
As to the second point (b), the color reproducibility is affected by the 
masking correction which is performed to compensate the discrepancies of 
the characteristics of the filters and of the toners from the ideal 
characteristics. 
The masking correction is usually performed by using linear masking 
coefficients which are determined in order to minimize the average color 
differences over the whole color reproduction region. However, the color 
difference between the original color and the reproduced color is not 
necessarily minimized in some parts in the color reproduction region and 
the errors in color reproduction and in gradation may become larger. Then, 
it is said the secondary masking processing including secondary terms such 
as DR.sub.2, DG.sub.2, DB.sub.2, DR.cndot.DG, DG.cndot.DB and DB.cndot.DR 
is better. However, this needs a complicated and larger circuit. 
In order to solve this problem, a color image read apparatus disclosed in 
Japanese patent laid open Publication No. 16,875/1990 has a plurality of 
masking coefficients each for an important color besides a masking 
coefficient for an image ordinary as to hues and when a user specifies a 
hue, the masking coefficient of the hue can be used. However, it is 
troublesome for a user to specify a hue. Sometimes, a user may specify an 
erroneous hue. Further, it is not necessarily natural that portions of 
different characters are intermingled in an image. 
As to the third point (c), it is better to change a data processing 
technique according to the characteristics of an image. Edges have to be 
emphasized for images such as characters and narrow lines, while half-tone 
images such as photographs have to be smoothed. Selective control between 
edge emphasis and smoothing has been carried out in some prior arts by 
detecting an edge for an input image data in order to discriminate a 
character/photograph portion for a monochromatic image. For example, the 
density of the center pixels and its density gradient are obtained at 
least in the two directions and contrast is emphasized. 
However, simple edge emphasis is not necessarily performed well for a 
full-color image because the image density changes according to the hue 
and the saturation. For example, when color changes from white to red, 
edge emphasis may be performed, while when color changes from red to cyan, 
edge emphasis has not to be performed because the hue changes anomalously 
at an edge as in color ghost phenomenon. An image of human facial skin is 
especially affected by the processing according to region discrimination. 
In a method disclosed in Japanese Patent laid open Publication No. 
171,067/1988, the gradient and the gradient direction of an edge are 
detected for a center pixel for each read color or for each reproduction 
color in a color image, and the data of the read color or the reproduction 
color is corrected according to the detection. However, in this method, 
because the correction is performed for each color, the edge emphasis is 
performed even for a hue change. Thus, the hue in an image does not 
necessarily change naturally. 
It has been considered that the image reproducibility may be enhanced if 
the data processing technique is changed according the characteristics of 
full-color image. For example, regions for character images and those for 
half-tone images are designated and the data processing is changed 
appropriate for each region. In this method, a user has to designates the 
regions. Further, it is troublesome for a user to designate the 
characteristics of an image in detail. The discrimination of a specified 
color is used for example for a color change function provided in a 
printer to change the specified color to another color. In order to 
prevent the error of the color discrimination of the specified color, for 
example a mask processing with use of majority decision for decision 
results is tried. However, the improvement of the color discrimination is 
only a little. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an image forming 
apparatus wherein the reproducibility of full-color image is improved. 
It is another object of the present invention to provide an image forming 
apparatus wherein the pureness of achromatic colors and the saturation of 
chromatic colors can be improved simultaneously for a full-color image. 
It is a further object of the present invention to provide an image forming 
apparatus wherein the deterioration of image at an edge portion can be 
prevented. 
The characteristics such as hue, saturation, edge portion or flat portion 
of full-color image data are decided for each pixel based on read data of 
three primary colors. The decision is performed on the data which have 
been subjected to filtering, and the processing of the image data such as 
smoothing or edge emphasis are changed according to the detected 
characteristics. For example, a plurality of masking coefficients is 
stored in the memory and the masking coefficient is selected according to 
the detected hue. The color data is decreased from the color data and the 
black data is generated for improving the pureness of black, while the 
amounts of the decrease of the color data and of the generated black data 
are controlled according to the detected saturation. The edge is decided 
only when the direction of the gradients of the three color data agree 
with each other. Further, edge emphasis or the smoothing is performed by 
comparing the edge detection quantity with threshold values only when the 
edge is detected. 
An advantage of the present invention is that errors in the decision of the 
characteristics of image can be reduced to improve the reproducibility of 
image by extracting the characteristics of image in a local region around 
a central pixel under processing. 
Another advantage of the present invention is that the precision of region 
decision can be improved, especially for a color image, with use of filter 
processing of input data according to the purpose of the object of region 
decision, to improve color reproduction, the pureness of black, the 
sharpness of characters and narrow lines and the reduction of image noises 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be explained below in the 
following order: 
(a) structure of digital color copying machine 
(b) image data processing 
(b-1) structure of image data processor 
(b-2) region discrimination and outline of image data processing 
(c) density conversion 
(d) black generation 
(e) automatic control of under color remove/black painting in region 
discrimination (decision of achromatic/chromatic color) 
(e-1) object of automatic control of under color remove/black painting 
(e-2) smoothing 
(e-3) decision of chromatic/achromatic color 
(f) automatic masking control in region discriminator (hue decision) 
(f-1) object of automatic masking control 
(g) color correction 
(h) automatic control of edge emphasis/smoothing in region discriminator 
(edge decision) 
(h-1) object of edge emphasis/smoothing 
(h-2) edge detection 
(i) MTF correction 
(a) Structure of Digital Color Copying Machine 
Referring now to the drawings, wherein like reference characters designate 
like or corresponding parts throughout the views, FIG. 1 shows a schematic 
structure of a digital color copying machine which consists mainly of an 
image reader 100 for reading a document image and a main body 200 for 
reproducing the document image. 
In FIG. 1, a scanner includes an exposure lamp 12, a rod lens array 13 to 
collect reflection light from a document put on a platen 15 and a contact 
type CCD color image sensor 14 to convert the collected light to an 
electric signal. The scanner 10 is driven by a motor 11 to move in the 
direction (subscan direction) of the arrow shown in FIG. 1. The optical 
image of the document illuminated by the exposure lamp 12 is converted by 
the image sensor 14 into a multi-level electric signal of red (R), green 
(G) and blue (B). The electric signal is converted by a read signal 
processor 20 to gradation data of yellow (Y), magenta (M), cyan (C) or 
black (K). Then, a print head 31 performs gamma correction of the 
gradation data and a dither processing if necessary, and it converts the 
corrected data to a digital drive signal to drive a laser diode 221 (not 
shown) in the print head 31. 
A laser beam emitted from the laser diode 221 according to the gradation 
data exposes a photoconductor drum 41 driven to be rotated, via a 
reflection mirror 37 as shown with a dot and dash line. Thus, an image of 
the document is formed on the photoconductor of the drum 41. The 
photoconductor drum 41 has been illuminated by an eraser lamp 42 and has 
been sensitized uniformly by a sensitizing charger 43 for each copy before 
the exposure. When the exposure is performed onto the photoconductor in 
the uniformly charged state, an electrostatic latent image is formed on 
the photoconductor drum 41. Then, one of developers 45a-45b of yellow, 
magenta, cyan and black toners is selected to develop the latent image. 
The developed image is transferred by a transfer charger 46 to a paper 
wound on a transfer drum 51. 
The above-mentioned printing process is repeated four times for yellow, 
magenta, cyan and black. At this time, the scanner 10 repeats the scanning 
in synchronization with the motion of the photoconductor drum 41 and the 
transfer drum 51. Then, the paper is isolated from the transfer drum 51 
with the operation of an isolation claw 47, the image is fixed by a fixer 
48 and the paper is carried out to a paper tray 49. In this process, a 
paper is supplied from a paper cassette 50 and is chucked at the top of 
the paper by a chucking mechanism 52 on the transfer drum 51 in order to 
prevent a shift of position on the image transfer. 
FIGS. 2 and 3 show a whole block diagram of the control system of the 
digital color machine. The image reader 100 is controlled by an image 
reader controller 101. The controller 101 controls the exposure lamp 12 
via a drive I/O 103 according to a position signal from a position 
detection switch 102 which indicates the position of a document on the 
platen 15 and controls a scan motor driver 105 via a drive I/O 103. The 
scan motor 11 is driven by the scan motor driver 105. 
On the other hand, the image reader controller 101 is connected via a bus 
to an image controller 106. The image controller 106 is connected to the 
CCD color image sensor 14 and the image signal processor 20. The image 
signal from the CCD color image sensor 14 is processed by the image signal 
processor 20. 
The main body 200 includes a printer controller 201 for controlling the 
copying action and a print head controller 202 for controlling the print 
head 31. The printer controller receives analog signals from various 
sensors 44, 60 and 203-205 for automatic image density control. Various 
data inputted with an operational panel 206 are sent to the printer 
controller 201 via a parallel I/O 207. The printer controller 201 is 
connected to a control ROM 208 storing a control program and a data ROM 
217 storing various data. The printer controller 201 controls a copying 
controller 210 and the display panel 211 according to the data from the 
operational panel 206 and the data ROM 209 under the contents of the 
control ROM 208. Further, the printer controller 201 controls high voltage 
units 214 and 215 for the grid voltage of the sensitizing charger 43 and 
for the developer bias voltage of the developer 45a-45d. 
The print head controller 202 acts according to the control program stored 
in the control ROM 216. The print head controller 202 is connected to the 
image signal processor 202 of the image reader 100 via an image bus and 
performs gamma correction on the basis of the image signal received via 
the image data bus with reference to a conversion table stored in the data 
ROM 217. Further, a dither processing is performed if necessary to express 
gradation. Then, the print head controller 202 controls the laser diode 
controller 220 via the drive I/O 218 and a parallel I/O 216, and the laser 
diode controller 220 controls the emitting of the laser diode 221. 
Further, the print head controller 202 is synchronized with the printer 
controller 201 and with the image signal processor 20 to each other via 
buses. 
(b) Image Signal Processing 
(b-1) Structure of Image Data Processing 
FIG. 4 shows a perspective view of a reading device, in which a surface of 
a document 91 is illuminated by the light source (halogen lamp) 12 having 
an optical spectrum of three wavelengths (R, G and B). The light reflected 
from the document 91 is focused with the rod lens array 13 linearly on the 
light-receiving plane of the CCD sensor 14. The optical system including 
the rod lens array 13, the light source 12 and the CCD color image sensor 
14 is moved in the direction of the arrow shown in FIG. 1, and the optical 
information of the document 91 is converted to an electrical signal by the 
CCD color image sensor 14. 
FIG. 5 shows a block diagram of the image signal processor 20 which 
processes the image signal from the CCD color image sensor 14 via the 
image signal processor 20 to the print head controller 202 as explained 
below. 
In the image signal processor 20, the image signal obtained by the 
photoelectric conversion by the CCD sensor 14 is converted to multi-value 
digital image data of R, G and B by an A/D converter 61. A clock generator 
70 generates clock signals to be sent to the CCD color image sensor 14 and 
to the A/D converter 61. The converted image data is subjected to shading 
correction by a shading correction part 62, and then the image data is 
converted to density data according to logarithmic conversion by a density 
converter 63. Further, a true black data K' is generated from the density 
data by a black generator 64. 
On the other hand, the characteristic of the image data after the shading 
correction is extracted for each pixel on a local region including the 
pixel to classify into flat density portions, edge portions and 
intermediate portions. 
FIG. 6 shows a circuit of the region discriminator 65. After an edge 
detector 84 processes the image data R, G and B received from the shading 
correction part 62 in order to detect an edge, an MTF correction 
controller 85 sends 2-bit filter select signals FA.sub.0 and FA.sub.1 to 
an MTF correction part 67. On the other hand, after a smoothing processor 
81 processes the image data R, G and B for filtering, the image data are 
processed by an under color remove/black paint (UCR/BP) controller 82 and 
by a color correction masking controller 83 to send 2-bit (four steps) 
achromatic-chromatic color decision signals US.sub.1, US.sub.0 and two-bit 
(four kinds) masking coefficient select signals MS.sub.0, MS.sub.1 to a 
color correction processor 66. 
In the color correction processor 66, black color data is generated and 
masking processing is performed at the same time in accordance to the 
achromatic/chromatic decision signals US.sub.1 and US.sub.0 and the 
masking coefficient select signals MS.sub.1 and MS.sub.0 received from the 
region discriminator 65. That is, a black data is generated from the read 
density data and the latters are converted to data of the three 
reproduction colors. Further, in the MTF correction part 67, a digital 
filter is selected in accordance to the filter select signal FS.sub.1 and 
FS.sub.0 from the region discriminator 65 to perform smoothing or edge 
emphasis. 
Next, a magnification change and remove part 68 may change the 
magnification, if necessary. Further, the color balance is adjusted by a 
color balance part 69 to send data to the print head controller 202. 
A signal M/C to indicate monochromatic mode or full color mode is sent from 
the print head controller 202 to the image signal processor 20. 
As shown in FIG. 7, a register 87 sets various control parameters according 
to a data bus D.sub.7 - D.sub.0, an address bus MA.sub.2 - MA.sub.0, a 
chip select signal CS.sub.1 and a write signal WR received from the print 
head controller 202. Then, the register 87 sends control parameters REF0, 
REF1 and REF2 to the under color remove/black paint controller 82, control 
parameters REF3 and REF4 to the MTF correction controller 85 and a control 
signal EDG to the MTF correction part 67. 
(b-2) Region Discrimination and Outline of Image Data Processing 
The outline of region discrimination and processings in relation therewith 
are explained before detailing each processings of the image signal 
processing. In this embodiment, the region discriminator 65 extracts 
following three characteristics from the read data of R, G and B for each 
pixel in a local region including the pixel at the center: (a) 
achromatic/chromatic color (achromatic/chromatic color decision signal US, 
refer section (e-3)), (b) edge detection (filter select signal FS, refer 
section (h-2)), and (c) hue decision (masking coefficient select signal 
MS, refer section (f-2)). Then, the color correction processor 66 
optimizes a black amount (USR/BP ratios) according to the decisions on the 
achromatic/chromatic color and on the edge detection, while the MTF 
correction part 67 performs smoothing or edge emphasis with spatial 
digital filter processing. 
(1) Achromatic color and flat density: The black amount is increased and 
the data of the reproduction colors M, C and Y are smoothed according to 
the result of achromatic/chromatic color decision. 
(2) Edge portion irrespective of achromatic/chromatic color: The result of 
achromatic/chromatic color decision is canceled, the black amount is 
decreased and edge emphasis is performed on the reproduction colors C, M 
and Y. 
(3) Chromatic color and flat density: The black amount is decreased and 
smoothing is performed on the reproduction colors C, M and Y. 
(4) Others: The black amount is intermediate and no edge emphasis and no 
smoothing are performed. 
Further, the color correction processor 66 has linear masking coefficients 
in accordance with a plurality of color groups, so that the masking 
processing is performed by using linear masking coefficients of the color 
group which includes the hue according to the result of hue decision. 
In the monochromatic mode, the black amount is chosen to be zero and the 
linear masking coefficients for monochromatic mode are used. 
(c) density conversion 
The density converter 63 converts the output data of the CCD color image 
sensor 14 so that the document density (OD) is linear for naked eyes. The 
output of the CCD color image sensor 14 has a linear photoelectric 
conversion characteristic according to the intensity of incident light 
(=document reflectance OR). On the other hand, the document reflectance OR 
and the document density OD has a relation of - log OR=OD. Then, the 
nonlinear read characteristic of the CCD color image sensor 14 is 
converted to the linear characteristic by using a reflectance/density 
conversion table. To be concrete, a reflectance/density conversion table 
346 is used to convert the read data for R, G and B of a central pixel 
under processing to density data DR, DG and DB. (d) black generation 
Data C', M', B' and K' of cyan, magenta, blue and black for full-color 
reproduction are made for each scan successively, and a full-color image 
is reproduced with use of total four scans. Black printing is performed 
because pure black is hard to be reproduced by the overlap of cyan, 
magenta and yellow toners owing to the effects of spectral characteristics 
of each toner. Then, in the full color copying machine of this embodiment, 
the reproducibility of black is improved and a full color image is 
realized by using the subtractive mixture of data Y', M' and B' and the 
black painting according to black data K'. 
The black generator 64 generates the black amount K from the components R, 
G and B of red, green and blue which represent the lightness on the 
document as follows: Because the data DR, DG and DB received from the 
density converter 63 are density data of R, G and B components, they are 
equal to the components C', M' and Y' of cyan, magenta and yellow which 
are complementary colors of red, green and blue read by the CCD sensor 14. 
Therefore, the minimum of DR, DG and DB corresponds to the overlap of C', 
M' and Y' on the document, and the minimum can be taken as the black data 
K'. Thus, the black generator 64 detects the black data K'=MIN (DR, DG, 
DB). 
As explained later, the black data K' obtained from the black data is used 
when the data of reproduction colors are derived in the color correction 
part 66. That is, the data C', M' and Y' are subtracted by 
.alpha..cndot.K', while .beta..cndot.K' is sent as the black amount K, 
wherein .alpha. designates a UCR ratio and .beta. designates a BP ratio. 
In order to improve the reproducibility of black, the parameters .alpha. 
and .beta. can be changed after smoothing by four steps according to the 
characteristics of a local region including the central pixel under 
processing. 
Next, the black generator 64 shown in FIG. 9 is explained. A comparator 301 
compares a red data DR with a green data DG, and a 2-input multiplexer 302 
sends the smaller value between DR and DG to a comparator 303. The 
comparator 303 compares the input value with a blue data DB, and a 2-input 
multiplexer 304 sends the smallest value among DR, DG and DB to a 2-input 
multiplexer 305. The 2-input multiplexer 305 sends the minimum (C: full 
color mode) or "00" (N: monochromatic mode) according to the signal M/C. 
The signal M/C determines the image output mode. If the signal is "L" 
level, the mode is full color mode and the minimum is outputted. On the 
other hand, if the signal is "H" by level, the mode is monochromatic mode 
and K' is set as an appropriate value for an actual output image. In this 
embodiment, K' is set always to be zero in the monochromatic mode and the 
data K' for black paint is cleared. Delay circuits 306, 307 and 308 are 
used to adjust timings. It is to be noted that though the reference signs 
C', M' and Y' of the output data are changed from those DR, DG and DB of 
the input data, they represent the same data substantially. 
(e) Automatic Control of Under Color Remove/Black Paint in Region 
Discrimination (Decision of Achromatic/Chromatic Color) 
(e-1) Object of Automatic Control of Under Color Remove/Black Paint 
As explained above, the black generator 64 detects K'=MIN (DR, DG, DB) as 
the black data K' and the color correction part 66 subtracts 
.alpha..cndot.K' from C', M' and Y' and sends .beta..cndot.K' as the K 
amount when the data K is calculated. The UCR ratio .alpha. determines the 
black quantity and the BP ratio .beta. decreases the color data. The 
UCR/BP ratios affect the saturation of color reproduction and the 
visibility of achromatic color. 
The reproducibility of achromatic color is improved if the UCR/BP ratios 
(-.alpha./.beta.) are increased because achromatic color is reproduced 
with pure black K'. On the contrary, the saturation of chromatic color is 
lowered because the output ratio of K' increases. Therefore, the 
improvement of the visibility of achromatic color may be compatible with 
that of the saturation of chromatic color by controlling the UCR/BP ratios 
according to the decision whether the color is achromatic or not. 
However, the decision is liable to err if the decision is performed on the 
read data of R, G and B (primary colors), and the erroneous decision 
causes the image quality to deteriorate. The causes of the erroneous 
decision include errors due to Moire patterns of dot document caused by 
the image reader system and due to errors and noises both on reading a 
document of uniform density and on reading a portion where the hue and 
lightness change gradually. 
First, smoothing processing (method of moving averages with weighting) on 
the read data R, G and B of a central pixel under processing is performed 
in a local region including the pixel. Then, the levels of R, G and B are 
compared to decide if the color of the pixel is an achromatic color or 
not. Further, in order to improve the reproducibility of black, the 
parameters .alpha., .beta. are changed at four steps (refer to FIG. 11) 
according to the characteristics of the region after the smoothing 
processing, and the UCR/BP ratios are increased as the color becomes more 
achromatic. 
(e-2) Smoothing 
Smoothing processing is performed for automatic under color remove/black 
paint of a region of 5.times.5=25 pixels including the central pixel under 
processing. In the smoothing processor 81 shown in FIG. 10, a method of 
moving averages with weighting addition is performed for the central pixel 
under processing for each color with use of 5.times.5 filter 344 on the 
8-bit R, G and B data (level 0-255) normalized with shading correction by 
the shading correction part 62. That is, first, data of four lines are 
stored in four line memories 340, 341, 342 and 343. Then, smoothing is 
performed with the filter 344 for smoothing processing on the center pixel 
by using the data R2 (G2, B2), R3 (G3, B3), R4 (G4, B4) and R5 (G5, B5) of 
the four lines and the data R1 (G1, B1) of the line on which data is being 
received. Next, the smoothed data RS (GS, BS) are sent to the under 
color/black paint part controller 82 and to the color correction masking 
controller 83. In the filter 344, the weighting is given gradually with 
the central pixel under processing at the center as shown in the numerals 
in the filter. 
By using the smoothing processing with the spatial frequency filter 344, it 
becomes possible to prevent a spurious resolution of pixels for high 
frequencies and to extract lower frequency components. Then, the following 
advantages can be obtained: (a) noise reduction in an image of uniform 
density, (b) reduction of Moire pattern of dot document, and (c) smoothing 
in an image where the hue, the lightness and the saturation change 
gradually. Thus, the precision of decision is improved. The data RS (GS, 
BS) smoothed as mentioned above are used for hue decision (automatic 
masking control) in the under color remove/black paint controller 82 and 
for achromatic/chromatic decision (automatic UCR/BP control) in the color 
correction masking controller 83. 
(e-3) Decision of Chromatic/Achromatic Color 
The decision of achromatic color (black) in the under color remove/black 
paint part 82 is performed by using that the read data R, G and B of three 
colors become almost equal to each other for an achromatic color. FIG. 11 
displays the distribution of domains between chromatic color and 
achromatic color for the decision. The decision is performed for each 
pixel. In FIG. 11, the data RS, GS and BG of a pixel which have been 
smoothed are classified in four distribution domains from achromatic color 
to chromatic color, as shown in Table 1: a range KLVL0 is confined by two 
solid lines (GS-REF0 &lt;RS, BS&lt;GS+REF0), a range KLVL1 is confined by two 
dot and dashed lines (GS-REF1&lt;RS, BS&lt;GS+REF1) and a range KLVL2 is 
confined by two dashed lines (GS-REF2&lt;RS, BS&lt;GS+REF2). Then, the 
achromatic/chromatic color decision signal US.sub.1, US.sub.0 is generated 
as shown in Table 1. If the data is in a range displayed as KLVL0, the 
color is decided to be a chromatic color, while if the data is outside the 
range displayed as KLVL2, the color is decided to be a chromatic color. 
Further, two domains are provided between them. Then, the UCR/BP ratios 
are changed stepwise according to the value 0, 1, 2 and 3 of the 2-bit 
decision signal US.sub.0, US.sub.1. The UCR/BP ratios increase as the 
color become more achromatic. 
TABLE 1 
______________________________________ 
______________________________________ 
FS.sub.0 
##STR1## 
##STR2## 
##STR3## 
US.sub.1, 0 
-.alpha./.beta. 
______________________________________ 
L L -- -- 0 -.alpha..sub.0 /.beta..sub.0 
achro- 
matic 
color 
H L -- 1 -.alpha..sub.1 /.beta..sub.1 
.uparw. 
H H L 2 -.alpha..sub.2 /.beta..sub.2 
.dwnarw. 
H H H 3 -.alpha..sub.3 /.beta..sub.3 
chroma- 
H -- -- -- tic color 
______________________________________ 
-.alpha..sub.0 .ltoreq.-.alpha..sub.1 .ltoreq.-.alpha..sub.2 
.ltoreq.-.alpha..sub.3. 
.beta..sub.0 .gtoreq..beta..sub.1 .gtoreq..beta..sub.2 
.gtoreq..beta..sub.3. 
(1.gtoreq..alpha..sub.0-3, .beta..sub.0--3 .gtoreq.0). 
In the circuit of the under color remove/black paint controller 82 shown in 
FIG. 12, it is decided which distribution domain the data RS, RG, RB for 
each pixel belong to. In this decision, a prescribed value is added and 
subtracted from the data GS of reference color to determine the level 
difference from the other two colors. That is, the subtracter 361 
subtracts REF0 from GS and the adder 362 adds REF0 to GS to get GS-REF0 
and GS+REF0. Then, these values are compared with RS, GS and BS by four 
comparators in the twelve comparators 367. Then, the results are sent to a 
NAND gate 369, and KLVL0="L" is sent to a table (ROM) 372 when GS-REF0 
&lt;RS, BS&lt;GS+REF0. Similarly, the subtracter 363 subtracts REF1 from GS and 
the adder 364 adds REF1 to GS to get GS-REF1 and GS+REF1. Then, these 
values are compared with RS, GS and BS by four comparators in the twelve 
comparators 367. Then, the results are sent to a NAND gate 370, and 
KLVL1="L" is sent to the table (ROM) 372 when GS-REF1&lt;RS, BS&lt;GS+REF1. 
Similarly, the subtractor 365 subtracts REF2 from GS and the adder 366 
adds REF2 to GS to get GS- REF2 and GS+REF2. Then, these values are 
compared with RS, GS and BS by four comparators in the twelve comparators 
367. Then, the results are sent to a NAND gate 370, and KLVL2="L" is sent 
to the table (ROM) 372 when GS-REF2&lt;RS, BS&lt;GS+REF2. The table 372 sends 
2-bit achromatic/chromatic color decision signal US.sub.1, US.sub.0 
according to the selection table in Table 1. 
In the table 372, the decision is canceled if the filter select signal 
FS.sub.0 supplied from the MTF correction controller 85 is equal to "L". 
That is, the achromatic/chromatic decision signal US.sub.1, US.sub.0 ="3" 
is sent for decreasing the black amount. The filter select signal FS.sub.0 
is sent if the edge detection amount in the main scan direction or subscan 
direction is more than the prescribed level REF3. This is intended to 
decrease the ratio of errors in the achromatic/chromatic color decision. 
That is, because the pixel is at an edge portion in this case, the 
achromatic/chromatic color decision is liable to err. Then, the 
achromatic/chromatic decision is set beforehand not to be performed for a 
pixel which is decided to be at an edge portion. In other words, the under 
color remove/black paint control is optimized only for a portion of an 
image where the image density is relatively uniform, and this prevents the 
image deterioration due to errors of the decision. 
As explained above, in the under color remove/black paint controller 82 and 
the color correction masking controller 83, various select signals 
(US.sub.1, US.sub.0, MS.sub.1, MS.sub.0, FS.sub.1, FS.sub.0) are generated 
to determine correction parameters and they are sent to the color 
correction part 66 and the MTF correction part 67. 
(f) Automatic Masking Control in Region Discriminator (Hue Decision) 
(f-1) Object of Automatic Masking Control 
Masking calculation is performed in the MTF correction part 37 in order to 
reproduce an image from full-color input data. Linear masking coefficients 
are determined in order to minimize the average color difference over the 
whole color reproduction region. However, the color difference is not 
necessarily minimized in some parts in the color reproduction region and 
the errors for color reproduction and for gradation may become large. 
Then, it is said the second-order masking processing including second 
order terms such as DR.sup.2, DG.sup.2, DB.sup.2, DR.cndot.DG. DG.cndot.DB 
and DB.cndot.DR is better, as mentioned above. However, this needs a 
complicated and larger circuit. 
Then, though this embodiment used the linear masking processing, a 
plurality of linear masking coefficients is provided in the color 
correction processor 66 for the four groups (primary color group, 
complementary color group and two intermediate groups). The color 
correction masking controller 83 decides the image data, and the masking 
coefficient is selected so as to minimize the color difference in the 
decided hue. Thus, the color reproducibility in this processing is similar 
to the secondary masking processing. 
Further, in the masking processing, the data RS, GS and BS which have been 
smoothed by the smoothing processor 81 (FIG. 10) are used in order to 
prevent the errors in the masking as in the case of the 
achromatic/chromatic decision. 
(f-2) Hue Decision 
The hue decision performed by the color correction masking controller 83 
uses a color correction table (ROM) 368 as shown in FIG. 12. The upper 
3-bits of the RS, GS and BS data are inputted to the address A.sub.8 - 
A.sub.0 of the color correction table 368, and the 2-bit masking 
coefficient select signal MS.sub.1, MS.sub.0 are outputted. 
The hue is classified into an R, G, B, W system (primary colors), a C, M, 
Y, BK system (complementary colors) and two intermediate groups. In a cube 
shown in FIG. 13, RS, GS and BS are taken as the three coordinate axes. 
The apexes represent pure components of cyan (C), green (G), blue (B) and 
white (W). Therefore, the R, G, B, W group is located in four small cubes 
including the apexes of (R), (G), (B) and (W). The C, M, Y, BK group is 
similar to the R, G. B, W group. One of the intermediate groups is located 
adjacent to the cubes of the C, M, Y, BK group, while the other of the 
intermediate groups is located adjacent to the cubes of the R, G, B, W 
group. As shown in FIG. 13, for example, in the (R), (M), (BK), (B) plane, 
the area (I) belongs to the C, M, Y, BK group, the area (II) belongs to 
the intermediate group close to the C, M, Y, BK group, the area (III) 
belongs to the intermediate group close to the R, G, B, W group, and the 
area (IV) belongs to the R, G, B, W group. 
As explained above, the masking is divided largely into the R, G, B masking 
and the C, M, Y masking. This is because the two sets of (R, G, B) and (C, 
M, Y) distribute hues sparsely at an appropriate degree. That is, when a 
color sample including hues distributed sparsely at an appropriate degree 
is used, the masking coefficients do not have values not so different from 
each other. Therefore, when the decision on the (R, G, B) group and the 
(C, M, Y) group are performed, the erroneous decision does not cause large 
trouble. Further, the intermediate area between the two groups are divided 
into two groups in order to reduce troubles even if an erroneous decision 
happens. 
The color correction table 368 decides what group the color of the pixel 
belongs to according to the input data RS, GS and BS. If the result is 
(I), MS.sub.1,0 ="3" is outputted; if the result is (II), MS.sub.1,0 ="2" 
is outputted; if the result is (III), MS.sub.1,0 ="1" is outputted; and if 
the result is (IV), MS.sub.1,0 ="0" is outputted. 
(g) Color Correction 
The color correction processor 66 corrects according to the transmission 
characteristics of each filter R, G and B in the CCD color image sensor 14 
and the reflection characteristics of each toner C, M and Y in the printer 
to make match to the ideal color reproduction. For example, the 
transmission characteristics of G filter shown in FIG. 14 and the 
reflection characteristics of magenta toner include non ideal wavelength 
region displayed with hatch in contrast to ideal characteristics. In order 
to correct this discrepancy from the ideal characteristics, the color 
correction processor 66 performs linear correction according to the 
following masking equation besides the above-mentioned black paint: 
##EQU1## 
EQU K'=MIN (DR, DG, DB). (2) 
Because the printing is performed successively on the four colors, cyan, 
magenta, yellow and black, the masking equation (1) is calculated by one 
line for each printing. 
In the circuit of the color correction processor 66 shown in FIG. 16, black 
paint control and color masking processing are performed by using the 
parameter UCR/BP ratios (-.alpha./.beta.) determined in the color 
correction register 826 in correspondence to the correction parameters 
determined in the region discriminator 65. The various kinds of 
coefficients are set in the color correction register displayed in detail 
in FIG. 17 according to the characteristics of the region. 
As displayed in FIG. 17, the color correction register 826 sends masking 
coefficients of the three colors (A.sub.ci, B.sub.ci, C.sub.ci, A.sub.mi, 
B.sub.mi, C.sub.mi, A.sub.yi, B.sub.yi, B.sub.yi), which are an element 
(M).sub.ji (j=c, m, y; i=1, 2, 3) of 3 .times.3 matrix M.sub.k, and UCR/BP 
ratios (-.alpha./.beta.) and d. The first masking coefficient M.sub.0 
(k=0) to be selected when MS.sub.1,0 ="0" makes the color difference small 
for primary colors R, G, B, while the fourth masking coefficient M.sub.3 
(k=3) to be selected when MS.sub.1,0 ="3" makes the color difference small 
for complementary colors C, M, Y. Further, the second masking coefficient 
M.sub.2 (k=1) is obtained as (2/3)M.sub.0 +(1/3)M.sub.3 with weighting for 
primary colors, while the third masking coefficient M.sub.3 (k=2) is 
obtained as (1/3)M.sub.0 +(2/3)M.sub. 3 with weighting for complementary 
colors. That is, the masking coefficients of the intermediate groups are 
set by mixing the masking coefficients for primary colors and for 
complementary colors in order to reduce hue change due to erroneous 
decision on color reproduction. 
The following matrix shows the masking coefficient M.sub.0 for R, G and B: 
##EQU2## 
Further, the following matrix shows the masking coefficient M.sub.3 for C, 
M and Y: 
##EQU3## 
For comparison, an ordinary masking coefficient will also be shown below. 
##EQU4## 
In the color correction register shown in FIG. 17, a register 861 sends the 
above-mentioned four kinds of masking coefficients for each color to 
multiplexers 862, 863 and 864 according to the data bus (MD.sub.7 
-MD.sub.0), the address bus (MA.sub.4 -MA.sub.0), a chip select signal 
CS.sub.0 and write signal WR from the print head controller 202. On the 
other hand, the multiplexers 862, 863 and 864 select a kind of masking 
coefficient according to the select signal MS.sub.1,0 received from the 
color correction masking controller 83 and send it to the 2-input 
multiplexers 865, 866 and 867. Further, the 2-input multiplexers 865, 866 
and 867 also receive coefficients D.sub.c, D.sub.m, D.sub.y for 
monochromatic mode. The multiplexers 865, 866 and 867 select one of them 
according to the mode select signal M/C. On the other hand, the four kinds 
of UCR/BP ratios are selected by a multiplexer 868 according to the signal 
US.sub.1,0. 
The constant "d" displayed in FIG. 17 is used to improve the visibility at 
a low density. However, it is not explained in detail here for the brevity 
of explanation. In the above-mentioned processing, "d" is set to be zero 
for the brevity of explanation. 
The color correction processor shown in FIG. 16 consists of the black paint 
part 82 and the color correction masking part 83. In the black paint part 
82, when the under color is removed to print C, M, Y colors, the black 
data K' received from the black generator 64 is multiplied with the UCR 
ratio (-.alpha.) received from the color correction register 826. Then, 
the product (-.alpha..cndot.K') is added with the complementary color data 
C', M' and Y' by adders 821, 822 and 823 to send the results as under 
color remove values C.sub.1, M.sub.1 and Y.sub.1. 0n the other hand, when 
black paint is controlled for printing black, the black quantity K' is 
multiplied with the BP ratio .beta. received from the color correction 
register 926 by an adder 824 to sent the product (.beta..cndot.K') via a 
limiter 834 to an adder 835. 
In the color correction masking part, multipliers 831, 832 and 833 
multiplies the data C.sub.1, M.sub.1, Y.sub.1 with the masking 
coefficients (A.sub.c -C.sub.c, A.sub.m -C.sub.m, A.sub.y -C.sub.y) 
received from the color correction register 826. Then, the obtained 
multiplied values C.sub.2, M.sub.2, Y.sub.2 are added in an adder 835 to 
give a data VIDEO1. At this time, the output from a limiter 834 is cleared 
as "00", and the adder 835 sends the result of the addition of C.sub.2, 
M.sub.2 and Y.sub.2. 
On the other hand, when black paint is controlled, the color correction 
register 826 sets "00" in the multipliers 831, 832 and 833. Therefore, 
C.sub.2, M.sub.2 and Y.sub.2 are cleared and only K.sub.1 (=K.sub.2) is 
sent through the limiter 834 as the VIDEO1 data. 
Examples of the masking correction effect will be displayed below. FIG. 18 
shows document colors (represented as open circles) and reproduction 
colors (represented as solid circles) in the uniform color space of CIE 
1976 by using L*a*b* system when the ordinary masking coefficient is used. 
In FIG. 18, the a*-b* plane displays hue and saturation and the L* 
direction perpendicular to the a*-b* plane displays lightness.) The 
difference between the document color and the reproduction color 
corresponds the color difference. In this case, the average color 
difference of only R, G and B is 10.5334, while that of only C, M and Y is 
4.0029. 
FIG. 19 displays document colors (open circles) and reproduction colors 
(solid circles) in the a*-b* plane when the masking coefficient M.sub.0 
for R, G and B is used. In this case, the average color difference of only 
R, G and B is 3.8576 which is much smaller than that obtained with use of 
the ordinary masking coefficient. The color difference of only C, M and Y 
is 12.1797. Further, FIG. 20 displays document colors (open circles) and 
reproduction colors (solid circles) in the a*-b* plane when the masking 
coefficient M.sub.3 for C, M and Y is used. In this case, the average 
color difference of only C, M and Y is 2.43782 which is much smaller than 
that obtained with use of the ordinary masking coefficient. As explained 
above, the color difference of a hue can be decreased by selecting an 
appropriate masking coefficient. 
When monochromatic mode is set with the M/C signal, the color reproduction 
is performed with use of a single color. The monochromatic reproduction 
means that the density information according to the sensitivity (relative 
luminous sensitivity) with which a man senses light when toners of one of 
C, M, Y, K, R (M+Y), G (B+Y) and B (C+M) are used. Therefore, the relative 
luminous sensitivity information (MC) may be obtained with use of linear 
processing with the masking coefficient, as in the above-mentioned masking 
processing. 
EQU MC=D.sub.c .cndot.C'+D.sub.m .cndot.M'+D.sub.y .cndot.Y'. 
That is, the color correction register 826 sets D.sub.c, D.sub.m and 
D.sub.y as the masking coefficients, and the data MC is sent as the data 
VIDEO1. The masking coefficients are determined according to the kind of 
toners in correspondence with the sensitivity. 
As explained above, the black paint is not carried out in this case. That 
is, in the black generation part 64 shown in FIG. 9, K'="00" is generated 
always in the monochromatic mode. 
(h) Automatic Control of Edge Emphasis/Smoothing in Region Discriminator 
(Edge Decision) 
(h-1) Object of Edge Emphasis/Smoothing 
In general, as to a monochromatic image, character/photograph automatic 
discrimination is performed according to the density change or density 
distribution of image. Then, edge emphasis is performed for a character 
image, while smoothing is performed for a photograph image. Thus, the 
sharpening and smoothing of image can be made compatible to optimize the 
MTF correction. 
However, as already mentioned above, as to a color image, simple edge 
emphasis is not necessarily performed well because the image density 
changes according to the hue and the saturation. For example, when color 
changes from white to red, edge emphasis may be performed, while when 
color changes from red to cyan, edge emphasis has not to be performed 
because the hue changes anomalously at an edge. An image of human facial 
skin is especially affected by such processing. Therefore, the control has 
to be performed by extracting only a change in the lightness of image. 
(h-2) Edge Detection 
In the edge detection part 84 shown in FIG. 10, the edge detection is 
performed on the 8-bit R, G, B data (level 0-255) normalized for shading 
correction by the shading correction part 62 as to an region around a 
central pixel under processing, for each color, both in the main scan 
direction and in the subscan direction. That is, data of four lines are 
stored successively in the four line memories 340, 341, 342 and 343. Then, 
by using the data R2 (G2, B2), R3 (G3, B3), R4 (G4, B4) and R5 (G5, B5) of 
the four lines and the data R1 (G1, B1) of the line on which data is being 
received, an edge is detected with a filter 345 for detecting an edge in 
the main scan direction and with another filter 346 for detecting an edge 
in the subscan direction, on the center pixel in the central line. Next, 
the output data RE1 (GE1, BE1) and RE2 (GE2, BE2) for the two directions 
are sent to the MTF correction controller 85. 
In the above-mentioned edge detection with use of the filters 345 and 346, 
the gradient and the gradient direction are extracted for the two 
directions. The gradient means the absolute value of the output data RE1 
(GE1, BE1) and RE2 (GE2, BE2) and the gradient direction means the sign 
(plus or minus) of the gradient. In an edge portion where the lightness 
changes rapidly, a hue change such as color ghost phenomenon may occur. 
Therefore, the output data are used to extract a portion where an error in 
achromatic/chromatic color decision is liable to occur and to select a 
region where the MTF correction has to be performed. 
As explained above, the data of the central pixel under processing is sent 
to the density converter 63 to be changed to the density data DR (DG, DB) 
with the reflection light quantity/density conversion table 347. 
FIG. 21 shows the circuit of the MTF correction controller 85 to control 
the MTF automatic control. In this circuit, an flat density area, an edge 
and the intermediate area between the flat density area and the edge are 
selected according to the signal from the edge detector 84. A flat density 
area means an area where the edge detection quantity of R, G, B data is 
smaller than a threshold value (REF3) both in the main scan direction and 
in the subscan direction. Then, the filter select signal FS.sub.0 ="L" is 
sent. On the other hand, an edge area means an area where the edge 
detection quantity of R, G, B data is larger than a threshold value REF4 
in the two directions and the gradient directions in the two directions 
agree with each other. Then, the filter select signal FS.sub.1 ="L" is 
sent. In order to prevent errors due to hue change, an edge is detected as 
to lightness change between achromatic colors (such as white, black-like 
base level, and black characters and narrow lines ) and as to a change 
between an achromatic color and a chromatic color (such as white, ground 
level such as color patch, and red/blue characters and narrow lines). If 
both filter select signals are not "L", the area is in the intermediate 
area. 
In the MTF correction control circuit shown in FIG. 21, edge detection 
quantities RE1, GE1 and BE1 in the main scan direction are converted to 
absolute values RE1', GE1' and BE1' by the absolute value detection 
circuits 381, 382 and 383. The absolute values RE1', GE1' and BE1' are 
compared with the threshold value REF3 by the comparator 390, and if all 
the absolute values are smaller than the threshold value REF3, a signal is 
sent via a negative logic AND gate 391 to a negative logic AND gate 395. 
The absolute values RE1', GE1' and BE1' are also compared with the 
threshold value REF4 by the comparator 390, and if all the absolute values 
are smaller than the threshold value REF4, a signal is sent via a negative 
logic AND gate 392 to a negative logic AND gate 396. On the other hand, 
the gradient determination circuit 384 detects the gradient (sign) of the 
edge from the edge detection quantities RE1, GE1 and BE1, and the sign is 
sent to the negative logic AND gate 396. Therefore, the AND gate 366 sends 
a filter select signal FS.sub.1 (="L") via an OR gate 398 if the edge 
detection quantity in the main scan direction is larger than the threshold 
value REF4 and the gradient directions of the R, G and B data agree with 
each other (or if the pixel is decided to be at an edge portion). 
Similarly, edge detection quantities RE2, GE2 and BE2 in the subscan 
direction are converted to absolute values RE2', GE2' and BE2' by the 
absolute value detection circuits 385,386 and 387. The absolute values 
RE2', GE2' and BE2' are compared with the threshold value REF3 by the 
comparator 390, and if all the absolute values are smaller than the 
threshold value REF3, a signal is sent via a negative logic AND gate 393 
to the negative logic AND gate 395. Therefore, the AND gate 395 sends a 
filter select signal FS.sub.0 if edge detection quantities are smaller 
than the threshold value REF3 both in the main scan direction and in the 
subscan direction (flat density portion). Similarly, the absolute values 
RE2', GE2' and BE2' are also compared with the threshold value REF4 by the 
comparator 390, and if all the absolute values are smaller than the 
threshold value REF4, a signal is sent via a negative logic AND gate 394 
to the negative logic AND gate 397. On the other hand, a gradient 
determination circuit 388 detects the gradient (sign) of the edge from the 
edge detection quantities RE2, GE2 and BE2, and the gradient direction is 
sent to the negative logic AND gate 397. Therefore, the AND gate 367 sends 
a filter select signal FS.sub.1 (="L") via the OR gate 398 if the edge 
detection quantity in the subscan direction is larger than the threshold 
value REF4 and the gradient directions of the R, G and B data agree with 
each other (or if the pixel is decided to be at an edge portion). 
FIG. 22 displays schematically the edge detection quantity (GE1) with use 
of the filter 345 when a G image data changes in the main scan direction. 
The edge detection quantity is compared with the threshold values REF3 and 
REF4, and filter select signals FS.sub.0 and FS.sub.1 are sent according 
to the comparison results. The signal FS.sub.1 is sent if either of the 
gradient signals SINA and SINB is "L". 
The threshold values REF3 and REF4 used in the MTF correction controller 85 
can be adjusted with the setting of sharpness externally (refer to FIG. 
7). For example, if a user intends to intensify sharpness, the threshold 
values REF3 and REF4 are set smaller. 
In this embodiment, REF3 is set to be smaller than REF4. However, REF3 may 
be set to be larger than REF4 if necessary. 
(i) MTF Correction 
The filter select signals FS.sub.1 and FS.sub.0 set by the MTF correction 
controller 85 are used to select a spatial filter in the MTF correction 
part 66. If FS.sub.0 ="L" (flat density portion) or if no edge is detected 
on the data R, B, G in the main scan and subscan directions for each 
color, the smoothing processing is performed on the data C, M, Y, K 
converted from the R, G, B data, and the achromatic/chromatic color 
decision is allowed. For an image of low saturation, if the 
achromatic/chromatic color decision is allowed, a large change in the K 
quantity seems to be random image noises. Therefore, the MTF correction 
part performs smoothing so as not to take attention. (This is also a 
reason that the achromatic/chromatic decision is classified in four steps. 
However, this is insufficient, and the noises are reduced further for 
pixels decided to be achromatic colors.) Further, image noises and Moire 
patterns due to reading can be also reduced and smoothing becomes possible 
of a portion such as a photograph where the lightness, saturation and hue 
change gradually. 
If FS.sub.1 ="L" (edge portion), the output of the Laplacian filter 324 is 
allowed, and the edge emphasis of an image is performed by adding with the 
central pixel under processing. Therefore, the sharpening of image can be 
performed at an edge portion of achromatic color without increasing the 
UCR/BP ratios and the visibility is improved. 
The MTF correction part 66 shown in FIG. 23 performs edge emphasis and 
smoothing with use of 2-dimensional digital filter of FIR. First, data of 
four lines are stored in four line memories 320, 321, 322 and 323 
successively. Then, the data of four lines and the data of a line which is 
being received are processed with a 5.times.5 digital filter 324 for 
second differentials (for edge emphasis) and with a 5.times.5 digital 
filter 325 for smoothing to be sent to a multiplier 326 and to a 2-input 
multiplexer 328, respectively. The multiplier 326 sends a product of the 
output of the digital filter 324 and an EDG value to a 2-input multiplexer 
327, which sends the product or "00" according as the filter select signal 
FS.sub.1 ="L" (edge portion) or "H". On the other hand, the multiplexer 
328 selects the output smoothed by the digital filter 325 or the output of 
the pixel from the line memory 321 not smoothed by the digital filter 325 
according as the filter select signal FS.sub.0 = "L" (flat density 
portion) or "H" to send to an adder 329. The adder 329 adds the two inputs 
to send the result as a signal VIDEO2, 
The secondary differential filter 324 used for edge emphasis detects an 
edge emphasis quantity of image, and edge emphasis is performed by adding 
the result of the linear transformation of the edge emphasis quantity 
obtained by the filter with the central pixel under processing (original 
image +secondary differential). That is, if FS.sub.1 ="L" (edge portion), 
the adder 329 adds the data for edge emphasis is added to the data of the 
central pixel under processing. 
On the other hand, the filter 325 for smoothing with a method of moving 
averages with weighting addition of peripheral pixels reduces image noises 
and results smooth image data. (The method of moving averages prevents 
spurious resolutions such as Moire patterns by using filter processing.) 
That is, if FS.sub.0 ="L" (flat density portion), only the smoothed data 
are sent from the adder 329. 
The EDG value which affects edge emphasis in the MTF correction part 66 can 
be adjusted externally by setting sharpness (refer to FIG. 7). For 
example, the EGE value may be increased when the sharpness is intended to 
be increased. 
The MTF processing explained above performs the sharpening and smoothing 
not of the reproduction data C, M, Y, but of the read data R, G, B. This 
is because the similar processing on the reproduction data performs edge 
emphasis even for a portion where the hue changes to deteriorate the color 
reproduction. Then, the MTF correction is performed selectively by 
extracting a change in lightness of the read data R, G, B. 
Although the present invention has been fully described in connection with 
the preferred embodiments thereof with reference to the accompanying 
drawings, it is to be noted that various changes and modifications are 
apparent to those skilled in the art. Such changes and modifications are 
to be understood as included within the scope of the present invention as 
defined by the appended claims unless they depart therefrom.