JP3245883B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reproducing apparatus such as a copying machine or a printer which converts read data of red, green and blue into data of reproduced colors of cyan, magenta and yellow to form a multi-color image. About.

[0002]

2. Description of the Related Art In a printer or the like which reproduces an image in full color, red, green and blue digital image data R, G and B read from an original are reproduced in three colors cyan, magenta and yellow C, M and Y for color reproduction. And reproduce the image. For this reason, data processing for converting red, green, and blue digital data obtained by scanning a document into three-color data for image reproduction is performed.

[0003]

In the data processing of a full-color image, it is necessary to consider both the vividness of black and the color saturation. That is, in reproducing black in a full-color image, cyan C,
Even if magenta M and yellow Y are superimposed to reproduce black, it is difficult to reproduce clear black due to the influence of the spectral characteristics of each toner. Therefore, the reproducibility of black is improved and full color is realized by the subtractive color mixture method using the reproduced color data Y, M, and C and the black printing using the black data K. However, in this method, the sharpness of black improves as the degree of black printing increases, but the saturation of chromatic colors decreases. Therefore, it is necessary to achieve both the improvement of the achromatic color sharpness and the improvement of the chromatic color saturation.

Therefore, it is conceivable to determine whether to perform black printing by determining whether the printing is achromatic or chromatic. But,
When it is simply determined whether the read color of red, green, or blue is an achromatic color or a chromatic color, there are many erroneous determinations, and image deterioration is severe. The causes are halftone originals and moire in the reading system, originals and hues with uniform density, errors in the reading system in portions where the brightness changes gradually, and noise.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus that achieves both improvement in achromatic color sharpness and chromatic color saturation in a full-color image.

[0006]

An image forming apparatus according to the present invention comprises an input means for inputting red, green, and blue digital data obtained by scanning an original, and a reading color input by the input means. Black generating means for obtaining the minimum value of digital data, smoothing means for smoothing digital data of red, green and blue reading colors inputted by the input means, and digital data smoothed by the smoothing means for each pixel. Saturation determining means for determining the degree, processed in parallel with the determination of the saturation, edge determining means for determining the edge of the data is not smoothed, and when the edge is determined by the edge determining means,
Regardless of the determination result by the saturation determination unit, a predetermined first coefficient and a second coefficient are selected, and when the edge determination unit determines that the image is not an edge, the first coefficient according to the determination result by the saturation determination unit is determined. Using a selecting means for selecting a coefficient and a second coefficient, and a first coefficient and a second coefficient selected by the selecting means, a product of a minimum value and a first coefficient obtained by the black generating means is created as black data; The undercolor removal black printing means for subtracting the product of the minimum value and the second coefficient from the read color digital data, and the read color data processed by the undercolor removal black printing means are converted to cyan and magenta. And data conversion means for converting data into yellow reproduction color data. Preferably, the determination unit determines the saturation based on a level difference between three colors of red, green, and blue.

[0007]

The black generation means finds the minimum value of the digital data of the read color input by the input means. Since the saturation determination unit determines the saturation of the digital data obtained by smoothing the input digital data of the read color, the determination accuracy of the saturation is improved. In addition, the optimum first and second coefficients can be selected in consideration of the edge determination. Thereby, undercolor removal and black printing can be appropriately performed.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A digital color copying machine according to an embodiment of the present invention will be described below in the following order with reference to the accompanying drawings. (a) Configuration of digital color copier (b) Image signal processing (b-1) Configuration of image signal processing unit (b-2) Results of area determination and outline of image signal processing (c) Density conversion unit (d) Black generation unit (e) Under color removal / black printing automatic control in area discrimination unit (achromatic chromatic color judgment) (e-1) Purpose of under color removal / black printing automatic control (e-2) Smoothing process ( e-3) Achromatic / chromatic color judgment (f) Automatic masking control (hue judgment) in area discriminating section (f-1) Purpose of automatic masking control (f-2) Hue judgment (g) Color correction processing section (h) (H-1) Purpose of automatic edge enhancement / smoothing control (n-2) Edge detection (i) MTF correction unit

(A) Configuration of Digital Color Copier FIG. 1 is a longitudinal sectional view showing the overall configuration of a digital color copier according to an embodiment of the present invention. The digital color copier includes an image reader unit 100 for reading a document image, and a main body unit 20 for reproducing an image read by the image reader unit.
It is roughly divided into 0.

In FIG. 1, a scanner 10 includes an exposure lamp 12 for irradiating a document, a rod lens array 13 for condensing light reflected from the document, and a contact type CCD for converting the condensed light into an electric signal. A color image sensor 14 is provided. The scanner 10 uses the motor 1 when reading a document.
1, moves in the direction of the arrow (sub-scanning direction), and scans the original placed on the platen 15. The image of the document surface irradiated by the exposure lamp 12 is photoelectrically converted by the image sensor 14. The multi-valued electrical signals of the three colors R, G, and B obtained by the image sensor 14 are read by the read signal processing unit 20 into yellow (Y), magenta (M), and cyan.
(C) or black (K). Next, the print head unit 31 corrects the input gradation data according to the gradation characteristics of this photoconductor (γ
After performing correction and dither processing as necessary, the corrected image data is D / A-converted to generate a laser diode drive signal, and the drive signal generates a laser diode 221 (see FIG. (Not shown).

The laser diode 2 corresponding to the gradation data
The laser beam generated from 21 exposes the photosensitive drum 41, which is driven to rotate, via a reflecting mirror 37, as shown by the dashed line in FIG. As a result, an image of the document is formed on the photoconductor of the photoconductor drum 41. Photoconductor drum 41
Before the exposure for each copy
And is charged by the charging charger 43. When exposure is performed in this uniformly charged state, an electrostatic latent image is formed on the photosensitive drum 41. Only one of the yellow, magenta, cyan, and black toner developing units 45a to 45d is selected to develop the electrostatic latent image on the photosensitive drum 41. The developed image is transferred by the transfer charger 46 onto a copy paper wound around the transfer drum 51.

The above printing process is repeated for yellow, magenta, cyan and black. At this time, the scanner 10 repeats the scanning operation in synchronization with the operations of the photosensitive drum 41 and the transfer drum 51. Thereafter, the copy paper is separated from the transfer drum 51 by operating the separation claw 47, is fixed through the fixing device 48, and is discharged to the discharge tray 49. The copy paper is fed from a paper cassette 50, and the leading end thereof is chucked by a chucking mechanism 52 on the transfer drum 51, so that no positional deviation occurs during transfer.

FIGS. 2 and 3 are block diagrams showing the entire control system of the digital color copying machine. Image reader unit 100
Is controlled by the image reader control unit 101. The image reader control unit 101 controls the exposure lamp 12 via the drive I / O103 according to a position signal from the position detection switch 102 indicating the position of the document on the platen 15, and also controls the drum I / O 103 and the parallel I / O 103. O
A scan motor driver 105 is controlled via the control unit 104. The scan motor 11 is a scan motor driver 10
5 driven.

On the other hand, the image reader control unit 101 is connected to the image control unit 106 by a bus. Image control unit 1
Reference numeral 06 is connected to each of the CCD color image sensor 14 and the image signal processing unit 20 via a bus. An image signal from the CCD color image sensor 14 is input to an image signal processing unit 20 described later and processed.

The main unit 200 includes a printer control unit 201 for controlling the general copying operation and a print head control unit 202 for controlling the print head unit 31.
Analog signals from the various sensors 44, 60, and 203 to 205 for automatic density control are input to the printer control unit 201. In addition, the printer control unit 2 is input via the parallel I / O 207 by key input to the operation panel 206.
01 is input with various data. Printer control unit 201
Is a control ROM 208 in which a control program is stored.
And a data ROM 209 storing various data. The printer control unit 201 includes various sensors 44 and 6
0, 203 to 205, operation panel 206 and data R
The data from the OM 209 controls the copy control unit 210 and the display panel 211 in accordance with the contents of the control ROM 208. Further, in order to perform automatic density compensation control, the grid of the charger is connected via the parallel I / O 212 and the drive I / O 213. High voltage unit 214 for voltage generation
And the high-voltage unit 215 for generating a developing device bias voltage.

The print head control unit 202 controls the control RO.
It operates according to the control program stored in the M216, is connected to the image signal processing unit 20 of the image reader unit 100 by an image data bus, and operates based on an image signal input through the image data bus. The gamma correction is performed with reference to the contents of the data ROM 217 storing the gamma correction conversion table. Further, when the multi-valued dither method is used as the gradation expression method, dither processing is performed, and the drive I / The laser diode driver 220 is controlled via O218 and parallel I / O219. The laser diode 221 is a laser diode driver 22
The light emission is controlled by 0. The print head control unit 202 is connected to the printer control unit 201, the image signal processing unit 20, and the image reader control unit 101 via a bus and is synchronized with each other.

(B) Image Signal Processing (b-1) Configuration of Image Signal Processing Unit FIG. 4 is a perspective view of the reading unit. 3 wavelengths in the reading section
Light source (halogen lamp) 1 with (R, G, B) spectral distribution
2 irradiates the surface of the original 91, and the reflected light is linearly imaged by the rod lens array 13 on the light receiving surface of the close-contact CCD color image sensor 14 in a line shape. The optical system including the rod lens array 13, the light source 12 and the CCD color image sensor 14 is line-scanned in the direction of the arrow in FIG. 1 and converts the light information of the original 91 into electric signals by the CCD color image sensor 14.

FIG. 5 is a block diagram of the image signal processing section 20. Referring to FIG. 5, processing of image signals from the CCD color image sensor 14 to the print head control section 202 via the image signal processing section 20 will be described. Will be described.
In the image signal processing unit 20, the image signal photoelectrically converted by the CCD color image sensor 14 is converted into an A / D signal.
The converter 61 converts the data into R, G, B multi-valued digital image data. The clock generator 70 generates a clock and transmits the clock to the CCD sensor 14 and the A / D converter 61. The converted image data is reflection data of the original 91 after being subjected to predetermined shading correction by the shading correction unit 62, so that the density conversion unit 63
It is converted to density data of the actual image by performing log conversion. Further, the black generator 64 generates true black data K ′ from the R, G, B data.

On the other hand, with respect to the image data which has been subjected to the shading correction, the area discriminating section 65 extracts, for each pixel, the characteristics of the image for the local area to which the pixel belongs, and obtains a flat density portion, an edge portion, and an intermediate portion of the image. Is performed. FIG. 6 shows a circuit configuration of the area determining unit 65. The image data R, G, and B output from the shading correction unit 62 are detected by the edge detection unit 84, and then the MTF correction control unit 85 outputs the 2-bit filter selection signals FS ₁ and FS ₀ to the MTF correction unit. Send to 67. on the other hand,
The image data R, G, B are subjected to the filtering process in the smoothing processing unit 81, and then the under color removal / black printing (UCR
/ BP) control unit 82 and color correction masking control unit 83, and two bits (4
Achromatic / chromatic color determination signals US ₁ , US ₀ and 2 bits
It outputs (four types) masking coefficient selection signals MS ₁ and MS ₀ .

In the color correction processing section 66, black data generation processing and masking processing are performed in accordance with the achromatic / chromatic color determination signals US ₁ and US ₀ and the masking coefficient selection signals MS ₁ and MS ₀ from the area determination section 65. At the same time. That is, black data is generated, subtracted from the read data, and the read data is converted into data of three reproduction colors of C, M, and Y. Further, the MTF correction section 67 selects a digital filter corresponding to the filter selection signals FS ₁ and FS ₀ from the area determination section 65 and performs a smoothing (smoothing) process or an edge emphasis process. Next, the magnification / movement unit 6
At 8, the magnification is changed. Further, the color balance section 69 adjusts the color balance and outputs data to the print head control section 202. A signal M / NC indicating whether the mode is the single color mode or the full color mode is sent from the print head control unit 202 to the image signal processing unit 20.

In FIG. 7, a register 87 includes a data bus (D _{7 to} D ₀ ), an address bus (MA _{2 to} MA ₀ ), and a chip selection signal NCS from the print head control unit 202.
_1. Various control parameters are set by the write signal NWR. Then, the control parameters REF0, REF
1 and REF2 are sent to the under color removal / black printing control unit 82, and the control parameters REF3 and REF4 are set to the MTF correction control unit 8
5 and sends the control parameter EDG to the MTF correction section 67.

(B-2) Area Determination Result and Outline of Image Signal Processing Each processing of the image signal processing will be described in detail below. Before that, an outline of the area determination and the processing corresponding thereto will be described. In the present embodiment, the following three features are extracted from the read data of R, G, and B in the local area around the pixel in the area determination unit 65 for each pixel. (a) Achromatic or chromatic (achromatic / chromatic color judgment signal US)
(See section (e-3)), (b) Edge detection (filter selection signal F
S) (see (h-2)), (c) Hue determination (masking coefficient selection signal MS) (see section (f-2)). And achromatic or chromatic,
In accordance with the determination result of the edge detection, the amount of black (UCR ratio / BP ratio) is calculated in the color correction processing unit 66 as follows.
Is optimized, and in the MTF correction unit 67,
Smoothing or edge enhancement is performed by spatial digital filter processing. (1) In the case of achromatic color and flat density: In accordance with the result of the achromatic / chromatic color determination, the black amount is increased, and the reproduced color M, C, Y data is smoothed. (2) Both achromatic and chromatic colors are edge portions: The result of the achromatic / chromatic color determination is canceled, the amount of black is reduced, and edge enhancement is performed for C, M, and Y, which are reproduced colors. (3) When the chromatic color and the density are flat: The amount of black is reduced according to the result of the achromatic / chromatic color determination, and the M, C,
Performs smoothing of Y data. (4) In other cases, the black amount is set to an intermediate value, and neither edge enhancement nor data smoothing is performed. Also, linear masking coefficients corresponding to a plurality of color groups are prepared in the color correction processing unit 66, and masking processing is performed using the linear masking coefficient of the color group to which the hue belongs in accordance with the result of the hue determination. Do. In the monochrome mode, the black amount is set to 0, and a linear masking coefficient for the monochrome mode is used.

(C) Density Conversion Unit The density conversion unit 63 converts the output data of the CCD color image sensor 14 into an original density (OD) as viewed from the human eyes.
Is converted to have a linear characteristic with respect to. CCD
The output of the color image sensor 14 has a photoelectric conversion characteristic that is linear with respect to the incident intensity (= original reflectance OR). On the other hand, the document reflectance (OR) and the document density (OD) are-
There is a relationship of logOR = OD. Therefore, using the reflectance / density conversion table, the CCD color image sensor 1
4 is converted into linear characteristics. More specifically, using the reflectance / density conversion table 347 of FIG.
Convert to R, DG, DB.

(D) Black generator Cyan, magenta, yellow,
Each of the black color data C ', M', Y ', K' is created for each scan by the frame sequential method, and full color is reproduced by a total of four scans. Here, the reason why black printing is also performed is that even if cyan, magenta, and yellow are superimposed to reproduce black, it is difficult to reproduce clear black due to the influence of the spectral characteristics of each toner. Therefore, in this full-color copying machine,
By the subtractive color mixture method using the data Y ', M', and C 'and the black printing using the black data K', the reproducibility of black is improved and full color is realized.

The black generator 64 calculates the black amount K from the red, green, and blue components R, G, and B representing the brightness on the document as follows. DR, DG, DB obtained from the density converter 63 are:
Since the respective density data of the R, G, and B components are the same, they correspond to the cyan, magenta, and yellow components C ′, M ′, and Y ′ that are the complementary colors of R, G, and B in the CCD reading unit. Therefore, as shown in FIG. 8, the minimum values of DR, DG, and DB may be black data K 'because C', M ', and Y' on the document are components in which colors are superimposed. Therefore, the black generation unit 64 detects black data K ′ = MIN (DR, DG, DB).

Then, as will be described later, when the color correction processing section 66 creates the reproduction color data C, M, Y, the data K ′ from the black generation section 64 is used, and C ′, M ′, Y '
When the black data K is created by subtracting α · K ′ from the data of (1), β · K ′ is output as the K amount. Where α
Is the UCR (under color removal) ratio described later, and β is B
P ratio. In order to improve black reproducibility, the parameters α and β are changed in four stages according to the characteristics of the local area including the target pixel after the smoothing process.

Next, the circuit of the black generator 64 shown in FIG. 9 will be described. The comparator 301 compares the red data DR and the green data DG, and the two-input multiplexer 302 outputs the smaller value of DR and DG to the comparator 303 based on the comparison result. This comparator 303
This output value is compared with the blue data DB, and the two-input multiplexer 304 determines the DR, DG, and DB based on the comparison result.
Is output to the two-input multiplexer 305. 2
The input multiplexer 305 is based on the signal M / NC,
The minimum value (full color mode) or 0 (single color mode) is output. That is, the signal M / NC determines the image output mode. At the “L” level, the full-color mode is output and the minimum value is output. However, at the “H” level, the monochrome mode is output. May be set to an appropriate value to optimize the output result. In the present embodiment, K ′ is fixed at 0, and the data K ′ for black printing is cleared. The delay circuits 306, 307, 308 are used for adjusting timing. The output data is read data R, G,
The display has been changed to C ', M', Y 'in the sense that B is data of cyan, magenta, and yellow which are complementary colors.
Substantially the same data is shown.

(E) Automatic undercolor removal / black printing control in area determination section (achromatic / chromatic color determination) The area determination section 65 includes undercolor removal / black printing automatic control, automatic masking control, edge emphasis / smoothing. An area determination process is performed for automatic control, and a correction parameter is output. Here, the undercolor removal / black printing automatic control will be described.

(E-1) Purpose of Under-Color Removal / Ink Printing Automatic Control (Achromatic / Achromatic Color Judgment) As described above, in the black generation unit 64, the black data K '
K ′ = MIN (DR, DG, DB) is detected. Then, the color correction processing unit 66 calculates α · from C ′, M ′, and Y ′.
To create data K by subtracting K ′, β · K ′ is K
Output as quantity. Here, α is the UCR ratio and determines the black amount. β is the BP ratio and lowers the color data. The UCR ratio / BP ratio has an effect on the saturation of color reproduction and the sharpness of achromatic colors.

The reproducibility of the achromatic color is determined by the UCR ratio / BP ratio (-
If α / β) is increased, the image is reproduced with pure black K ′, which is improved, but the saturation of the chromatic color is reduced due to an increase in the output ratio of K ′. Therefore, it is considered that by controlling the UCR ratio / BP ratio by determining whether or not there is an achromatic color, it is possible to achieve both an improvement in the clarity of the achromatic color and an increase in the saturation of the chromatic color.

However, simply determining whether the read data R, G, B (primary color system) is an achromatic color easily causes an erroneous determination, and conversely leads to image deterioration. The causes of the erroneous determination include an erroneous determination due to moire of a halftone original in the reading system, and an erroneous determination due to an error or noise of the reading system in a document having a uniform density, a hue, and a portion where brightness changes gradually.

Then, first, the read data R,
Smoothing processing (weighted moving average processing in the embodiment) is performed on a local region including the pixel for G and B. Next, the levels of R, G, and B are compared to determine the achromatic color. Further, in order to improve the black reproducibility, the parameters α and β are changed in four stages (see FIG. 11) corresponding to the characteristics of the region including the pixel of interest after the smoothing process. / BP ratio was increased.

(E-2) Smoothing Processing For the undercolor removal / black printing automatic control, first, a smoothing processing is performed on an area of 5 × 5 = 25 pixels centering on the target pixel. In the smoothing processing unit 81 shown in FIG. 10, 8 bits (levels 0 to 25) standardized by shading correction in the shading correction unit 62
The moving average of the weighted addition for the pixel of interest is performed for each color from the R, G, B data of 5) using the 5 × 5 filter 344. That is, first, the four line memories 340, 3
Four lines of data are sequentially stored in 41, 342, and 343. Then, the data R2 (G2, B2),
From R3 (G3, B3), R4 (G4, B4), R5 (G5, B5) and data R1 (G1, B1) of the line currently being output,
After the center pixel of the center line is smoothed by the smoothing filter 344, the smoothed data RS (GS, BS) is sent to the under color removal / ink printing control unit 82 and the color correction masking control unit 83. send. In the filter 344, weighting is performed gently centering on the target pixel.

By performing the smoothing process using the filter 344, it becomes possible to prevent the pseudo resolution of the high frequency of the pixel and to extract the low frequency component. This allows
(A) Removal of noise from images with uniform density, (b) Removal of moiré caused by the halftone dot image reading system, and (c) Hue, lightness, and saturation change gently for the area discrimination target. This has the effect of smoothing the image and improves the discrimination accuracy. The data RS (GS, BS) smoothed in this way is used for hue determination (automatic masking control) in the under color removal / ink printing control unit 82, and achromatic / chromatic color determination (UCR) in the color correction masking control unit 83. / BP automatic control).

(E-3) Achromatic / Achromatic Color Judgment The undercolor removal / black printing control unit 82 uses the fact that the read data of three colors is R ≒ G ≒ B in the achromatic (black) judgment. And go. FIG. 11 shows a distribution area diagram for this determination. This determination is made for each pixel. In FIG. 11, the smoothed data RS, GS, BS of each pixel is 2
Range KLVL0 (GS-RE)
F0 <RS, BS <GS-REF0) Range KLVL1 (GS-REF1 <RS, BS) delimited by two dashed lines
<GS-REF1) Range KL separated by two broken lines
VL2 (GS-REF2 <RS, BS <GS-REF2)
Are classified into four distribution areas from achromatic to chromatic, as shown in the selection table of Table 1, and achromatic / chromatic determination signals US ₁ and US ₀ are generated correspondingly. Here, if it is within the area indicated by KLVL0, it is determined to be achromatic, and if it is outside the area indicated by KLVL2, it is determined to be chromatic. However, two areas are provided in the middle. . The 2-bit achromatic / chromatic color determination signal US ₁ ,
Value stepwise changed in corresponding to the value 0, 1, 2, 3, represented by US ₀ UCR ratio / BP ratio (-α / β). This ratio is set to a higher value for an achromatic color.

[0036]

[Table 1]

The under color removal / ink printing control section 82 shown in FIG.
In the circuit of FIG. 1, the data RS, GS and BS of each pixel are
1 to determine which distribution area it belongs to, and determine achromatic color. In this determination, a certain constant value is added to or subtracted from the reference color data (GS), and the level difference from the other two colors is determined. That is, the subtractor 361 and the adder 362 perform subtraction and addition of REF0 on the GS data,
GS-REF0 and GS + REF0 are obtained. And 12
The four comparators out of the three comparators 367 compare these with RS, GS, and BS.
Then, the result is passed through NAND gate 369 to G
N if S-REF0 <RS, BS <GS + REF0
KLVL0 = “L” is output to the table (ROM) 372. Similarly, the subtractor 363 and the adder 364 perform subtraction and addition of REF1 on the GS data, and
-REF1, GS + REF1 is obtained. Then, four comparators in the comparator 367 compare with these RS and BS. And the result is NA
GS-REF1 <RS, B through ND gate 370
When S <GS + REF1, NKLVL1 = "L" is output to the table 372. Further, the subtractor 365 and the adder 366 subtract and add REF2 to the GS data to obtain GS-REF2 and GS + REF2. Then, the four comparators in the comparator 367 compare these with RS and BS.
Then, the result is passed through NAND gate 371 to G
N if S-REF2 <RS, BS <GS + REF2
KLVL2 = “L” is output to the table 372. In the table 372, according to the selection table in Table 1,
The 2-bit achromatic / chromatic color determination signals US ₁ and US ₀ are output.

Here, in the table 372, the MT
Filter selection signal FS input from F correction control unit 85
_{If 0} is "L" (edge portion), the above determination is canceled. That is, achromatic / chromatic color determination signals US ₁ , US ₀ = “3” for reducing the amount of black are output. Filter selection signal FS
₀ is output when the edge detection amount in the main scanning direction or the sub-scanning direction of the R, G, B data is equal to or higher than a certain level (REF3). In this case, since it is the edge of the image, it is considered that the achromatic / chromatic color determination is likely to be erroneous. Therefore, for the pixel determined to be the edge of the image, the achromatic / chromatic color determination is performed in advance. The processing is set not to be performed, and the rate of erroneous determination is reduced. In other words, undercolor removal / black printing control is optimized only in a portion where the image density is relatively uniform, so that image deterioration due to erroneous determination is eliminated.

As described above, the under color removal / black printing control unit 82 and the color correction masking control unit 83 shown in FIG.
Then, various selection signals (US ₁ ,
US ₀ , MS ₁ , MS ₀ , FS ₁ , FS ₀ ) and outputs them to the color correction processing section 66 and the MTF correction section 67.

(F) Automatic Masking Control (Hue Judgment) in Area Discrimination Unit (f-1) Purpose of Automatic Masking Control In order to reproduce full-color input data on an image, a masking calculation is performed in the MTF correction unit 37. The linear masking coefficient is set so that the average color difference is minimized over almost the entire color gamut.However, in a portion of the color gamut, the color difference is not always minimized, and the color reproduction error and the tone error are reduced. It gets bigger. So, DR, DG, D
In the B term, its squared term and DR • DG, DG • DB, DB •
Although it is said that the secondary masking process with the addition of the DR term is good, the circuit configuration becomes complicated and large-scale.
Therefore, in this embodiment, the linear masking process is used, but four groups (primary color group,
A plurality of linear masking coefficients corresponding to the complementary color group and the two intermediate groups) are prepared in the color correction processing unit 66, and the hue of the image data is determined by the color correction masking control unit 83. By selecting a masking coefficient that minimizes the color difference in the hue, color reproducibility comparable to that of the secondary masking process is obtained. In the masking process, similarly to the case of the achromatic / chromatic color determination, erroneous determination is less likely to occur by using the data RS, GS, and BS subjected to the smoothing process in the smoothing processing unit 81 in FIG.

(F-2) Hue Judgment The hue judgment in the color correction masking control unit 83 is shown in FIG.
As shown in FIG. 2, the correction is performed by a color correction table (ROM) 368. Here, the address A of the color correction table 368
_The three upper bits of the RS, GS and BS data are input to _{8 to} A ₀ , and correspondingly, 2-bit masking coefficient selection signals MS ₁ and MS ₀ are output.

The hues are R, G, B, W (primary color) and C,
They are classified into two groups of M, Y, and BK (complementary color) and an intermediate group therebetween. Here, as shown in FIG.
Consider a cube with S, GS, and BS data as coordinate axes.
Each vertex of the cube is cyan (C), green (G), blue (B), white (W)
Represents the pure component of Therefore, the R, G, B, and W groups are located in four small cubes including the vertices of (R), (G), (B), and (W). The same applies to the C, M, Y, and BK groups. One of the middle groups is located on the outer shape of a large cube surrounding the cube of the R, G, B, W group, and the other middle group surrounds the cube of the C, M, Y, BK group. Located on the outside of a large cube. As shown in the figure, taking the (R), (M), (BK) and (B) planes as an example,
(I) is a C, M, Y, BK group, and the region (II)
Is an intermediate group close to the C, M, Y, BK system,
(III) is an intermediate group close to the R, G, B, W group, and area (IV) is an R, G, B, W group. As described above, R, G, B-based masking and C, M, Y-based masking are roughly classified as follows. Both sets of (R, G, B) and (C, M, Y) appropriately set the hue. This is because they are sparsely distributed.
That is, when color samples that are appropriately sparsely distributed are used, the masking coefficient described later does not take a greatly different value. Therefore, when the (R, G, B) system and the (C, M, Y) system are discriminated, even if they are erroneously judged, there is no major problem. Further, the reason why the middle part is divided into two is to make it difficult for troubles to occur even if an erroneous determination is made.

In the color correction table 368, the input R
According to the S, GS, and BS data, it is determined which group of the cube is to belong to. If the determination result is (I), MS ₁ , ₀
= Output "3", when the (II), and outputs the MS _{1, 0} = "2", and outputs the, MS _{1, 0} = "1" when (III),
At (IV), MS ₁ , ₀ = "0" is output.

(G) Color Correction Processing Section The color correction processing section 66 includes the CCD color image sensor 14
The transmission characteristics of each of the filters R, G, and B and the reflection characteristics of each of the toners C, M, and Y in the printer unit are corrected, and the color reproducibility is matched to a characteristic close to ideal. Taking the G filter and the M toner as an example, as shown in the transmission characteristics of FIG. 14 and the reflection characteristics of FIG. 15, each characteristic of the G filter and the M toner is indicated by a hatched portion as compared with the ideal characteristics. There is a non-ideal wavelength region. In order to make this correction, the color correction processing unit 66 performs a linear correction based on the following masking equation, in addition to the black printing process described above. (Since printing is performed in a frame-sequential manner, this masking equation is executed line by line.

[0045]

(Equation 1)

K '= MIN (DR, DG, DB)

In the circuit of the color correction processing unit 66 shown in FIG. 16, a parameter U set by the color correction register 826 in accordance with the correction parameter determined by the area determination unit 65.
In accordance with the CR ratio / BP ratio (-α / β), black printing control and color correction masking are performed. The various coefficients are set in the color correction register 826 shown in detail in FIG.

As shown in the circuit diagram of FIG. 17, the color correction register 826 stores the masking coefficients (A _ci ,
B _ci , C _ci , A _mi , B _mi , C _mi , A _yi , B _yi , C _yi ) (this is 3
× 3 matrix M _k elements _{(M) ji (j = c} , m, y; i = 1,2,
3)) and the UCR ratio / BP ratio (−α / β) and d. Here, the first masking coefficient M ₀ (k = 0) selected when MS ₁ , ₀ = "0" is a coefficient for reducing the color difference with respect to the primary color system R, G, B. The fourth masking coefficient M ₃ (k = 3) selected when MS ₁ , ₀ = “3” reduces the color difference with respect to C, M, Y, which is a complementary color system. further,
Second masking coefficient selected when MS ₁ , ₀ = "1"
(k = 1) is weighted to the masking coefficient for the primary color system
A masking coefficient that is (2/3) M ₀ + (1/3) M ₃ is selected, and a third masking coefficient (k = 2) selected when MS ₁ , ₀ = "2" is , A masking coefficient of (1/3) M ₀ + (2/3) M ₃ weighted to the complementary masking coefficient. The reason why the masking coefficient of the intermediate group is set by mixing the masking coefficient for the primary color system and the masking coefficient for the complementary color system is to make the hue change due to erroneous determination in color reproduction less noticeable.

The next matrix shows the masking coefficients M ₀ for R, G and B.

(Equation 2) Further, the following matrix shows a masking coefficient M for C, M, and Y.
_{Shows 3} .

(Equation 3) For comparison, a conventional normal masking coefficient is shown below.

(Equation 4)

That is, the color correction register 826 shown in FIG.
In the circuit diagram of the level 861, a data bus (MD ₇ ~MD ₀₎ from the print head controller 202, an address bus (MA ₄ ~MA _0), and chip select signal NCS _0,
Data setting is performed by the NWR signal. That is,
The data of the above-described four types of matrix coefficients of each color are output to the multiplexers 862, 863, and 864, respectively. Multiplexer 862,863,864, corresponding to the selection signal MS _{1, 0} from the color correction masking control unit 83 selects one of the matrix coefficients, two-input multiplexer 86
5, 866 and 867. On the other hand, the coefficients Dc, Dm, and Dy for the monochrome mode are also input to the two-input multiplexers 865, 866, and 867. Multiplexer 865,
866 and 867 select and output one of them according to the mode selection signal M / NC. On the other hand, the four UCR ratios / BP ratios are selected by the multiplexer 868 by the signals US ₁ , ₀ . The constant d is used to increase the saturation at a low density, but the description is omitted here. In the above processing, d = 0 is set for simplification of the description.

The circuit of the color correction processing unit 66 shown in FIG. 16 will be described. First, in the black color printing unit, at the time of under color removal control for performing C, M, and Y printing, the multiplier 824 outputs the signal from the black generation unit 64 to the black generation unit. Is multiplied by the UCR ratio (−α) from the color correction register 826. Then, the multiplied value (−α · K ′) is added to the complementary color data C ′, M ′, and Y ′ by the adders 821, 822, and 823, and the lower color removal values C ₁ and M are added.
Output as _1, Y _1. On the other hand, at the time of black printing control for printing K, the multiplier 824 multiplies the BP ratio β from the color correction register 926 and adds the multiplied value (β · K ′) via the limiter circuit 834 to the adder 834. 835.

Further, the color correction masking unit uses a linear masking process with a simple circuit configuration. At the time of undercolor removal control, the multipliers 831, 832, and 833 apply data to the data C ₁ , M ₁ , and Y ₁ . Each masking coefficient (A _{c to} C _c , A _{m to} C _m , A _{y to} C _y ) from the color correction register 826 is multiplied. Then, the multiplied values C ₂ , M ₂ , and Y ₂ are added to the adder 83.
5 and output as VIDEO1 data. At this time, the output from the limiter circuit 834 has been cleared to “00”, and the adder 835 outputs the addition result of C ₂ , M ₂ , and Y ₂ .

On the other hand, at the time of black printing control, the color correction register 826 sets “00” in the multipliers 831, 832 and 833, so that C ₂ , M ₂ and Y ₂ are cleared and K ₁ (= K ₂ )
Only the data is output as VIDEO1 data through the limiter 834.

In order to show the masking correction effect, first, FIG.
8 shows the original color (represented by a white circle) and the reproduction color (represented by a black circle) when the above-described ordinary masking coefficient is used, according to CIE19.
This is expressed by the L * a * b * color system of the 76 uniform color space. (The a * -b * plane shown in the figure indicates hue and saturation, and the L * direction perpendicular to the paper surface indicates lightness.) The difference between the original color and the reproduced color corresponds to the color difference. Here, the average dye of only R, G, and B is 10.5335, and the average color difference of only C, M, and Y is 4.0029. Figure 19 is a diagram showing the above-mentioned R, G, original color when using the masking coefficient M ₀ for B (open circles) and reproduced color (black circle) in a * -b * plane. here,
The average color difference of only R, G, and B is 3.8576, and the hue shift is smaller than in the case of a normal masking coefficient. The average color difference of only C, M, and Y is 12.1797. Further, FIG. 20, the above-described C, M, original color when using the masking coefficient M ₃ for Y (open circles) and reproduced color (black circles) a
It is the figure represented by the * -b * plane. Here, the average color difference of only C, M, and Y is 2.43782, and the hue shift is smaller than that in the case of a normal masking coefficient. In addition,
The average color difference of only C, M, and Y is 12.7757. As clearly shown above, the selected masking coefficient reduces the color difference of the corresponding hue.

When the monochrome mode is set by the M / NC signal, color reproduction is performed in a single color. Reproduction by a single color means that density information based on sensitivity (relative luminous efficiency) of sensing brightness as a human sense is expressed as C, M, Y, K, R (M + Y), G (B +
Y) or B (C + M). Therefore, similar to the masking process, the relative luminosity information
(MC) may be created by linear processing of the masking coefficients DR, DG, and DB. _{MC = D c · C '+} D m · M' + D y · Y ' that is, the color correction register 826, Dc as a masking coefficient, Dm, to set the Dy, thereby, VIDEO1
MC data is output as data. The masking coefficients Dc, Dm, and Dy are determined according to the relative luminous efficiency depending on the type of the toner. At this time, as described above, the black printing process is not performed. That is, in the black generation unit 64 shown in FIG. 9, K ′ = “00” in the monochrome mode.
Is output.

(H) Automatic Edge Enhancement / Smoothing Control in Area Discrimination Unit (h-1) Purpose of Automatic Edge Enhancement / Smoothing Control Generally, for a monochromatic image, text / photographs are taken according to the density change or density distribution of the image. Automatic identification is performed, edge enhancement is performed on a character image, and smoothing processing is performed on a photographic image, thereby achieving both sharpening and smoothing of the image, and optimizing the MTF correction.

However, in the case of a color image, mere edge enhancement changes the image density even with changes in hue and saturation, and thus such identification does not always work well. For example, when changing from white to red, the edge may be emphasized. However, when changing from red to cyan, the hue changes strangely at the edge, so it is better not to emphasize the edge. Skin color has a particularly large effect. Therefore, it is necessary to extract and control only the change in image brightness.

(H-2) Edge Detection In the edge detection unit 84 shown in FIG. 10, a pixel of interest is obtained from 8-bit R, G, B data (levels 0 to 255) standardized by shading correction in the shading correction unit 62. Edge detection in both the main scanning and sub-scanning directions is performed for each color in the area around. That is, first, four lines of data are sequentially stored in the four line memories 340, 341, 342, and 343. Then, the data R2 (G2, B2), R3 (G3, B3), and R4 (G4, B
4), R5 (G5, B5) and data R of the line currently being output
1 (G1, B1), the MTF correction control is performed after the data of the center pixel of the center line is detected by the filter 345 for detecting edges in the main scanning direction and the filter 346 for detecting edges in the sub-scanning direction. The output data RE1 (GE1, BE1) and RE2 (GE2, BE2) in both directions are supplied to the unit 85.
Send each.

In the edge detection using the filters 345 and 346 described here, the amount of inclination of the pixel of interest and the inclination direction in two directions of the main scanning direction and the sub-scanning direction are extracted. Here, the inclination amount is determined by the output data RE1 (GE1,
BE1) and RE2 (GE2, BE2) are the absolute values, and the inclination direction is the sign (positive or negative). A hue change represented by a color ghost phenomenon occurs at an edge portion of the image (a portion where the brightness changes rapidly). For this reason, the output data is extracted by the MTF correction control unit 85 to extract a portion where the erroneous determination of the achromatic / chromatic color determination is likely to occur, and selectively extracts the MTF.
It is used for dividing the area to be corrected.

The data of the pixel of interest on the center line is sent to the density conversion unit 63 as described above.
Density data D using reflectance / density conversion table 347
Converted to R (DG, DB).

FIG. 21 shows a circuit of the MTF correction control unit 85 for performing the MTF automatic control.
Based on a signal from the edge detection unit 84, the image is divided into a flat density portion, an edge portion, and an intermediate portion thereof. The density flat portion is defined as R, G, and R in both the main scanning and sub-scanning directions.
The threshold value (RE
F3) An area that is equal to or less than That is, the density flat portion is a region where the brightness change is small for any color data. At this time, a filter selection signal FS ₀ = “L” is output. Conversely, the edge portion means that the edge detection amount of R, G, B data is equal to or greater than a certain threshold value (REF4) in either the main scanning direction or the sub-scanning direction, and R, G, This is an area where the inclination directions of the B data match. At this time, a filter selection signal FS ₁ = “L” is output. Here, in order to prevent misjudgment due to hue change, change in lightness between achromatic colors (white ← → ground level such as black ← → black characters / thin lines) or change between achromatic colors and chromatic colors (white ← → Extract a background level such as a color patch →→ red / blue characters, thin lines) as edges. If both filter selection signals are not "L", it is an intermediate part.

In the MTF correction control circuit shown in FIG. 21, the edge detection amounts RE1, GE detected in the main scanning direction
1 and BE1 are absolute value detection circuits 381 and 38, respectively.
Are converted into absolute values RE1 ', GE1', and BE1 'by the second and the third 383. The absolute values RE1 ', GE1', BE1 'are compared with a threshold value REF3 in a comparator 390. When the total absolute value is smaller than the threshold value REF3, the absolute value RE1', GE1 ', BE1' is passed through a negative logical AND gate 391 to a negative logical AND gate 395.
Is sent to Absolute values RE1 ', GE1', BE1 '
Is similarly set to the threshold value RE in the comparator 390.
F4, and when the total absolute value is larger than the threshold value REF4, the negative logical AND gate 392 is passed to the negative logical AND gate.
A signal is sent to gate 396. On the other hand, the edge detection amount RE
1, GE1 and BE1 detect the inclination (sign) at the edge in the inclination discrimination circuit 384, and the sign is negative logical AND.
Output to gate 396. Therefore, the negative logic AND gate 396 determines that the edge detection amount is greater than the threshold value REF4 in the main scanning direction and that the inclination directions of the R, G, and B data match (when the edge portion is determined). Via a negative logic OR gate 397 to the filter selection signal FS ₁ (=
“L”) is output.

Similarly, the edge detection amounts RE2, GE2, BE2 detected in the sub-main scanning direction are calculated by the absolute value detection circuits 385, 386, 387, respectively.
Converted to BE2 '. This absolute value RE2 ', GE2', B
E2 ′ is the threshold value REF in the comparator 390.
3, and if the total absolute value is smaller than the threshold value REF3, a signal is sent to the negative logic AND gate 395 via the negative logic AND gate 393. Thus, a negative logic AND gate 395, the edge detection amount in both the main scanning direction and sub-scanning direction and outputs a filter selection signal FS ₀ if the threshold REF3 smaller (uniform density portion). Similarly,
The absolute values RE2 ', GE2', BE2 'are
Compared to threshold REF4 at 0, a signal is sent to negative logic AND gate 397 via negative logic AND gate 394 if the total absolute value is greater than threshold REF4. On the other hand, in the edge detection amounts RE2, GE2, BE2, the inclination (sign) at the edge is detected by the inclination determination circuit 388, and the sign is output to the negative logic AND gate 396. Therefore, the negative logic AND gate 396 has an edge detection amount larger than the threshold value REF4 in the sub-scanning direction.
And when the inclination directions of the R, G, B data match
The filter selection signal FS ₁ (= “L”) is output to the (edge) through the negative logic OR gate 397. FIG. 22 schematically shows the edge detection amount (GE1) using the filter 345 when the G image data changes in the main scanning direction as shown on the lower side. This edge detection amount (GE1) is compared with threshold values REF3 and REF4 as shown on the upper side, and filter selection signals FS ₀ and FS ₁ are output in accordance with the results.
Note that FS ₁ is output when _one of the inclination signals NSINA and NSINB is “L”.

Threshold value R in MTF correction control section 85
EF3 and REF4 can be externally adjusted by sharpness setting (see FIG. 7). For example, to increase the sharpness, REF3 and REF4 may be set low.

In this embodiment, REF3 <REF4
REF3> RE according to the purpose of the processing.
F4 may be set.

(I) MTF correction unit The filter selection signal F set by the MTF correction control unit 85
S ₁ and FS ₀ are selection signals of the spatial filter of the MTF correction unit 66. FS ₀ = no edge in both main scanning and sub scanning directions for read data R, G, B for any color
In the case of “L” (density flat portion), smoothing processing is performed on the data converted to C, M, Y, and K data, and achromatic / chromatic color determination is permitted. Even if the achromatic / chromatic color determination is permitted, an image with low saturation looks like random image noise when the amount of K changes drastically. Therefore, the MTF correction unit 67 performs a smoothing process to make the change inconspicuous. (It is for this reason that the achromatic / chromatic color determination is classified into four levels, but this is still insufficient, and measures have been taken to further reduce noise even for pixels determined to be achromatic. Also, image noise and moire caused by the reading system are reduced,
This makes it possible to smooth portions where the brightness, saturation, and hue gradually change as in a photograph.

When FS ₁ = “L” (edge portion), the output of the Laplacian filter 324 is permitted, the addition with the pixel of interest is performed, and the edge enhancement processing of the image is performed. Therefore, the sharpness of an achromatic edge portion is improved because the image is sharpened without increasing the UCR ratio / BP ratio.

The MTF correction unit 66 shown in FIG.
Edge enhancement and smoothing are performed by using an R two-dimensional digital filter. First, four lines of data are sequentially stored in the four line memories 320, 321, 322, and 323. The data of each line and the data of the line currently being output are processed by a 5 × 5 digital filter 324 for secondary differentiation (for edge enhancement) and a 5 × 5 digital filter 325 for smoothing. And output to the multiplier 326 and the two-input multiplexer 328, respectively. The multiplier 326 outputs the product of the output value of the digital filter 324 and the value EDG to the two-input multiplexer 327, and outputs the product to the two-input multiplexer 32.
7 is a filter selection signal FS ₁ = “L” (edge),
The output value or “00” corresponding to “H” is added to the adder 3.
29. On the other hand, the multiplexer 328 compares the smoothed output value of the digital filter 325 and the output value of the target pixel from the line memory 321 that have not been smoothed with the filter selection signal FS ₀ = “L” (density flat portion), “H”. And outputs it to the adder 329. The adder 329 adds the two input values and outputs the result as a signal VIDEO2.

The secondary differential filter 324 used for edge enhancement detects the amount of edge enhancement of the image. The edge enhancement is performed by linearly converting the edge enhancement amount obtained by this filter and the center pixel. (Original image + second derivative). That is, when FS ₁ = “L” (edge portion), the adder 329 adds the edge-emphasized data to the data of the target pixel.

On the other hand, the filter 325 used in the smoothing process reduces image noise by using a moving average by weighted addition of peripheral pixels, and creates smooth image data. (The weighted addition prevents pseudo-resolution such as moiré due to filter processing.) That is, FS ₀ = “L”
In the case of (density flat portion), only the data subjected to the smoothing process is output from the adder 239.

The EDG value affecting the edge emphasis in the MTF correction section 66 can be externally adjusted by setting the sharpness (see FIG. 7). For example, when it is desired to increase the sharpness, the EDG value may be increased.

In the MTF processing described above, the read data R, G, and B are sharpened and smoothed. This is because if the same processing is performed on the reproduced color data C, M, and Y, the edge enhancement is performed even on the hue change portion, and conversely, the color reproducibility is deteriorated. Therefore, the MTF correction is selectively performed by extracting a change in the brightness of the read data.

[0073]

Since the saturation is determined after smoothing the input digital data, erroneous determination of the saturation is reduced. Therefore, the determination accuracy is improved for an image with uniform density and hue such as a color patch. Further, since the black printing process is performed stepwise in the density uniform portion of the halftone dot document, the image noise is reduced and the image changes naturally. Also,
By performing the determination on the read color data that has been subjected to the smoothing processing, the erroneous determination of the saturation is further reduced, and the edge determination is taken into account, so that the reproduction can be performed more accurately.

[Brief description of the drawings]

FIG. 1 is a sectional view of a full-color copying machine.

FIG. 2 is a block diagram of a part of a control system.

FIG. 3 is a block diagram of another part of the control system.

FIG. 4 is a perspective view of a reading unit.

FIG. 5 is a block diagram of an image signal processing unit.

FIG. 6 is a circuit diagram of an area determining unit.

FIG. 7 is a circuit diagram of a register for setting various parameters.

FIG. 8 is a graph showing a relationship between density data and a black amount K ′.

FIG. 9 is a circuit diagram of a black generation unit.

FIG. 10 is a circuit diagram of a smoothing processing unit and an edge detection unit.

FIG. 11 is a graph showing four regions from an achromatic color to a chromatic color.

FIG. 12 is a circuit diagram of an undercolor removal / ink printing control unit and a color correction control unit.

FIG. 13 is a diagram of a cube having a hue distribution.

FIG. 14 is a graph showing characteristics of a G filter.

FIG. 15 is a graph showing characteristics of M toner.

FIG. 16 is a circuit diagram of a color correction processing unit.

FIG. 17 is a circuit diagram of a color correction register.

FIG. 18 is a diagram illustrating a color difference when a normal masking coefficient is used.

FIG. 19 is a diagram showing a color difference when using R, G, B masking coefficients (M ₀ ).

FIG. 20 is a diagram illustrating a color difference when a C, M, and Y masking coefficient (M ₃ ) is used.

FIG. 21 is a circuit diagram of an MTF correction control unit.

FIG. 22 is a diagram illustrating an example of edge detection.

FIG. 23 is a circuit diagram of an MTF correction unit.

[Explanation of symbols]

65: region determination unit, 66: color correction processing unit, 67: M
TF correction section, 81: smoothing processing section, 82: under color removal / ink printing control section, 83: color correction masking control section, 84: edge detection section, 85: MTF correction control section,
US ₁ , US ₀ … achromatic / chromatic color determination signal, MS ₁ , MS ₀ …
Masking coefficient selection signal, FS _1, FS ₀ ... filter selection signal.

Continuation of the front page (56) References JP-A-63-296947 (JP, A) JP-A-63-46069 (JP, A) JP-A-63-177683 (JP, A) JP-A-1-188171 (JP) , A) JP-A-2-249365 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 1 / 40,1 / 46,1 / 60

Claims (2)

(57) [Claims] An input unit for inputting red, green, and blue digital data obtained by scanning an original; a black generating unit for obtaining a minimum value of digital data of a read color input by the input unit; Means for smoothing the digital data of the read colors of red, green and blue inputted by the means; and, for each pixel, a saturation determining means for determining the saturation of the digital data smoothed by the smoothing means; and Edge determining means for determining an edge for data which is processed in parallel with the determination and which is not smoothed; and a first coefficient which is determined in advance when the edge determining means determines that the edge is an edge, regardless of the determination result by the saturation determining means. And the second coefficient are selected, and if the edge is determined to be not an edge by the edge determining means, the determination result by the saturation determining means Selection means for selecting the first coefficient and the second coefficient in accordance, with the first coefficient and the second coefficient selected by the selection means,
Under color removal black printing means for creating a product of the minimum value and the first coefficient obtained by the black generation means as black data, and subtracting the product of the minimum value and the second coefficient from the digital data of the read color. An image forming apparatus comprising: data conversion means for converting the read color data processed by the undercolor removal black printing means into cyan, magenta, and yellow reproduced color data. 2. The image forming apparatus according to claim 1, wherein the determining unit determines the saturation based on a level difference between three colors of red, green, and blue.