JPH0276760A - Tone correction of digital color image formation device and device for tone correction - Google Patents

Abstract

PURPOSE:To obtain a color output image of constantly stable quality by providing the first process which outputs an incorporated gray scale signal as a gray scale image from a color image output section, the second process which reads the gray scale image and the third process which corrects an output tone from the image output section based on the data which is read and the gray scale signal. CONSTITUTION:The gray scale signal generated by a gray scale signal generation circuit 34 is output through a color image output section, and consequently, a gray scale chart is obtained which contains information reflecting characteristics under the existing circumstances of the color image output section. Then the gray scale chart is read from a color image input section to obtain a difference between the color image data which is read and the original gray scale signal generated by the gray scale signal generation circuit 34. Next, the contents of a RAM for conversion in an output tone conversion circuit are corrected so that the variations in the characteristics of the color image output section are corrected. Thus it is possible to correct the color tone or misaligned gradation of a copy in the color image output section with high accuracy, if any change in the circumstances occurs.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is applicable to digital color copying machines, color facsimile machines, color printers, etc., which have an output section that outputs images using laser xerography, inkjet, thermal transfer, etc. The present invention relates to a tone correction method and apparatus for a color image forming apparatus.

(Conventional technology) Conventional color j! As an example of an image forming apparatus, a digital color copying machine reads a color original 1 and generates density signals Da, Do of the three primary colors of blue, green, and red, as shown in FIG.
, DR, and the color image input section 2 outputs the density signals Da, DQ, DR as M (Y).

Bitmap data Y and M corresponding to the amounts of magenta (M), cyan (C), and black (K) color materials.

It is comprised of a color image processing section 3 that converts into C and K, and an m-image output section 4 that outputs a color copy 6 using laser xerography, inkjet, thermal transfer, etc. based on these color material signals.

(Problems to be Solved by the Invention) In conventional color image forming apparatuses, the color tone and gradation of the output image output from the image output section vary depending on the environment such as temperature and humidity at the time of output, and Therefore, it is difficult to always obtain the same output image. For example, when using a conventional digital color copying machine, when the same original is reprinted at different dates and times, the resulting output images have very different tones and gradations even though the original images are the same. There was a problem.

This is because the reading density output value of the image input unit and the output value (Y, M, C, %) of the image output unit fluctuate due to changes in the environment such as temperature and humidity and are not constant.

SUMMARY OF THE INVENTION An object of the present invention is to provide a tone correction method and apparatus that can always obtain a color output image of stable quality even when there are changes in the environment such as temperature and humidity.

(Means for Solving the Problems) A tone correction method for a digital color image forming apparatus according to the present invention includes a first process of outputting a built-in grayscale signal as a grayscale image from a color image output section, and a first process of outputting a built-in grayscale signal as a grayscale image from a color image output section; The present invention is characterized by comprising a second process of reading an image, and a third process of correcting the output tone of the color image output section based on the data read in the second process and the gray scale signal.

Further, the tone correction device of the digital color image forming apparatus according to the present invention for carrying out the method includes a tone correction device for outputting a reference gray scale signal as a gray scale image via an image output unit of the image forming device. grayscale signal generation means for generating a signal; grayscale image input means for optically reading the grayscale image output from the color image output section; and generation of a read signal of the read grayscale image and the grayscale signal generation means. and an image output section tone correction means for correcting the color image signal supplied to the color image output section of the image forming apparatus based on the gray scale signal.

As a gray scale image input means, a dedicated input means using a spectrophotometer etc. may be prepared, but if the digital color image forming apparatus has a color image manual section, instead of providing a dedicated manual means. The color image input section can be used for

According to another aspect, the present invention can be configured to correct not only the tone of the color image output section but also the color image input section.

That is, according to another aspect, the present invention includes a first process of reading a reference grayscale image for input tone correction, and a predetermined value of the equivalent neutral density converter for the data read in the first process. a second process of correcting the tone of the color image input section by correcting the conversion characteristic of the equivalent neutral density conversion section based on the conversion characteristic; A third process for outputting a grayscale image from the output section, a fourth process for reading the output grayscale image, and a color image outputting section based on the data read in the fourth process and the grayscale signal. and a fifth process for modifying the output tone.

In addition, the tone correction device of the digital color image forming apparatus of the present invention for carrying out the method uses an input signal read by the image processing unit of the image forming apparatus) and an equivalent neutral value of the read signal of the grayscale image for tone correction. an image input section tone correction means for correcting the conversion characteristics of the equivalent neutral density conversion section based on the predetermined conversion characteristics of the density conversion section; In order to output a scaled image, a grayscale signal generation means for generating a grayscale signal, and a read signal of the grayscale image from the image output section read by the image input section of the image forming apparatus and the grayscale signal generation means. Based on the generated grayscale signal,
and image output section tone correction means for correcting the color image signal supplied to the image output section of the image forming apparatus.

(Function) The tone correction method of the present invention includes a grayscale signal generating means, and generates a grayscale image by using the grayscale signal generated by the grayscale signal through the color image output section of the image forming apparatus. Reflects the characteristics of the color image output section in the current environment in the grayscale image,
Collects information for tone correction of the color image output section. This grayscale image containing information for tone correction is read by the grayscale input means, and by comparing it with the signal generated by the grayscale trust generation means, a deviation from the initial value of the characteristics of the color image output section is detected. and make corrections.

In this way, the present invention measures the characteristics of the color image output unit in its usage state, detects deviations from the initial values, and makes corrections for the deviations. Even if there are fluctuations in the characteristics of the image output section, this can be corrected with high precision and a stable color output image can always be obtained.

When using the present invention in a color image forming apparatus that does not have an input section for optically reading images, such as a color printer, it is possible to use a device that inputs a gray scale image, such as a spectrophotometer. It is necessary to add a dedicated input means suitable for this purpose. On the other hand, if the color image forming apparatus has a manual part for optically reading images, such as a color copying machine or a color facsimile machine, the input part can be used as a gray scale input means. Therefore, the configuration for tone correction becomes simple.

However, the color image input section of such a color image forming apparatus is subject to environmental conditions such as temperature and humidity, exposure lamp and CCD.
Due to changes over time, the output value may not always remain constant. If the grayscale image for correction is configured to be read using the color image input section of the color imager, if there is a change in the characteristics of the readout output as described above, the grayscale image for correction will not be included in the grayscale image for correction. In this case, it becomes impossible to accurately detect information on characteristic fluctuations of the color image output unit, and there is a risk that the accuracy of tone correction of the color image output unit may decrease.

An aspect of the present invention to avoid this is to correct the tone of the color image input section and then correct the tone of the color image output section. In this way, when correcting the tone of the color image output section, the characteristics of the color image input section have been corrected, and the grayscale image for correction of the color image output section can be accurately corrected by the color image input section. It becomes possible to read it.

Furthermore, by performing tone correction on the color image input section in addition to tone correction on the color image output section, it is possible to obtain a copy output with stable quality of the negative layer.

(Example) Hereinafter, the present invention will be explained in detail with reference to examples thereof.

FIG. 1 shows an embodiment in which the present invention is applied to a digital color copying machine, and shows the circuit configuration of a portion corresponding to the color image processing section 3 of the digital color copying machine shown in FIG. In FIG. 1, the part surrounded by a broken line shows an example of a detailed configuration of a tone modification circuit added to implement the tone modification method of the present invention. .29 and 36
37.38.39, the copying machine mode is switched to the tone correction mode, and this tone correction circuit is electrically connected to the color image processing section 3, so that its tone correction function is activated. When used as a copier, multiplexers 27, 28, 29 and 36.
37.38.39, the copier mode is set and the tone correction circuit is electrically disconnected from the copier.

The color image input section 2 (FIG. 7) optically reads the document and inputs the red (R), green (G), and blue (B) density signals of the document into the first It supplies the color image processing unit 3 shown in the figure, for example, a light source lamp and COD.
A light source lamp illuminates an original set on a platen, and the reflected light is received by a line sensor such as a COD. That CCD is R
1G and B color filters are arranged dot-sequentially on photosensitive pixels, and each color has a resolution of 400 to 5 oospr.

The amount of reflected light for each color is calculated from the reflected light ff1W of the white reference plate received at the start of scanning - the reflected light I obtained relative to this.
R+ is A/D converted and then logarithmically converted to the R and G of the original pixel of about 8 bits.

B　1sIll　D　Output as I.

D1=-l og (R+/VV+) i=R1
The G+B color image processing unit 3 performs equivalent neutral density processing (END processing), smoothing/sharpening filter processing, masking processing, and undercolor removal processing (υCR) using each circuit described below.
processing), output tone conversion processing (TRC processing), dither processing, and other image processing to produce Y, M, and C corresponding to the amount of coloring material of the color image output section 4.

K bitmap data is output to the color image output section 4.

The equivalent neutral density processing circuit (END processing circuit) 6°7.8 receives density signals D11.8 manually input from the color image input section 2.
Equivalent neutral concentration processing is performed on Do and DR. In this equivalent neutral density processing, the grayscale red, green, and blue densities are projectively transformed into grayscale visual densities, so that when the input document is gray, the red, green, and blue densities are equal. Adjust accordingly.

The output E after equivalent neutral concentration processing is the concentration D as shown in the formula below.
It becomes a function of I.

E+=f (D+) r i=R+ cl B smoothing・
The sharpening circuits 9, 10, and 11 perform filter processing on the output E after the equivalent neutral density processing.

For example, when a document is composed of halftone dots, low-frequency moiré and high-frequency noise may occur due to interference between the sampling period during reading and the halftone dot period of the document. Therefore, by performing filter processing in the smoothing/sharpening circuit 9°to, ti, the moiré and noise are prevented, and the M T F of the degraded image is thereby prevented.
(Modular tr-ansfer fun
ctlon) to improve sharpness.

The masking circuits 12, 13.14 perform masking processing on the filtered signals supplied by the smoothing/sharpening circuits 9.10.11.

This masking process corrects color turbidity that occurs due to the influence of unnecessary absorption density of the output coloring material used in the color image output section 4.

For example, by correcting color turbidity using a 3×3 matrix operation as shown below, the input signal Ea+EO+E8 is
Signal NEW corresponding to yellow, magenta, and cyan output color materials
, E, EC.

The black signal generation circuit 15 has EV, Ek4. A certain amount of a signal equal to EC is replaced with a black signal EK.

For example, the black signal generation circuit 15 is connected to the masking circuit 12.
, 13.14, the minimum value of the signals Ev, E□Ec is determined, and this is multiplied by a certain coefficient α to calculate the black signal EK□.

E,'=αmin (Ey, E+m+ Ec) The under color removal circuits 16, 17.18 each output the signals E v +EX, E
(The black signal EIl' generated by the black signal generation circuit 15 from +
Output signal E V ' with the undercolor removed by reducing the amount of
r EM ′ I E Co .

EI'=EJ-rcEx" * EJ) (J=Y1M
r C) The signal EV'+EM'+E♂ after removing the undercolor is sent to the multiplexer 3B, 37.38 in the copier mode.
The signal EK' from the black signal generating circuit 15 is supplied to the output tone converting circuit 22 via the multiplexer 39.

Output tone conversion circuit (TRC circuit) 19, 20° 21.
22 are density signals EJ' (j=Y.

M, C, K) as a signal ffi P corresponding to the halftone dot area ratio.
J= fJ(EJ') (j=Y1M,C,K)
It is configured as a look-up table stored in RAM that reads the corresponding P by Elo. In the present invention, the conversion characteristics are modified by modifying the lookup table as described below.

The dither processing circuits 23, 24, 25.26 convert the signals ffi PJ corresponding to the halftone area ratio output from the output tone conversion circuits 19, 20, 21.22 into bitmaps of each color using a dither matrix or the like. This is a circuit that converts data into data Y, M, C, and K.

The multiplexers 27.28.29 are circuits for switching between the copying machine operating mode and the tone correction mode, and in the copying machine operating mode, the masking circuits 12.13.1
The output of 4 is sent to the black signal generation circuit 15 and the under color removal circuit 1.17.
．． 18 inputs and in tone modification mode,
The outputs of the masking circuits 12, 13.14 are connected to data thinning circuits 30.31.32.

The data thinning circuits 30, 31.32 reduce the data by appropriately thinning out the data from the masking circuits 12, 13, 14, thereby reducing the capacity of the RAM 33 (as well as increasing the speed of arithmetic processing for tone correction). It's meant to be faster.

The RAM 33 is a circuit that stores the outputs of the data thinning circuits 30, 31, and 32.

The gray scale signal generation circuit 34 generates a density level of 0 in order to output the reference gray scale signal as a gray scale image via the color image output section of the image forming apparatus.
This circuit generates a gray scale signal (El) from to n.

The CPU 35 reads the grayscale signal E output from the grayscale signal generation circuit 34 and the grayscale image output by the image output unit as a -H color copy of the grayscale signal, and reads the signal E1 obtained by using the image input unit. A modified tone function is obtained from the output tone conversion circuit 1119, 20.degree. 21, and arithmetic processing is performed to replace the conversion look-up table of the output tone conversion circuit 1119.20.degree. 21 with the corrected data.

The multiplexers 3B, 37, and 38 are circuits that switch between the copying machine operating mode and the tone correction mode, and in the copying machine operating mode, the undercolor removal circuit! '6.17.1
8 and the output of the black signal generation circuit 15 are connected to the output tone conversion circuits 19, 20, 21 and 22, and in the tone correction mode, the gray scale signal generation circuit 34
Connect the output of to the data thinning circuit 30.31.32,
The input of the output tone conversion circuit 22 is connected to ground.

The tone correction method using the apparatus of this embodiment configured as above and the operation of the apparatus will be explained.

FIG. 2 shows the operational flow of tone correction processing.

To perform tone correction, first, the operator instructs grayscale output using the tone correction start switch 47 provided on the operation panel, and then the multiplexers 36 and 37
．． 38 connects the output of the gray scale signal generation circuit 34 to the output tone conversion circuits 19, 20, and 21. Also, at this time, the color image input section 2 of the color copying machine is rendered inactive.

The gray scale signal generation circuit 34 generates gray scale signals Ev, E14 .
Ec (EY=EM=EC) is generated (Figure 2 ■).

The grayscale signals EY, EM, EC are subjected to TRC processing in output tone conversion circuits 19, 20, 21, then dithered, and then color-1! ii) The image output unit 4 prints out a correction grayscale chart (correction grayscale image) 48 as shown in FIG. 3 (FIG. 2). This correction grayscale chart 48 is
It is influenced by the characteristics of the color image output section 4 in the current environment, and therefore includes information on how far the characteristics of the color image output section 4 in the current environment deviate from the default values.

Next, as shown in FIG. 4, the printed correction grayscale chart 48 is set at a predetermined position on the platen of the color image input unit 2 (for example, at the position of an A4 original). By pressing the gray scale manual command switch, multiplexers 27, 28.
29 connects the outputs of the masking circuits 12, 13.14 to the data thinning circuits 30, 31, and 32, and the color image input section 2 scans the set correction gray scale to obtain 0 to n levels of density Da, Read Do, D11 (Figure 2 ■).

Then, for the densities DB, Do, and DR, the END processing circuits 6.7 and 8, the smoothing fist sharpening circuit 9゜10.11
．． The masking circuits 12, 13, and 14 perform the respective processing to obtain the values E, , E, , corresponding to the END of Y, M, and C.
Find EC (Figure 2 ■).

The signals EV, EM, and EC are sent to the data thinning circuit 30.
At 31.32, the data is appropriately thinned out by sampling at regular intervals to reduce the number of data, and then stored in the RAM 33. By reducing the amount of data in this way,
The capacity of the RAM 33 can be reduced, and the processing time for performing tone correction can be shortened.

The CPU 35 reads the density level 0- stored in the RAM 33.
The decimated signals Ev, EM within each level of n
, EC to find the average value EYI E, , Ec (Figure 2 ■). The initial values El of levels 0 to n generated by the gray scale signal generation port w134 and the data E, (
=Ev, EM, EC), the modified tone function E
Find +=g(E+) (Figure 2 ■).

FIG. 5 is a graph for explaining the tone correction process, and the first quadrant is the TRC characteristic at the time of initial setting.

The second quadrant is the output characteristic of the color image output section 4, and shows the relationship between the END vMc value of the gray scale signal and the ENDYM (+ value) of the gray scale read by the color image input section 2. The output characteristic of the output unit 4 in the current environment, and the broken line shows the characteristic at the time of initial setting.

The third quadrant has characteristics of the color image input section, END processing, and masking processing, and is an inverse function of the first quadrant.

The fourth quadrant shows curves for TRC correction. In this fourth quadrant, the solid line is i= of the modified tone function E + = g(E,) obtained by the CPU 35.
This is a plot of the case of Y, which is an inverse function of the current output characteristic (solid line in the second quadrant) of the color image output unit 4. The dashed straight line is the initial setting.

The product of the function in the fourth quadrant and the function in the first quadrant becomes the corrected TRC characteristic. Therefore, the conversion look-up tables in the RAMs of the tone output conversion circuits 19, 20, and 21 are rewritten to ones with corrected TRC characteristics (FIG. 2).

Note that although FIG. 5 shows the case where i=Y,
Similar graphs can be drawn for M and c.

According to this embodiment, by outputting the gray scale signal generated by the gray scale signal generation circuit 34 via the color image output section 4, information reflecting the characteristics of the current environment of the color image output section 4 is output. After obtaining a grayscale chart including the grayscale chart, the color image input section 2 reads the grayscale chart, determines the deviation from the original grayscale signal generated by the grayscale signal generation circuit 34, and determines the characteristics of the color image output section 4. The contents of the conversion RAM of the output tone conversion circuit are corrected to compensate for the changes, so even if changes occur in the environment, it is possible to maintain accuracy (correction) of deviations in tone and gradation of copies of the color image output section. ,
It is possible to maintain stable output quality.

Furthermore, in this embodiment, since the correction grayscale chart is read by the color image input section 2 for a copying machine, a special measurement input section such as a spectrophotometer is not required for this purpose.

FIG. 6 shows another embodiment of the present invention, in which the tone variation of the color image input section 2 is corrected before the tone variation of the color image output section 4 is corrected.

That is, the output value of the color image input section 2 of the color copying machine does not always remain constant due to environmental conditions such as temperature and humidity, and changes over time in the exposure lamp, CCD, etc. There is no problem when reading the correction grayscale chart 48 output from the color image output section 4 using a dedicated input means that can accurately read the grayscale, such as a spectrophotometer. When the gray scale chart 48 is configured to be read using the color image input unit 2, if there is a change in the characteristics of the read output as described above, the color image output unit 4 included in the correction gray scale chart 48 Therefore, the accuracy of the tone correction of the color image output unit 4 may deteriorate.

Therefore, in this embodiment, tone correction is also performed in the color image input section 2, and even if the color image input section 2 of the copying machine is used to read the correction gray scale chart 48, accurate tone correction can be performed. This is how it was done. Furthermore, in this embodiment, in addition to the tone correction of the color image output section 4, by also performing the tone correction of the color image input section 2, it is of course possible to obtain a copy output with stable quality of the - layer. . Note that for tone correction in the color image manual section 2, the technology disclosed in Japanese Patent Application No. 1983-84865 "Density Correction Apparatus for Color Image Forming Apparatus" previously filed by the present applicant may be used. Can be done.

The main points of this embodiment that differ from the embodiment of FIG. 1 are as follows:
A multiplexer 40.41 that switches between correction of the characteristics of the color image input section 2 and tone correction of the color image output section 4.
．． 42, and 43.44°45 are added, and E
The function to modify the conversion characteristics of the ND processing circuit 6.7°8 is C.
This has been added to the PU35.

In this embodiment, first, in order to correct changes in the characteristics of the color image input section 2, a grayscale chart for input section correction prepared in advance is read by the color image input section 2, and the END processing circuit 6 of the color image processing section 3 is read. , 7.8.

When making this correction, multiplexers 40, 41, 42, and 43, 44 should be
．． 45 is set in the position shown, color, ii! Image ii: The reading output signal DalDQ+DR of the grayscale chart for input section correction from the human power section 2 is input to the data thinning circuits 30, 31, and 32, and the number of data is reduced to R.
It is stored in AM33.

The CPU 35 receives the signal f5J stored in the RAM 33.
, that is, the average value f5 of each gray scale step from ,Dc,,DR. ．． Find f5o and f5R. And the f5a, f5o, fiR and END processing times/g
Initial data of equivalent neutral concentration conversion characteristics of 6,7°8 Er=
Based on r(DJ), a modified tone function fc (D+) is determined, and the equivalent neutral density conversion characteristic of the END processing time w16.7.8 is corrected.

Note that the configuration and operation of the tone correction of the color image output section 4 are the same as those in the embodiment shown in FIG. 1, so a description thereof will be omitted.

(Effects of the Invention) As described in detail above, the present invention measures the characteristics of the color image output unit in its usage state, detects deviation from the initial value, and corrects the deviation. - Even if there are fluctuations in the characteristics of the color image output unit due to changes in humidity or other environments, this can be corrected with high accuracy and a stable color output image can always be obtained.

If the color image forming device has an input section that optically reads images, such as a color copying machine or a color facsimile device, the input section can be used as a grayscale input means, and therefore the tone The configuration for modification becomes easy.

Furthermore, in the aspect of the present invention in which the tone of the color image output section is corrected after the tone correction of the color image input section, when the tone correction of the color image output section is performed, the characteristics of the color image input section are not corrected. This allows the color image input section to accurately read the gray scale chart for color image output section correction, thereby making it possible to obtain stable color image output with higher quality.

Furthermore, by performing tone correction on the color image input section in addition to tone correction on the color image output section, it is possible to obtain a copy output with stable quality of the negative layer.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of an apparatus for carrying out the tone modification method of the present invention. FIG. 2 is a flow diagram showing the operation of the tone correction process of the apparatus of the embodiment shown in FIG. FIG. 3 is a diagram showing an example of a gray scale chart for correction. FIG. 4 is a top view of the color image input section 2. FIG. 5 is a graph for explaining the principle of tone correction processing. FIG. 6 is a block diagram showing another embodiment of an apparatus for implementing the tone modification method of the present invention. FIG. 7 is a block diagram showing the general configuration of a color copying machine. 1...Color original, 2...Color image output section, 3.
...Color image processing section, 4...Color image output section, 5
...Color copy, 6,7.8...Equivalent neutral density processing circuit (EN-D processing circuit), 9.10. li...Smoothing compound sharpening circuit! 2, 13.14... Masking circuit, 15... Black signal generation circuit, te, t7.18.
...Undercolor removal circuit, 19.20.21゜22...Output tone conversion port [(TRC circuit), 23.24.25.2
6 river dither processing circuit, 27°28.29.3B, 37.
38.39.40゜41.42.43.44.45...
- Multiplexer, 30, 31. 32... Data thinning circuit, 33... RAM, 34... Gray scale signal generation circuit, 35... CPU. Figure 2 3g! ! ! 48 No. 4 15gI Y No. 7vA

Claims (5)

[Claims] (1) A first process that outputs the built-in grayscale signal as a grayscale image from the color image output section of the image forming device; a second process that reads the output grayscale image; and a second process that reads the output grayscale image. A tone correction method for a digital color image forming apparatus, comprising a third process of correcting an output tone of a color image output section based on data and the gray scale signal. (2) grayscale signal generating means for generating a grayscale signal in order to output the reference grayscale signal as a grayscale image via an image output unit of an image forming apparatus; and a grayscale signal output from the image output unit. A grayscale image input means for optically reading an image; and a color signal supplied to an image output section of an image forming apparatus based on a read signal of the read grayscale image and a grayscale signal generated by the grayscale signal generation means. 1. A tone correction device for a digital color image forming apparatus, comprising: an image output section tone correction means for applying correction to an image signal. (3) The tone correction device for a digital color image forming apparatus according to claim 2, wherein a color image input section provided in the digital color image forming apparatus is used as the gray scale image input means. (4) a first process of reading a reference gray scale image for input tone correction; and the equivalent neutral density based on the data read in the first process and the predetermined conversion characteristics of the equivalent neutral density converter. a second process that corrects the tone of the color image input section by correcting the conversion characteristics of the conversion section; and a third process that outputs the built-in grayscale signal as a grayscale image from the color image output section of the image forming apparatus. a fourth process of reading the output grayscale image; and a fifth process of correcting the output tone of the color image output section based on the data read in the fourth process and the grayscale signal. A tone correction method for a digital color image forming apparatus, comprising: (5) Based on the read signal of the gray scale image for input tone correction read by the image input unit of the image forming apparatus and the predetermined conversion characteristics of the equivalent neutral density converter, convert the conversion characteristics of the equivalent neutral density converter. image input unit tone correction means for correction; grayscale signal generation means for generating a grayscale signal in order to output the reference grayscale signal as a grayscale image via an image output unit of the image forming apparatus; A color image is supplied to the image output section of the image forming apparatus based on the read signal of the gray scale image from the image output section read by the image input section of the apparatus and the gray scale signal generated by the gray scale signal generating means. 1. A tone correction device for a digital color image forming apparatus, comprising: an image output section tone correction means for applying correction to a signal.