JP2003324619A - Image processing equipment and its control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein when gradation correction to make accumulated color difference of a patch image linear is performed, density inversion is generated when there is low precision of in-plane uniformity of the patch, uniform gradation characteristics cannot be ensured in the L*a*b* space, when the color difference is accumulated, as it is, and color-matching precision is lowered. <P>SOLUTION: Concerning the patch image, showing a plurality of gradations based on a monotonically increasing image signal, a density value and chromaticity information of each patch are obtained (S904). Density difference between adjacent patches in the increasing direction of the image signal is calculated (S905). A patch, wherein the difference value is positive, is extracted (S906). About the extracted patch, on the basis of the chromaticity information, the color difference between the adjacent patches in the increasing direction of the image signal is accumulated, and accumulated color difference is calculated (S908). A table for gradation correction is formed, in such a manner that the accumulated color difference becomes linear with respect to the image signal (S909). <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus and its control method for improving color reproducibility of a formed image in an image forming apparatus.

[0002]

2. Description of the Related Art Recent copying machines have been connected to a network together with a printer or the like and have been installed in an MFP (MultiFunction Printe
Some are used as r). In such an environment, it is possible to match the colors of the images to be printed or the colors of the images to be printed on the display such as CRT with the devices connected to the network. Many are done.

Various color management processes are known as such color matching techniques. For example, ICC
In color management using the (International Color Consortium) profile, calibration is performed by creating a device-specific ICC profile (multidimensional LUT) such as a printer or a copying machine. For example, a personal computer (PC) creates print data by performing color conversion using a profile created in advance, and outputs the print data to a device corresponding to the profile. This makes it possible to match the color of the image printed out from the device with the appearance of the color of the image displayed on the display device or the like.

In order to create such a profile unique to the apparatus, dedicated software, a colorimeter, etc. are commercially available, so that even general users can obtain the output color of an image forming apparatus such as a printer as a target color. The environment for matching with is being prepared.

As another calibration method, there is also known a method in which the contents of a gamma LUT relating to gradation are changed to obtain desired gradation characteristics without performing color conversion by a multidimensional LUT of an ICC profile. There is.

As described above, the color management is an effective method for suppressing the difference in output color between a plurality of devices of the same model or between different models, and its applicable range is not limited to the example described above. For example, by matching the color printed by the printer with the color printed by the offset printing machine, the printer can be used for color calibration of printing. In this case, color management as shown in FIG. 22 is performed on the application of the PC by preparing ICC profiles for the printing machine and the printer.

The contents of the printing ICC profile and the printer ICC profile shown in FIG. 22 are based on the color measurement of a patch using a colorimeter. Both profiles are respectively press and printer independent color spaces (eg CIE L *
It is created by associating with the a * b * color space), which allows the printing color of the printing press to match the printing color of the printer. The color management module (CMM) running on the PC can create print data for print color calibration by performing color conversion using these profiles.

As described above, since the color management environment such as the colorimeter, the application, and the profile creation software is prepared, the method of calibrating the color of the printing machine using the electrophotographic printer is mainly used in the design industry. Is spreading to.

[0009]

However, in the color proofing method of the printing machine using the above-mentioned printer, the IC
Since the gradation characteristics that take into consideration the device-independent color space (L * a * b * color space) such as the PCS (Profile Connection Space) of the C profile are not used, the color matching accuracy by the ICC profile is sufficient. I couldn't withdraw.

Further, since the ICC profile is generally a multi-dimensional LUT with limited grid points (for example, 33 × 33 × 33 points), it has uniform gradation characteristics in the L * a * b * color space. There is
It is desirable in terms of interpolation accuracy. In order to realize such a uniform color space, it is effective to apply a known cumulative color difference linear gradation characteristic, but there is a problem in terms of in-plane uniform accuracy. For example, even when the input information (halftone dot%) is increasing but the density is decreasing, a color difference occurs, and thus the cumulative color difference increases. Even if the gradation characteristic that makes the accumulated color difference linear is adopted in such a state, it is not possible to secure the uniform gradation characteristic in the L * a * b * space, that is, the color matching accuracy is improved. I couldn't let you do it.

The present invention has been made to solve the above problems, and an image processing apparatus and a control method therefor capable of improving color matching accuracy by adjusting gradation characteristics of a printer linearly with accumulated color difference. The purpose is to provide.

[0012]

As one means for achieving the above object, the image processing apparatus of the present invention has the following configuration.

That is, with respect to a patch image showing a plurality of gradations based on a simply increased image signal, a measured value for each patch,
And a measuring means for obtaining chromaticity information, a difference means for calculating a difference between the measured values between adjacent patches in the increasing direction of the image signal, and a gradation value based on the difference value between the patches and the chromaticity information. Table creating means for creating a gradation correction table so that the characteristic is a linear cumulative color difference.

For example, the table creating means may include a patch extracting means for extracting a patch having a positive difference value, and the extracted patch in the increasing direction of the image signal based on the chromaticity information. Accumulating means for calculating a cumulative color difference by accumulating color differences between adjacent patches, and creating a gradation correction table so that the cumulative color difference is linear with respect to the image signal. To do.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail below with reference to the drawings.

<First Embodiment><ApparatusConfiguration> FIG. 1 is a diagram showing a schematic configuration of a four-color full-color laser beam printer (hereinafter referred to as a printer), which is an image forming apparatus according to the present embodiment.

The printer shown in FIG. 1 has a document reading section 7 for reading a document image and a read image signal for a read image signal.
An image processing unit 209 that performs image processing such as gradation correction, and a printer unit 2 that forms a visible image on the recording paper P based on the image signal.
It consists of 00 and.

Each of the printer units 200 has a magenta
There are four image forming stations for forming images of (M), cyan (C), yellow (Y), and black (BK). Each image forming station includes an electrophotographic photosensitive member (hereinafter referred to as "photosensitive drum") 1a, 1b, 1c, 1d that is an image holding member rotatably supported in the direction of the arrow in the figure,
Along the rotation direction, the chargers 12a, 12b, 12c, 12d, the developing devices 2a, 2b, 2c, 2d, and the cleaners 4a, 4b, 4c, 4d.
And so on.

Developing devices 2a, 2b, 2c, 2d and cleaners 4a, 4
Below the photosensitive drums 1a, 1b, 1c, 1d between b, 4c, 4d, a transfer belt 31 is provided so as to be in contact with them. The transfer belt 31 sequentially conveys the recording paper P, which is a recording medium, to the respective photosensitive drums 1a, 1b, 1c, 1d. The images formed on the photosensitive drums 1a, 1b, 1c, 1d in each image forming station are transferred to the recording paper P on the transfer belt 31 by the transfer chargers 3a, 3b, 3c, 3d.

Further, the printer unit 200 includes a plurality of paper feed units, that is, paper feed cassettes 61b, 61c, 61d and an arrow R in the figure.
A manual paper feed tray 61a that can be pulled out in the direction of 61a and a large-capacity paper deck 61e are provided, and recording paper P can be loaded in each.

While the recording paper P is supported on the transfer belt 31 and passes through each image forming station, the toner images of the respective colors formed on the photosensitive drums 1a, 1b, 1c and 1d are sequentially transferred. It When this transfer process is completed, the recording paper P
Is conveyed to the fixing device 5 by a conveyor belt 62 which is separated from the transfer belt 31 and serves as recording paper guide means.

The fixing device 5 includes a fixing roller 51 which is rotatably supported, a pressure roller 52 which rotates while being pressed against the fixing roller 51, a release agent applying device 53, and a roller cleaning device. Fixing roller 51 and pressure roller 52
A heater such as a halogen lamp is disposed inside each. A thermistor (not shown) is in contact with each of the fixing roller 51 and the pressure roller 52, and the fixing roller 51 and the pressure roller 52 are controlled by controlling the voltage applied to each heater via a temperature adjusting device (not shown). The surface temperature of is adjusted. The pressure value of the pressure roller 52 and the surface temperature of the fixing roller can be made variable by the fixing control mechanism 60.

The fixing roller 51 is in contact with a releasing agent applying device 53 for applying silicone oil as a releasing agent on the surface thereof, and the recording paper P is conveyed by a conveying belt 62 to press the fixing roller 51. The toner is prevented from adhering to the surface of the fixing roller 51 when passing through the roller 52.
Further, the release agent application device 53 is connected to an application amount control device 63 that controls the application amount of silicone oil applied to the surface of the fixing roller 51.

A drive motor (not shown) for driving the fixing roller 51 and the pressure roller 52 includes a fixing roller 51 and a pressure roller for pressing and heating the conveying speed of the recording paper P, that is, both front and back surfaces of the recording paper P. A speed control device 64 for controlling the rotation speed with 52 is connected. As a result, the unfixed toner image on the surface of the recording paper P is melted and fixed, and a full-color image is formed on the recording paper P. The recording paper P on which the full-color image is fixed is separated from the pressure roller 52 by a separation claw (not shown).

Reference numeral 7 denotes a document reading unit (hereinafter referred to as a reader unit), which optically scans a document placed on a document table and reads it by a CCD (not shown) to obtain an image signal of each color.

An image processing unit 209 performs image processing such as gradation correction, which is a feature of this embodiment, on the image signal of each color read by the reader unit 7. This image processing unit
Details of the processing in 209 will be described later.

An operation display 300 of the laser beam printer is used to input commands from the operator and notify the operator of the state of the apparatus.

Cumulative Color Difference Gradation Characteristic and Correction Method The method of creating the cumulative color difference gradation employed in this embodiment will be described below.

First, in the LUT off state, that is, in the original engine state in which gradation correction has not been performed, patches of 64 gradations for each color are output. Then, the output 64 gradation patch is placed on the reader unit 7 to read the image, and the chromaticity and the density are calculated.

The reader unit 7 is also used during normal copying, and the red (R) and green read here are used.
The RGB luminance signals of (G) and blue (B) are converted into L * a * b * chromaticity information by a chromaticity calculation mechanism described later.

On the other hand, regarding the method of calculating the density information, similarly, the density is calculated from the RGB luminance signal via the LOG converter. In general, the relationship between the emission brightness (Io), the reception brightness (I), and the density (D) is represented by D = -log10 (I / Io), and thus the density can be calculated by one-dimensional conversion.
Therefore, in the present embodiment, the brightness of red (R) → cyan (C), green (G) → magenta (M), blue (B) → yellow (Y) −
Uses a density conversion table. In addition, black (BK)
As for, the G → BK table was adopted by using the G signal at the center position in the visible light region, which has a wide spectral sensitivity distribution.

On the other hand, the chromaticity information is calculated by three-dimensional direct mapping of RGB → L * a * b * which is similar to the ICC profile. The L * a * b * colorimetric condition when creating this direct mapping coefficient is to improve the color matching accuracy using the ICC profile. The field of view is 45 ° / 0 ° (or 0 ° / 45 °).

In this embodiment, the color difference and the density difference are calculated using the chromaticity information and the density information calculated as described above, but the density difference is calculated first. Since 64 gradation patches are used in this embodiment, the density difference ΔD _n (n = 0 to 63) among all 63 patches is calculated by the following formula.

ΔD _n = (D _{n + 1} −D _n ) At this time, when all ΔD are positive (plus),
Proceed to color difference information calculation. On the other hand, when there is a negative (minus) ΔD, it is highly likely that in-plane unevenness or the like was detected when reading the gradation patch, so it was used when calculating the negative ΔD (n The patch information on the (+1) side is discarded, and the density difference from the next (n + 2) patch is calculated again. Then, it is determined whether the newly calculated density difference is positive (0 ≦ ΔD) or negative (ΔD <0), and if negative, the above operation is repeated. Then, if the density difference becomes positive, the process proceeds to the calculation process of the color difference information in the patch.

The color difference calculation processing in this embodiment will be described in detail below. Since 64 gradation patches are used here, the color difference between all 63 patches ΔE _n (n = 0 to 6
3) is calculated by the following formula.

[0036] _{ΔE n = ((L * n} + 1 -L * n) 2 + (a * n + 1 -a * n) 2
+ (B * _{n + 1-} b * _n ) ² ) ^0.5 Here, the accumulated color difference values ΣΔE _n are obtained by accumulating the obtained inter-patch color differences as shown in FIG. If the relationship between the cumulative color difference value ΣΔE _n and the input signal value shows a linear characteristic, the L * a * b * color space becomes uniform, and the color matching accuracy can be improved.

Hereinafter, it will be described with reference to FIGS. 3, 4 and 5 that the linear gradation characteristics of the cumulative color difference show uniform gradation characteristics in the L * a * b * color space.

FIG. 3 shows a single color from the white point to CMY, and RGB.
Image signals of up to two colors (image signal for RGB / 2),
It is a figure showing the relation of a cumulative color difference value. According to the figure, CM
It can be seen that if the cumulative color difference of the Y single color is a linear gradation characteristic, the secondary color RGB also exhibits a linear gradation characteristic of the cumulative color difference.

FIG. 4 is a diagram showing the outermost peripheral movement relationship of the a * -b * plane, which is Y → R, M → R, M → B, C → B, C → G, Y →.
G, the relationship between each signal change amount and the cumulative color difference is shown. According to the figure, if the monochromatic cumulative color difference gradation characteristics are linear, almost uniform gradation can be obtained even in the outermost circumference (a * -b * plane).

FIG. 5 shows the relationship between the halftone dot% from the white point to the three CMY colors BK (if CMY is 10%, the halftone dot% is 10%) and the cumulative color difference, and the cumulative BK single color. It is a figure which shows a color difference. It can be seen from the drawing that even with respect to the gradation in the shadow portion, if the gradation characteristic of the cumulative color difference is linear, uniform gradation characteristic can be obtained in the L * a * b * color space.

Here, FIG. 6 verifies the color matching accuracy of the printer in which the gradation correction of the present embodiment described above is executed, and the copying machine in which the conventional density linear gradation correction is executed. The results are shown. Hereinafter, this verification method will be described.

First, DDCP (Direct Digital Color Proof
er) The IT87 / 2 (928 patch) chromaticity value output on a compatible device and its ICC profile were prepared. afterwards,
In the printer under test, the cumulative color difference linear obtained as described above, the density linear used in the CMS which is the conventional gradation characteristic, the input signal-the upward convex in the density correspondence,
An ICC profile was created for each of the three gradation characteristics. Note that the well-known absolute col is used as the color conversion method.
adopted orimetric.

The application for ICC creation is also
Three profiles, Canon profile creation application and two other commercially available products, are prepared. One DDCP machine (target) and each gradation, application,
The color difference of 928 patches was obtained through a total of 6 ICC profiles (destinations). Note that FIG. 6 shows the verification result by the profile creation application manufactured by Canon Inc.

In the graph of FIG. 6, the horizontal axis represents the color difference Δ.
E, the vertical axis represents the cumulative occurrence probability, and the area where the cumulative occurrence probability is divided by a graph and the area up to ΔE = 10 are called the color reproduction index. The higher the percentage of the color reproduction index, the higher the color matching accuracy.

According to the verification results of the color matching accuracy as described above, it has been found that the color matching accuracy is improved in the order of cumulative color difference linear> density linear> upward convex in any application.

FIG. 7 shows a graph of input signal-cumulative color difference at the time of density linear gradation. The graph shown in the figure cannot be said to be linear, and it can be seen that the information amount of the signal is considerably deviated. In such a biased color space (gradation), it is difficult to interpolate in the L * a * b * color space as described in the above problem. That is, since the number of signals is limited, an interpolation mismatch occurs due to a portion having a small amount of information, resulting in a decrease in color matching accuracy.

As described above, in order to improve the color matching accuracy under a limited number of signals, L * a * b *
It is important that the gradations are uniform in the color space, and therefore it can be seen that the gradation characteristic of linear cumulative color difference is effective.

Description of Image Processing Section The processing in the image processing section 209 in this embodiment will be described in detail below. FIG. 8 is a block diagram showing a schematic configuration of the image processing unit 209.

In FIG. 8, the CCD 210 in the reader unit 7 is
The original image is read at 600 dpi, and the read image is input to the image processing unit 209 as an RGB signal. The RGB signal input to the image processing unit 209 is converted into a digital RGB signal by the A / D converter 102.
Converted to a signal.

The shading correction unit 103 corrects the illumination light amount, the light amount unevenness generated in the lens optical system, and the sensitivity unevenness of the pixels of the CCD 210. The scaling unit 104 enlarges or reduces the read image. The input direct mapping unit 105 is a device-independent color space L * a * b * for input RGB signals.
Convert to a signal on the color space. The output direct mapping unit 106 converts the L * a * b * signal into a specified CMYK signal.

The resolution converter 107 converts the 600 dpi image signal into 1
Although the resolution is converted to 200 dpi, the resolution conversion can be turned on / off under the control of the CPU 110. Image formation pattern processing unit 1
Reference numeral 08 has a multi-valued function based on the well-known line growth type dither method, dot concentration type dither method, etc., and an image forming pattern is selected under the control of the CPU 110. The CMYK signals output from the image forming pattern processing unit 108 are output to the printer unit 200.
Sent to. Although the image forming pattern processing unit 108 also performs processing using a LUT for correcting the gamma characteristic of the printer unit 200, this LUT processing is generally performed before pattern processing such as matrix calculation.

The density conversion unit 120, which receives the RGB signal output from the scaling unit 104, converts the RGB luminance information into CMY density information and sends it to the LUT generation unit 121. The L * a * b * signals output from the input direct mapping unit 105 are also input to the LUT generation unit 121. In the LUT generation unit 121, the positive / negative information of the density difference based on the input density information and the color difference information between the patches based on the L * a * b * information are obtained, and from these two information,
A gradation correction table (LUT) is created so that the cumulative color difference with respect to the number of signals has a linear output gradation characteristic. LUT generator
The LUT created in 121 is uploaded to the image formation pattern processing unit 108.

Based on the control program stored in the ROM 111, the CPU 110 uses the RAM 112 as a work memory to comprehensively control each component of the image processing unit 209. For example, the resolution conversion unit 108 and the image forming pattern. It also controls the parameter setting in the processing unit 109 and the like. The CPU 110 also has a network I / F for communicating with the operation / display unit 114 and external devices.
It controls 113 and inputs / outputs image information and device information. That is, the CPU 110 is a processor that controls the entire system.

The display / operation unit 114 plays a role of transmitting information input by the user to the CPU 110. The HDD 115 is a hard disk drive and stores system software, general image data, and output image data. HDD1
The contents of 15 can be set by the user.
A raster image processor (RIP) 116 expands the PDL code into a bitmap image and sends it as an L * a * b * or CMYK signal before and after the output direct mapping unit 106.

The density converter 120 has a table for converting the input RGB luminance signals into R → C, G → M, and B → Y, which are in a complementary color relationship.
It has a table of G → BK different from G → M.

Details of LUT Generation Process Hereinafter, with respect to the LUT generation process in the LUT generation unit 121,
This will be described in detail with reference to the flowchart of FIG.

When the automatic gradation correction is instructed (S901), the color laser beam printer according to the present embodiment causes each photosensitive drum 1
The contrast potential is determined by the surface potential sensor (not shown) on a, 1b, 1c, 1d and the photosensor that detects the toner patch image on each drum to guarantee the maximum density (S90
2). Since this processing is described in detail in Japanese Patent Laid-Open No. 10-028229, detailed description will be omitted here.

Then, with the LUT off, the CMYK colors are evenly distributed.
The latent image, development, transfer, and fixing processes are performed on the 4-gradation patch, and an image is formed on recording paper P and discharged (S9
03). The output 64-gradation patch is placed on the platen of the reader unit 7 by the user, and the image is read in accordance with the user instruction from the operation display 300 (S904). The image data of 64 gradations read by the reader unit 7 is converted from RGB luminance signals into density signals (CMY) and chromaticity information (L * a * b *).

In the LUT generator 121, the density difference between the patches is positive (0 ≦ ΔD _n ) based on the density signal value.
It is determined whether or not (S905, S906). If all are positive, proceed to the next step (S907).

On the other hand, if there is a non-positive density difference, the patch information (D _{n + 1} ) is skipped and the next patch information (D _{n + 2} ) is referred to. This skip processing is performed until the density difference becomes positive or the patch density reaches the maximum value (255, 100%)
By repeating until (S915 to S927), the reference patch (density difference detection patch) group to be sampled is determined. In the present embodiment, the skip processing is repeated until the patch density reaches the maximum value (S916, S921, S926), but since the patch of the present embodiment has 64 gradations, the current processing patch is The above skip process may be controlled to be repeated until the 64th position is reached.

By this skip processing, the density value of each patch in the reference patch group shows a characteristic of simple increase with respect to the image signal value. When the reference patch group is determined in this way, the process proceeds to step S907.

In step S907, the color difference between the patches in the reference patch group is calculated (S907). At this time, for the (D _{n + 1} ) patch whose density difference is negative,
Since it has already been excluded from the reference patch, the present embodiment is characterized in that the gradation is corrected without using the chromaticity information about this patch.

Then, the color difference between the patches is integrated to calculate a cumulative color difference (S908), and a correction table (LUT) is created so that the cumulative color difference has a linear gradation (S909). Specifically, as shown in FIG. 10, by inversely converting the relationship between the input gradation and the output gradation, it is possible to maintain a gradation with a linear cumulative color difference with respect to the input signal. Note that the solid line in FIG. 10 shows the gradation characteristics when the gradation correction LUT is turned off (not used), and the broken line shows the characteristics that should be set as an LUT, which is the gradation characteristics shown by the solid line reversed. In this way, if the cumulative color difference is corrected so as to have a linear gradation characteristic, a linear relationship is obtained in the L * a * b * space, and color matching accuracy can be improved.

The correction LUT created by the LUT generator 121 as described above is uploaded to the image forming pattern processor 108, and thereafter, until automatic gradation correction is executed again,
An image is formed with this gradation characteristic (S910 to S914).

As described above, in the printer of this embodiment, by adopting the gradation characteristic evenly distributed in the color space (L * a * b *) that can be reproduced by itself,
Color matching accuracy can be improved.

Further, in order to realize the above L * a * b * uniform gradation characteristic, the cumulative color difference is calculated by excluding the inversion portion of the density difference between the patches, thereby further improving the color matching accuracy. To be done.

<Second Embodiment> The second embodiment of the present invention will be described below.
An embodiment will be described.

FIG. 11 is a view showing the schematic arrangement of a four-color full-color laser beam printer (hereinafter referred to as a printer), which is an image forming apparatus according to the second embodiment, and is similar to the above-described first embodiment. The same number is attached to the configuration of
The description is omitted.

The difference from the first embodiment is that the reader section is not provided and only the printout function is provided. Further, as a patch input mechanism at the time of gradation correction, a color patch sensor is provided after fixing, an RGB luminance signal is input as in the first embodiment, and gradation correction is performed based on the accumulated color difference information and density difference information. It is characterized by

The second embodiment is characterized in that the operability for the user is improved as compared with the gradation correction method described in the first embodiment. That is, even in a printer without the copying function, the operability is improved while improving the color matching accuracy.

The differences from the first embodiment will be described in detail below.

In the printer of the second embodiment, the first
A color patch sensor 16 was provided after fixing instead of the reader unit 7 in the embodiment. Further, since the patch output sample (recording paper) for tone correction is stored in the dedicated sample storage section 70, it is not touched by the user. The reason for adopting such a configuration is that even if a patch for gradation correction is output on a recording sheet, the patch image does not need to be confirmed by the user's eyes, and it is mixed with important output materials. This is because it is only troublesome for the user, such as being lost or having to be scrapped. Further, it is effective that the service person does not have to bother the user even for checking the gradation correction status.

The color patch sensor 16 is provided exclusively for automatic gradation correction processing, and its schematic configuration is shown in FIG. As shown in the figure, the color patch sensor 16
On the recording paper passing between the fixing roller 51 and the pressure roller 52, sensing is possible only in the portion where the patches of each color of CMYK are present. That is, the brightness value for each patch is obtained by detecting the reflected light from the patch portion.

FIG. 13 is a block diagram showing the schematic arrangement of the image processing unit 209 in the second embodiment. In the figure, the same numbers are assigned to the configurations that perform the same functions as in FIG. 8 shown in the first embodiment, and detailed description thereof is omitted. In FIG. 13, since the RGB luminance signal input from the CCD 210 (reader unit) in FIG. 8 is input from the patch sensor 16 after fixing, the shading correction unit and the scaling unit are deleted. . There are no major differences for other mechanisms.

The LUT generation process in the LUT generation unit 121 of the second embodiment will be described in detail below with reference to the flowchart of FIG. In the figure, the above-mentioned first
The same steps as those in FIG. 9 according to the embodiment are denoted by the same step numbers, and detailed description will be omitted.

In the printer of the second embodiment, when the automatic gradation correction is instructed (S901), the photosensitive drums 1a, 1b,
The contrast potential is determined by the surface potential sensor (not shown) on 1c and 1d and the photosensor that detects the toner patch image on each drum to guarantee the maximum density (S902).

Then, with the LUT off, the CMYK colors are evenly distributed.
The latent image, development, transfer, and fixing processes are performed on the 4-gradation patch to form an image on the recording paper P (S903). Then, the 64-gradation patch formed on the recording paper P is read by the color patch sensor 16 for each CMYK color (S14
01). The image data of 64 gradations read by the color patch sensor 16 is converted from RGB luminance signals into density signals (CMY) and chromaticity information (L * a * b *).
The recording paper P on which the patch is formed is sent to and stored in the sample storage unit 70 without being discharged outside the machine.

Then, in the LUT generator 121, a gradation correction table (LUT) is created based on the density signal and the chromaticity information by the same procedure as in the first embodiment described above.

As described above, according to the second embodiment, it is possible to improve the color matching accuracy while improving the operability even in the printer that does not have the reader unit.

Further, by providing the sample storage section 70, a service person can easily judge the automatic gradation correction status.

<Third Embodiment> The third embodiment of the present invention will be described below.
An embodiment will be described.

The third embodiment is characterized in that the in-plane uniformity accuracy is determined based only on the chromaticity information without determining whether or not the density information is reversed on the formed patch. According to such a configuration, it is not necessary to provide a LOG converter (brightness-density conversion table) required for each color for brightness-density conversion, so that the cost is reduced. Also, BK
With regard to the above, since the in-plane uniform accuracy is confirmed using the brightness information, it is not necessary to perform a special calculation, and the configuration is simplified. Also, for each CMY color, the in-plane uniform accuracy is confirmed using the saturation information, but since the calculation is performed from the L * a * b * space that is input, the calculation formula will change even if the input sensor changes. Product development becomes easy.

The brightness-density conversion table must be experimentally created every time the toner, the sensor, the fixing condition, etc. are changed, but in the third embodiment, it is calculated from the chromaticity. It is also advantageous in terms of cost and shortening of development time.

The configuration of the printer in the third embodiment is the same as that in the above-described second embodiment, and the description thereof will be omitted.

Cumulative Color Difference Gradation Characteristics and Correction Method In the third embodiment, the cumulative color difference is replaced by using the brightness difference information and the saturation difference information instead of the density difference information discriminated in the second embodiment. Create a gradation correction table. Less than,
The theory of creating the cumulative color difference gradation correction table in the third embodiment will be described.

FIG. 15 shows a CMYK monochromatic gradation characteristic locus, with the L * (brightness) component on the vertical axis and the a * and b * components on the horizontal axis. According to the figure, it can be seen that the L * component has a high correlation with the BK color. Therefore, for the BK color, by using the brightness difference information instead of the density difference information used in the second embodiment,
It is possible to judge the in-plane uniformity. On the other hand, for each color of CMY, although there is some correlation with the brightness difference, it is difficult to say that the in-plane uniformity can be detected with high accuracy using the brightness difference information because the depth direction is not sufficient. Particularly for the Y color, since the difference in brightness between the maximum density part and the white point is extremely small, it is more difficult to detect the in-plane uniformity based on the brightness difference information.

FIG. 16 is a diagram in which the color space diagram shown in FIG. 15 is projected from the lightness direction. According to the figure, it is understood that the monochromatic locus tends to continue from the center to the outside. This locus represents the direction of saturation, indicating that it becomes brighter toward the outside. Therefore, for each CMY color, the in-plane uniformity may be determined using this saturation information.

The method of extracting the lightness from the chromaticity (L * a * b *) and the method of calculating the saturation will be described below.

In the third embodiment, as in the first and second embodiments described above, the RGB luminance signal is input and the chromaticity information (L
* a * b *) calculates brightness and saturation.

The lightness is used for BK gradation correction, but since L * of the chromaticity information is lightness information, no special calculation is performed. On the other hand, for saturation information, a * b *
It is represented by the distance from the center (a *, b *) = (0,0) in the color plane. That is, the saturation C can be calculated by the following formula.

C = (a * ² + b * ² ) ^{0.5 In} the third embodiment, the saturation calculation is performed for each patch in this way, and the saturation difference between patches is the same as the density difference between patches in the second embodiment. Make sure that is a plus. That is, similar to the case based on the density difference, if the saturation difference between patches is not positive, it is not added to the cumulative color difference calculation.

Description of Image Processing Unit FIG. 17 is a block diagram showing the schematic arrangement of the image processing unit 209 in the third embodiment. In the figure, as compared with the configuration shown in FIG. 13 in the second embodiment, a saturation calculation unit 122 is used instead of the density conversion unit 120 that converts luminance information into density information.
Is provided. Further, there is no particular change in the configuration of the LUT generation unit 121 that is the tone correction table creation mechanism, but in the second embodiment, the density difference information between patches is calculated by inputting the density difference information. BK in three embodiments
Is used to determine whether or not the patch chromaticity information is used for calculating the cumulative color difference by using the lightness difference inequality sign for CMY and the saturation difference inequality sign for each CMY color.

Since the cumulative color difference calculating method and the table creating method are the same as those in the first and second embodiments described above, detailed description thereof will be omitted.

Details of LUT Generation Process Hereinafter, the LUT generation process in the LUT generation unit 121 of the third embodiment will be described in detail with reference to the flowchart of FIG. In the figure, the same step numbers are given to the same processes as those in FIG. 14 in the above-described second embodiment,
Detailed description is omitted.

Also in the printer of the third embodiment, when the automatic gradation correction is instructed (S901), the photosensitive drums 1a, 1b,
The contrast potential is determined by the surface potential sensor (not shown) on 1c and 1d and the photosensor that detects the toner patch image on each drum to guarantee the maximum density (S902).

After that, with the LUT off, the CMYK colors are evenly distributed.
The latent image, development, transfer, and fixing processes are performed on the 4-gradation patch to form an image on the recording paper P (S903). Then, the 64-gradation patch formed on the recording paper P is read by the color patch sensor 16 for each CMYK color (S14
01). The image data of 64 gradations read by the color patch sensor 16 is converted from RGB luminance signals into chromaticity information (L * a * b *). The recording paper P on which the patch is formed is sent to and stored in the sample storage unit 70 without being discharged outside the machine.

Then, the lightness information L and the saturation information C are extracted from the patch information converted into the chromaticity signal in order to confirm the in-plane uniform accuracy. For the lightness information L, the L * component may be adopted as it is, and for the saturation information C, it may be calculated based on the above arithmetic expression.

Then, regarding the BK patch portion, it is confirmed whether or not the difference in brightness (L * _n )-(L * _{n + 1} ) between patches is positive, and if all are positive, go to the next step (S907). Go forward (S18
01, S1802). On the other hand, if there is a non-positive lightness difference, the lightness information (L * _{n + 1} ) that calculated the lightness difference between the patches is skipped, and the next patch information (L * _{n + 2} ) is referred to. This skip processing is repeated until the lightness difference becomes positive or the patch density reaches the maximum value (255, 100%) (steps S1803 to S927).

On the other hand, for the CMY color patch portion, the saturation
Saturation difference between patches using information ΔC = C _{n + 1}-C_n Calculate
Then, it is confirmed whether or not this ΔC is positive. All of
If the saturation difference between the patches is positive, the next step
Proceed to (S907), but if there is a negative saturation difference,
Saturation information (C_{n + 1}) Skip
And refer to the following patch information. Color this skip processing
Until the difference becomes positive or the maximum patch density (2
55, 100%) (steps S1803 to S92)
7).

By the above processing, the reference patch group to be sampled is determined. When the reference patch group is determined, the process proceeds to step S907, and thereafter, by the same procedure as that of the above-described first embodiment, the gradation correction table (L
UT).

As described above, according to the third embodiment, the in-plane uniformity is calculated by using the lightness signal which does not require a special calculation process and the easily calculated saturation information instead of the density information. Make a decision. This made it necessary for each color
With a simple configuration that does not require a LOG converter, highly accurate determination is possible.

By reflecting this determination result at the time of gradation correction, highly accurate gradation correction can be executed.

<Fourth Embodiment> The fourth embodiment of the present invention will be described below.
An embodiment will be described.

The fourth embodiment is characterized in that the printer for executing the high-accuracy gradation correction described in each of the above-described embodiments has a structure capable of further cost reduction. More specifically, in the printer, the first to third
The patch colorimetric mechanism provided in the embodiment is unnecessary, and the image correction table is created using a commercially available colorimeter.

The printer according to the fourth embodiment does not have a copying function as in the above-described second embodiment, but is further characterized by not having a color patch sensor after fixing. That is, since it has a general printer configuration, detailed description thereof is omitted here.

Description of Image Processing Unit FIG. 19 is a block diagram showing the schematic arrangement of the image processing unit 209 in the fourth embodiment. In this figure, as compared with the configuration shown in FIG. 17 in the third embodiment, the color patch sensor 16 is deleted, so the input direct mapping unit 105 is deleted. Further, a chromaticity input unit 123 for newly inputting chromaticity information from an external colorimeter is newly provided. The fourth embodiment is characterized in that a gradation correction table is created based on the input chromaticity information using the brightness difference, the saturation difference, and the accumulated color difference information.

Commercially Available Colorimeter It is assumed in the fourth embodiment that the user already has a commercially available colorimeter.

Generally, a user who is particular about color matching precision usually outputs an image after color conversion by developing an ICC profile or the like independently and incorporating it in a controller or a PC. ,
It is considered that he owns a high-precision colorimeter for creating a high-precision ICC profile. Therefore, it is possible to substitute the colorimetric function of the printer by the colorimeter. Therefore, in the printer of the fourth embodiment, the color patch sensor for color measurement is deleted to realize the cost reduction targeting such a user.

In general, a commercially available colorimeter uses L * from spectral reflectance.
It calculates a * b * data and has higher accuracy than L * a * b * data obtained by direct mapping calculation from RGB data. Below, from the spectral reflectance data in a commercial colorimeter
The method of calculating L * a * b * will be described. The steps a to g described below are defined by CIE (International Commission on Illumination).

A. Obtain the spectral reflectance R (λ) of the sample (380n
m-780 nm).

B. Color matching functions x (λ), y (λ), z (λ) and standard light spectral distribution SD50 (λ) are prepared.

[0112] e. Integration of color matching function y (λ) and standard light spectral distribution SD50 (λ) Σ {SD50 (λ) × y (λ)} f. XYZ calculation X = 100 × Σ {SD50 (λ) × y (λ)} / Σ {R (λ) × SD50 (λ) × x (λ)} Y = 100 × Σ {SD50 (λ) × y (λ)} / Σ {R (λ) × SD50 (λ) × y (λ)} Z = 100 × Σ {SD50 (λ) × y (λ)} / Σ {R (λ) × SD50 (λ) × z (λ)} g.L * a * b * calculation L * = 116 × (Y / Yn) ^1/3 -16 a * = 500 {(X / Xn) ¹ /3-(Y / Yn) ^1/3 } b * = 200 {(Y / Yn) ^1/3 -(Z / Zn) ^1/3 } When Y / Yn> 0.008856, if Xn, Yn, Zn are standard light tristimulus values, (X / Xn) ^1/3 = 7.78 (X / Xn) ^1/3 +16/116 (Y / Yn) ^1/3 = 7.78 (Y / Yn) ^1/3 +16/116 (Z / Zn) ^1/3 = 7.78 (Z / Zn) ^1/3 +16/116 The above x (λ), y (λ) and z (λ) are usually described by adding a bar (−) to the upper part of each.

Details of LUT Generation Process The LUT generation process in the LUT generation unit 121 of the fourth embodiment is shown in the flowchart of FIG. This figure is almost the same as FIG. 18 in the above-described third embodiment, but a 64-gradation color patch formed on the recording paper P in steps S901 to S903 is colorimetrically owned by the user in step S2001. The only difference is that color measurement is performed by a device and the color measurement data is input again to the printer. This step S200
FIG. 21 shows the concept of color measurement processing in 1.

Then, based on the input chromaticity information of L * a * b *, the brightness difference is detected for BK and the saturation difference is detected for CMY as in the third embodiment, and the in-plane uniformity is detected. The accuracy is judged and the gradation correction LUT is generated.

As described above, according to the fourth embodiment, by entrusting the colorimetric function of the printer to an external device, it is possible to realize highly accurate gradation correction while realizing further cost reduction. it can.

In the fourth embodiment, an example in which an automatic scanning colorimeter is used has been described, but if it is possible to output chromaticity information (L * a * b *) of a patch, it is a handy type. Alternatively, it may be an XY table type. That is, it goes without saying that the present invention is within the scope of the present invention regardless of the configuration of the colorimeter.

[0117]

Other Embodiments The present invention is not limited to the above-described first to fourth embodiments, and the following forms are also applicable.

For example, it is possible to use the brightness difference information as a method of discriminating the in-plane uniform accuracy for BK and the density difference information for other CMY.

Further, an input-to-output gradation correction table is created by performing the gradation correction according to any of the above-described embodiments on an application installed in a PC or the like, and the image of a printer or the like is created from the PC. To the forming device,
The table may be uploaded.

Further, the reader unit and the patch sensor unit incorporated in the printer may be of any structure capable of detecting the spectral reflectance characteristic like a commercially available colorimeter in order to perform highly accurate detection.

Even when the present invention is applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), a device including one device (for example, a copying machine, a facsimile). Device).

Further, an object of the present invention is to supply a storage medium having a program code of software for realizing the functions of the above-described embodiments to a system or device, and to supply the computer (or CPU or MPU) of the system or device. It goes without saying that is also achieved by reading and executing the program code stored in the storage medium.

In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention.

A storage medium for supplying the program code is, for example, a floppy (registered trademark) disk, a hard disk, an optical disk, a magneto-optical disk, a CD.
-ROM, CD-R, magnetic tape, non-volatile memory card, RO
M etc. can be used.

Further, not only the functions of the above-described embodiments are realized by executing the program code read by the computer, but also the OS (operating system) running on the computer based on the instructions of the program code. ) Etc. do some of the actual processing,
It goes without saying that the processing includes the case where the functions of the above-described embodiments are realized.

Further, after the program code read from the storage medium is written in the memory provided in the function expansion board inserted into the computer or the function expansion unit connected to the computer, based on the instruction of the program code, The CPU provided in the function expansion board or function expansion unit performs some or all of the actual processing,
It goes without saying that the processing includes the case where the functions of the above-described embodiments are realized.

[0127]

As described above, according to the present invention, the color matching accuracy can be improved by adjusting the gradation characteristics of the printer linearly with the cumulative color difference.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of a printer according to an embodiment of the invention.

FIG. 2 is a diagram showing a concept of cumulative color difference calculation in the present embodiment.

FIG. 3 is a diagram showing characteristics of cumulative color difference between a single color and a secondary color.

FIG. 4 is a diagram showing characteristics of cumulative color difference at the outermost periphery on a * b * space.

FIG. 5 is a diagram showing characteristics of accumulated color difference from the outermost peripheral point to the lowest lightness point on the a * b * space.

FIG. 6 is a diagram showing a relationship between gradation characteristics and color matching accuracy.

FIG. 7 is a diagram showing cumulative color difference characteristics at the time of density linear gradation characteristics.

FIG. 8 is a block diagram showing a schematic configuration of an image processing unit in the present embodiment.

FIG. 9 is a flowchart showing a process of generating a gradation correction LUT in this embodiment.

FIG. 10 is a diagram showing an example of creating a gradation correction LUT in the present embodiment.

FIG. 11 is a diagram showing a schematic configuration of a printer according to a second embodiment.

FIG. 12 is a diagram showing a schematic configuration of a color patch sensor according to a second embodiment.

FIG. 13 is a block diagram showing a schematic configuration of an image processing unit in the second embodiment.

FIG. 14 is a flowchart showing a tone correction LUT generation process in the second embodiment.

FIG. 15 is a diagram showing a monochromatic gradation locus on the L * a * b * space in the third and fourth embodiments.

FIG. 16 is a diagram showing a monochromatic gradation locus on an a * b * space in the third and fourth embodiments.

FIG. 17 is a block diagram showing a schematic configuration of an image processing unit in the third embodiment.

FIG. 18 is a flowchart illustrating a tone correction LUT generation process according to the third embodiment.

FIG. 19 is a block diagram showing a schematic configuration of an image processing unit in a fourth embodiment.

FIG. 20 is a flowchart showing a tone correction LUT generation process in the fourth embodiment.

FIG. 21 is a diagram showing a schematic configuration of a colorimeter in a fourth embodiment.

FIG. 22 is a diagram showing the flow of a general color management process.

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) G06T 5/00 100 H04N 1/40 101E H04N 1/407 B41J 3/00 A 1/60 BF term (reference) 2C187 AC07 AF03 BF10 GA01 GA03 2C262 AB11 BA09 BA16 BA19 BB03 BB05 BC01 BC03 FA13 GA02 5B057 AA11 BA02 CA01 CA08 CA12 CA16 CB01 CB08 CB12 CB16 CC01 CE11 CE17 CE18 CH07 CH08 5C077 LL19 MM27 MP08 PP15 PP32 PP33 PP36 PP37 PP48 PQ12 PQ23 TT02 5C079 HB01 HB03 HB08 HB12 LA12 LB02 MA04 MA10 MA11 NA03 NA05 PA03

Claims (16)

[Claims] 1. A measuring means for obtaining a measurement value and chromaticity information for each patch of a patch image showing a plurality of gradations based on a simple increase image signal, and the patch between adjacent patches in the increasing direction of the image signal Difference means for calculating the difference between the measured values, and table creating means for creating the gradation correction table so that the gradation characteristics are linear in the cumulative color difference based on the difference value between the patches and the chromaticity information. An image processing apparatus having. 2. The table creating means includes a patch extracting means for extracting a patch having a positive difference value, and the extracted patch based on the chromaticity information.
Accumulating means for calculating a cumulative color difference by accumulating color differences between adjacent patches in the increasing direction of the image signal, and a gradation correction table so that the cumulative color difference is linear with respect to the image signal. The image processing apparatus according to claim 1, wherein 3. The image forming device according to claim 1, further comprising image forming means for forming an image after performing gradation correction on the input image signal based on the gradation correction table. Image processing device. 4. The image processing apparatus according to claim 3, wherein the image forming unit forms the patch image without using the gradation correction table. 5. The image processing apparatus according to claim 4, wherein the image forming unit forms the patch image on a recording medium. 6. The measurement means obtains density information as a measurement value for each patch.
The image processing device according to any one of 1. 7. The image processing apparatus according to claim 6, wherein the measuring unit measures the brightness of each patch and obtains the density information and the chromaticity information based on the brightness. 8. The apparatus further comprises a discharging means for discharging the recording medium on which the patch image is formed by the image forming means, and the measuring means optically reads the recording medium discharged by the discharging means. The image processing apparatus according to claim 7, wherein the brightness of the patch image is measured by the following. 9. The storage device further comprises a storage unit for storing the recording medium on which the patch image is formed by the image forming unit in the image processing apparatus, and the measuring unit before storing in the storage unit. The image processing apparatus according to claim 7, wherein the brightness of the patch image is measured by optically reading a recording medium. 10. The image processing apparatus according to claim 1, wherein the measuring unit obtains lightness information and saturation information as measurement values for each patch. 11. The measuring means measures the brightness of each patch, obtains the chromaticity information based on the brightness, and obtains the lightness information and the saturation information based on the chromaticity information. The image processing apparatus according to claim 10. 12. The image processing apparatus according to claim 10, wherein the measuring means is a chromaticity measuring device, and obtains the lightness information and the saturation information based on the measured chromaticity information. 13. A measuring step for obtaining a measurement value and chromaticity information for each patch for a patch image showing a plurality of gradations based on a simply increasing image signal; A difference step of calculating a difference between the measured values, and a table creation step of creating a gradation correction table so that the gradation characteristics are cumulative color difference linear based on the difference value between the patches and the chromaticity information. A method of controlling an image processing apparatus, comprising: 14. The table creating step comprises: a patch extracting step of extracting a patch having the positive difference value; and a patch extracting step for the extracted patch based on the chromaticity information.
A step of accumulating color differences between adjacent patches in the increasing direction of the image signal to calculate a cumulative color difference, and a gradation correction table so that the cumulative color difference becomes linear with respect to the image signal. 14. The control method for the image processing apparatus according to claim 13, wherein 15. A program which, when executed on a computer, causes the computer to operate as the image processing apparatus according to any one of claims 1 to 12. 16. A recording medium on which the program according to claim 15 is recorded.