JP3325765B2 - Output gradation adjustment method for image output device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for adjusting an output gradation of an image output apparatus such as an image forming apparatus for forming an image based on image data representing a gradation of a pixel density, and an output floor. The present invention relates to an image output device having a configuration for automatically adjusting a key.

[0002]

2. Description of the Related Art Conventionally, a color original has been optically read by a CCD (Charge Coupled Device) scanner or the like, and red (R),
2. Description of the Related Art Digital color copiers have been used which convert signals into three primary color signals based on additive color mixture of green (G) and blue (B) and form a color copy image of a document based on these signals. The R, G, and B three primary color signals output from the scanner are converted into three primary color data by subtractive color mixing of cyan (C), magenta (M), and yellow (Y), which are complementary colors of these signals. The three primary color data is, for example, data of 8 bits and 256 gradations for each color, and represents the density of each color component. Black (BK) data is generated based on the three primary color data of C, M, and Y.

For example, the surface of a photoconductor is scanned by a laser beam modulated based on C data, and an electrostatic latent image corresponding to cyan is formed on the surface of the photoconductor. The electrostatic latent image is developed into a toner image using cyan toner, and the toner image is transferred to copy paper. Similarly, magenta, yellow, and black toners are transferred to the M data, Y data, and BK data in an overlapping manner, and finally, the toner is heated and fixed to achieve color copying.

[0004] Due to the nature of the copying machine, the original and the copied image can be easily compared with each other, so that the requirements for the reproducibility of the original are strict. However, there are individual differences in development characteristics and photoconductor sensitivity characteristics among a plurality of copying machines, and these characteristics are also affected by the environment in which the copying machine is used. Further, the developing characteristics are different for each of the colors cyan, magenta, yellow and black. Therefore, in order to faithfully reproduce the color of the original,
It is necessary to individually adjust each of the four color toners for each copying machine.

In general, such adjustment is made by copying a standard color original on which a standard color original is formed, and visually comparing the copy with the standard color original. Then, a gradation correction curve for correcting the gradation of each color of cyan, magenta, yellow, and black is created, and a table corresponding to the curve is stored in a backup memory in the copying machine.

[0006]

However, in the case of manual adjustment as described above, the adjustment work is complicated.
The adjustment takes a long time, and the output image after the adjustment may vary due to individual differences between the adjustment operators. Accordingly, an object of the present invention is to solve the above-mentioned technical problem and to provide an output gradation adjustment method capable of automating at least a part of an output gradation adjustment operation of an image output device.

Another object of the present invention is to provide an image output apparatus having a structure for automatically adjusting the output gradation.

[0008]

According to a first aspect of the present invention, there is provided an image processing apparatus for correcting input image data in accordance with a gradation correction curve and outputting an image based on the corrected image data. A method for adjusting an output gradation of an output device, comprising: setting initial gradation correction curve data in the image output device; and outputting predetermined test image data corresponding to a plurality of gradations to the image output device. and step by entering a step of outputting a test image corresponding to the data for the test image and reading et allowed by an optical reading means a test image which is the output, to get the first reading data, Based on the first read data, the test image
Estimate zero-point input data corresponding to the density value of the background part of the image
Then, the value near the estimated zero input data is set to the lowest value.
As a step to create a new set of test image data
And the initial test tone correction
The curve data is input to the set image output device, and the second time
Forming a test image by performing image output of
And the test image formed by the second image output
Is read by the optical reading means, and the second reading is performed.
Acquiring the acquired data, the first and second read data, reference output curve data representing a reference output gradation for the input image data, and the initial gradation correction curve data. Calculating the gradation correction curve data suitable for the image output device to match the input / output characteristics of the image output device with the reference output curve data;
Setting the gradation correction curve data in the image output device.

In this method, a test image is formed based on the test image data with the initial gradation correction curve data set. By reading this test image by the optical reading means, the first image corresponding to the test image data is read .
Read data is obtained. In addition, the first formed test
The zero point input data is estimated based on the test image. zero
Point input data is equivalent to the density value of the background part of the test image.
Input data to be input. Then, the zero input data
A test image data set with the nearby values as the minimum
Test image based on this new test image data.
Formed again. Light the test image formed this second time
The second reading by reading by the
Data is obtained. Correspondence between test image data and first read data , and test image formed for the second time
The correspondence between the image data and the second read data is nothing but the input / output characteristics of the image output device in which the initial gradation correction curve data is set. Therefore, new gradation correction curve data is calculated based on the first and second read data, the initial gradation correction curve data, and the reference output curve data. As a result, it is possible to obtain gradation correction curve data suitable for the input / output characteristics of the image output device. By setting the gradation correction curve data in the image output device, the adjustment of the output gradation of the image output device is completed. Depending on the input / output characteristics of the image output device
Very low density for relatively small test images
An image may be formed. In this case, the first
For small values of test data,
Read data cannot be obtained. So, the above zero
Input data is estimated, and values near this zero input data
Is set as the minimum value. I
Therefore, the test image based on this new test image data
Image (the test image formed for the second time) to optical reading means
Therefore, the read data obtained by reading are all significant.
Value. Read data obtained in this way
With the read data for the test image formed the first time
, The number of significant read data increases
To find more appropriate gradation correction curve data.
Wear. Note that, in order to make the gradation expression of the low density portion excellent.
In the second set of test image data,
It is preferable to include a lot of corresponding data.

The calculation of the gradation correction curve data based on the first and second read data, the initial gradation correction curve data, and the reference output curve data can be automated by data calculation processing. Most of the gradation adjustment work can be automated. And the first one
Since the tone correction curve data is calculated based on the second read data, the second read data, the initial tone correction curve data, and the reference output curve data, there is no variation among adjustment operators. Therefore, the output gradation of the image output device can be surely adjusted to the optimum state.

[0011] Claim image output apparatus having a configuration for implementing the first invention described, as described in claim 2, an image output device for outputting an image based on input image data A tone correction curve storage means for storing tone correction curve data, and correcting the input image data according to the tone correction curve data stored in the tone correction curve storage means, and outputting the corrected data. Tone correction means;
Image output means for outputting an image based on the data corrected by the gradation correction means, reference output storage means for storing reference output curve data representing a reference output gradation for the input image data, and the gradation correction curve Means for setting initial gradation correction curve data in a storage means, and a method for setting a predetermined test image corresponding to a plurality of gradations in a state where the initial gradation correction curve data is set in the gradation correction curve storage means. It provides data to the gradation correction means, and means for outputting a test image corresponding to the data for the test images from the image output unit reads images optically, and optical reading means outputs the read data Optically reading the test image
Based on the read data at the time of reading
Zero point input data corresponding to the density value of the background part of the test image
And reduce the value near the estimated zero-point input data to the minimum.
Test to create a new test image data set as a value
Image data creating means, and the initial tone correction curve data
Is set in the tone correction curve storage means,
Test created by the above test image data creation means
The image data is input to the tone correction means, and the second image
Means for performing image output to form a test image, the read data, and the image formed by the second image output.
The test image was read by the optical reading means.
Second read data, the reference output curve data,
A tone correction curve calculation for calculating tone correction curve data suitable for the image output device based on the initial tone correction curve data so as to match the input / output characteristics of the image output device with the reference output curve data. means, Ru der those characterized in that it comprises means for setting the gradation correction curve data calculated by the tone correction curve calculating means to the gradation correction curve storage means.

[0012]

[0013]

[0014]

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described.
This will be described in detail with reference to the accompanying drawings. 1. FIG. 1 is a simplified cross-sectional view showing an internal configuration of a digital color copying machine to which an embodiment of the present invention is applied. This color copier has a scanner unit 1 as an optical reading unit for reading an original, and a printer unit 2 as an image output unit for processing a signal from the scanner unit 1 to form a color image. . A transparent plate 3 on which an original is set is provided above the scanner unit 1. Above the transparent plate 3, an automatic document feeder 4 is further provided. The automatic document feeder 4 has a pair of drive rollers 5 and 6 and an endless belt 7 wound around the pair of drive rollers 5 and 6. The document feeder 4 feeds a plurality of documents (not shown) set on the document tray 8 one by one onto the transparent plate 3 and sets them at predetermined positions.

Below the transparent plate 3, a scanning reading unit 13 equipped with a light source 11 and a one-dimensional color CCD (charge coupled device) image sensor 12 obtains a driving force from an optical motor 14 and moves along the transparent plate 3. Reciprocates. As a result, illumination scanning of the original placed on the transparent plate 3 is achieved. In the course of the illumination scanning, light generated from the light source 11 is reflected on the surface of the document, and the reflected light enters the image sensor 12 via the lens 15. As a result, the output of the image sensor 12 output according to the time series becomes a signal representing an image formed on the front surface of the document.

The color image sensor 12 outputs three primary color signals of red, green and blue. This signal is converted into digital data by an analog / digital converter (not shown) provided in the scanner unit 1, and further, C and C corresponding to the three primary colors of subtractive color mixing, cyan (C), magenta (M), and yellow (Y). It is converted to M, Y, data. This data is, for example, 8-bit data. Therefore, the C, M, and Y data are each 25
Data representing the density of each color component of each pixel in six gradations.

The C, M and Y data are stored in the printer unit 2
Given to. The printer unit 2 performs predetermined processing on the given C, M, and Y data, and creates BK data corresponding to black in addition to the three primary color data. Then, video signals respectively corresponding to the C, M, Y and BK data are created, and given to the laser scanning unit 21 in this order.

The laser beam 22 generated from the laser scanning unit 21 is guided to a photoreceptor 23 having a cylindrical shape, and the photoreceptor 23 is exposed. The photoconductor 23 rotates around its axis in the direction of arrow 24, and the photoconductor 23 before exposure is exposed.
Is uniformly charged by the charger 27. Therefore, an electrostatic latent image corresponding to the modulation applied to the laser beam 22 is formed on the surface of the photoconductor 23 by the exposure with the laser beam 22.

The electrostatic latent image is developed into a toner image by a developing unit 25, and the toner image is copied on a copy sheet (a paper sheet) wound around a straight cylindrical transfer drum 26 disposed close to the surface of the photoconductor 23. (Not shown). The surface of the photoconductor 23 after the transfer of the toner image is cleaned by the cleaning device 28. The developing unit 25 includes developing cartridges 25C, 25M, 25Y, and 25B holding cyan, magenta, yellow, and black toners, respectively.
K, which are arranged in this order from above and are held
And an elevating mechanism 31 that moves the holder 30 up and down. With this configuration, the laser scanning unit 2
The developing cartridges 25C, 25M, 25Y, and 25BK are switched to contact the photosensitive member 23 in response to the C, M, Y, and BK video signals given to the first unit. Thus, cyan, magenta,
Yellow and black toner images are sequentially formed.

The transfer drum 26 has an arrow 35 around its axis so that the peripheral speed is equal to the peripheral speed of the photoconductor 23.
It is driven to rotate in the direction. A transfer device 36 is provided inside the transfer drum 26 for transferring the toner on the surface of the photoconductor 23 to a copy sheet wound around the transfer drum 26 by high-frequency discharge. Further, a pair of separators 37 for facilitating the separation of the copy paper by corona discharge is disposed downstream of the transfer unit 36 in the rotation direction of the transfer drum 26. A separation claw 38 for separating the copy sheet on which the toner image has been transferred from the transfer drum 26 is provided further downstream than the separator 37.

Around the transfer drum 26, a cleaning device 39 for cleaning the surface of the transfer drum 26 after the copy paper is separated is provided.
The separation claw 38 and the cleaning device 39 are configured to be able to freely contact and separate from the transfer drum 26. The copy sheet separated from the transfer drum 26 by the separation claw 38 is guided to the fixing unit 41 by the transport unit 40, and the fixing process of the toner image on the surface thereof is performed. The copy sheet on which the toner image has been fixed is discharged outside the apparatus through a discharge path 42.

Copy sheets are stacked in cassettes 52 and 53, fed out by paper feed rollers 54 and 55, and guided to a transport path 58 by transport rollers 56 and 57. Then, after fine adjustment of the paper feed timing is performed by the registration roller 59 in the vicinity of the transfer drum 26,
Paper is fed toward the transfer drum 26. The fed copy sheet is gripped by a clip mechanism (not shown), and is wound around the transfer drum 26 as the transfer drum 26 rotates.

The copy paper wound around the transfer drum 26 is held on the surface of the transfer drum 26 before the transfer of the cyan, magenta, yellow and black toner images is completed. Further, during a period in which the cyan, magenta and yellow toner images are formed on the copy sheet, the separation claw 38 and the cleaning device 39 are retracted to a position separated from the transfer drum 26. When the three color toner images are transferred to the copy sheet, the separation claw 38 and the cleaning device 39 come into contact with the transfer drum 26, and
The discharge at 7 is performed.

When the leading edge of the copy sheet onto which the black toner image of the fourth color has been transferred reaches the separation claw 38, the grip of the clip mechanism is released. Then, the copy paper separated by the separation claw 38 is guided to the fixing unit 41 via the transport unit 40. Of course, when a single color copy is performed using any one color toner, the separation claw 38 and the cleaning device 39 are brought into contact with the transfer drum 26 from the beginning, and the copy sheet does not reach the cleaning device 39. 2. Electrical Configuration of Digital Color Copier FIG. 2 is a block diagram showing an electrical configuration of a main part of the above color copier. From the scanner unit 1, C, M and Y
Is generated. These data are provided to a black generation unit 61, where the C, M, and Y data are corrected and BK data corresponding to black toner is generated.

The C, M, Y, and BK data are subjected to a so-called masking process or the like in the color correction unit 62,
It is provided to the selector section 63. The selector section 63 selects and outputs any one color data corresponding to a signal to be supplied to the laser scanning unit 21, and supplies the selected data to the gradation correction section 64. The gradation correction unit 64 performs gradation correction for each color according to the sensitivity characteristics of the photoconductor 23 and the development characteristics of the developing unit 25. That is, the data for each color is individually increased or decreased.

The data after the gradation correction is applied to a printer output unit 66 that creates a signal to be applied to the laser scanning unit 21 described above. Control and calculation of each unit are performed by a CPU (central processing unit) 70. The CPU 70 includes a bus 71
, And the bus 71 includes the black generation unit 6 described above.
1, a color correction unit 62, a selector unit 63, a gradation correction unit 64, and the like are connected. The CPU 70 is also connected to a ROM 72 storing an operation program and the like, a RAM 73 used as a work area and the like, and a RAM 74 with a backup power supply storing gradation correction curve data representing a gradation correction curve. The gradation correction curve is a curve referred to when the gradation correction unit 64 executes the gradation correction processing, and corresponds to the correspondence between the input gradation from the selector 63 and the output gradation to be given to the printer output unit 66. Is represented. The process of appropriately setting the tone correction curve data is the output tone adjustment process. The processing by the tone correction unit 64 is actually realized by software processing executed by the CPU 70 with reference to the tone correction curve in the RAM 74.

Further, a signal is inputted to the CPU 70 from, for example, an operation unit 75 provided on the upper surface of the scanner unit 1. The operation unit 75 is provided with an adjustment mode key (not shown). When the adjustment mode key is operated, the operation mode is shifted to an adjustment mode for creating gradation correction curve data. In the adjustment mode, C, M, Y and BK
Four types of test data corresponding to the respective colors are provided to the printer output unit 66, and four image forming operations are executed on a trial basis. As a result, a test image for each color is formed. 3. Test Image Used for Output Tone Adjustment FIG. 3 is a diagram showing an example of a test image formed when adjusting the output tone. The adjustment of the output gradation is executed sequentially for each of the four colors of cyan, magenta, yellow and black, for example. Therefore, a 16-level gray scale is formed for the color to be adjusted. Specifically, 16 rectangular areas TP in which the density gradually increases
, TP2,..., TP16 are formed. One rectangular area is composed of, for example, 200 pixels vertically and 512 pixels horizontally, and each pixel in one rectangular area is formed based on equal data. In FIG. 3, the level of the density of each region is represented by the density of oblique lines. 4. Output Gradation Adjustment Process FIG. 4 is a flowchart for explaining the flow of the output gradation adjustment process, and FIG. 5 is a diagram for explaining the principle of the output gradation adjustment process. The output gradation adjustment processing is performed in the RAM 7
4 is achieved by setting appropriate gradation correction curve data. More specifically, the input / output characteristics (tone correction curve) of the tone correction unit 64 are determined so that the density of the image read by the scanner unit 1 is appropriately reproduced in the output image of the printer unit 2. Can be

First, as the gradation correction curve data to be referred to by the gradation correction section 64, initial gradation correction curve data corresponding to the initial gradation correction curve L1 shown in FIG. Step S1). As the initial gradation correction curve data, for example, data corresponding to a linear straight line in which the input data and the output data are directly proportional may be set, or the gradation correction curve data set at that time may be set. May be adopted as the initial gradation correction curve data. In the adjustment performed at the copying machine production stage, a straight line in which the input data and the output data are directly proportional is adopted as an initial gradation correction curve, and is set at the time when the copying operation is adjusted by a serviceman. It is considered preferable that the gradation correction curve is adopted as the initial gradation correction curve.

Next, the CPU 70 reads out the test image data stored in the ROM 72 in advance, and inputs the data to the gradation correction unit 64. As a result, a test image as shown in FIG. 3 is formed on the copy paper (step S2). The test image data is, for example, such that the input image data is 8 bits, 2 bits,
If it expresses the density in 56 gradations, ｛10,22,34,4
6, 58, 70, 82, 94, 106, 125, 144, 163, 182, 201, 220, 239}. Although the test image is a single color gray scale, as in the case of the full color copy, cyan,
It is preferable that transfer operations of magenta, yellow, and black toners are performed. This is because the density is different between the case where the number of times of transfer is one and the case where the number of times of transfer is four, even for a monochrome image. By performing the transfer operation four times, a state close to the actual use state of the copying machine is realized.
The test images of cyan, magenta, yellow, and black may be formed at once in different areas on one sheet of paper.

The formed test image is placed on the transparent plate 3 on the upper surface of the copying machine main body 1, and is read by the scanner unit 1 (step S3). As a result, FIG.
As shown in (1), read data corresponding to each of the test image data is obtained. For example, the rectangular area TPi (i = 1, 2, 3,...,
In each area of 16), an average value of data of a plurality of pixels in a square area of 16 pixels × 16 pixels may be used as read data. As a result, the influence of uneven density between pixels can be eliminated.

Next, test image data is newly created in order to output the test image again (step S4).
The newly created test image data is different from the test image data for the first test image. A test image is output using the newly created test image data (snap S5), and read data of each density region of the test image is obtained (step S6). In this way, a large amount of read data can be obtained by outputting and reading the test image twice.

Next, the obtained read data is shown in FIG.
Is compared with the reference output curve data corresponding to the reference output curve L2 shown in FIG. The reference output curve L2 is a curve in which input data to the gradation correction unit 64 is associated with density data that should be originally obtained. Data corresponding to the reference output curve L2 is stored in the ROM 72 in advance. . In the copying machine in which the output gradation adjustment processing is completed, a test image is formed based on the test image data, and when the test image is read by the scanner unit 1, the relationship between the test image data and the read data is as follows. , The reference output curve L2.

The CPU 70 calculates initial gradation correction curve data,
Based on the read data and the reference output curve data, candidate point data serving as a basis for creating gradation correction curve data is calculated (step S7). More specifically, the input value of the reference output curve L2 corresponding to the read data is used as the input data, and the output value of the initial gradation correction curve L1 corresponding to the test image data corresponding to the read data is used as the output data. Then, candidate point data composed of a pair of input data and output data is determined.

The candidate point data is as shown in FIG. That is, the graph of the candidate point data is arranged below the graph of the reference output curve L2, the graph of the initial gradation correction curve L1 is arranged to the left of the graph of the candidate point data, and the graph of the initial gradation correction curve L1 is The graph of the read data is arranged in the. In this case, straight lines are drawn horizontally and vertically from the read data points, and these straight lines are referred to as the reference output curve L.
2 and each intersection with the initial gradation correction curve L1 are bent at a right angle, and the intersection of the two bent straight lines is set as a candidate point.

For example, it is assumed that the output value of the initial gradation correction curve L1 for the test image data td is io. It is also assumed that the read data for the test image data td is rd. On the other hand, in the reference output curve L2, when the input gradation data is bi, the output gradation is rd. In this case, the gradation correction unit 64 determines that the input data b
If the output data io is output for i, a pixel having an appropriate density of the gradation rd is output. Therefore, if the output data io is associated with the input data bi, this point becomes a candidate point to be placed on the gradation correction curve.

Based on the candidate point data thus obtained, the CPU 70 interpolates between the candidate points according to a predetermined program in the ROM 72 or controls the output data to increase monotonically with an increase in the input data. To adjust,
The gradation correction curve data is created by adjusting the output data with respect to the input data so as not to be abrupt (step S8).

Similar processing is performed for each of the four colors of cyan, magenta, yellow and black, and the gradation correction curve data for the four colors is set in the RAM 74 (step S9), whereby the output gradation adjustment processing is performed. Complete. As the copy mode, for example, a character mode suitable for copying a character image, a map mode suitable for copying a map image,
When a photo mode suitable for a copying machine for photographic images and a character / photo mode suitable for a copying machine for images in which characters and photographs are mixed can be set, gradation correction for four colors is performed for each mode. Curve data is obtained and RAM 74
Is preferably set to. In this case, the ROM 7
In 2, a reference output curve is stored in advance for each color in each mode. 5. Data Creation for Second Test Image FIG. 6 is a diagram for explaining test image data creation processing for the second test image executed in step S4 of FIG. In creating the test image data, the zero point input data is estimated. Zero input data is
This is image data corresponding to the density (background data) of the background portion of the test image. The background data is obtained by sampling the output of the scanner unit 1 corresponding to the background portion when reading the test image for the first time.

In forming the first test image, the RO
The predetermined test image data stored in M72 is applied, but depending on the characteristics of the photoconductor 23 and the toner,
It may not be possible to form an image with a sufficient density for relatively small image data. Therefore, zero point input data estimation processing is performed based on the result of reading the first test image.

FIG. 6A shows an example of the relationship between the test image data corresponding to the first test image and the read data. In this example, the test image data “10”, “2”
For “2” and “34”, the output of the scanner unit 1 is the background data (= 0), and for the next test image data “46”, the read data is larger than the background data for the first time. Become. Therefore, the zero input data is
It is between "34" and "46".

Therefore, according to the following procedures 1 to 3 , the CPU 70 creates test image data for forming a second test image. Procedure 1 The point at which the read data first becomes larger than the background data is point B, and the point immediately before it is point A. Further, a point corresponding to the test image data larger than the point B and at which the read data becomes larger than the point B for the first time is defined as a point C. If the read data corresponding to the minimum test image data “10” is larger than the base data, this point is point B, so the origin (0, 0) is point A.

Step 2 Next, the inclination of the line segment AB connecting the points A and B,
The inclination of a line segment BC connecting the point and the point is obtained. Based on the inclinations of these line segments AB and BC, zero point input data is estimated by one of two methods, and data for the second test image is created.

That is, the inclination of the line segment AB is the inclination of the line segment BC.
6b, the state is as shown in FIG.
Input data is considered to exist between points A and B
You. Then, it corresponds to points a, b, and c that divide line segment AB into four equal parts.
Data value a_X, B_X, C _XFor the second test image
Data is the first three data. On the other hand, the line segment AB
When the inclination is smaller than the inclination of the line segment BC, FIG.
It will be like the following. In this case, zero input data
Is an extension of the line segment BC, and (read data) = (base data)
Is considered to be near point D, which is the intersection with the straight line
You. Therefore, points that divide the line segment BD into four equal parts are referred to as points a, b, and c.
And a data value a corresponding to these points_X, B_X, C_XBut
The first three data of the second test image data
You. That is, the data a near the zero-point input data_XBut 2
It is the minimum value of the data set for the test image for the second time.

Step 3 By estimating the zero-point input data as described above, the first three data a of the second test image data are obtained.
_X, b _X, the c _X is determined, other data should be the second test image data is determined. Specifically, a set of data {16,28,40,52,64,76,88,100,112,133,152,17 that is determined not to overlap with the first test image data
Among 1,190,210,229,248}, data larger than the data c _X is employed as the test image data. However, the total number of data including the data a _X , b _X , and c _X is 16
Is exceeded, the total number of data becomes 16
Large data is excluded from the test image data.

The second test image data created in this way contains a lot of data of the low gradation part. Therefore, the test images formed based on the first and second test image data are respectively read by the scanner unit 1.
, It is possible to obtain particularly detailed read data in the low gradation portion. Therefore, in the low gradation part, many candidate point data can be obtained.
For example, in the reproduction of a halftone image, the expression of the low gradation part is extremely important. Therefore, it is necessary to obtain more candidate point data in the low gradation part to create more appropriate gradation correction curve data. Can be. 6. FIG. 7 is a flowchart for explaining the tone correction curve data creation process in step S8 of FIG. First, reverse correction of candidate point data is performed (step S8).
1). Since the larger the input image data is, the larger the output image data must be, the tone correction curve must be a monotonically increasing curve in which the output data monotonically increases as the input data increases. Therefore, the candidate points are sequentially traced in ascending order of the input data. If the magnitude relation of the input data and the magnitude relation of the output data are reversed, the candidate point data is corrected. More specifically, the inversion is eliminated by increasing or decreasing the output data while keeping the input data of the candidate point data as it is.

Next, based on the candidate point data subjected to the reverse rotation correction, interpolation for giving output data values corresponding to all input data values between the candidate points is performed, and a provisional gradation correction curve is obtained. A certain first preliminary gradation correction curve is created (step S82). For example, if the image data is 8 bits (25
6 gradations), and the printer output unit 66 outputs 10 bits (1
It is assumed that the density of each pixel can be expressed by inputting image data of (024 gradations). In this case, 0 is set for each value of the input data in the range of 0 to 255.
Output data values in the range of 1023 are associated.
However, the lower limit value “0” of the output data is associated with the lower limit value “0” of the input data, and the upper limit value “102” of the output data is associated with the upper limit value “255” of the input data.
"3" is associated with it.

The first preliminary gradation correction curve is created based on the candidate point data subjected to the reverse rotation correction. As a result of the above interpolation processing, the first preliminary gradation correction curve has a monotone increasing curve. May occur in some places. Therefore,
A process for correcting such a portion to a monotonically increasing curve is performed. Thus, a second preliminary gradation correction curve, which is a second temporary gradation correction curve, is obtained. (Step S
83).

Next, a process for correcting the high gradation area portion of the second preliminary gradation correction curve is performed (step S8).
4). This correction is intended to alleviate a sharp change in the slope of the second preliminary gradation correction curve in the high gradation region and create a third preliminary gradation correction curve that is a third provisional gradation correction curve. This is the process. This process is based on the empirical fact that the output data should change smoothly with changes in the input data. If the gradient of the gradation correction curve changes abruptly, there is a possibility that the reproduction of a halftone image will be hindered. That is, a so-called pseudo contour may be generated in the halftone image.

Step S8 is applied to the third preliminary gradation correction curve.
As a result of the correction processing of No. 4, there may be a case where a portion that is not a monotonically increasing curve occurs. Therefore, processing for correcting such a portion to a monotonically increasing curve is performed, and a fourth preliminary gradation correction curve, which is a fourth temporary gradation correction curve, is created.
(Step S85). Then, finally, the fourth preliminary gradation correction curve is corrected to alleviate a sudden gradation change or gradation jump of each part of the curve (step S86).
A gradation correction curve to be referred to by the gradation correction unit 64 is completed. A table representing this gradation correction curve is stored in a RAM
74. The gradation correction section 64 refers to a table of gradation correction curves based on the values of the input image data, and reads out the output image data from this table. 7. FIG. 8 is a diagram for explaining the reverse rotation correction performed in step S81 in FIG. The reverse rotation correction is performed according to the following procedure.

Procedure 1 As described above, candidate point data is composed of a set of input data and output data. Therefore, first, the candidate point data is sorted in ascending order based on the value of the input data. Then, a candidate point corresponding to the third input data from the smallest is set as a point of interest (indicated by a symbol “◎” in FIG. 8).

Step 2 Next, a point of interest for a candidate point immediately before the point of interest (indicated by a symbol “○” in FIG. 8) and a candidate point immediately after the point of interest (indicated by a symbol “●” in FIG. 8). Is examined. More specifically, the area of the output data is
As shown in FIG. 8A, an area SB smaller than the output data of the immediately preceding candidate point, an area SA between the output data of the immediately preceding candidate point and the output data of the immediately succeeding candidate point, and an output of the immediately succeeding candidate point It is divided into areas SC larger than the data, and it is determined to which area the output data of the point of interest belongs.

[0052] FIG. 8 (b), the output data of the point of interest is a diagram illustrating a case belonging to the area SA between immediately before and the output data of the candidate point immediately. In this case, no correction is made to the data of the point of interest. FIG. 8C is a diagram showing a case where the output data of the target point is smaller than the output data of the immediately preceding candidate point and therefore belongs to the area SB. In this case, the output data of the target point is pulled up to a line segment 81 connecting the immediately preceding candidate point and the immediately following candidate point.

FIGS. 8 (d) and 8 (e) are views showing the processing when the output data of the target point is larger than the output data of the immediately succeeding candidate point and therefore belongs to the area SC. In this case, a candidate point immediately before the immediately preceding candidate point (indicated by the symbol “△” in FIG. 8) is also referred to. More specifically, a straight line 82 passing through the immediately preceding candidate point and the preceding candidate point is assumed.

If the target point is below the straight line 82 as shown in FIG. 8D, that is, the output data of the target point is smaller than the output data on the straight line 82 corresponding to the input data of the target point. Is smaller, the output data of the target point is not corrected. This is because there is a high possibility that there is an error in the data of the candidate point immediately after the point of interest.

On the other hand, as shown in FIG. 8E, when the point of interest is above the straight line 82, that is, when the output data of the point of interest is higher than the output data on the line 82 corresponding to the input data of the point of interest. Is larger, a correction is made to lower the output data of the point of interest. More specifically,
The output data of the target point is modified based on the ratio of the differences a1 and a2 between the input data of the target point and the input data of the candidate points immediately before and after the target point. That is, regarding the differences b1 and b2 between the output data of the corrected point of interest and the output data on the straight lines 82 and 81 corresponding to the input data of the point of interest,
The following equation holds.

B1: b2 = a1: a2 Step 3 When the processing of the above step 2 for a certain point of interest is completed,
The point of interest is moved to the next candidate point, and the process of procedure 2 is executed. If the point of interest is the final candidate point, the process ends.

The origin (0,0) and the final point (25)
5, the upper limit of the output data) are indispensable points, and these two points are added as candidate points. 8. Interpolation of Candidate Points FIGS. 9 and 10 are diagrams for explaining an interpolation process (step S82 in FIG. 7) of candidate point data subjected to reverse rotation correction. The X axis is input data, and the Y axis is output data.

The procedure for creating the first preliminary gradation correction curve by interpolating the candidate point data is as follows. Procedure 1 First, as shown in FIG. 9 (a), the first candidate point (origin) is set to point A, the second candidate point is set to point B, and point A is formed on the quadratic curve 83 where point B is a pole. And point B are interpolated.

Procedure 2 Next, among the point B and the candidate points whose distance in the X-axis direction from the point B is greater than 4, the candidate point closest to the point B is defined as a point C,
A straight line 84 interpolates between points B and C. As shown in FIG. 9B, when the inclination of the straight line 84 passing through the points B and C is 1 or more (when the angle with respect to the X axis is 45 degrees or more), the X coordinate of the point B is obtained. A line 85 between the point α on the quadratic curve corresponding to the X coordinate obtained by subtracting 4 from the point B and the point β on the straight line BC corresponding to the X coordinate obtained by adding 4 to the X coordinate of the point B is further drawn.
To interpolate. Then, the point β is set as the point B again. As a result, the line between the quadratic curve 83 and the straight line 84 is smoothly connected by the straight line 85, and a rapid change in gradation is alleviated.

Procedure 3 Further, point B is set at point A, point C is set at point B, the candidate point on the right is set at point C, and the candidate point on the right is set at point D. The interpolation processing shown in FIG. That is,
First, an angle ∠ABC between the straight line AB and the straight line BC is obtained. If the angle ∠ABC is 120 degrees or more, the processing shown in FIG. 10A is performed, and if the angle ∠ABC is less than 120 degrees, the processing shown in FIG. 10B is performed.

In the processing shown in FIG. 10 (a), the space between BCs is interpolated by a straight line 86, and the midpoint of the line segment AB and the midpoint of the line segment BC are re-interpolated by a straight line 87. The middle point is again designated as point B. In the processing shown in FIG.
A quadratic curve 88 passing through points and C and a quadratic curve 89 passing through points B, C and D are assumed. Then, between the points B and C is interpolated by the curve 100 passing between the two quadratic curves 88 and 89. In particular,
Difference a between input data and each input data at points B and C
The interpolated output data is determined such that the ratio of 1, a2 becomes equal to the difference b1, b2 between the interpolated output data and each output data on the curves 88 and 89.

Step 4 If the point D is not the final candidate point, the procedure returns to the step 3. If the point D is the final candidate point, a straight line is interpolated between the points C and D, and the process ends. Thus, the first preliminary gradation correction curve is completed. 9. Correction to Monotone Increasing Curve FIG. 11 is a diagram for describing processing for correction to a monotonic increasing curve (corresponding to steps S83 and S85 in FIG. 7). The X axis is input data, and the Y axis is output data.
The correction to the monotone increasing curve is performed on the first preliminary gradation correction curve and the third preliminary gradation correction curve. FIG.
FIG. 11A shows an example of the first or third preliminary gradation correction curve before the change, and FIG. 11B shows the second or third preliminary gradation correction curve obtained as a result of the processing.
Alternatively, an example of a fourth preliminary gradation correction curve is shown.

The processing for correcting the monotone increasing curve is executed according to the following procedure. Procedure 1 First, a point of interest is set at the origin (0, 0) of the processing target curve (first or third preliminary gradation correction curve). Procedure 2 Next, while increasing the value of the input data by one, the point of interest is sequentially shifted, and a maximum point is searched. The maximum point is a point that is larger than the Y coordinate of the Y coordinate of the target point is a point after the straight. If the maximum point is not found, the processing for correcting the monotonically increasing curve ends.

Step 3 When the maximum point is found, the following processing is performed. That is, as shown in FIG. 11B, the maximum point is point A, and the minimum point that appears first after point A is point B. Further, points on the processing target curve after point B, and points A and Y
The first point at which the coordinates are equal is point C. Further, the first point after the point C and having a Y coordinate larger than the Y coordinate of the point C by the difference between the respective Y coordinates of the points C and B is defined as the point D.

Between the points A and C, a straight line AD and a straight line AC
Is corrected to a straight line 90 that bisects the interval between. The curve 91 between points C and D is corrected to a curve 91 that bisects the line between the straight line AD and the curve to be processed. Thus, the area between the points A and D is corrected to a monotonically increasing curve. Step 4 Set the point of interest to the point on the right of point D, and return to step 2. D
If the point is the final point (the point of the input data “255”), the process ends. 10. FIGS. 12 and 13 are diagrams for explaining a curve correction process (corresponding to step S84 in FIG. 7) relating to a high gradation region. The X axis represents input data and the Y axis represents output data.

It has been empirically clarified that, through the interpolation process of the candidate points, a portion where the slope of the curve extremely changes appears in the high gradation region. Therefore, a process for alleviating such a sudden change in inclination is performed as follows. Procedure 1 First, the coordinate system is compressed so that the scales of the X axis and the Y axis are equal. For example, the image data given from the selector unit 63 (see FIG. 2) to the gradation correction unit 64 expresses the density of each pixel in 256 gradations from 0 to 255, and the printer output unit 66 1024
In the case where a signal for expressing the density of each pixel by gradation is given to the laser scanning unit 21, the gradation correction unit 64 converts 8-bit input image data into 10-bit output image data. become. In this case, the Y axis corresponding to the output data is compressed to 1/4.

Step 2 The last point (255, 255) of the second preliminary gradation correction curve on the compressed coordinate plane is set to point C, the candidate point on the left of point C is set to point B, and the left of point B is set to point B. The next candidate point is point A. In this case, the candidate points indicate points on the second preliminary gradation correction curve corresponding to the input data of the candidate point data.

Step 3 With the point C fixed, the points A and B are shifted to the left by one point (tracing the immediately preceding candidate point), and the angle ∠ABC
The first point A and point B at which the size of is smaller than 150 degrees are searched. FIG. 12A shows that the angle ∠ABC is 150
FIG. 12B shows the case where the angle ∠ABC is 1
The case where the angle is smaller than 50 degrees is shown.

From the empirical fact, the gradation correction curve to be finally set generally has a small change ratio of the output data with respect to the input data in the high gradation region, and has a small ratio with respect to the input data in the middle gradation region. It has been found that the rate of change of the output data is relatively large. Therefore, near the transition from the middle gradation region to the high gradation region, a sharp change in the slope of the curve occurs. The procedure 3 finds a place where the inclination changes suddenly in this way.

However, when the X coordinate of the point A becomes smaller than 170, the search is terminated, the compression of the coordinate system is released, and the correction processing for the high gradation area ends. Step 4 When the angle ∠ABC is smaller than 150 degrees, the length of the line segment BC connecting the points B and C is set to a, and the radius r of the circle indicating the range of interpolation is calculated based on the following equation. I do.

[0071]

(Equation 1)

Then, as shown in FIG. 12 (c), a point A ', which is the intersection of a circle 92 (circle representing the range of interpolation) of radius r centered on point B and the second preliminary gradation correction curve. , And circle 9
The point C ', which is the intersection of 2 and the line segment BC, is determined. Equation 1 above
Is given by r = a when ∠ABC = 90 degrees, and ∠A
It is determined that r = a / 2 when BC = 150 degrees. This alleviates a sudden change in the slope of the curve. For example, although in the case of ∠ABC = 90 ° corresponds to the case where the change in slope of the curve is greatest, in this case, by the radius r of the circle indicating a range of complementation are a, is between smooth co Will be possible.

Step 5 In accordance with the inclination of the straight line AB, the inclination of the straight line BC, and the inclination of the straight line A'B, one of the following procedures 6 or 7 is performed. That is, if the inclination of AB> the inclination of BC and the inclination of AB> the inclination of A'B or the inclination of AB <the inclination of BC and the inclination of AB <the inclination of A'B, the processing of the procedure 6 is performed. If the above condition is not satisfied, the process of step 7 is performed.

Step 6 As shown in FIG. 13A, a line 93 is interpolated between the points A 'and C'. Step 7 As shown in FIG. 13 (b), an arc 94 (centered at an intersection D between a straight line passing through the point A 'and orthogonal to the straight line A'B and a straight line passing through the point C' and orthogonal to the straight line BC '). Interpolation between the points A 'and C' is performed by the arcs which are in contact with the straight lines A'B and BC at the points A 'and C', respectively.

Step 8 The point A 'is set as the point C, the candidate point on the left of the point C is set as the point B, and the candidate point on the left of the point B is set as the point A, and the procedure returns to the step 3. 11. Reduction of gradation change / gradation skipping The processing in step S86 in FIG. 7 is processing for making a gradation correction curve a smooth curve.
It is performed in the following procedure.

Procedure 1 The slopes of the line segment AB and the line segment BC are obtained based on the data of three points A, B, and C where the input data is continuous. Step 2 A change in the inclination is obtained based on the difference between the inclinations of the line segments AB and BC.

Step 3 When the change in the inclination is 5 or more, the average value of the Y coordinates (output data) of the points A and B is converted to the Y coordinate of the point B (output data).
And Step 4 Steps 1 to 3 are repeated until the variation value of the slope becomes less than 5 everywhere on the curve. 12. Example of Processing FIGS. 14 and 15 are diagrams showing actual processing results when a tone correction curve for magenta is created. FIG.
4 (a) shows read data obtained by reading a test image by the scanner unit 1, FIG. 14 (b) shows an initial gradation correction curve, and FIG. 14 (c) shows a reference output curve. FIG. 14D shows candidate point data obtained based on the read data, the initial gradation correction curve, and the reference output curve. From FIG. 14D, it is understood that a sufficient number of candidate points are obtained in the low gradation part as a result of forming and reading the test image twice.

The candidate point data shown in FIG. 14 (d) is subjected to the inversion correction processing to obtain the candidate point data shown in FIG. 15 (a). By performing a process of changing to a monotonically increasing curve, a process of correcting a high gradation portion, etc., a monotonically increasing curve shown in FIG. 15B is finally created as a gradation correction curve. 13. Conclusion As described above, according to the present embodiment, the gradation correction curve data is automatically calculated and set based on the read data obtained by reading the test image, the initial gradation correction curve data, and the reference output curve data. You. Therefore, most of the output gradation adjustment work can be automated, and appropriate adjustment can be reliably performed irrespective of the skill of the adjustment operator.

Further, the zero-point input data is estimated based on the read data of the first test image, and the second test image is obtained by using new test image data that minimizes the value near the zero-point input data. Is formed and read. As a result, a large number of significant read data can be obtained, and when the test image is formed only once, or the data for the second test image is set without considering the read data of the first test image. Compared to when
It is possible to calculate more appropriate gradation correction curve data. In particular, since the data for the second test image includes a relatively large amount of data corresponding to the low gradation part, there is an advantage that the output gradation in the low gradation part is appropriately adjusted.

Further, the step of creating gradation correction curve data using the candidate point data obtained based on the read data, the initial correction curve data, and the reference output curve data includes:
Reverse point correction of candidate point data, correction processing to monotonous increase curve,
It merely includes relatively simple processing such as interpolation between candidate points using a linear curve and a quadratic curve. Therefore, the calculation of the gradation correction curve data does not require a long time, so that the output gradation adjustment processing can be completed in a short time. 14． Other Embodiments In the above embodiment, a color digital copying machine has been described as an example. However, the present invention is not limited to a color digital copying machine, but may be based on image data such as a monochrome copying machine or a color printer. And can be widely applied to a device that outputs an image.

In addition, various changes can be made within the technical scope described in the claims.

[0082]

According to the first or second aspect of the present invention, it is possible to automate the operation of the gradation correction curve data by a data operation process. Most of the work can be automated. As a result, the output gradation adjustment operation can be completed in a short time, and the image output device can be surely adjusted to an appropriate state regardless of the skill of the adjustment operator. In addition, the read data of the test image
To the extent that significant read data can be obtained based on
Test image data for the second test image formation is created.
Is done. As a result, the tone correction curve data
A large number of significant read data can be used. This
Thereby, more appropriate gradation correction curve data is calculated.
Increases the accuracy of the output gradation adjustment process
be able to.

[0083]

[Brief description of the drawings]

FIG. 1 is a simplified cross-sectional view showing an internal configuration of a color digital copying machine to which an embodiment of the present invention is applied.

FIG. 2 is a block diagram showing an electrical configuration of a main part of the color digital copying machine.

FIG. 3 is a diagram illustrating a test image.

FIG. 4 is a flowchart illustrating a flow of an output gradation adjustment process.

FIG. 5 is a diagram for explaining the principle of creating a gradation correction curve.

FIG. 6 is a diagram illustrating a process of creating data for a second test image.

FIG. 7 is a flowchart illustrating a flow of a process for creating gradation correction curve data based on candidate point data.

FIG. 8 is a view for explaining a candidate point reverse rotation correction process;

FIG. 9 is a diagram for explaining a candidate point interpolation process;

FIG. 10 is a diagram for explaining a candidate point interpolation process.

FIG. 11 is a diagram for explaining a process for correcting a curve to a monotonically increasing curve;

FIG. 12 is a diagram for explaining a correction process of a high gradation area portion.

FIG. 13 is a diagram for describing a correction process for a high gradation area portion.

FIG. 14 is a diagram showing an actual example of creating a gradation correction curve.

FIG. 15 is a diagram illustrating an actual example of creating a gradation correction curve.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Scanner part 2 Printer part 64 Gradation correction part 66 Printer output part 70 CPU 72 ROM 73 RAM 74 RAM with backup

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-1-61172 (JP, A) JP-A-7-162682 (JP, A) JP-A-7-203207 (JP, A) JP-A-6-162207 62251 (JP, A) JP-A-4-179370 (JP, A) JP-A-63-214077 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 1 / 40-1 / 409 H04N 1/46 H04N 1/60

Claims (2)

(57) [Claims] 1. A method for correcting input image data according to a gradation correction curve and adjusting an output gradation of an image output device for outputting an image based on the corrected image data, comprising: Setting curve data in the image output device; inputting predetermined test image data corresponding to a plurality of gradations to the image output device, and outputting a test image corresponding to the test image data; the test image is the output reading <br/> preparative et allowed by an optical reading means, acquiring the first read data, based on the first read data, the base of the test image
Estimate the zero-point input data corresponding to the density value of the
Assuming that the value near the estimated zero input data is the lowest value,
Creating a new set of test image data; and adding the new test image data to the initial tone correction curve data.
Data is input to the image output device where the
Outputting the test image to form a test image, and displaying the test image formed by the second image output.
The second reading data is read by the optical reading means.
Acquiring the data, the first and second read data, reference output curve data representing a reference output gradation for the input image data, and the initial gradation correction curve data. Calculating the gradation correction curve data suitable for the image output device so that the input / output characteristic of the image output device matches the reference output curve data; and setting the gradation correction curve data in the image output device. An output gradation adjustment method characterized by including: 2. An image output apparatus for outputting an image based on input image data, comprising: a gradation correction curve storage means for storing gradation correction curve data; Tone correction means for performing correction in accordance with the tone correction curve data stored in the means, and outputting corrected data; image output means for outputting an image based on the data corrected by the tone correction means; Reference output storage means for storing reference output curve data representing reference output gradation for image data; means for setting initial gradation correction curve data in the gradation correction curve storage means; In the state set in the gradation correction curve storage means, predetermined test image data corresponding to a plurality of gradations is provided to the gradation correction means, and the test image data corresponding to the test image data is provided. Means for the test images output from the image output unit reads images optically, when the optical reading means outputs the read data, the test image is read by the above optical reading means
Of the test image based on the read data
Estimate the zero input data corresponding to the degree value, and
The value near the point input data is set as the minimum value and a new test
Test image data creation means for creating a set of image data
And the initial tone correction curve data is stored in the tone correction curve storage means.
Create the above test image data with the column set
The test image data created by the
Input to the correct means and let the second image output
Means for forming the image , the read data, and the second image output.
The read test image is read by the optical reading means.
Based on the second read data, the reference output curve data, and the initial gradation correction curve data, so that the input / output characteristics of the image output device match the reference output curve data. Means for calculating gradation correction curve data suitable for the output device; means for setting the gradation correction curve data calculated by the gradation correction curve calculation means in the gradation correction curve storage means; An image output device comprising: