JPH06292010A - Corrected gradation curve generator - Google Patents

Abstract

PURPOSE:To obtain a corrected gradation curve generator in which a gradation conversion curve to correct a change over the time of the image forming device or dispersion between the devices is obtained with simple calculation at a limited ROM capacity and by which a gradation conversion characteristic at a desired density area is corrected optionally. CONSTITUTION:Gradation conversion tables each comprising 10 curves to execute correction of a gradation characteristic with respect to a reference gradation curve A, that is, full gradation correction gradation curves B (B0-B9), low density area correction gradation curves C1 (C10-C19) and high density area correction gradation curves C2 (C20-C29) are prepared. Then a desired correction gradation conversion curve (conversion table) is selected to vary a gradation conversion characteristic of an entire gradation area, a highlight area and a shadow area is selected with respect to the reference gradation curve A among the 10 gradation characteristic conversion tables and gradation characteristic is sequentially corrected to obtain a characteristic conversion gradation curve E.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital type copying machine,
Used in an image processing device of an image forming apparatus such as a printer or a fax machine, a correction gradation curve is generated to correct a gradation conversion characteristic of a reference gradation curve for gradation conversion of image data read by a document reading device. Regarding the device.

[0002]

2. Description of the Related Art In digital color copying machines and color scanners, it is necessary to ensure consistency with the density gradation of a recorded image on a transfer paper output from a printer based on image data obtained by scanning an original having gradation. In order to capture, a gradation conversion device that performs gradation conversion of image data is provided. However, in general, it is not easy to set an appropriate gradation conversion curve when performing gradation conversion by accurately grasping the characteristics of the density distribution of the original, and in the past, setting of the gradation conversion curve often relied on operator experience. It was It is desired to develop a technique for expressing the characteristics of the density distribution of the original image with simple parameters and automatically setting a gradation conversion curve that conforms to the empirical rule according to the parameters. Therefore, for example, in Japanese Patent Laid-Open No. 12245/1990, model curve generating means for inputting data instructing a density range of an original image and generating a model curve whose curved state is determined according to the density range of the original image On the gradation conversion coordinate plane, a correction means for generating the model curve so as to pass through a predetermined highlight point and shadow point is provided, and the curved state of the model curve is at least an upward convex state and a downward convex state. A gradation conversion curve generating device is disclosed in which, from a group of states including a convex state, one that conforms to the empirical rule is automatically selected according to the density range of the original image.

[0003]

However, since the gradation conversion curve generating device disclosed in the above-mentioned prior art requires complicated calculation, it cannot be used in an image forming device such as a copying machine. Since the calculation takes time, a relatively large amount of ROM capacity is required to make the user wait during the conversion process or to store the gradation conversion curve, which increases the cost of the device. Had its drawbacks. Further, since the curved state of the model curve used when modifying the gradation conversion curve at the time of initial setting of the device is selected from the state group including the upward convex state and the downward convex state, The curved shape of the gradation conversion curve after the correction was restricted, and it was difficult to make a small correction in a predetermined gradation area.

The present invention has been made in view of the above-mentioned problems in the prior art, and has a limited ROM capacity, and a gradation conversion curve for correcting the temporal change of the image forming apparatus and the variation between machines by a simple calculation. It is an object of the present invention to provide a corrected gradation curve generation device capable of obtaining the above-mentioned characteristics and capable of arbitrarily correcting the gradation conversion characteristics of a desired density region.

[0005]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides an overall gradation correction gradation curve B for changing the gradation conversion characteristics of all density areas and an gradation conversion characteristics of a specific density area. (N ≧ 1) of specific density region correction gradation curves Ci (i = 1, 2, ..., N) are stored in advance, a predetermined reference document is read by a document reading device, and an image is read. The density of the image recorded on the recording paper is detected by the forming device, and the whole gradation correction gradation curve B and the specific density range correction gradation are compared with the reference gradation curve A for gradation conversion of the image data read by the document reading device. A plurality of gradation conversion characteristics are corrected based on the curve Ci.

[0006]

The reference document is read prior to the document reading operation by the document reading device, and the recording image is recorded on the recording paper by the image forming device. When the density of the image recorded on the recording paper is detected, according to the detected density data,
With respect to the reference gradation curve A, a plurality of gradation conversion characteristics are corrected based on the whole gradation correction gradation curve B and the specific density range correction gradation curve Ci.

[0007]

Embodiments Embodiments in which the present invention is applied to an electrophotographic copying machine (hereinafter simply referred to as a copying machine) which is an image forming apparatus will be described. First, the outline of the mechanism of the copying machine main body 101 of the embodiment will be described with reference to the mechanism diagram shown in FIG. In FIG. 2, a 120 mmφ organic photoconductor (OPC) drum 102, which serves as an image carrier, is arranged in the central portion of the copying machine main body 101, and is surrounded by a charging charger 103 for charging the surface of the photoconductor drum 102. A laser optical system 104 that irradiates a laser beam emitted from a semiconductor laser on the surface of the uniformly charged photosensitive drum 102 to form an electrostatic latent image; and supplies toner of each color to the electrostatic latent image for development. Then, a black developing device 105 for obtaining a toner image for each color and three color developing devices 10 for yellow Y, magenta M, and cyan C
6, 107, 108, the intermediate transfer belt 10 for sequentially transferring the toner images of the respective colors formed on the photosensitive drum 102.
9, a bias roller 110 that applies a transfer voltage to the intermediate transfer belt 109, and a cleaning device 11 that removes toner remaining on the surface of the photosensitive drum 102 after transfer.
1. A charge removing unit 112 for removing charges remaining on the surface of the photosensitive drum 102 after transfer is sequentially arranged. Around the intermediate transfer belt 109, a transfer bias roller 113 for applying a voltage for transferring the transferred toner image to the transfer material and a belt cleaning device 114 for removing the toner image remaining after being transferred to the transfer material. Is provided. Further, a fixing device 116 that heats and pressurizes the toner on the transfer material to fix the toner on the transfer material is arranged at the downstream end of the conveyor belt 115 that conveys the transfer material separated from the intermediate transfer belt 109. A discharge tray 117 for discharging the fixed transfer material is attached to the outlet of the fixing device 116. Above the laser optical system 104 of the copying machine main body 101, a contact glass 118 as a document placing table, and an exposure lamp 11 for irradiating a document placed on the contact glass 118 with scanning light.
9, a reflection mirror 121 that guides the reflected light from the document to the imaging lens 122, and a CC as a photoelectric conversion element that photoelectrically converts the reflected light that is imaged and entered by the imaging lens 122.
An image sensor array 123 including a D (Charge Coupled Device) is arranged. The image signal obtained by converting the image information of the original document into an electric signal by the image sensor array 123 is sent to the laser optical system 104 through an image processing device described later to control the laser oscillation of the semiconductor laser therein.

Next, the electrical component of the copying machine will be described with reference to FIG. 3 which shows the outline of the electrical component of the copying machine. As shown in FIG. 3, the copying machine has a main control unit (CPU) that controls the entire copying machine.
A ROM 131 and a RAM 13 that are provided with a main control unit 130 and store predetermined control information.
2 is attached. Further, the main control unit 130 has a laser optical system control unit 134, a power supply circuit 135, an optical sensor 136, and an interface I / O 133.
A toner concentration sensor 137, an environment sensor 138, a potential sensor 139, a toner replenishing circuit 140, and an intermediate transfer belt driving unit 141 are connected to each other. The laser optical system controller 134 adjusts the laser output of the laser optical system 104. Further, the power supply circuit 135 applies a predetermined charging discharge voltage to the charging charger 103 and also develops the developing devices 105, 106, 107, 108 for the respective colors.
To the bias roller 110 and the transfer bias roller 113. The optical sensor 136 includes a light emitting element such as a light emitting diode and a light receiving element such as a photo sensor. The optical sensor 136 is disposed in the vicinity of the transfer position of the photoconductor drum 102 in the vicinity thereof and is a toner image of a detection pattern latent image formed on the photoconductor drum 102. In addition to detecting the amount of adhered toner and the amount of adhered toner on the background for each color, the so-called residual potential of the photoconductor drum 102 after static elimination is detected. This photoelectric sensor 1
The detection output signal from 36 is applied to a photoelectric sensor control unit (not shown). The photoelectric sensor control unit obtains a ratio between the toner adhesion amount in the detection pattern toner image and the toner adhesion amount in the background portion and compares the ratio value with a reference value to detect a change in the image density, and the toner density sensor 1
The control value of 37 is corrected. Further, the toner concentration sensor 137 detects the toner concentration in the developing devices 105 to 108 based on the change in magnetic permeability of the developer existing in the developing devices 105 to 108. The toner concentration sensor 137 compares the detected toner concentration value with the reference value, and when the toner concentration falls below a certain value and is in a toner shortage state, a toner replenishment signal of a magnitude corresponding to the shortage is supplied. It has a function of outputting to the circuit 140. The potential sensor 139 detects the surface potential of the photoconductor 102, which is an image carrier, and the intermediate transfer belt drive unit 141 controls driving of the intermediate transfer belt 109. A developer containing M toner and a carrier is contained in the M developing device 107, and is stirred by the rotation of the agent stirring member 204M. The developer regulating member regulates the amount of developer drawn up onto the developing sleeve 201M. The developer supplied onto the developing sleeve 201M is magnetically carried by the developing sleeve 201M and moves as a magnetic brush in the rotating direction of the developing sleeve 201M. Although not shown in FIG. 3, the same applies to developing devices of other colors.

FIG. 1 is a block diagram of an image processing apparatus including an image processing unit. The configuration of the image processing apparatus will be described below. In FIG. 1, 401 is a scanner, 4
Reference numeral 02 is a shading correction circuit, 403 is an RGBγ correction circuit, 404 is an image separation circuit, 405 is an MTF correction circuit, 406 is a color conversion-UCR processing circuit, 407 is a scaling circuit, 408 is an image processing (create) circuit, and 409 is 409. M
A TF filter, 410 is a γ conversion circuit, 411 is a gradation processing circuit, and 412 is a printer. The image processing unit is a part of the image processing apparatus shown in FIG. 1 excluding the scanner 401 and the printer 412. The original placed on the contact glass 118 is separated into three colors of R, G, and B by the scanner 401 and read. The shading correction circuit 402 corrects unevenness of the image sensor of the image sensor array 123, uneven illumination of the light source of the exposure lamp 119, and the like. In the RGBγ correction circuit 403, the image signal read by the scanner 401 is converted from reflectance data to brightness data. The image separation circuit 404 determines the character portion and the photograph portion, and determines the chromatic color and the achromatic color. The MTF correction circuit 405 particularly corrects the deterioration of the MTF characteristic in the high frequency region of the image signal. The color conversion-UCR processing circuit 406 corrects the difference between the spectral characteristics of the input R, G, and B color separation characteristics and the output Y, M, and C color data, and colors necessary for faithful color reproduction. Data Y,
A color correction processing unit that calculates the values of M and C, and Y, M, and
And a UCR processing unit for replacing a component in which the three colors of C overlap with Bk (black). The above color correction processing is shown in FIG.
It can be realized by executing a matrix operation such as 2. In FIG. 12, R , G , and B are 3 of R, G, and B.
Represents the color complement. The values of the matrix coefficient aij are the input system color data (R, G, B) and the output system color data (Y, M, C).
Is determined by the spectral characteristics of. Although the first-order masking equation is used in this embodiment, color correction can be performed with higher accuracy by using a quadratic term such as B ² or BG , or a higher-order term. Further, the arithmetic expression may be changed depending on the hue, or the Neugeber equation may be used. Whichever method is used, three complementary color components Y, M,
C can be obtained from the values of the complements B 1 , G and R of the three color components (or the three color components B, G and R may be used). On the other hand, the UCR process can be performed by calculating using the following equation. Y ′ = Y−α · min (Y, M, C) M ′ = M−α · min (Y, M, C) C ′ = C−α · min (Y, M, C) Bk = α · min (Y, M, C) In the above equation, α is a coefficient that determines the amount of UCR, and α
When = 1, 100% UCR processing is performed. For example, by setting α≈1 in the high density portion and α≈0 in the highlight portion, the image in the highlight portion can be smoothed. Note that α may be a constant value.

The scaling circuit 407 performs vertical and horizontal scaling, and the image processing (create) circuit 408 performs repeat processing. Further, the MTF filter 409 performs processing for changing the frequency characteristic of the image signal, such as edge enhancement and smoothing according to the user's preference such as a sharp image or a soft image. In the γ conversion circuit 410, the printer 41
The image signal is corrected according to the characteristics of 2. Also,
It is also possible to simultaneously perform processing such as background removal. The gradation processing circuit 411 performs dither processing or pattern processing. Interfaces (I / F) 413 and 414 are provided for processing image data read by the scanner 401 by an external image processing device or the like, and outputting image data from the external image processing device by the printer 412. . Image processing CPU 4 for controlling the above-mentioned image processing circuit
15 and ROM416 and RAM417 are connected by BUS418. The image processing CPU 415 is a serial I /
Through F, it is connected to the system controller 419 and an external host computer 420 as necessary, and receives command signals from an operation unit (not shown).

FIG. 4 is a block diagram of a laser conversion circuit of the printer 412. Look-up table (LU
In T) 451, γ conversion can be performed on 8-bit image data. The 8-bit image signal input to the pulse width modulation circuit (PWM) 452 is converted into 4-valued pulse width data based on the higher 2-bit signal, and the intensity modulation circuit (PM) 453 lower-order 6-bit signal. Based on 64
The value is intensity-modulated. Laser diode (LD)
454 emits light based on the modulated drive signal. The photo detector (PD) 455 detects the light emission intensity of the LD 454, and the laser optical system controller 134 corrects the light amount for each dot based on the detection result. The maximum value of the intensity of the laser light can be changed to 8 bits (256 steps) independently of the image signal. The write frequency of the LD454 is 18.6MH _z, 1 pixel of the scan time is 53.8Nsec.

The gradation curve correction processing for changing the gradation conversion characteristic of the gradation conversion processing performed by the γ conversion circuit 410 shown in FIG. 3 will be described below. With respect to the reference gradation curve A that is the reference, the correction gradation curve that changes the entire gradation conversion characteristics is the whole gradation correction gradation curve B, and the correction gradation curve that changes the gradation conversion characteristics in the highlight area (low density area) is the low density area. A characteristic in which a gradation curve for changing the gradation conversion curve C1 for the gradation area (high density area) and a gradation curve for changing the gradation curve for the high density area C2 and the reference gradation curve A are converted by the gradation curve B for the whole gradation correction curve C. Let E be the conversion gradation curve and E = B
Notated as (A). The gradation characteristic conversion of E = B (A) can be expressed as follows by using the programming language C specifically. This program can also be expressed as follows using a function transform (Transform). Listing 2. typedef int Table [256]; Table A, B, E; Table Transform (Table Transformed, Table Transformer, Table Original) {int i; for (i = 0; 1 <= 255; i ++) Transformed [i] = Transformer [ Original [i]]; return Transformed;} main () {E = Transform (E, B, A);} Further, the gradation characteristic conversion using the low-density correction gradation curve C1 and the high-density correction gradation curve C2 The programming language C for executing can be expressed as follows. Alternatively, the final main function may be expressed as follows. Listing 4. main () {E = Transform (E, C2, Transform (E, C1, Transform (E, B, A)));} In this case, the above program can be expressed as follows using the function transformation described above. . Listing 5. typedef int Table [256]; typedef Table TransformTable [9]; Table A, E; TransformTable B, C1, C2; int i1, i2, i3; main () {E = Transform (E, C2 [i3], Transform ( E, C1 [i2], Transform (E, B [i1], A);} FIG. 5 shows a reference gradation curve A, and all gradation correction gradation curves B (B0 to B9) for correcting gradation characteristics corresponding thereto. 6 is a graph showing the gradation characteristics of a low-density range correction gradation curve C1 (C10 to C19), a high-density area correction gradation curve C2 (C20 to C29), and a characteristic conversion gradation curve E. 10 shown in FIG.
A gradation characteristic conversion table (TransformTable) made up of a set of books (only the maximum convex upward, the maximum convex downward, and the straight curved shape is shown for the sake of illustration) is prepared. From the conversion table, for the reference gradation curve A, select the number (i1, i2, i3) of the correction gradation conversion curve (conversion table) that changes the gradation conversion characteristics of the entire gradation area, highlight area and shadow area. . This selection is made by the CPU connected to the image processing unit shown in FIG.
The selection signal is output based on the image data obtained by reading the reference document from 415, but the selection may be instructed from the operation unit of the copying machine.

The data of the characteristic conversion gradation curve E converted from the reference gradation curve A by the whole gradation correction gradation curve B is calculated in advance and stored in the ROM 416 shown in FIG. Can be transformed as follows. Listing 6. typedef int Table [256]; typedef Table TransformTable [10]; Table E; TransformTable A, C1, C2; int i1, i2, i3; main () {E = Transform (E, C2 [i3], Transform (E, C1 [i2], A [i1]);} Note that the correction gradation curves B, C1, for converting the gradation conversion characteristics
C2 may include the identity transformation I. 8 in this embodiment
Since a bit image signal is handled, the gradation characteristic conversion in this case is n = I (n) for an image signal n of 0 to 255 values. The display of the identity conversion I using the programming language C can be expressed by a conversion table having the following arrangement (the middle is omitted) as in the above. Table NoChange = {0,1,2, ..., 254,255}; / * ── * / Alternatively, it may be expressed by the following program. NoChange [n], which is the result of gradation conversion of the image signal n with this conversion table NoChange, is the same as the original image signal n. That is, in the case of identity conversion, since the result of gradation conversion and the result of non-conversion are the same by definition, it is not necessary to perform the gradation conversion operation.

By the way, in the above-mentioned gradation characteristic conversion operation, the index of the largest value (that is, i1 = i2 = i3 =
When the correction gradation curve of 9) is the highest convex of the respective correction gradation curves, the correction gradation curves B, C1 and C2 are 3
The characteristic conversion gradation curve E which is the highest convex among the characteristic conversion gradation curves which is the gradation characteristic conversion by the two correction gradation curves is limited to the characteristic conversion gradation curve EMAX obtained by the gradation characteristic conversion of the following program. Listing 8. TransformTable A, C1, C2; Table E MAX; int i1, i2, i3; main () {i1 = i2 = i3 = 9; E MAX = Transform (E MAX, C2 [i3], Transform (E MAX, C1 [i2], A [i1]);} However, the characteristic conversion gradation curve E When a characteristic conversion gradation curve E which is convex above MAX is required, it cannot be represented by the gradation characteristic conversion of this program. In such a case, as described below, the corrected gradation curve B,
By repeating the gradation characteristic conversion by C1 and C2 as many times as necessary, it is possible to obtain a characteristic conversion gradation curve which is further convex. FIG. 6 shows a case in which the standard gradation curve A is converted by the whole gradation correction gradation curve B9 to a gradation characteristic A1 and the whole gradation correction gradation curve B9 is further changed in gradation characteristics. The program for this gradation characteristic conversion is as follows. Listing 9. int i1, i2, i3, i4, i5, i6; void main () {E = Transform (E, C2 [i3], Transform (E, C1 [i2], Transform (E, B [i1], A); E = Transform (E, C2 [i6], Transform (E, C1 [i5], Transform (E, B [i4], E);) By the way, at the point on the reference gradation curve A, all gradation correction gradation curves B The value changes only by the conversion curves C1 and C2
Assuming that the point whose value does not change is x and the target point after the tone characteristic conversion of point x is k, then as an example of the simplest case, how to use the whole tone correction tone curve B and the reference tone curve A Whether the gradation characteristics should be converted can be known by the following algorithm.

Listing 10. / * After the gradation characteristic conversion, give the point k through which the characteristic conversion gradation curve should pass, and find the number of all gradation correction gradation curve B. indexOF B: Number of the correction gradation curve to be used in all gradation correction gradation curve B count: Number of times to repeat the gradation characteristic conversion. * / void get _B _table _number (int index, int k, int * indexOF _B, int * count) {int i, j, value; i = 5; / * All gradation correction gradation curve B [representing the identity conversion curve 5] is the center of the table. When convex upward, i> 5, when convex downward i <5. * / J = 0; / * initial number of iterations is 0, and when convex upward, j> 0, convex downward In case of j <0. * / Value = A [index]; if (k> B [i] [value]) {/ * When convex upward * / / * While increasing the value of index i by 1 All gradation corrections with k <= B [i] [value] Search for gradation curve B. When the target k is above the point on the whole gradation correction gradation curve B [9], the index i is returned to the central value 5 (i = 5) and the number of repetitions j is increased by one. At that time, the new initial value of the value value, which is the reference for the gradation characteristic conversion, is converted into the gradation characteristic value B [9] by the gradation correction curve B [9], which is the highest convex of all gradation correction correct gradation curve B. ] [value]. * / i ++; while (k> B [i] [value]) {i ++; if (i> 9) {++ j; i = 5; value = B [9] [value];}}} else {/ * Convex downward * / / * Decrease the value of index i one by one and search for all gradation correction gradation curve B such that k> B [i] [value]. When the target k becomes smaller than the lower limit of the total gradation correction gradation curve B, the index i is returned to the center value 5 (i = 5) and the number of repetitions j is decreased by one. At that time, the value B [0] [value] obtained by converting the new initial value of the value value, which is the reference for the gradation characteristic conversion, by the all-gradation-corrected gradation curve B [0], which is the lowest convex of the all-gradation compensated gradation curve B ] * / i--; while (k <B [i] [value]) {i--; if (i <0) {--j; i = 5; value = B [0] [value];}} } * IndexOf_B = i; * count = j;} Using this value, the characteristic conversion gradation curve E is obtained as follows.

Listing 11. int index, k, indexOf _B, count, i2, i3; Table E, A, A1; TransformTable B; void main () {int i; for (i = 0; i <= 255; i ++) A1 [(i) ] = A [(i)]; get _B _table _number (k, & indexOf B, &count); if (count> 0) for (i = 0; i <count; i ++) A1 = Transform (B [9], A1 ); else for (i = 0; i>count; i--) A1 = Transform (B [0], A1); E Transform (B [indexOf _B], A1); E = Transform (C2 [i3] , Transform (C1 [i2], E));} Similarly, the transformation curves C1 and C2 are obtained. that time,
In order to obtain the value of the low density correction gradation curve C1, the value changes at the point on the reference gradation curve A by the low density correction gradation curve C1 and by the high density correction gradation curve C2. It is desirable not to do so or to select the target point k with a small change amount. In addition, the high density range correction gradation curve C2
In order to determine which correction gradation curve should be used, the value changes at the point on the reference gradation curve A by the high density correction gradation curve C2 and the low density correction gradation curve C
It is desirable to select a target point k whose value does not change or whose change amount is small depending on 1.

After the standard gradation curve A is converted by the whole gradation correction gradation curve B, the gradation characteristics are converted, and then the low density region correction gradation curve C1 is obtained.
Alternatively, when the gradation characteristic conversion is performed by the high-density correction gradation curve C2, the influences of the correction gradation curves C1 and C2 differ depending on the shape of the characteristic conversion gradation curve E after the gradation characteristics conversion is performed by the whole gradation correction gradation curve B. . That is, as shown in FIG. 6, when the gradation characteristic conversion by the low-density correction gradation curve C1 affects the gradation conversion characteristics of the image signal with density data of 64 or less, the gradation characteristic conversion is performed by the shape of the characteristic conversion gradation curve E. The degree of influence of is different. Therefore, in actuality, when it is desired to change the gradation curve only in the low density area, the gradation conversion characteristics may change up to the intermediate density area or the high density area. In this case, it is impossible to change the gradation conversion characteristic of the specific density region, which is the original purpose of the corrected gradation curves C1 and C2. Therefore, by changing the gradation characteristic of the low-density correction gradation curve C1 for converting the gradation curve of the highlight area to the gradation characteristic by the characteristic conversion gradation curve E, it is possible to prevent the gradation conversion characteristics of an undesired density area from changing. be able to.

FIG. 7 shows another gradation conversion method in which the gradation characteristic conversion method with respect to the reference gradation curve A is different. An example of this gradation conversion program is shown in Listing 12.
Shown in. Listing 12. Listing 11. The difference is with the latter part of the last line of the program. The style of this part is selected according to the desired correction gradation curve. Listing 12. int index, k, indexOf _B, count, i2, i3; Table E, A; TransformTable B; void main () {int i; for (i = 0; i <= 255; i ++) A1 [i] = A [ i]; get _B _table _number (k, & indexOf_B, &count); if (count> 0) for (i = 0; i <count; i ++) A1 = Transform (B [9], A1); else for (i ＝ 0; i ＞ count; i--) A1 ＝ Transform (A1, B [0], A1); E ＝ Transform (E, B [indexOf _B], A1); E = Transform (E, C2 [i3] , Transform (E, E, C1 [i2])); Next, still another gradation conversion method will be described. 8 and 9 are diagrams for explaining the grayscale conversion method. In the figure, the start and end points of the corrected gradation curve for an 8-bit image signal are P0 = (0,0) and P1 = (255,25), respectively.
5) In order to generate the total gradation correction gradation curve B for changing the entire gradation conversion characteristic, the start point P0 and the end point P are generated.
A straight line L that intersects with the straight line P0P1 connecting 1 and a control point P3 that is on the straight line L and has a distance d from the intersection of the straight line P0P1 and the straight line L as a parameter, and a quadratic Bezier curve are used. FIG. 8 shows that when the straight line P0P1 and the straight line L1 go straight,
FIG. 9 shows a case where the straight line L2 is parallel to the vertical axis. When the center point of the line segment P0P1 in the example of FIG. 8 is PC,
The control point P3 is P3 (d) ＝ PC + (-d / √2, d / √2) ＝ (127.5) with the distance d from the center point PC ＝ (PO + P1) / 2 ＝ (127.5,127.5) as a parameter. -d / √2, 127.5 + d / √2). Using this, the gradation conversion curve P (d, t) giving the total gradation correction gradation curve B is P (d, t) = P0 ・ t ² + 2 ・ P3 (d) ・ t ・ (1-t) + It is given by P1 · (1-t) ² . The control point P3 (d) in the example of FIG. 9 is given by P3 (d) = PC + (0, d) with the difference d of the y coordinate component from the center point PC as a parameter. In this embodiment, the center point PC of the line segment P0P1
However, the reference point may be any other point, and the reference point may be selected in accordance with the required whole gradation correction gradation curve B.

Next, a gradation conversion method for changing the gradation conversion characteristics of a specific density area such as a highlight area and a shadow area will be described. Similar to the above-described generation of the correction gradation curve, a straight line L that intersects the straight line P0P1 and the straight line P0P1, and control points P3 to 3 existing on the straight line L and having a distance d from the intersection point of the straight lines P0P1 and the straight line L as a parameter. Generate a corrected tone curve using the following Bezier curve. 10 and 11 show examples in which the straight line L1 is orthogonal to the straight line P0P1 and the straight line L2 is parallel to the vertical axis. As shown in FIG. 10, a corrected gradation curve that changes the gradation characteristic of the highlight area is generated as follows. Start point P0, end point
Let P1 be P0 = (0,0) and P1 = (255,255), respectively, and let the first control point P2 be P2 = (64,64). The control point P3 in the example shown in FIG. 10 is the distance from the intersection of the straight line P0P1 and the straight line L1.
Using d as a parameter, P3 (d) = (32,32) + (-d / √2,
d / √2). Similarly, the control point P3 in the example shown in FIG. 11 is P3 (d) = (32,32) + (0, d) using the distance d from the intersection of the straight line P0P1 and the straight line L2 as a parameter.
Therefore, the gradation conversion curve P (d, t) that gives the corrected gradation curve that changes the gradation characteristics of the highlight area is P (d, t) = P0 ・ t using the start point P0, the end point P1 and the control points P2, P3. ^{It is given by 3} + 3 · P2 · t ² · (1-t) +3 · P3 (d) · t · (1-t) ² + P1 · (1-t) ³ . In this embodiment, the end point coordinates are P1 = (2
55,255), but the end point coordinates are other coordinate points on the line segment m [(0,0)-(255,255)] such as P1 = (64,64), and the area between them is the highlight area, Alternatively, as in the shadow area, a correction gradation curve for changing the gradation conversion characteristic of a specific density area is calculated, and the gradation characteristic conversion is performed as an identity conversion for the line segment that is not included in the line segment P0P1 on the line segment m. It is also possible to do.

[0020]

As described above, according to the first aspect of the present invention, the whole gradation correction gradation curve B for changing the gradation conversion characteristics of the entire density area and the gradation conversion characteristics of the specific density area are changed. The characteristic value data that gives the n specific density range correction gradation curves Ci are stored in advance, and a plurality of gradation conversions based on the entire gradation correction gradation curve B and the specific density range correction gradation curve Ci are performed with respect to the reference gradation curve A. Since the characteristics are corrected, it is possible to obtain a gradation conversion curve that corrects the temporal variation of the image forming apparatus and the variation between the machines by a simple calculation with a relatively small memory capacity, and at the same time, obtain a desired density region. The gradation conversion characteristics can be modified arbitrarily. According to the invention described in claim 2, after the gradation conversion characteristic of the reference gradation curve A is corrected based on the whole gradation correction gradation curve B, a plurality of gradation conversion characteristics based on the specific concentration range correction gradation curve Ci are obtained. Since the correction is performed, it is possible to effectively correct the gradation conversion characteristics in the entire gradation density region. According to the invention of claim 3, after the gradation conversion characteristic of the reference gradation curve A is corrected at least once based on any one of the specific density area correction gradation curves Ck of the specific density area correction gradation curve Ci, the whole is corrected. Gradation correction The gradation conversion characteristic is corrected based on the gradation curve B, and further, the specific concentration range correction gradation curve Ci (i ≠ k
Since the gradation conversion characteristic is corrected at least once based on (4), the gradation conversion characteristic in the specific density range can be effectively corrected. According to the invention of claim 4, after the correction of the gradation conversion characteristics is completed, the gradation conversion characteristics are corrected again. Therefore, even if the number of stored characteristic value data groups is small, By repeating the correction once for the gradation conversion curve that has been corrected, the required gradation conversion curve can be obtained. According to the fifth aspect of the present invention, the start point P0, the end point P1, and the straight line P0P1 connecting the start point P0 and the end point P1 of the whole gradation correction gradation curve B or the specific concentration range correction gradation curve Ci.
By a quadratic or cubic Bezier curve defined by one or two control points that exist on the straight line L that intersects with the control point P2 and have the distance d from the intersection point of the straight line P0P1 and the straight line L as a parameter. Since the whole gradation correction gradation curve B or the specific density range correction gradation curve Ci is generated, the gradation conversion characteristic curve can be expressed by simple parameters, and the gradation conversion curve having a smooth gradation characteristic can be obtained. Can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of an image processing apparatus of a copying machine according to an exemplary embodiment of the present invention.

FIG. 2 is a mechanism diagram showing an outline of a mechanism of a copying machine main body.

FIG. 3 is a configuration diagram showing an outline of an electric component section of the copying machine.

FIG. 4 is a block diagram of a laser conversion circuit of the printer.

FIG. 5 is a graph showing the gradation characteristics of a standard gradation curve, an overall gradation correction gradation curve for performing gradation characteristic correction for the reference gradation curve, a low density correction gradation curve, a high density correction gradation curve and a characteristic conversion gradation curve. .

[FIG. 6] All gradation correction gradation curves B9 with respect to the reference gradation curve
7 is a graph showing a characteristic conversion gradation curve obtained by further converting the gradation characteristics by the all gradation correction gradation curve B9 to the gradation curve A1 converted by the gradation characteristics.

FIG. 7 is a graph showing another gradation conversion method in which the gradation characteristic conversion method with respect to the reference gradation curve is different.

FIG. 8 is an explanatory diagram related to a method of generating an entire gradation correction gradation curve.

FIG. 9 is an explanatory diagram relating to another method of generating an entire gradation correction gradation curve.

FIG. 10 is an explanatory diagram for describing a gradation conversion method that changes the gradation conversion characteristic of a specific density region.

FIG. 11 is an explanatory diagram for explaining another gradation conversion method that changes the gradation conversion characteristic of a specific density region.

FIG. 12 is a mathematical diagram showing a matrix arithmetic expression for executing color correction processing.

[Explanation of symbols]

 101 Copier main body 102 Photoconductor drum 104 Laser optical system 123 Image sensor array 401 Scanner 405 MTF correction circuit 410 γ conversion circuit 411 Gradation processing circuit 412 Printer 413, 414 Interface

Claims (5)

[Claims] 1. A reference gradation for gradation conversion of image data read by a document reading device by detecting a density of an image recorded on recording paper by an image reading device by reading a predetermined reference document. In the correction gradation curve generating device for generating the correction gradation curve for correcting the gradation conversion characteristic of the curve A, the whole gradation correction gradation curve B for changing the gradation conversion characteristic of the entire density area and the highlight area or the shadow area , Characteristic value data for giving n (n ≧ 1) specific gradation region correction gradation curves Ci (i = 1, 2, ..., N) for changing the gradation conversion characteristic of the specific density region are stored in advance, For the standard gradation curve A, the whole gradation correction gradation curve B
And a correction gradation curve generation device for correcting a plurality of gradation conversion characteristics based on the specific density range correction gradation curve Ci. 2. The corrected gradation curve generating device according to claim 1, wherein after the gradation conversion characteristic of the reference gradation curve A is corrected based on the whole gradation correction gradation curve B, it is based on the specific density range correction gradation curve Ci. A corrected gradation curve generating device characterized by correcting a plurality of gradation conversion characteristics. 3. The corrected gradation curve generating device according to claim 1, wherein a reference based on any one of the specific density range correction gradation curves Ci of the specific density range correction gradation curve Ck (k = 1,2, ..., n). After correcting the gradation conversion characteristics of the gradation curve A at least once,
All gradation correction The gradation conversion characteristic is corrected based on the gradation curve B, and further the gradation conversion characteristic is corrected at least once based on the specific density range correction gradation curve Ci (i ≠ k). Gradation curve generator. 4. The corrected gradation curve generation device according to claim 1, wherein after the correction of the gradation conversion characteristics is completed, the correction gradation curve generation is repeated again. apparatus. 5. The corrected gradation curve generating device according to claim 1, wherein the whole gradation correction gradation curve B or the specific concentration range correction gradation curve C is used.
i exists on a straight line L that intersects a start point P0, an end point P1, and a straight line P0P1 connecting the start point P0 and the end point P1.
The whole gradation correction gradation curve B or the above-mentioned specification by a quadratic or cubic Bezier curve defined by one or two control points including a control point P2 whose parameter is the distance d from the intersection of 0P1 and the straight line L A corrected gradation curve generation device characterized by generating a density range corrected gradation curve Ci.