JPH05153396A - Gradation curve set-up method - Google Patents

Abstract

PURPOSE:To easily correct the gradation of an area having a large slope in an intial gradation curve area by correcting this curve based on the correction characteristic secured from an accumulated density histogram. CONSTITUTION:The image data read by a scan reading head are set up like the initial gradation curve and stored in a 1st frame memory 35 via an A/D converter 31, a color arithmetic circuit 32, a switching circuit 33, and a thinning circuit 34. An information processing part fetches the color components Y, M and C stored in the memory 35 to another memory of the information processing part. Then a CPU carries out a prescribed operation to produce a density histogram. Based on this histogram, the picture element numbers of each class are successively added together in the order of lower classes for production of an accumulated density histogram. Then the initial gradation curve is corrected by the correction characteristic secured from the accumulated density histogram.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradation curve setup method for a gradation conversion device used in the field of image processing such as a color scanner for plate making. Therefore, the present invention relates to a gradation curve setup method suitable for obtaining a final gradation curve by making corrections.

[0002]

2. Description of the Related Art In an image processing apparatus such as a plate-making color scanner, JP-A-61-20042 proposed by the applicant as an apparatus for setting up a gradation curve for creating a halftone image (reproduction image) from an original image. A plate-making color scanner disclosed in, for example, the publication is known. As shown in FIG. 12, a plate-making color scanner disclosed in the above publication includes a scanning reading device 101, an image processing device 102,
A scanning recording device 103 and an information processing device 104 are provided.

The scanning reading device 101 reads image data of an original image (not shown) mounted on an input cylinder.

The image processing apparatus 102 uses a look-up table for setting up a gradation curve, or image data from data of B, G, and R color components to Y,
It is provided with a color operation circuit for converting data into M, C, and K color components. The image processing apparatus 102 converts the input image data read by the scanning reading apparatus 101 into
Output image data is formed by processing in the look-up table or the color calculation circuit.

The scanning recording device 103 has a dot generator for converting the output image data formed by the image processing device 102 into a halftone dot signal, and is attached on the output cylinder based on the halftone dot signal. A halftone image is exposed and recorded on a photosensitive material (not shown).

The information processing device 104 calculates a gradation curve to be set up in the lookup table of the image processing device 102 based on the input image data read by the scanning reading device 101. The information processing device 104 can also correct the gradation curve by operating a keyboard or the like.

By the way, in the above plate-making color scanner, an initial gradation curve is set up in the lookup table of the image processing apparatus 102 as follows.

First, a prescan is performed, and the relationship between the density value of each pixel and the number of pixels giving this density value is obtained based on the input image data of the original image read by the scanning reading device 101.

Then, based on the image data, a density histogram showing the appearance frequency of pixels corresponding to the density class value is obtained.

Next, the cumulative density histogram as shown in FIG. 13 is obtained by sequentially accumulating the appearance frequencies of the pixels in the density histogram in ascending order of the class value. In addition, in FIG. 13, the cumulative value of the appearance frequency is represented by a relative frequency. In addition, in FIG.
Is the concentration value and RN is the cumulative relative frequency.

Next, the cumulative relative frequencies RNH and RNS for determining the optimum highlight density DH and shadow density DS are determined. These cumulative relative frequencies RNH and RNS are obtained empirically from a large number of sample original images prepared in advance, and the value of the cumulative relative frequencies RNH is usually 1
%, And the value of the cumulative relative frequency RNS is about 98%.

By applying the cumulative relative frequencies RNH and RNS to the cumulative density histogram of FIG. 13,
The class values of the densities corresponding to the cumulative relative frequencies RNH and RNS are obtained, and these class values are defined as the highlight density DH and the shadow density DS. Output halftone values corresponding to the highlight density DH and the shadow density DS are set to determine the highlight point HL and the shadow point SD.

Next, an initial gradation curve passing through the highlight point HL and the shadow point SD is calculated.
The initial gradation curve may be a gradation curve having standard characteristics selected in advance according to the original image.
It may be a gradation curve obtained as proposed in Japanese Patent No. 575. This JP-A-63-425
In Japanese Patent Laid-Open No. 75-75, a second gradation curve is obtained based on the density histogram obtained by prescanning an original image, and this second gradation curve is combined with a standard first gradation curve prepared in advance. As a result, the third gradation curve that is the initial gradation curve is obtained.

Next, the output image based on the image data whose gradation has been converted according to the initial gradation curve is displayed on a color monitor (not shown). Then, the operator operates the keyboard or the like of the information processing device 104 while looking at the color monitor to adjust the calculated initial gradation curve as necessary.

Next, the initial gradation curve set as described above is set up in the look-up table of the image processing apparatus 102.

After the initial gradation curve is set up in the look-up table, the original image is scanned by the scanning and reading device 101 to obtain a duplicate image whose gradation is converted according to the set initial gradation curve. ..

In the plate-making color scanner, etc.,
The initial gradation curve is set up as described above, but before the final printing of the duplicate image is completed, a proof copy of the duplicate image is created using the above initial gradation curve and this proof print is made. It is common to show "" to the client and ask whether it is right or wrong.
Then, after the gradation curve is modified according to the client's request, the final printing process is started.

As described above, in general, the initial gradation curve often needs to be modified according to the instruction from the client. However, although the initial gradation curve is set up almost automatically as described above, the work of correcting the once set initial gradation curve as instructed by the client is conventionally performed. The reality is that we rely only on the experience of the operator.
The above-mentioned instructions from the client include correction of highlight points and shadow points, partial change of tone, and the like. Among them, in order to respond to the instruction regarding the change of condition, the work by a particularly skilled operator is required. Conventionally, the matters that the operator has judged when changing the condition as described above are generally the following A to D.
4 points shown in.

A. Selection of a plate for correcting the gradation curve, that is, selection of which color component of the color components Y, M, C, K is to be corrected.

B. The determination of the density area to be corrected, that is, the determination of which area of the highlight area, the intermediate area and the shadow area of the gradation curve is to be modified.

C. Judgment whether to put out condition or reduce.

D. Degree of correction, that is, determination of how much the correction should be made in a direction that gives a tone or in a direction that reduces it.

[0023]

[Problems to be Solved by the Invention] However, the above A
It is necessary to have a considerable amount of experience to make the determinations of items A to B among the determinations of to D while looking at the screen of a color monitor or the like. Therefore, if the operator makes all of the judgments A to D, it takes a lot of time to obtain a gradation curve that satisfies the client's request, or a large number of skilled operators must be secured. The problem arises.

The present invention has been made in view of the above circumstances, and the correction of the initial gradation curve for obtaining a desired duplicate image can be performed by a relatively simple operation. It is intended to provide a gradation curve setup method.

[0025]

According to a first aspect of the present invention, there is provided a gradation curve setup method for setting up a gradation curve in a gradation conversion section of an image processing apparatus, which is based on image data of an original image. The process of setting up the initial gradation curve,
A step of creating a duplicate image using the initial gradation curve; a step of designating a corrected portion in the duplicate image with respect to the image of the original image; a step of obtaining a density histogram based on image data of the corrected portion; The method includes a step of obtaining a cumulative density histogram based on the density histogram, and a step of obtaining a correction characteristic for correcting the initial gradation curve into the final gradation curve based on the cumulative density histogram.

According to a second aspect of the present invention, there is provided a gradation curve setup method according to the first aspect, wherein the initial gradation curve is set up in a first look-up table of the gradation converting section and the correction characteristic is set to the first look-up table. It further includes the step of setting up the second lookup table in the latter stage of the lookup table.

A gradation curve setup method according to a third aspect of the present invention further comprises the step of synthesizing the initial gradation curve and the correction characteristic to obtain a final gradation curve.

[0028]

According to the gradation curve setup method of the first aspect, the cumulative density histogram obtained on the basis of the image data of the corrected portion becomes a histogram in which the gradient of the change of the cumulative density is large in the area where the appearance frequency is high in the corrected portion. .. That is, by correcting the initial gradation curve according to the correction characteristic obtained based on the cumulative density histogram, the gradation of the area corresponding to the area having the large gradient in the area of the initial gradation curve is increased or decreased, and the correction is performed. You can put the tone on or off.

According to the gradation curve setup method of the second aspect, output image data is obtained by correcting the image data whose gradation is converted by the initial gradation curve with the correction characteristic.

According to the gradation curve setup method of the third aspect, the output image data can be directly obtained from the input image data by the final gradation curve.

[0031]

1 is a schematic block diagram of a plate-making color scanner to which an embodiment of the present invention is applied.

This plate-making color scanner produces a halftone dot image for each color component on a photosensitive material E attached to an output cylinder 6 based on image data of a color original F such as a color film attached to an input cylinder 1. To form. This plate-making color scanner includes, in addition to the input cylinder 1 and the output cylinder 6, a scanning reading head 2, an image processing section 3, a dot gelator 4, a scanning recording head 5, a gradation conversion image display device 7 and an information processing section. Eight.

The scanning reading head 2 is a known scanning reading head that reads an image of a color original F attached to the input cylinder 1 rotating at high speed for each pixel while moving in the axial direction of the input cylinder 1. .. That is, the scanning reading head 2 scans the color original document F, and the color components B, G, and
Obtain an analog density signal for each R. This analog density signal is sent to the image processing unit 3.

The image processing unit 3, as shown in FIG.
A D converter 31, a color calculation circuit 32, a switching circuit 33, a pixel thinning circuit 34, a first frame memory 35, and a gradation conversion unit 36 are provided.

The A / D converter 31 converts the analog density signal sent from the scanning reading head 2 into digital input image data.

The color calculation circuit 32 converts the digital input image data from the data of B, G, R color components into Y, M,
Convert to data with C and K color components. The image data output from the color calculation circuit 32 is given to the switching circuit 33.

The switching circuit 33 is a circuit for switching the data flow between the prescan for setting up the initial gradation curve and the main scan for creating a duplicate image. When the pre-scan is selected, the switching circuit 33 causes the color calculation circuit 32 to operate.
Is connected to the thinning circuit 34, and when the main scan is selected, the color operation circuit 32 is connected to the gradation converting section 36.

The thinning circuit 34 thins out a part of the image data input through the switching circuit 33 when the switching circuit 33 selects the prescan side, and the thinning circuit 34 stores it in the first frame memory 35. give.

The first frame memory 35 stores the input image data supplied via the thinning circuit 34.
The input image data stored in the first frame memory 35 is sent to the first look-up table 36A of the gradation conversion unit 36, which will be described later, and is also sent to the information processing unit 8 as necessary.

The tone converting section 36 has a first look-up table 36A and a second look-up table 36B.

In the first look-up table 36A,
An initial gradation curve for gradation conversion of image data input via the thinning circuit 34 is set up. The initial gradation curve set up in the first look-up table 36A is obtained by the information processing section 8 based on the image data read by the scanning reading head 2. The information processing section 8 calculates the initial gradation curve by a known method disclosed in the above-mentioned Japanese Patent Laid-Open No. 63-42575 or Japanese Laid-Open Patent Publication No. 2-157758 by the present applicant. Further, the calculated initial gradation curve can be adjusted by the information processing unit 8.

In the second lookup table 36B,
Correction characteristics for correcting the initial gradation curve set up in the first look-up table 36A are set up. This correction characteristic is set up in order to obtain a duplicate image that adapts to the client's instructions, etc., and the input value at the stage of proof printing that creates a duplicate image according to the initial gradation curve:
The output value is set up to be 1: 1.
The correction characteristics set up in the second look-up table 36B when the initial gradation curve is corrected by a client's instruction or the like at the stage of proof printing are as follows in the information processing unit 8 described later with reference to FIG. Guided by. The output image data corrected by the second look-up table 36B is given to the dot generator 4.

The dot gelator 4 includes the image processing unit 3
Output image data of the Y, M, C, and K color components output from the second look-up table 36B is converted into a halftone dot signal, and this halftone dot signal is sent to the scanning recording head 5.

On the basis of the halftone dot signal, the scanning recording head 5 moves in the axial direction of the output cylinder 6 and, at the same time, halftone dots on the photosensitive material E attached on the output cylinder 6 which rotates at a high speed. The image is exposed and recorded.

The gradation-converted image display device 7 includes a second frame memory 71, a signal conversion circuit 72, and a D / A converter 73.
And a color monitor 74. The second frame memory 71 uses the Y,
The output image data of M, C, K color components is stored. The output image data stored in the second frame memory 71 is called as needed, converted into B, G, and R color components in the signal conversion circuit 72, and then converted into an analog signal by the D / A converter 73. By doing so, the image converted for the duplicate image in the gradation converting unit 36 can be displayed on the color monitor 74.

The information processing section 8 includes a CPU 8A and this CPU
It has a console 8B and a memory 8C connected to 8A. The console 8B includes a keyboard cursor, a mouse, or the like, which is a part of an instructing unit for a correction point, which will be described later, and can read the coordinates on the image displayed on the color monitor 74. The CPU 8A can specify a rectangular area in the image displayed on the color monitor 74 based on the two coordinates designated by the console 8B. Also, this information processing unit 8
Calculates the initial gradation curve and the correction characteristic as described above.

Next, the outline of the plate making process using the plate making color scanner shown in FIGS. 1 and 2 will be described with reference to the flow chart shown in FIG.

First, in step S1, color separation conditions are set up. Here, as one of the color separation conditions, the color components Y, M, C, and
Set up an initial gradation curve for each K. The initial gradation curve set up in the first lookup table 36A is calculated based on the image data obtained by the prescan. As described above, the method of setting up the initial gradation curve is well known from Japanese Patent Application Laid-Open No. 63-42575, Japanese Patent Application Laid-Open No. 2-157758, etc., by the applicant of the present invention, and therefore its explanation is omitted here.

Next, in step S2, gradation conversion is performed using the initial gradation curve set up in the first lookup table 36A in step S1. The image data whose gradation has been converted in this way is
Is stored in the frame memory 71.

In step S3, the image data is read from the second frame memory 71 and displayed on the color monitor 74.

In step S4, the output image displayed on the color monitor 74 is viewed to determine whether it is necessary to adjust the initial gradation curve.

If it is determined in step S4 that the initial gradation curve needs to be adjusted, the process proceeds to step S5. In step S5, the initial gradation curve set up in the first look-up table 36A is adjusted by operating the console 8B or the setup section of the scanner body while looking at the color monitor 74. After performing such adjustment, the process proceeds to step S6.

In step S6, the color monitor 74 displays an image based on the image data whose gradation is converted by the adjusted initial gradation curve. After the display, the process returns to step S4.

If it is determined in step S4 that the output image of the color monitor 74 is viewed, it is determined that the adjustment of the initial gradation curve is completed, the process proceeds to step S7.

In step S7, the switching circuit 33 is switched to the main scan side to perform the main scan, so that Y,
A halftone dot image is exposed and recorded on the photosensitive material E attached to the output cylinder 6 for each of the M, C and K color components. As a result, a printing plate for each of the Y, M, C and K color components is created.
In this case, in the plate-making color scanner of FIG. 1, in the gradation converting unit 36, the output image data of the first look-up table 36A passes through the second look-up table 36B. However, the second lookup table 36B
By setting up a characteristic of input value: output value of 1: 1, the output of the first look-up table 36A, that is, the image data that has been gradation-converted by the initial gradation curve is directly converted to this gradation conversion. It is taken out as output image data of the unit 36.

In step S8, proof printing is performed using the printing plate thus prepared.

In step S9, the proof print created in step S8 is shown to the client, and the client is instructed as to whether correction is necessary. In this step S9, when the instruction of "no correction" is received from the client, the process proceeds to step S12. If the client has instructed "correction", the process proceeds to step S10.

In step S10, the correction characteristics are set up in the second look-up table 36B in order to obtain a duplicate image that matches the instruction of the client issued in step S9. This correction characteristic setup is performed according to steps S61 to S68 shown in FIG. After setting up the correction characteristics in step S10, the process proceeds to step S11.

In step S11, the same main scan as in step S7 is performed, and a halftone image is exposed and recorded on the photosensitive material E attached to the output cylinder 6 for each of the Y, M, C and K color components. However, in the main scan in step S11, the gradation conversion unit 36 corrects the output of the first look-up table 36A by the correction characteristic set up in the second look-up table 36B. That is, the gradation conversion unit 36 uses the final gradation curve obtained by synthesizing the initial gradation curve set up in the first look-up table 36A and the correction characteristic set up in the second look-up table 36B to obtain a color calculation circuit. The input image data, which is the output of 32, is gradation-converted into output image data.

After the printing plates for Y, M, C, and K have been created in step S11, the process returns to step S8.

When the instruction of "no correction" is received from the client in step S9, the process proceeds to step S12.

In step S12, the same printing plate as the printing plate used for the proof printing for which the "no correction" instruction has been received from the client in step S9 is adopted as the printing plate for main printing.

The outline of the plate making process performed using the plate making color scanner is as described above.

Next, the second look-up table 3
A procedure for obtaining the correction characteristic set up in 6B will be described (FIG. 4).

First, in step S61, Y, M, and gradation converted according to the initial gradation curve are converted.
The output image data with the C and K color components is read. Then, as shown in FIG. 5, an image A based on the read output image data is displayed on the color monitor 74. Figure 5
The image A shown in is an image in which the person B is drawn in the center and the background is a substantially uniform dark background.

Next, in step S62, the gradation correction portion in the image A displayed on the color monitor 74 is designated using the console 8B. This gradation correction location is determined in accordance with the client instruction made in step S9.

For example, when the client gives an instruction to put out the tone of the skin portion of the person B in the image A, as shown by the broken line in FIG.
The gradation correction location Aa that surrounds only the skin part of the person B is specified.

In the image A, when an instruction is given to clarify the contrast between the head of the person B and the background portion, as shown by the broken line in FIG. 6B, the head of the person B is shown. Area surrounding the boundary between the background and background
Specify as b.

In the image A, when an instruction is given to clarify the contrast between the whole person B and the background portion, as shown by the broken line in FIG. An area obtained by cutting the outer peripheral portion of the background portion is designated as the gradation correction location Ac.

In step S63, a density histogram of the gradation correction location designated in step S62 is created. This density histogram is obtained by the scanning reading head 2
Is created by the information processing unit 8 based on the image data of the gradation correction portion obtained by performing the pre-scan. The image data read by the scanning reading head 2 passes through the A / D converter 31, the color calculation circuit 32, the switching circuit 33, and the thinning circuit 34 in the same manner as when the initial gradation curve is set up. It is given to the memory 35 and stored in the first frame memory 35. The information processing section 8 fetches the image data of the color components Y, M, and C stored in the first frame memory 35 into the memory 8C, and based on the image data fetched into the memory 8C, a predetermined calculation in the CPU 8A A density histogram is created by performing.

The calculation for obtaining the density histogram is performed as follows, for example.

(1) First, the density values DY, DM, and DC for the color components Y, M, and C of each pixel forming the gradation correction portion
Ask for.

(2) Next, the density values DY, DM, DC
Are accumulated for each color component Y, M, C and for each class having a predetermined width.

By performing the calculations shown in (1) and (2) above, for example, a density histogram for each color component Y, M, C as shown in FIG. 7 is created. This FIG. 7 corresponds to FIG.
As shown in (a), the density histogram is illustrated when the gradation correction location Aa is designated to change the tone of the skin portion of the person B. In the density histogram of FIG. 7,
For the color component C, the appearance frequency of pixels is high in the highlight region HS, and for the color components M and Y, the appearance frequency of pixels is high in the intermediate region MS.

Note that the density histogram of FIG. 7 is represented by approximation with a curve by making the class width of density values sufficiently small. Specifically, the density histogram of FIG. 7 expresses the density values DY, DM, and DC for the respective color components Y, M, and C by 8-bit signals, so that the entire density range is "2 ⁸ -1 = 255". It is created by dividing the number into "." And calculating the frequency of appearance of each class value.

In this embodiment, the correction characteristic for the color component K is not obtained, and therefore the density histogram of the color component K is not obtained in step S63. This is because the ratio of the color component K is extremely small, and even if the correction characteristic of the color component K is not set up, the finish of the duplicate image is not significantly affected. However, for the color component K, the other color components Y, M,
Similar to C, the correction characteristic may be obtained by the procedure shown in and after step S63.

In step S64, the above step S63 is performed.
Based on the density histograms for each of the color components Y, M, and C obtained in step 1, a cumulative density histogram for each of the color components Y, M, and C as shown in FIG. 8 is created. This cumulative density histogram is created by sequentially adding the number of pixels of each class of the density histogram in ascending order of class. In FIG.
The cumulative number of pixels in each class is converted into a cumulative relative frequency with the value corresponding to the number of pixels of the entire gradation correction portion set to 255, and a point P0 of cumulative relative frequency = "0" and a cumulative total attitude number =
The straight line S connecting the point P1 of "255" has an inclination of 45 degrees.

In the cumulative density histogram of FIG. 8, the gradient is larger than the straight line S in the area where the frequency of appearance of pixels in the density histogram is high, and as a result, it is a curve. As shown in FIG. 6A, when the human skin part is designated as the gradation correction location, the cumulative density histogram of the color component C has a large gradient in the highlight area HS, and the cumulative density histogram of the color components M and Y. Is the intermediate region M
The slope is large at S. As described above, the cumulative density histogram created only by the image data of the gradation correction portion forms a characteristic curve in which the gradient becomes large in the area where the frequency of appearance of pixels is high in the density histogram of FIG.

That is, in the density histogram, if the area where the frequency of appearance of pixels is high is the shadow area SS, the area where the gradient is larger than the straight line S in the cumulative density histogram is also the shadow area. In the density histogram, the shadow area SS and the highlight area H
When the appearance frequency of the pixel of S is higher than the appearance frequency of the pixel of the intermediate area MS, the gradient becomes larger than the straight line S in both the highlight area and the shadow area of the cumulative density histogram.

In step S65, it is selected whether or not the tone at the gradation correction location is to be produced, and a reference characteristic curve as shown in FIG. 9 is created. When the tone is set, as shown in FIG. 9A, the curve formed by the cumulative density histogram shown in FIG. 8 is directly used as the reference characteristic curve. Further, in the case of suppressing the tone, as shown in FIG. 9B, a curve obtained by inverting the curve formed by the cumulative density histogram with the straight line S as a symmetry line is used as a reference characteristic curve.

In step S66, the above step S65 is performed.
A plurality of characteristic curves that are close to the reference characteristic curve are created based on the reference characteristic curve obtained in step 1 and the standard characteristic line obtained by converting the initial gradation curve at a ratio of 1: 1.

The characteristic curves C1 to C4 shown in FIG.
Reference characteristic curve and standard characteristic straight line C of color component C shown in (a)
It was created based on 0 and. In the characteristic curves C1 to C4 shown in FIG. 10, the characteristic curve C2 is shown in FIG.
It matches the reference characteristic curve shown in (a). The remaining characteristic curves C1, C3, C4 are, for example, the standard characteristic straight line on the line where the distance between the standard characteristic straight line C0 and each of the characteristic curves C1, C3, C4 is orthogonal to the standard characteristic straight line C0. C0 and the reference characteristic curve (characteristic curve C2
) And the standard characteristic straight line C0 have the same ratio at any position.
In this case, the characteristic curve C2 can be regarded as the ratio being "1". The ratio is a standard characteristic line C0.
The greater the degree of the distance between the characteristic curves C1 to C4 and the greater the degree of the distance between the standard characteristic line C0 and the reference characteristic curve, the more the tone correction point is operated.

The characteristic curves C1 to C4 shown in FIG.
In this step S66,
With respect to the color components M and Y, four characteristic curves which are approximate to the reference characteristic curves of these color components M and Y and whose ratios correspond to the characteristic curves C1 to C4 are created. FIG. 11 shows characteristic curves M1 to M4 (Y1 to Y4) created by approximating the reference characteristic curve of the color component M (Y) shown in FIG. 9A. In FIG. 11, M0 and Y0 are color components M,
It is a standard characteristic straight line of Y.

The characteristic curves created as described above are the characteristic curves C1, M1 and Y1 as the first characteristic group, the characteristic curves C2, M2 and Y2 as the second characteristic group, and the characteristic curve C3,
M3 and Y3 are the third characteristic group, and characteristic curves C4 and M4
, Y4 as the fourth characteristic group, respectively, memory 8C
Remember.

In step S67, an appropriate characteristic group is selected from the first to fourth characteristic groups created in step S66. The selection of this characteristic group is performed as follows.

First, the image data for each color component Y, M, C stored in the memory 8C in the step S63 is used by using the characteristic curves C1, M1, Y1 of the first characteristic group stored in the memory 8C. to correct.

Next, the output image based on the corrected image data is displayed on the color monitor 74.

Thereafter, using the remaining characteristic curves of the second to fourth characteristic groups, the output image based on the corrected image data is displayed on the color monitor 74 for each characteristic group.

Then, the most preferable output image is selected from the output images corresponding to the first to fourth characteristic groups, and the characteristic group corresponding to this output image is selected.

In step S68, the characteristic curve for each color component of the characteristic group selected in step S67 (for example, characteristic curves C3, M3, Y3) is used as the correction characteristic of the initial gradation curve to determine the second color of the corresponding color component. Set up in the lookup table 36B.

As described above, the correction characteristic set up in the second look-up table 36B is the curve of the cumulative density histogram for each color component Y, M, C or its approximate curve. Therefore, the correction characteristic automatically has a large gradient in a region in which the frequency of appearance of pixels constituting the gradation correction location is high. That is, the correction characteristic set up in the second look-up table 36B automatically specifies which area of the initial gradation curve is adjusted for each of the color components Y, M, and C.

When the output image data based on the initial gradation curve set up in the first look-up table 36A is corrected by the correction characteristics as described above, the output of the second look-up table 36B is as follows.
It is possible to obtain output image data capable of producing the tone of the gradation correction portion rather than the output image data of the first lookup table 36A. That is, using the gradation conversion unit 36 in which the initial gradation curve is set up in the first look-up table 36A and the correction characteristics obtained in steps S61 to S68 are set up in the second look-up table 36B, the steps are performed. By performing the main scan for the final printing in S12, it is possible to obtain an output image in which the tone of the tone correction portion is more tuned than in the initial setup.

As is clear from the above, the above step S
When the correction characteristics are set up in the second look-up table 36B by the routines 61 to 68, the operator simply indicates the gradation correction area, and the operator can specify the gradation curve to adjust the color components Y, M, and C for each plate. It is possible to automatically obtain the selection result and the determination result of the region where the characteristic is moved in the gradation curve. That is, the operator does not need to perform the selection work or the determination work that requires the highest degree of skill. The operator need only perform the work of deciding whether to give or reduce the tone in the region where the gradation curve is moved based on the instruction of the client, and the work of deciding the degree of giving or reducing the tone while looking at the color monitor 74. Good.

In the above embodiment, the color operation circuit 32 is provided on the primary side of the gradation conversion unit 36. However, the plate operation color scanner is provided with the color operation circuit on the secondary side of the gradation conversion unit. In the same manner, the correction characteristics for correcting the initial gradation curve can be obtained in exactly the same manner. In this case, the initial gradation curve set up in the first lookup table is the initial gradation curve for the color components R, G, B. Also, the correction characteristics set up in the second lookup table are
It is obtained based on the cumulative density histogram for each color component R, G, B.

Further, in the above embodiment, in step S63, the density values DY, DM, and DC are accumulated for each color component Y, M, and C, and for each class of a predetermined width, so that the color components Y, M, and C are classified. I am creating a density histogram of
The density histogram does not necessarily have to be the density histogram obtained as in the embodiment.

The density histogram is, for example, a density histogram for each color component in which the average density value of the color components Y, M, and C for each pixel is obtained, and the class of the predetermined width is used as the class related to the average density value. Good. In this case, the class values of the density histogram and the cumulative density histogram are common class values for all color components.

The density histogram is a density histogram common to the color components Y, M, and C, which shows the relationship between the average density value of the color components Y, M, and C and the number of pixels giving the average density value. You may. In this case, the cumulative density histogram obtained based on this density histogram is also the cumulative density histogram of the color components Y, M, and C related to the average density value. Therefore, the correction characteristic obtained based on this cumulative density histogram specifies only the correction area in the initial gradation curve.

Further, in the above embodiment, the first look-up table 36A for setting up the initial gradation curve and the second look-up table for correcting the output of the first look-up table 36A are set up.
Gradation conversion section 3 having a lookup table 36B
6, the gradation of the image data is converted. However, the gradation conversion unit 36 may be provided with only one lookup table for setting up the gradation curve. In this look-up table, an initial gradation curve by initial setup is set up, and after the correction characteristics are calculated, instead of the initial gradation curve, a final gradation curve obtained by combining the initial gradation curve and the correction characteristics. To set up. The composition of the initial gradation curve and the correction characteristics is
It may be performed by the information processing unit 8.

In addition, the concept of proof printing is DDCP.
(Direct digital color proofer) is also included.

Further, in the above embodiment, the color monitor 74
Although the gradation correction portion is specified above, the gradation correction portion may be specified by another specifying means such as a digitizer.

[0101]

According to the first to third aspects of the present invention, when the initial gradation curve is corrected, it is not necessary for the operator to determine the gradation correction area in the initial gradation curve. Is relatively easy to perform without requiring the work of a skilled operator.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a plate-making color scanner that implements a gradation curve setup method according to the present invention.

FIG. 2 is a configuration diagram showing an outline of an image processing unit 3 of the plate-making color scanner of FIG.

FIG. 3 is a flowchart showing an outline of a plate making process.

FIG. 4 is a flowchart showing a correction characteristic setup process.

FIG. 5 is a diagram showing an image based on output image data.

FIG. 6 is a diagram showing an example of designation of a gradation correction location.

FIG. 7 is a diagram showing a density histogram of a gradation correction portion.

FIG. 8 is a diagram showing a cumulative density histogram of a gradation correction location.

FIG. 9 is a diagram showing a reference characteristic curve.

FIG. 10 is a diagram showing a plurality of characteristic curves regarding a color component C.

FIG. 11 is a diagram showing a plurality of characteristic curves concerning color components M and Y.

FIG. 12 is a schematic configuration diagram of a conventional plate-making color scanner.

FIG. 13 is a cumulative density histogram for calculating an initial gradation curve.

FIG. 14 is an explanatory diagram of a gradation curve.

[Explanation of symbols]

 36 gradation conversion unit 36A first look-up table 36B second look-up table A image Aa, Ab, Ac gradation correction place

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[Procedure amendment]

[Submission date] January 30, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction content]

By applying the cumulative relative frequencies RNH and RNS to the cumulative density histogram of FIG. 13,
The class values of the densities corresponding to the cumulative relative frequencies RNH and RNS are obtained, and these class values are defined as the highlight density DH and the shadow density DS. The output dot% values corresponding to the highlight density DH and the shadow density DS are set to determine the highlight point HL and the shadow point SD (see FIG. 1).
4) .

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Name of item to be corrected] Fig. 14

[Correction method] Change

[Correction content]

FIG. 14

Claims (3)

[Claims] 1. A method of setting up a gradation curve in a gradation conversion unit of an image processing apparatus, comprising a step of setting up an initial gradation curve based on image data of an original image, and a duplicate image using the initial gradation curve. A step of creating, a step of designating a modified portion in the duplicate image with respect to the original image, a step of obtaining a density histogram based on the image data of the corrected portion, and a step of obtaining a cumulative density histogram based on the density histogram A gradation curve setup method comprising: a step of obtaining a correction characteristic for correcting an initial gradation curve to a final gradation curve based on the cumulative density histogram. 2. The initial gradation curve is set up in a first look-up table of the gradation conversion unit, and the correction characteristic is set in a second look-up table at a stage subsequent to the first look-up table. The gradation curve setup method according to claim 1, further comprising a step of setting up. 3. The gradation curve setup method according to claim 1, further comprising a step of synthesizing the initial gradation curve and the correction characteristic to obtain a final gradation curve.