JPH03169166A - Plate making color scanner and picture signal processing method used therefor - Google Patents

Abstract

PURPOSE:To attain color resolution of an original quickly and accurately by decomposing the color of an original with a picture signal input means so as to input an RGB picture signal and correcting the inputted 1st RGB picture signal into a 2nd RGB picture signal by a picture signal correction means in response to the characteristic of the original. CONSTITUTION:An original is color-resolved by a picture signal input means 1 and a 1st RGB picture signal is inputted and a 1st RGB picture signal is corrected into a 2nd RGB picture signal in response to the characteristic of an original by a picture signal correction means 2. The 2nd RGB picture signal is subjected to color correction or gradation correction processing by a picture signal conversion means 8 and converted into a picture signal CMYK for print and an exposure means 9 forms a picture on an exposure film based on the converted picture signal CMYK. Thus, the original color is resolved accurately and quickly.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is a plate-making process that scans a document and separates it into image signals of C (cyan), M (magenta), Y (yellow), and K (black). This is related to color scanners. (Prior Art) A color scanner for plate making is used to create an original plate for printing from a manuscript, and its general configuration is shown in Fig. 4. In the figure, in the image signal input unit 20, the original is scanned through a color separation filter and separated into three primary colors, and then R (red), G (green),
It is converted into a B (blue) image signal, and the converted image signal is amplified by a logarithmic amplification circuit. The amplified RGB image signals are converted in the density range adjustment section 21 so as to correspond to the density range determined by the set highlight (minimum density) and shadow (maximum density) values. The RGB image signal whose density range has been adjusted in this way is converted into a CMYK image signal in the color correction section 22, and then processed in the gradation correction section 23, such as changing the gradation color and gray balance.

In the exposure section 24, based on the CMYK image signals inputted from the gradation correction section 23, an image corresponding to each signal is formed on the exposure film.

(Problem to be Solved by the Invention) By the way, in the above-mentioned color scanner for plate making, it is necessary to set decomposition conditions such as setting highlights and shadows and selecting a gradation curve for each manuscript according to its contents. The reality is that setting these decomposition conditions is generally difficult and relies on the experience and intuition of skilled operators. Here, one of the decomposition conditions that is difficult to set is determining the gradation curve. What makes it difficult to determine this gradation curve is that, as shown in Figure 3, the gradation curve is not set for the RGB signals obtained directly from the original, but rather for CMY signals that have already been range-adjusted and color-converted.
This is because it is set for the K signal.

The present invention has been made in view of the above-mentioned problems, and is a color scanner for plate making that corrects RGB image signals obtained by scanning a document according to the VF characteristics of the document, and facilitates setting of decomposition conditions. The purpose is to provide an image signal processing method used therein. (Means for Solving the Problems) To achieve this object, the present invention provides an image signal input means for scanning an original through a color separation filter and inputting a first RGB image signal, and an image signal input means for inputting a first RGB image signal. an image signal modifying means for modifying the input RBG image signal according to the characteristics of the document and outputting a second RGB image signal; and a second RGB image signal from the image signal modifying means. CM for printing by performing color conversion including color correction and gradation correction on the CB image signal
Provided is a color scanner for plate making, comprising an image signal converting means for outputting YK image signals, and an exposing means for forming an image on an exposed film based on the CMYK image signals output from the image signal converting means. do.

Furthermore, the present invention scans the original through a color separation filter to obtain first RGB image signals, and then scans the original through a color separation filter.
The image signal is corrected into a second RGB image signal according to the characteristics of the document, and the corrected second RGB image signal is converted according to the characteristics of the printing material to produce a CMYK image signal for printing. The present invention provides an image signal processing method for use in a color scanner for plate making.

(Function) In the color scanner for prepress of the present invention, the image signal input means color-separates the original and inputs the first RGB image signal, and the input first RGB image signal is processed by the image signal correction means. is corrected into a second RGB image signal according to the characteristics of the document, and this second RGB image signal is subjected to processing such as color correction and gradation correction in an image signal conversion means, and is then converted into a second RGB image signal for printing. The CMYK image signals are converted into CMYK image signals, and an exposure means forms an image on the exposed film based on the converted CMYK image signals.

Further, in the image processing method used in the color scanner for prepress of the present invention, the first RGB image signal obtained by scanning the original through a color separation filter is converted into a second ROB image according to the characteristics of the original. This second R
C for printing according to the characteristics of the printing material from the image signal of GB
MYK image signals are created, but when converting from 20RGB image signals to CMYK image signals for printing, there is no need to consider the characteristics of the original, and it is based only on the fixed characteristics of the printing material. Decomposition conditions are set.

(Example) An example of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing an example of a color scanner for plate making according to the present invention. In the figure, 1 is an image input unit that reads a document set on an input drum (not shown) with an optical head, separates it into the three primary colors of RGB through a color separation filter, and converts the separated optical data into RGB with a phototube. It is converted into an image signal, and the converted image signal is amplified by a logarithmic amplification circuit. 2 is a brightness signal separator, which separates the brightness signal L from the RGB signals of each pixel input from the image signal input section 1.
Separate. 3 is a histogram measuring device, which measures the frequency of the value of the brightness signal L of each pixel separated by the brightness signal separator 2 and calculates a histogram. 4 is look amp
It is a table (LUT) creator, and creates a look amplifier table 5 that converts the brightness signal L based on the data calculated by the histogram counter 3. Reference numeral 6 denotes a switch switch for switching a switch for inputting the brightness signal L to either the histogram counter 6 or the look amplifier table 5. Adders 7a, 7b, and 7c add the RGB signals from which the brightness signal L is separated and the signal output from the run amplifier table 5. 8 is an image signal conversion section, which includes adders 7a, 7b,
Processing such as color correction, gradation correction, and gray balance is applied to the RGB image signal input from 7c to create CMYK for printing.
Create an image signal of Reference numeral 9 denotes an exposure section, which forms an image on an exposure film set on an exposure drum (not shown) based on the CMYK image signals output from the image signal conversion section 8, and creates separation plates for each color. do. Next, the operation of the color scanner for prepress shown in Figure 1 will be explained. First, in the image signal input section 1, a pre-scan of the document set on the input drum is performed. This pre-scanning is performed to set up look-up table 5, and is different from the main scanning performed to create disassembled plates for printing; it scans the original at a coarse resolution of about 5 lines/m. This is done by scanning. Next, a brightness signal separator 2 generates a brightness signal L from the RGB signals of each pixel of the document obtained by prescanning. The brightness signal L can be determined by the following equation using α, β, and γ as coefficients.

L=α・G+β・R+γ・B However, α+β+γ−1 In this case, the luminance signal Y in the NTSC system is Y = 0
．． 59G +0.3OR +0.1)B, the coefficients in the above equation are α=0.59, β-0.30
γ=0. 1), but since the RGB signal of the prepress scanner shown in FIG. 1 and the ROB signal of the NTSC system do not closely match, the above coefficient α. It is preferable to select values for β and T that are suitable for the RGB signals of the plate-making skimmer to which the present invention is applied. The frequency of the brightness signal L of each pixel created in this way is measured by a histogram counter 3 to obtain a histogram. At this time, by turning the changeover switch 6 to the contact 6a side, the brightness signal L is transferred to the histogram counter 3.
is input. Next, the look-up table generator 4 calculates the average value and standard deviation of the histogram obtained by the histogram counter 3 to obtain a normal distribution.
A conversion curve g such that this normal distribution corresponds to the above-mentioned histogram is determined, and the value of RUNOK AMP TEF' NORE 5 is set.

FIG. 2 is an explanatory diagram showing the relationship between the above-mentioned histogram, normal distribution, and lI1tA conversion curve g.The operation of the look-up table creator 4 will be explained in detail based on the diagram below. Now, suppose that the histogram counter 3 has obtained a histogram 1) shown in the fourth quadrant of the coordinate system shown in FIG. In this fourth quadrant, the horizontal axis represents the value of the brightness signal L, and the vertical axis represents the frequency. The look-up table creator 4 calculates the mean value and standard deviation of the histogram 11, and obtains a normal distribution 12 having the same mean value and standard deviation as the calculated mean (11! and standard deviation). l
2 is shown in the second quadrant of the coordinate system of FIG. 2. In the second quadrant, the vertical axis shows the value of the brightness signal L', and the horizontal axis shows the frequency. Here, let a certain value of the brightness signal L be X, and the histogram l1
Let S be the area of the part less than or equal to X, and similarly, the normal distribution l
2, the area of the portion below the value X' of the brightness signal L' is assumed to be S'. Then, the first
Let P be a point on the quadrant. Point P for value X of brightness signal L
is uniquely determined, and the conversion curve g can be obtained by finding the locus of point P when X is changed. Data is written to the memory so that the horizontal axis coordinate value of each point of the obtained conversion curve g is used as an address, and the vertical axis coordinate value is the content of the corresponding address. This will set Look Anobu Table Nore 5. When the cents of the values in the look-up table 5 are completed as described above, the selector switch 6 is turned to the contact 6b side, and the image signal input section l is turned on.
Perform the main scan of the manuscript. In this main scan, the document is scanned at a predetermined magnification, and the obtained pixel-by-pixel RG
The brightness signal L is removed from the B signal by the brightness signal separator 2. That is, from the brightness signal separator 2, R-L, G-
L, BL, signals are output, and adders 7a, 7b, 7
c. Adder? In a, 7b, and 7c, the signal from the brightness signal separator 2 and the brightness signal L' from the look-up table 5 are added. Here, in the look-up table 5, the difference between the input value L and the output value L' is set as ΔL (-L'-L), and the adder 7a,
The signals output from 7b and 7c are R'G'
B', then R' = R + ΔL G' - G + ΔL B' = B + ΔL. In this case, the above ΔL is the original input from the image signal input section 1! It represents the difference in brightness from a standard original, that is, the shading characteristics of the original. By adding ΔL to the original RGB signal, the characteristics of the original are corrected, and the standard original signal R'G'B' is obtained. Furthermore, since the above ΔL is used to correct the density of the original to the standard original density, by adding ΔL to the original RGB signal, the density range is also adjusted at the same time, which is similar to a conventional scanner. There is no need to set highlight and shadow density.

The signals R'G'B' outputted from the adders 7a, 7b, and 7c in this way are subjected to color correction in the image signal converter 8,
Processing such as gradation correction and gray balance is performed.

In color correction in the image signal converter 8, first, R',
The data of G' and B' are Y. ．． The data is converted into M and C data, and BK data is created from the converted Y and MSC data. Then, the data of R'G', B' are changed to Y, M,
There are two main methods for converting to C data.

The first method is to define a color solid in which the density values of Y, M, and C are placed on one coordinate axis of a three-dimensional orthogonal coordinate system, and to store the converted Y, M, and C values in a storage device corresponding to this color solid. , C is a method of storing data. R', G', B before conversion
' data as an address value, access one point in the color solid defined in the storage device, and read out the Y, M, C data stored for this one point to perform color conversion. It will be done. In this method, storing Y5M, C data for all points in the color solid in the storage device would require a storage device with a huge storage capacity, so only Y, M and M data for the representative points in the color solid are stored in the storage device. , C, and if an intermediate value between adjacent representative points is required, it is desirable to obtain it by interpolation.

The second color correction method uses a masking equation, and is a simple method that uses the l-order masking equation expressed by the following equation.

Y=a,,R'+a+zG'+. ,a. B'
M- ax,R' + aztG' + a23
B'C=as, R' + asxG' + as2B'
Here, a. ~ays is called the first-order masking coefficient,
These are parameters that affect the converted Y, M, and C values, and these nine parameters are set so that the image expressed in the R'G'B' system and the image expressed in the CMY system are equivalent. be done.

Typically, nine parameters are determined from the ratio of the highest concentrations of R', G', and B'.

This method of using a first-order masking equation is simple, but if the error due to color correction is large, a second-order masking equation may be used to reduce the error.

In the quadratic masking equation, R, G. ．． In addition to the linear term B, Rg, G2, Bz and RG,
Quadratic terms CB and BR enter the equation. Usually, the secondary masking coefficient that is the parameter in this case is
It is set by selecting some pixels as sampling pixels and performing calculations using the method of least squares so that the error between the RGB system representation and the CMY system representation for the same sampling pixel is minimized.

Next, in the gradation correction of the image signal converter 8, gradation curves fO to f4 as shown in FIG. 3 are used. In Fig. 3, fO is a standard characteristic curve, fl is a highlight emphasis curve, f2 is a shadow emphasis curve, f3 is a highlight/shadow emphasis curve, and f4 is a halftone emphasis curve. When using fO and correcting the gradation, select f1 to f4 depending on the gradation part you want to emphasize.
The curve may be used as appropriate. This gradation correction is performed by a gradation circuit or a RAM incorporating a look-up table. Note that the gradation curve is not limited to the one shown in FIG. 3, and can be set arbitrarily. Next, Y after color correction and gradation correction are completed.
Gray balance processing is performed on the MC signal.
In gray balance processing, the differences of each CMY signal are corrected so that when CMY is equal, it becomes gray. Here, color correction, gradation correction in the image signal conversion unit 8,
Processing such as gray balance is performed on R'G'B' signals that have already been corrected to standard original shading. Therefore, the processes such as color correction, gradation correction, gray balance, etc. need not be performed according to the characteristics of the document, but may be performed based on the characteristics of printing materials such as ink and paper.

As a result, for printing materials under the same conditions, if you first set the processing conditions of the image signal converter 8, there is no need to set decomposition conditions such as highlight/shadow settings and gradation curve selection for each document. .

CMY output from the image signal converter 8 in this way
Each of the K image signals is input to the exposure section 9, and each signal is applied to an exposure device (not shown). Then, an image is formed on an exposure film set on an exposure drum by this exposure device, and separation plates corresponding to CMYK image signals are created. (Effects of the Invention) As described above, in the color scanner for plate making of the present invention, the original is color-separated by the image signal input means, and the first R
A GB image signal is input, and the input first RGB image signal is corrected to a second RGB image signal according to the characteristics of the document by the image signal correction means, so that the standard RGB image signal is not used at the RGB stage. The image signal conversion means performs processing such as color correction and gradation correction on the second RGB image signal to create a CMYK image signal for printing. The separation conditions of the scanner are determined by the characteristics of printing materials such as ink and paper, and when separating the colors of a document with a scanner, it is a task that requires skill, such as setting highlights and shadows for each document and selecting a gradation curve. This provides an excellent effect in that color separation of a document can be performed accurately and quickly without the need to perform color separation.

[Brief explanation of the drawing]

FIG. 1 schematically shows the structure FIi. Figure 2 is an explanatory diagram showing the histogram obtained from the histogram counter 4 shown in Figure 1, its normal distribution, and the relationship between these and the gradation conversion curve, and Figure 3 is an example of the gradation conversion curve. Characteristic diagram shown, 4th
The figure is a schematic diagram showing the general configuration of a color scanner for plate making. ■−・−・One image signal input section 2−・One brightness signal separator 3−−−−−−Histogram counter 4−−・−・− Look up table creator 5−
------ Look amplifier table 6 --- All changeover switches 7 a, 7 b, 1c --- Adder 8 --- Image signal converter 9 --- One exposed area

Claims (3)

[Claims] (1) Scan the original through a color separation filter to obtain the first R
an image signal input means for inputting a GB image signal; and an image signal modification means for modifying the RBG image signal input from the image signal input means according to the characteristics of the document and outputting a second RGB image signal. The second RGB image signal from this image signal correction means is subjected to color conversion including color correction and gradation correction to produce CMY for printing.
A plate making device comprising: an image signal conversion means for outputting a K image signal; and an exposure means for forming an image on an exposed film based on the CMYK image signal output from the image signal conversion means. color scanner. (2) a brightness signal separation means for separating a brightness signal from an image signal obtained by scanning a document; a histogram creation means for creating a histogram of the brightness signal output from the brightness signal separation means; and a histogram creation means. and look-up table creation means for creating a look-up table for converting the first image signal into the second image signal based on the histogram created by ) color scanner for prepress. (3) Scan the original through a color separation filter to obtain the first R
After obtaining the GB image signal, the first RGB image signal is modified into a second RGB image signal according to the characteristics of the document, and the modified second RGB image signal is applied to the printing material. An image signal processing method used in a color scanner for plate making, characterized in that a CMYK image signal for printing is created by converting according to characteristics. (4) The plate making according to claim (3), characterized in that the first RGB image signal is corrected to the second RGB image signal based on a brightness signal obtained by scanning a document in advance. An image signal processing method used for color scanners.