JP3489787B2 - Image processing device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus which performs optimum color correction according to the type of original.

[0002]

2. Description of the Related Art Generally, there are two types of originals having a pattern used in a color copying machine, that is, a silver halide photograph and a halftone photograph. By the way, even if a silver salt photograph and a halftone dot photograph appearing to be the same color on an original, the signal values are different when the original is read and input by a general color scanner, that is, the scanner does not read the same color. This is a phenomenon mainly caused by the difference in the dyes of silver salt photographs and halftone photographs.

Therefore, when the same processing, for example, color correction, is performed on a silver halide photograph and a halftone photograph, it is not possible to expect satisfactory color reproduction from any photographic original. A general color correction coefficient for a copying machine is calculated based on information obtained by reading a color patch output by the color printer of the copying machine itself with a color scanner. For this reason, the dyes of the silver salt photographic original and the color printer are greatly different from each other, resulting in a problem that the color reproduction of the silver salt photographic is deteriorated.

[0004]

As a method for solving this, for example, a color correction coefficient for silver halide photography and a color correction coefficient for halftone photography are set and designated by the user (button operation).
There is a method to switch the color correction coefficient according to (Xerox Technical Report NO7, 1992, pp30-
31). However, this method is troublesome because the user has to judge whether the "picture" to be copied is a silver salt photograph or a halftone photograph each time the operation is performed, and the beginner user can determine the type of photograph. There is also the possibility of making a mistake.

Therefore, the present applicant first uses prescanning to determine whether the original type, in particular, whether the design is a silver salt photograph or a halftone photograph, or whether both are included by cutting and pasting. Then, based on the result, an image processing apparatus is proposed in which the color correction coefficient is switched to perform the optimum color correction on the original (Japanese Patent Application Nos. 4-344682 and 5-2.
22).

According to this apparatus, since the original is automatically judged, the problem in the above-mentioned method is solved, but the prescan requires a required time. For example, in the case of a four-color color copying machine of a frame-sequential system (one-drum system), it is necessary to perform one scan for pre-scan and five scans for four-color plate.

An object of the present invention is to determine the type of original document under the condition that the processing is completed by four scans without performing prescan in a four-color color copying machine of a frame sequential system (one drum system). An object of the present invention is to provide an image processing apparatus that switches the color correction coefficient based on the result and performs the optimum color correction on the original.

[0008]

In order to achieve the above object, according to the invention of claim 1 , an original is read out as digital signals of R, G and B, and K, C, M and Y are recorded in a frame sequential manner. In an image processing apparatus for obtaining a signal, the R, G, B
Color correction means for obtaining C, M, Y signals from signals, color correction coefficient group for photographic originals, color correction coefficient group for halftone originals, and color correction coefficient for photographic / halftone originals intermediate between them A means for storing at least two kinds of color correction coefficient groups in the group, and K from the R, G, B signals at the time of the first scan.
It is characterized in that it is provided with means for determining the type of the original at the same time as obtaining the plate recording signal, and means for selecting the color correction coefficient group based on the determination result during the second and subsequent scans.

According to the second aspect of the present invention, each color correction coefficient group is based on data obtained by reading a plurality of color patches of a photograph as R, G, B digital signals for the color correction coefficient group for a photo original. The color correction coefficient group for the halftone dot original is generated on the basis of the data read out as a plurality of R, G, and B digital signals for the color correction coefficient group for the halftone original, and the color for the photograph / halftone original is generated. The correction coefficient group is characterized in that a plurality of color patches composed of a photograph and a halftone dot having a predetermined ratio are generated based on the data read as R, G, and B digital signals.

According to a third aspect of the present invention, the means for determining the type of the original at the time of the first scanning determines whether or not the pixel of interest or the block of interest is a halftone dot region, and the number of the determined regions is determined. The feature is that the type of document is determined based on the.

According to a fourth aspect of the present invention, the means for determining the type of the original at the time of the first scanning determines whether or not the pixel of interest or the block of interest is a photographic region, and the number of the determined regions is determined. The feature is that the type of the document is determined based on the document.

According to a fifth aspect of the present invention, the means for determining the type of the original at the time of the first scanning determines whether or not the pixel of interest or the block of interest is a halftone dot area, and further, the pixel of interest or the block of interest. It is characterized in that it is determined whether or not is a photo area, and the type of the original is determined based on the number of the determined areas.

According to the sixth aspect of the present invention, it is characterized in that whether the pixel of interest or the block of interest is a halftone dot area is determined by detecting a pixel pattern corresponding to the center of the halftone dot.

According to a seventh aspect of the present invention, the determination as to whether the pixel of interest or the block of interest is a photographic area is made by detecting a pixel pattern corresponding to an intermediate level.

[0015]

The original determination circuit determines the type of original, that is, a halftone dot photograph original, a silver halide photograph original, and a halftone dot / silver halide photograph original, from the green signal at the first scanning. At the same time, the color printer is driven by the K signal from the black generation circuit to print the K plate first. The color correction coefficient storage unit stores a color correction coefficient group for each type of document. This color correction coefficient group is generated based on data obtained by inputting a photographic patch and a halftone patch with a scanner. During the second and subsequent scans (CMY plate image formation), a color correction coefficient group is selected based on the result of document determination, and the color correction process optimal for the document is performed.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the present invention, showing a configuration when applied to a color copying machine for printing in a frame sequential manner. In FIG. 1, 1 is a color scanner, 2 is a Log conversion circuit, 3 is a black generation circuit that generates a K signal, 4 is a color correction circuit, 5 is a UCR circuit, 6 is a color printer, 7 is a document determination circuit, and 8 is A color correction coefficient storage unit.

The present invention relates to original determination at the time of scanning and color correction processing based on the determination result. First, the schematic operation of the apparatus of FIG. 1 will be described, and then each processing block constituting the apparatus will be described in detail. I will describe.

The present invention is characterized by the order of printing. That is, the black (K) plate is printed first, and then cyan (C), magenta (M), and yellow (Y) are sequentially printed, and a total of four scans are performed. Here, K must be printed first, but C,
The printing order of M and Y is not limited to this.

At the time of the first scan (at the time of K plate image formation), the document determination circuit 7 operates. At the time of this K version image formation, UCR
The circuit 5 does not operate (that is, UCR through), and the K signal from the black generation circuit 3 drives the color printer 6. During the second and subsequent scans (2-4) (when printing C, M, Y plates)
In this case, a color correction coefficient is selected based on the document determination information obtained in the first scan, and R, G, B color correction is performed.

Each processing block will be described below. The color scanner 1 has a photoelectric conversion element such as a CCD element, reads an original, and separates it into three RGB colors.
Outputs a bit digital signal. This output signal is a reflectance linear signal.

The Log conversion circuit 2 uses the table to convert the reflectance linear signal into a density linear signal (R, G, B).
Convert to. In the density linear signal, white is (R,
G, B) = (0,0,0), and black is (R, G, B)
= (255,255,255).

In the black generation circuit 3, the density linear signals (R,
From G, B), a K signal is generated by the following calculation.
K0 = min (R, G, B) Formula (1) Further, by table conversion, K = f (K0) is generated.

FIG. 2 shows an example of a black amount conversion table. The main purpose of this table conversion is to highlight areas (K0 <
It is to try not to put black in 50) as much as possible.

The color correction circuit 4 removes the turbid component of the color separation filter of the scanner 1 and the turbid component of the ink. Hereinafter, in order to simplify the description, it is assumed that the color correction circuit 4 performs color correction using a linear linear equation. That is, in this color correction method, R, G, and B are signals after Log conversion (8 bits each), C, M, and Y are printer drive signals (8 bits each), and a0 to a3, b0 to b3, When c0 to c3 are color correction coefficient groups, the color correction coefficient group is represented by Expression (2).

[0025] The present embodiment is not limited to the above-described color correction method, and for example, a non-linear quadratic color correction method considering a quadratic term such as R square or G × B, or such a non-linear quadratic method. By using the result of the approximate expression, C is set at the representative point of the RGB space.
A memory interpolation method for performing color correction by setting MY values and interpolating the values (for example, Japanese Patent Publication No. 58-16180).
(See Japanese Patent Publication) and the like.

As described above, according to the present invention, the color correction coefficient group is selected in the second and subsequent scans based on the determination result of the original in the first scan, and the color correction processing suitable for the original is performed. It was done.

First, a color correction coefficient group prepared in advance, that is, a color correction coefficient group for halftone dot photographs, a color correction coefficient group for silver halide photographs, and a silver salt / halftone dot photographs are stored in the color correction coefficient storage unit 8. A method of setting the color correction coefficient group will be described.

(B) In order to set this color correction coefficient group,
A printer approximation formula (L *, a *, b * -C, M, Y) is created. That is, the C, M and Y signals are selected at appropriate sampling intervals, and the color mixture patch is output by the color printer 6. Then, this mixed color patch is measured with an appropriate colorimeter. Here, the measured colorimetric values are represented by L *, a *, and b *. As a result, the following data group is obtained.

[0029] Based on the above data group, for example, the coefficients (α0 to α9, β0 to β9, γ0 to γ9) of an approximate expression such as the following expression are determined by the least squares method.

[0030]

(B) Next, a color patch (for example, about 512 colors) consisting of a silver halide photograph is input from the color scanner 1 to obtain R, G, B signals of each patch after Log conversion. In practice, a plurality of pixels near the center of each patch are averaged (smoothed) for the purpose of reducing noise. On the other hand, a color patch consisting of a silver salt photograph was measured by the above-mentioned colorimeter (L
*, A *, b *). As a result, (R,
The relationship between (G, B) and (L *, a *, b *) is obtained.

(C) In the same manner as in (a) above, a color patch of a halftone picture is input and the relationship between (R, G, B) and (L *, a *, b *) of the patch is obtained. .

From the results of the above (a), (b) and (c), the color correction coefficient group (a0 to a3, b0 to b) of the equation (2) is obtained.
3, c0 to c3) are determined. That is, a group of color correction coefficients for silver salt photography (a0 to a3, b0 to b3, c0 to c
3); (R, G, B) and (L *, a *, b *) obtained in (b)
By using the relation of (3) and (R,
G, B) and (C, M, Y) data groups can be obtained. A group of color correction coefficients for silver salt photographs (a0 to a3, b0 to b3, c) is applied to this data group by using the least squares method so that the original color of the silver salt photograph matches the color printed color.
0 to c3) are determined.

Color correction coefficient group (a0 to a3,
b0 to b3, c0 to c3); (R, G, B) and (L *, a *, b *) obtained in (c)
By using the relation of (3) and (R,
G, B) and (C, M, Y) data groups are obtained. A color correction coefficient group (a0 to a3, b0 to b3, c0 to c3) for halftone dot photography is determined for this data group by using the least squares method.

A group of color correction coefficients for silver salt / halftone dot photography (a0-
a3, b0 to b3, c0 to c3); (R), (C), and (C, M,
Y) data group is selected with an appropriate ratio (here, 1 [512 patch]: 1 [512 patch]), and the least squares method is applied to this data group to color for silver salt / dot photography. A correction coefficient group (a0 to a3, b0 to b3, c0 to c3) is determined.

The color correction coefficient group for the halftone dot photograph, the color correction coefficient group for the silver salt photograph, and the color correction coefficient group for the silver salt / halftone dot photograph thus determined are respectively ROM or RAM.
It is written in the color correction coefficient storage unit 8 configured by.

In the UCR circuit 5, the K signal from the black generation circuit 3 is output as it is at the first scan (during K plate image formation). During the second and subsequent scans (2-4) (C,
When printing the M and Y plates), C by printing K
MY is subtracted. That is, Next, the document determination circuit 7 will be described. In the present embodiment, two determination methods are used as the original determination method. As will be described later, there are a case where the first determination method is used and a case where the first and second determination methods are combined. Examples have been disclosed.

The first determination method is a method of determining a halftone dot original. As this method, the paper “Image area separation method of mixed image of characters / designs (dots, photographs)” previously proposed by the present applicant (The Institute of Electronics, Information and Communication Engineers, Vol. J75-DI)
1 No. 1 pp39-47, January 1992), the technique of dot area detection by "peak pixel" detection is used.

That is, the peak pixel is detected by calculation. Specifically, as shown in FIG. 3, in a 3 × 3 block, the density level L of the central pixel is higher or lower than that of all the surrounding pixels, and a diagonal line across L and the central pixel. The density level a of the pair of pixels existing in
When b is | 2 × L−a−b |> TH (fixed threshold) for all four pairs, the central pixel is set as the peak pixel.

In the original decision circuit 7 of this embodiment, this peak pixel is detected by the first cancel, and it is decided by the count value whether or not the target original is a halftone dot. Further, after the peak pixel is detected, the peak pixel is counted for each block of a predetermined size (for example, 8 × 8 pixels), and when the count value is a predetermined number or more, the target block is set as a “halftone dot block”, and this halftone dot It may be possible to determine whether the target document is a halftone dot based on the count value of the block. In addition, as the input signal at the time of judgment,
g (green) signal is used.

The second determination method is a method for determining a photographic original. When a photographic original is read, many pixels have an intermediate level, and these pixels have some lumps. In this embodiment, by utilizing such a characteristic of the photo original, the pixels of the photo original are detected in the first scan,
Based on the count value, it is determined whether the target document is a photograph.

After detecting the photo pixels, the photo pixels are counted for each block of a predetermined size (for example, 8 × 8 pixels),
When the count value is a predetermined number or more, the block of interest may be a “photo block”, and it may be determined whether or not the target document is a photo based on the count value of the photo block.

As the input signal, the g (green) signal is ternarized with two threshold values, and an intermediate level pixel (th1>X> th) is obtained.
2) is matched with the pattern shown in FIG. Then, when all the 5 × 7 pixels are at the intermediate level, the flag is set with the central pixel as the photographic original pixel.

<First Embodiment when Applied to Color Copier> As the original determination circuit 7 in the first embodiment, the halftone original determination method described above is used. That is, the original determination circuit 7 detects peak pixels in the entire surface of the image (for example, A4 size) at the time of the first scanning, and the count value (P) thereof.
To calculate. This value P is compared with a predetermined threshold value TH1,
If P> TH1, the color correction coefficient group (a0-
a3, b0 to b3, c0 to c3) are output during the second and subsequent scans, and otherwise, a group of color correction coefficients for silver salt photography (a0 to a3, b0 to b3, c0).
A signal for selecting c3) is output.

If P> TH1, a signal for selecting the color correction coefficient group (a0 to a3, b0 to b3, c0 to c3) for halftone dot photography is output, and if not, silver salt / halftone screen Color correction coefficient group for dot photographs (a0 to a3, b0 to b3, c0
The combination may be changed so that a signal for selecting c3) is output. Further, instead of the peak pixels, the above-mentioned halftone dot blocks may be counted and used as the information for the document determination.

<Second Embodiment when Applied to Color Copier> In this embodiment, the above-described halftone dot original determination method and photographic original determination method are used as the original determination circuit 7. Figure 5
FIG. 3 is a block configuration diagram of a document determination circuit 7. That is, at the time of the first scan, the peak pixel detection unit 21 detects peak pixels, the counter 22 counts them, and the count value P is calculated.

At the same time, the photo original pixel detecting section 25 detects the photo original pixels, and the counter 26 counts them to calculate the count value S. This processing is performed for the entire screen (for example, A4 size) at each pixel or at a predetermined interval (for example, every 8 pixels). The two values P and S and a predetermined threshold value are compared by the comparators 23, 24, 27 and 28, and the comparison result is determined by the comprehensive determination unit 29, whereby the color correction coefficient for the second and subsequent scans. Determine the herd.

That is, if P> TH2 and S <TH5, a group of color correction coefficients for halftone dot photography (a0 to a3, b0).
b3, c0 to c3) is selected, and the signal S> TH is output.
4 and P <TH3, a signal for selecting a group of color correction coefficients for silver halide photography (a0 to a3, b0 to b3, c0 to c3) is output, and otherwise, silver salt / halftone dot photography. Color correction coefficient group (a0 to a3, b0 to b3, c0 to c3)
The signal for selecting is output.

Instead of the peak pixels and the photographic pixels, the above-mentioned halftone dot blocks and photographic blocks may be counted and used as the information for the document determination.

Then, during the second and subsequent scans, a color correction coefficient group is read from the color correction coefficient storage section 8 based on the previous selection signal, set in the color correction circuit 4, and a color suitable for the input document is read. Using the correction coefficient group, color correction processing is performed according to equation (2).

[0051]

As described above, according to the first aspect of the present invention, since the optimum color correction coefficient is set for each type of original, a silver halide photograph original, a halftone photograph original, and Optimal color reproduction is possible for a mixed original.

According to the second aspect of the present invention, since the color correction coefficient group is set based on the data input by the scanner for each of the photographic patch and the halftone dot patch, optimum color reproduction is possible.

According to the third aspect of the invention, it is automatically determined whether or not the original document to be copied is a halftone original document, and the optimum color correction coefficient group is selected based on the determination result. Therefore, optimum color reproduction is possible.

According to the invention described in claim 4 , it is automatically judged whether or not the original to be copied is a silver halide photograph original, and the optimum color correction coefficient group is selected based on the result of the judgment. Therefore, optimal color reproduction is possible.

According to the fifth aspect of the invention, it is automatically determined whether the original to be copied is a halftone original, a silver halide photograph original, or another original, and based on the determination result. Since the optimum color correction coefficient group is selected, optimum color reproduction is possible.

[0056] According to the invention of claim 6, wherein, the type of the document to be copied automatically and recognizes with high accuracy, since the selecting an optimum color correction coefficient group on the basis of the result,
Optimal color reproduction is possible.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 shows an example of a black amount conversion table.

FIG. 3 is a diagram illustrating detection of peak pixels.

FIG. 4 is an example of a pattern for detecting picture original pixels.

FIG. 5 is a block configuration diagram of a document determination circuit.

[Explanation of symbols]

1 color scanner 2 Log conversion circuit 3 Black generation circuit 4 color correction circuit 5 UCR circuit 6 color printer 7 Original judgment circuit 8 color correction coefficient storage

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-1-194571 (JP, A) JP-A-4-119765 (JP, A) JP-A-63-48071 (JP, A) JP-A-3- 54972 (JP, A) Satoshi Ouchi, 2 people outside, image area separation method of mixed areas of characters / pictures (dots, photographs), The Institute of Electronics, Information and Communication Engineers (DII), Japan, Electronic Information Communication Academic Society, January 25, 1992, Vol. J75-D-II, No. 1, p. 39-47 (58) Fields investigated (Int.Cl. ⁷ , DB name) G06T 1/00-11/80 H04N 1/60-1/62 B41J 2/525

Claims (7)

(57) [Claims] 1. An image processing apparatus for reading an original as digital signals of R, G, B and obtaining recording signals of K, C, M, Y in a frame sequential manner, in the image processing apparatus, from the R, G, B signals to C,
Of the color correction means for obtaining the M and Y signals, the color correction coefficient group for the photo original, the color correction coefficient group for the halftone dot original, and the color correction coefficient group for the photo / halftone original which is between them, Means for storing at least two or more types of color correction coefficient groups, means for determining the type of the original at the same time as obtaining a K-plate recording signal from the R, G, B signals at the time of the first scan, and second and subsequent times And a means for selecting the color correction coefficient group based on the determination result at the time of scanning. 2. The color correction coefficient group is generated on the basis of data obtained by reading out a plurality of color patches of a photograph as digital signals of R, G, and B for a color correction coefficient group for a photo original, and a halftone dot is generated. For the color correction coefficient group for the original, a plurality of color patches consisting of halftone dots are generated based on the data read out as R, G, B digital signals, and a photograph /
For a group of color correction coefficients for a halftone original, a plurality of color patches consisting of a photograph and halftone dots having a predetermined ratio are used as R, G, and B color patches.
The image processing apparatus according to claim 1 , wherein the image processing apparatus is generated based on the data read as the digital signal. 3. The means for determining the document type at the first scan determines whether or not the pixel of interest or the block of interest is a halftone dot region, and the document type is determined based on the number of the determined regions. The image processing apparatus according to claim 1, wherein 4. The means for determining the type of the original at the time of the first scan determines whether or not the pixel of interest or the block of interest is a photographic region, and determines the type of the original based on the number of the determined regions. the image processing apparatus according to claim 1, wherein the determining. 5. The means for determining the type of the original at the time of the first scanning determines whether or not the pixel of interest or the block of interest is a halftone dot region, and further, whether the pixel of interest or the block of interest is a photographic region. determines whether the image processing apparatus according to claim 1, wherein the determining the type of the document based on the number of each region the determination. Wherein said target pixel or target block of whether halftone regions determination, according to claim 3 or 5, wherein the performing by the detection pixel pattern corresponding to the center of the halftone dots Image processing device. 7. The determination of whether the target pixel or the block of interest is a photograph region, claim and performs the detection of the pixel pattern corresponding to an intermediate level 4
Alternatively, the image processing device according to item 5 .