JPH0614215B2 - Gradation correction method in color electronic copying system - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a gradation correction method in a color electronic copying system.

(Prior Art) As is well known, in the electronic copying method, when the image density of a document is plotted on the horizontal axis and the image density of a copied image is plotted on the vertical axis, the correspondence relationship between the two densities is plotted. However, the curve is called a gradation curve. Therefore, when the original has a continuous image density distribution from a low image density to a high image density, it is not possible to copy this original and reproduce the gradation of the original on the copied image over the entire density area.

However, most of the documents to be copied are generally office documents, and in such documents, the image density is rarely continuous from the low density region to the high density region.

On the other hand, the gradation curve changes according to the charging condition, the exposure condition, the developing condition and the transfer condition of the photoconductor, that is, the copying condition.

Therefore, by setting an appropriate gradation curve according to the image density distribution in the original to be copied, the gradation of the image on the original can be improved.

This is the same in the color electronic copying method, but in the color electronic copying method, the original is color-separated,
Since the selection and setting of the gradation curve must be made corresponding to the color separation image, it is difficult to correct the gradation by utilizing the principle as described above.

(Object) The present invention has been made in view of the above circumstances, and an object thereof is to provide a novel gradation correction method capable of improving gradation in a color electronic copying system. .

(Structure) Hereinafter, the present invention will be described.

In the color electronic copying system to which the present invention is applied, the original is color-separated and the neutral image is developed with black toner.

Although the original has a chromatic image and an achromatic image, the chromatic image portion will be referred to as a color image portion and the other portions will be referred to as neutral portions.

Further, a plurality of types of gradation curves are prepared in advance.

The original is color-separated and read by the solid-state image sensor prior to the color electronic copying process.

Based on the image information obtained by this reading, the pattern of density distribution in each color separated image of the color image portion and the neutral image is detected.

Then, a gradation curve for each of a plurality of visible images required to obtain one color copy is selected and set according to the pattern of the density distribution. That is, a gradation curve that best reproduces the pattern of the density distribution is selected and set for each of the plurality of visible images from the plurality of kinds of gradation curves prepared in advance.

FIG. 1 shows a color electronic copying apparatus 1 to which the present invention can be applied.
An example is shown. Since this figure is an explanatory diagram, the relative magnitude relationship of the dimensions of each part is not necessarily accurate.

The copier will first be briefly described, followed by a brief description of the color copier process and then how the invention may be embodied.

In FIG. 1, reference numeral 10 indicates a photoconductive photoconductor.
The photoconductor 10 has a drum shape and is rotatable in the arrow direction. As the material of the photoconductive layer, a photoconductive substance having a panchromatic spectral sensitivity such as As ₂ Se ₃ is used.

Around the photoconductor 10, a charger 12, an eraser 18, developing devices 20, 22, 24, 26, a holder 2 are provided.
8, a static eliminator 32, and a cleaner 34 are provided.

Reference numeral 16 indicates an original placing glass, which is an original O to be copied.
Are placed flat on the original placing glass 16.

The exposure optical system indicated by reference numeral 14 is composed of a lamp 140, a plane mirror 141, a roof mirror 142, a plane mirror 143, a lens 144, and a filter device F.

Further, the reading optical system denoted by reference numeral 40 is a mirror 40
1, a lens 402, and a single-plate color solid-state image pickup device 403, and the mirror 401 can selectively take a position indicated by a solid line by swinging.

A standard density plate 42 is provided at the right end of the document placing glass 16.

Further, in FIG. 1, reference numeral 30 is a transfer device, and reference numeral 36 is a fixing device. Further, reference symbol S indicates a transfer sheet.

Now, returning to the exposure optical system 14, when the original O is illuminated and scanned, the lamp 140 is caused to emit light, and the lamp 140 and the plane mirror 14 are moved.
1 and 1 are moved integrally to the left, and at the same time, the Dach mirror 142
Is moved to the left at a speed half of the moving speed of the plane mirror 141. Then, if the mirror 401 is in the position of the broken line, the image of the illumination portion of the document O is formed on the photoconductor 10 by the lens 144.

If the mirror 401 of the reading optical system 40 is in the position shown by the solid line, the image of the illumination section of the document O is a color solid-state image sensor.
Image on 403.

The filter device F has a red filter F1, a green filter F2, a blue filter F3, and a neutral density filter F4 (hereinafter abbreviated as ND filter F4), and each filter is selectively placed in the optical path of the exposure optical system. It can be placed.

The eraser 18 is composed of an LED array 181 and a converging optical transmitter array 182.

The holder 28 has a drum shape and holds the transfer paper S for transferring a visible image.
It is adapted to rotate in the direction of the arrow following the rotation of.

Now, in the color solid-state image pickup device 403, a large number of minute light receiving elements are closely arranged in one row and arranged in a direction orthogonal to the drawing of FIG. The individual light receiving elements are , And therefore one light-receiving element signals the image elements on the original at one time.

Each individual light receiving element is provided with a minute filter. The color of these filters is red,
There are three types, green and blue, which are cyclically arranged in the order of red, green, and blue. Therefore, looking at three adjacent light receiving elements, one of them is covered with a red filter, the other with a green filter, and the other with a blue filter.

Such three light receiving elements correspond to one pixel on the original, that is, an area of 125 μm × 125 μm on the original. Therefore, when the original O is read while being illuminated and scanned, the original O is separated into red, green, and blue for each pixel and read.

In FIG. 1, if the length of the document O in the left-right direction is l ₁ and the length in the direction orthogonal to the drawing is l ₂ , That is, the original O is matrix-formed in M rows and N columns and is color-separated and read. In this matrix of pixels, the pixel at the position of m rows and n columns is replaced with pixel (m, n)
Will be written.

On the other hand, the LED array 181 of the eraser 18 is 125 μm × 125
Micro light emitting elements (LEDs) having a light emitting area of μm are closely arranged in a direction orthogonal to the drawing of FIG. 1 so that these light emitting elements (hereinafter referred to as LEDs) can emit light in any combination. Has become.

When one of the LEDs emits light, a focused optical transmitter array 182
Due to the image forming action of the
Image on top. Therefore, the eraser 18 can erase the electrostatic latent image on the photoconductor 10 in pixel units.

The outline of the color copying process in the color electronic copying apparatus shown in FIG. 1 will be described below. In this process, the present invention has not been applied yet.

When the original O to be copied is placed on the original placing glass 16 as shown in the figure and the apparatus is operated, the mirror 401 of the reading optical system 40 for reading the original O is arranged in a solid line position, and then the original O is read. Are illuminated and scanned, and the color original O is read by the color solid-state image pickup device 403 for each of the three primary colors red, green,
It is separated into blue and signalized.

By the way, prior to reading the original O, the image of the standard density plate 42 is projected on the color solid-state imaging device 403, and the contents of the standard density plate 42 are read.

The standard density plate 42 is a 16-step gray scale,
The contents are as shown in Table 1.

The density numbers 1 to 16 are convenient indexes given to the corresponding density.

The read signal of the standard density plate 42 is sent to a microcomputer (not shown).

Then, the original O is read, but the original O is
It has a chromatic image and an achromatic image on a white background. this house,
As described above, the portion of the chromatic image is called the color image portion and the portion other than the color image portion is called the neutral portion.

Now, the read signal of the original O is composed of three kinds of signals for each pixel. That is, it is a signal read after being separated into red, green and blue colors. Therefore, in the pixel (m, n), the signals color-separated into red, green, and blue are R (m, n), G (m, n), and B (m, n), respectively.

Now, the read signal R (m, n) of the original O is sent to the above-mentioned microcomputer and compared with the contents of the standard density plate 42 inputted previously. Is converted to. That is, the signal R (m, n) is the concentration number L _R (m, n) and G (m, n) is the L _G (m, n).
Then, B (m, n) is converted to L _B (m, n).

Then, for each pixel, it is determined whether the pixel belongs to the color image portion or the neutral portion.

An achromatic image is the same regardless of the color separation.
For pixel (m, n), if L _R (m, n) =
L _G (m, n) and L _G (m, n) = L _B (m,
If it is n), the pixel (m, n) belongs to the neutral part, and if not, it belongs to the color image part.

Subsequently, the mirror 401 is placed in the position indicated by the broken line, the photoconductor 10 rotates, and uniform charging is performed by the charger.

Then, the original O is illuminated and the photoconductor 10 is exposed. At this time, the red filter of the filter device F is arranged in the exposure optical path. Therefore, the electrostatic latent image formed on the photoconductor 10 corresponds to an image that is color-separated into red. Therefore, this electrostatic latent image is referred to as a red color-separated latent image of the original O.

When this color-separated latent image passes through the erase section of the eraser 18, the microcomputer detects the LED array 18
The LED corresponding to the pixel belonging to the neutral section 1 is emitted. As a result, the latent image for color separation is erased at the latent image portion corresponding to the neutral portion.

After that, the electrostatic latent image is developed by the developing device 20 by a magnetic brush developing method using a cyan toner, that is, a toner colored in cyan (which has a complementary color relationship with red for color separation). Thus, a cyan visible image is formed on the photoconductor 10 and moves as the photoconductor rotates.

The transfer paper S is clamped at its front end by the holder 28 according to the sequence of the process, and is held so as to wind around the peripheral surface of the holder 28 by the rotation of the holder 28. Overlaid on the visual image. At this time,
The transfer device 30 charges the holding body 28 from the back side with electric charges having a polarity that electrically attracts the visible image, and transfers the visible image onto the transfer sheet S by an electric force. After the transfer of the visible image, the photoconductor 10 is destaticized by the destaticizer 32, and the residual toner is removed by the cleaner 34.

Then, the green filter F2 of the filter device F is installed in the exposure optical path, and the same process is repeated. The color-separated latent image formed at this time corresponds to the original image that has been color-separated into green, but the latent image corresponding to the neutral portion is erased by the eraser 18, and is developed with magenta toner by the developing device 22. The obtained magenta-colored visible image is transferred onto the transfer paper S so as to be superposed on the cyan-colored visible image.

The blue filter F4 of the filter device F and the developing device 2
A similar process is repeated with 4 and. Developing device 2
In 4, the yellow toner is used.

Finally, the ND filter F4 of the filter device F is disposed in the exposure optical path, and an electrostatic latent image (non-color separation latent image) corresponding to the original O is formed on the photoconductor 10 without color separation. . In this electrostatic latent image, the latent image portion corresponding to the color image portion is erased by the eraser 18.

This electrostatic latent image is developed by the developing device 26 using black toner.

When the black visible image thus obtained is transferred onto the transfer paper S, the transfer paper S is separated from the holding body 28 and fixed to the fixing device 16.
The toner image is fixed and discharged as a color copy outside the apparatus.

The above is the outline of the color copying process when the present invention is not implemented in the apparatus example shown in FIG.

The application of the present invention will be described below by taking this color copying process as an example.

In carrying out the present invention, a gradation curve for each of a plurality of visible images necessary for obtaining one color copy is selected and set according to the density distribution pattern.

In the example color copying process described above, the number of visible images required to obtain one color copy is four, which are visible images of cyan, magenta, yellow and black.

As described above, the gradation curve changes depending on the charging condition of the photoconductor, the exposure condition, the developing condition, and the transfer condition.
Here, consider a case where the exposure condition is changed.

Generally, the gradation curve under normal copying conditions is like the curve 2-1 in FIG.

A low image density region having a document image density of 0.5 or less on the horizontal axis will be hereinafter referred to as a highlight portion, and a high image density region having a density of 1.0 or more will be hereinafter referred to as a shadow portion. Looking at the gradation curve 2-1, there is a small inclination of the gradation curve in the highlight portion and the shadow portion in the gradation curve 2-1, so that the visible image obtained according to this gradation curve is the original The reproducibility of the gradation in the shadow part and highlight part of the image becomes worse.

However, the gradation of the intermediate image density area of the original is reproduced well.

Therefore, it is preferable to select the gradation curve 2-1 for a document having important information mainly in the intermediate image density area. The gradation curve 2-1 is also suitable for a document having a pure black image on a white background.

If the exposure amount during electrostatic latent image formation is sufficiently small,
A gradation curve like the gradation curve 2-2 is obtained. This gradation curve 2
In the case of -2, the reproducibility of gradation in the highlight part is good.

Also, when the exposure light amount during electrostatic latent image formation increases, the curve
A gradation curve like 2-3 is obtained. In this gradation curve 2-3,
Good reproducibility of gradation of image density higher than the intermediate image density.

By changing the exposure light amount variously, various other gradation curves can be obtained, but here, for the sake of simplicity of explanation, only the gradation curves 2-1, 2-2, 2-3 are assumed, An example will be taken in which one of the three gradation curves is selected according to the density distribution pattern.

Hereinafter, each step of the present invention will be described.

(B) Detection of pattern of density distribution As described above, in the apparatus example of FIG. 1, the color original is color-separated and read prior to the copying process. That is, from the pixel (m, n), the color separation read signal R (m,
n), G (m, n), and B (m, n) are obtained, and these are concentration number L _R (m,
n), L _G (m, n), and L _B (m, n).
These density numbers L _R (m, n) and
It corresponds to the image density in the color separation image. Then, using these density numbers, the color image part and the neutral image part were separated.

Then, at this stage, the information of the neutral image portion of the original O and the information of each color separated image of the color image portion are obtained. This information is given by the density number.

Therefore, for each of these four types of information, the sum of the pixels having the same density is taken for each density.

For example, taking the red color separated image of the color image portion as an example, the total number of pixels having a density of 1 in this red color separated image is
Let X _R (1). The total number of pixels having the density number i (i = 1 to 16) is written as X _R (i). Then, X _R (i) gives the pattern of the density distribution of the red color separated image. Similarly, the density distribution pattern of the green and blue color separation images is
X _G (i) and X _B (i) will be written, and the density distribution pattern of the neutral image part will be written as X _N (i).

Now, for example, it is assumed that the histogram of X _R (i) is as shown in FIG. 3 (I). Since the density number 9 corresponds to a density of 0.5 (see Table 1), in the case of the pattern shown in FIG. 3 (I), most of the image information is concentrated in the highlight area.

Therefore, in this case, the gradation curve is the curve 2-
I know I should choose 2.

Further, if the pattern of the density distribution of the neutral image part is as shown in FIG. 3 (II), it is easy to understand that the gradation curve 2-1 should be selected from the above-mentioned points. Will be done.

(B) Selection of gradation curve Pattern of image density distribution X _R (i), X _G (i), X _B (i), X _N (i) (i
= 1 to 16) is obtained, the next problem is how to select an appropriate gradation curve according to each pattern. There are various methods for this, but here, a method using a discriminant function will be given as an example.

The functions f _i (i), f ₂ (i), f ₃ (i), f ₄ (i) shown in FIG. ₄ are called discriminant functions. Of these functions, f _i (i) is the gradation curve 2-2,
f ₂ (i) is the gradation curve 2-3, and f ₃ (i) and f ₄ (i) are the gradation curves
Corresponds to 2-1. That is, if the pattern of the image density is f ₁ (i), the gradation curve 2-2 is selected to select the gradation of the information (concentrated on the highlight part) on the document. Can be reproduced well. Therefore, it is determined which discriminant function the image density distribution pattern X _R (i) or the like obtained by reading the document is similar to.

To do this specifically: As an example,
Take the pattern X _R (i).

At this time, (J = 1 to 4) is calculated, and the magnitude relationship among Y _R1 , Y _R2 , Y _R3 , and Y _R4 is checked, and the smallest one is obtained. what if,
If Y _R4 is the minimum, the pattern X _R is the discriminant function f
_The gradation curve 2-1 is selected as the one most similar to ₄ (i). If X _R (i) is as shown in FIG. 3 (I), X _R (i) is most similar to the discriminant function f ₁ (i), and the discriminant curve 2-2 is selected. Become.

The same applies to the patterns X _G (i), X _B (i), and X _N (i).

The process for carrying out the present invention in the apparatus shown in FIG. 1 will be briefly described below.

First, the original O is read, the color separation images of the color image portion and the neutral image portion are discriminated, and the pattern of the image density distribution of each of the color separation images and the neutral image portion is detected and these are detected. The gradation curve is selected according to

Now, for the sake of concreteness of the description, for each color separated image of red, green and blue of the color image part and the neutral image part,
It is assumed that the gradation curves 2-2, 2-2, 2-3, 2-1 are selected respectively.

Then, the microcomputer of the device first displays the gradation curve.
The exposure condition, that is, the light emission amount of the lamp 140 is set so that 2-2 is realized.

The copying process is then started.

First, the color separation of the original O by the filter F1 with red color and the development with cyan toner are performed, but the gradation curve of the cyan visible image transferred onto the transfer paper S is a curve 2-
It follows 2 (Fig. 2). Needless to say, the eraser 18 erases the portion of the electrostatic latent image corresponding to the neutral image.

Subsequently, the color separation of green by the filter F2, the erasure of the neutral image corresponding portion of the color separation latent image, the development with magenta toner, and the transfer of the visible image are performed.

Further, blue color separation by the filter F3, erase of the neutral image corresponding portion of the color separation latent image, development with yellow toner, and transfer of the visible image are performed. At this time, the exposure conditions are set so that the gradation curve 2-3 is realized.

Then, the microcomputer sets the exposure condition so that the gradation curve 2-1 is realized. Then, the electrostatic latent image is formed through the ND filter F4, the latent image portion corresponding to the color image portion is erased, the development with the black toner is performed, the black visible image is transferred, and the fixing copy of the color image is discharged. And the copying process ends.

(Effect) As described above, according to the present invention, it is possible to provide a novel gradation correction method in the color electronic copying system.

In this gradation correction method, an original is color-separated and read, and based on the read image information, a pattern of density distribution in each color-separated image of the color image part and the neutral image is detected, and a pattern is detected according to this pattern. Since the gradation curve that most closely reproduces is selected and set, appropriate gradation correction can be performed in color copying.

In the above description, there are three types of gradation curves to be selected, but four or more gradation curves may be prepared.

The original may be read by a plurality of (three) solid-state image pickup devices (for example, CCDs) having filters of three primary colors, or a well-known dot-fluorescent tube array may be used instead of the LED array of the eraser. Good.

[Brief description of drawings]

FIG. 1 shows a color electronic copying apparatus 1 in which the present invention can be implemented.
An explanatory view showing only an essential part of the example, and FIGS. 2 to 4 are views for explaining the present invention. 1 ... photoreceptor, F ... filter device, 18 ... eraser, 40 ... reading optical system

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Goo Ikeda 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd. (72) Inoue Kishio Sakai 1-3-6 Nakamagome, Ota-ku, Tokyo In Ricoh Co., Ltd. (72) Inventor Tsukasa Adachi 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd.

Claims (1)

[Claims] 1. In a color electronic copying system in which an original is color-separated and a neutral image is developed with black toner, the gradation changes depending on copying conditions and determines the correspondence between the original image density and the copy image density. Multiple types of curves are prepared in advance, and color documents are color-separated prior to the color electronic copying process and read by the solid-state image sensor. Based on the read image information, the density distribution in each color-separated image and neutral image of the color image part Of each of the visible images required to obtain one color copy, and selects the gradation curve that best reproduces each density distribution pattern according to the above density distribution pattern. A gradation correction method characterized in that the gradation is set.