JP2679723B2 - Gradation correction method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradation correction method. In the well-known electrophotographic technique, when the image density of the original is plotted on the horizontal axis and the image density of the electrophotographic image is plotted on the vertical axis, the correspondence between the two densities is plotted. The relationship is not a straight line but a curve 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 the original and reproduce the gradation of the original on the copied image over the entire density area. Can not. However, most of the manuscripts generally copied are office documents, and there is almost no such manuscript in which the image density is continuous from the low efficiency region to the high density region. Further, the gradation curve changes according to the charging condition, exposure condition, developing condition and transfer condition of the photoconductor, that is, so-called image forming condition. Therefore, the gradation reproducibility on the copied image can be improved by selecting and setting an appropriate gradation curve according to the image density distribution on the original. This also applies to the color electronic copying system. However, in the color electronic copying system, the original is color-separated, and the gradation curve must be selected and set according to the color-separated image. It was difficult to correct the gradation using the above principle. SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and is a novel gradation correction capable of improving the gradation of an input color image. The purpose is to provide a method. According to the gradation correction method of the present invention, an image forming condition is set in a color image forming apparatus for developing with a plurality of color toners in accordance with color components of an input color image. Depending on the color components of the input color image, a plurality of types of gradation curves that determine the correspondence between the density of the input color image and the image forming density are prepared in advance. The pattern of each density distribution from the low density area to the high density area of each color component is detected, and the above-mentioned density distribution pattern is applied to each of a plurality of visible images required to obtain one color image. depending, to form a visible image that best reproduces the pattern of the concentration distribution
It is characterized in that the gradation curves are selected and set as described above . From the viewpoint of image conversion, the electrophotographic method is
The input image is once converted into an electrostatic latent image which is an intermediate image, and the electrostatic latent image is visualized to be converted into an output image. The gradation curve can be changed depending on the condition when converting the input image into the electrostatic latent image and / or the condition when converting the electrostatic latent image into the output image. In the color image forming apparatus, the input image has a plurality of color components, and each color component converted into an output image is superimposed to form an output color image. According to the present invention, the pattern of each density distribution of each color component of the input image from the low density area to the high density area is detected, and the visible image which best reproduces the density distribution pattern for each color component is detected. The gradation curves are selected and set so as to respectively form the . EXAMPLES Specific examples will be described below. FIG. 1 shows an example of a color electronic copying apparatus which is a color image forming apparatus to which the present invention can be applied. Since this diagram is an explanatory diagram, the relative magnitude relationship of the dimensions of the respective parts is not always accurate. In FIG. 1, a photoconductive photosensitive member designated by the reference numeral 10 has a drum shape and is rotatable in the arrow direction. As a material for the photoconductive layer, for example, a photoconductive substance having a panchromatic spectral sensitivity such as As ₂ Se ₃ is used. Around the photosensitive member 10, a charger 12, an eraser 18, and developing devices 20, 22, 24, 2 are provided.
6, a holder 28, a static eliminator 32, and a cleaner 34 are provided. The original glass designated by the reference numeral 16 is a flat glass on which the original 0 to be copied is placed. The exposure optical system indicated by reference numeral 14 includes a lamp 140, a plane mirror 141, and a roof mirror 1.
42, plane mirror 143, lens 144, filter device F
It consists of. Further, the reading optical system shown by reference numeral 40 is composed of a mirror 401, a lens 402, and a single plate type color solid-state image pickup device 403. It can be taken selectively. A reference density plate 42 is provided at the right end of the original placing glass 16. In FIG. 1, reference numeral 30 indicates a transfer device, reference numeral 36 indicates a fixing device, and reference numeral S indicates a transfer sheet. Returning to the exposure optical system 14, when the original 0 is illuminated and scanned, the lamp 140 is caused to emit light and the lamp 1
40 and the plane mirror 141 are integrally moved to the left, and at the same time, the roof mirror 142 is moved to 1/2 of the moving speed of the plane mirror 141.
Move to the left at the speed of. Then, if the mirror 401 is in the position of the broken line, the image of the illumination portion of the document 0 is formed on the photoconductor 10 by the lens 144. If the mirror 401 of the reading optical system 40 is placed in the position indicated by the solid line, the image of the illumination section of the document 0 is formed on the color solid-state image pickup device. The filter device F is a red filter F.
1, a green filter F2, a blue filter F3, a neutral density filter F4 (hereinafter abbreviated as ND filter F4), and each filter can be selectively arranged in the optical path of the exposure optical system. The eraser 18 includes an LED array 181 and
It is composed of a converging optical transmission body array 182. The holder 28 has a drum shape and holds the transfer paper S for transferring a visible image.
Following the rotation of the photoconductor 10, the photoconductor 10 rotates in the direction of the arrow. In the color solid-state image pickup element 403, a large number of minute light receiving elements are arranged in close contact with each other in a line in a direction orthogonal to the drawing of FIG. Each light receiving element is
Corresponding to image elements of 125/3 .mu.m.times.125 / 3 .mu.m on the manuscript 0, one light receiving element therefore signals the above-mentioned image elements on the manuscript at one time. Each of the individual light receiving elements is
It is covered with a minute filter. The colors of these filters are three colors of red, 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 is covered with a green filter, and the other is covered with a blue filter. Such three light receiving elements correspond to one on the original.
It corresponds to a pixel, that is, an area portion of 125 μm × 125 μm on the original. Therefore, if the original 0 is read together with the illuminating device, the original 0 is read by color-separating each pixel into red, green, and blue. That is, the color image on the original, which is the input color image, is read after being separated into the respective color components as described above. In FIG. 1, if the length of the document 0 in the left-right direction is l ₁ and the length in the direction orthogonal to the drawing is l ₂ , then M = 1
_1/125 [mu] m, as N = l ₂ / 125μm, document 0 is necessarily to be read by the color separation are matrixed in M rows and N columns. In this pixel matrix, the pixel at the position of m rows and n columns will be referred to as pixel (m, n). On the other hand, the LED array 18 of the eraser 18
1 is composed of minute light emitting elements (LEDs) having a light emitting area of 125 μm × 125 μm closely arranged in a direction orthogonal to the drawing of FIG. 1, and these light emitting elements (hereinafter referred to as LEDs) emit light in any combination. It can be done. When one of the LEDs is made to emit light, the LED that emits light due to the image forming action of the converging light transmitter array 182.
An image of the same size is formed on the photoconductor 10. Therefore, the eraser 18 can erase the electrostatic latent image on the photoconductor 10 in pixel units. The outline of image formation in the color electronic copying machine shown in FIG. 1 will be described below. When the original 0 having a color image 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 0 is read.
Is arranged in the position of a solid line, then the original 0 is illuminated and scanned, and the color image 0 is separated by the color solid-state imaging device 403 into three primary color components of red, green, and blue, and signals are obtained. Be converted. Before reading the original 0, the image of the reference density plate 42 is projected on the color solid-state image pickup element 403, and the contents of the reference density plate 42 are read. The reference density plate 42 is a 16-step gray scale, and the density number:
1 to 16 correspond to the concentrations as follows. Concentration number: 1 2 3 4 5 6 7 8 Concentration: 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Concentration number: 9 10 11 12 13 14 15 16 Concentration: 0.50 0.60 0.70 0.80 0.90 1.00 1.20 1.40 Concentration number: 1 to 16 is a convenient index given to the corresponding concentration. The read signal of the reference density plate 42 is sent to a microcomputer (not shown). Next, the manuscript 0 is read,
The document 0 has a chromatic image and an achromatic image on a white background.
Of these, the chromatic image portion is called a color image portion, and the portion other than the color image portion is called a neutral portion. The read signal of the original 0 is composed of three kinds of signals for each pixel. That is, it is a signal read by color separation into red, green and blue. Therefore, in the pixel (m, n), the signals color-separated into red, green, and blue are defined as R (m, n), G (m, n), and B (m, n), respectively. The reading signal R (m, n) of the original 0 is sent to the microcomputer and compared with the previously input contents of the reference density plate 42, and the density number: 1 to 1
16. That is, the signal R (m, n) is converted into the density number L _R (m, n), and G (m, n) is converted into the density number L.
_G (m, n) and B (m, n) are converted into density numbers L _B (m, n). Next, for each pixel, it is determined whether the pixel belongs to the color image portion or the neutral portion. The achromatic image is the same regardless of the color separation, so if we look at the pixel (m, n), if L _R (m,
n) = L _G (m, n) and L _G (m, n) = L
It can be seen that if _B (m, 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 the charger 12 uniformly charges the photoconductor 10. Subsequently, the document 0 is illuminated and the photoconductor 1
An exposure of 0 is performed. At this time, the red filter F1 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. This electrostatic latent image is called a red color separation latent image of the original 0. When this color-separated latent image passes through the erase portion of the eraser 18, the microcomputer outputs L corresponding to the pixels belonging to the neutral portion of the LED array 181.
Make the ED emit light. 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 line 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 (complementary color relationship with red for color separation). Thus, a cyan visible image is formed on the photoconductor 10 and moves as the photoconductor 10 rotates. The transfer paper S is clamped at its front end by the holder 28 in accordance with 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 visible image. At this time, the transfer device 30 charges the holding body 28 from the back side by the electric charge of the polarity that electrically attracts the visible image, and transfers the visible image onto the transfer sheet S by an electric force. The photoconductor 10 after the transfer of the visible image is neutralized by the static eliminator 32, and is cleaned by the cleaner 34.
The residual toner is removed with. Next, the green filter F2 of the filter device F is installed in the exposure optical path, and the same process as above is repeated. The color-separated latent image formed at this time corresponds to the green color component, but the latent image portion corresponding to the neutral portion is erased by the eraser 18 and developed by the developing device 22 with magenta toner, and the obtained magenta visible image is transferred. It is transferred onto the paper S so as to be superposed on the cyan visible image. The same process is repeated using the blue filter F3 of the filter device F and the developing device 24, and the developing device 24 develops with yellow toner. Finally, the ND filter F4 of the filter device F is installed in the exposure optical path, and an electrostatic latent image (non-color separation latent image) corresponding to the original 0 is formed on the photoconductor 10 without color separation. To be done. In the non-color separation latent image, the line image portion corresponding to the color image portion is erased by the eraser 18. The electrostatic latent image after erasing is developed by the developing device 26 using black toner, and when the black visible image thus obtained is transferred onto the transfer paper S, the transfer paper S is held by the holder 28. Is separated and sent to the fixing device 36, where the toner image is fixed and discharged as a color copy image to the outside of the device. The above is the outline of image formation by the apparatus shown in FIG. The application of the present invention in the above image formation will be described below. In carrying out the present invention, a gradation curve for each of a plurality of visible images necessary for obtaining one color image is selected and set according to the density distribution pattern. In the case of the image forming process described above, four visible images, that is, cyan, magenta, yellow, and black visible images are required to obtain one color image. As described above, the gradation curve changes depending on the charging condition, exposure condition, developing condition and transfer condition of the photoconductor.
Here, a case where the exposure curve is changed to change the gradation curve will be described. Generally, the gradation curve under normal copying conditions is as shown by curve 2-1 in FIG. Hereinafter, the density of the original image density shown on the horizontal axis of FIG.
A low image density area of 5 or less is called a "highlight portion", and a high image density area of a density of 1.0 or more is called a "shadow portion". Looking at the gradation curve 2-1, since the inclination of the gradation curve is small in the highlight portion and the shadow portion,
In the visible image obtained according to the gradation curve 2-1, the reproducibility of the gradation of the shadow portion and the highlight portion of the original becomes poor. However, the gradation of the intermediate image density area of the original is reproduced well. Therefore, the gradation curve 2-1 is preferably selected in the case of a document having important information mainly in the intermediate image density area or a document having a pure black image on a white background. When the amount of exposure is made sufficiently small at the time of forming the electrostatic latent image, the gradation curve becomes as shown by the gradation curve 2-2 in FIG. The gradation curve 2-2 has good reproducibility of gradation in the highlight portion. On the contrary, when the exposure amount is large, the gradation curve becomes like the gradation curve 2-3 in FIG.
-3 has good reproducibility of gradation of image density of intermediate density or higher. Various gradation curves can be obtained by changing the exposure light amount, but here, for the sake of simplicity of explanation, only the gradation curves 2-1, 2-2 and 2-3 are assumed and the density is assumed. An example will be described in which one of these three gradation curves is selected according to the distribution pattern. (B) Detection of Pattern of Density Distribution As described above, in the apparatus example of FIG. 1, a color image is separated into individual color components and read before the copying process. That is, the color separation read signal: R from the pixel (m, n)
(M, n), G (m, n), B (m, n) are obtained, and these are number of concentrations: L _R (m,
n), L _G (m, n), L _B (m, n). Number of densities: L _R (m, n), etc.
n) corresponds to the image density in the color-separated image. Therefore, at this stage, the information of each component image of the document 0 is obtained, and the information is given by each density number. Therefore, for each of these four types of information,
The number of pixels in the same density portion is counted for each density number. For example, taking a red component image as an example, the number of densities: i (i = 1
The total number of pixels of 16 to 16) is represented by X _R (i). Then
X _R (i) gives a pattern of the density distribution of the red component image (giving how the number of pixels having the density number: i changes depending on the density number: i). Similarly, the density distribution pattern of the green and blue component images is calculated as X _G
(I), X _B (i), and the concentration distribution in the neutral portion is represented by X _N (i). When X _R (i) is represented by a histogram, a pattern as shown in FIG. 2B can be obtained, for example. In this case, the density number: 9 corresponds to the density: 0.5, so (B)
It can be seen that in the case of the pattern of the density distribution as described above, most of the image information is concentrated in the highlight area. Therefore, in this case, it is understood that the gradation curve 2-2 in FIG. 2A should be selected as the gradation curve. If the pattern of the density distribution in the neutral portion is as shown in FIG. 2C, in this case, the gradation curve 2-1 in FIG. 2A should be selected. It will be easily understood. (B) Selection of gradation curve Pattern of image density distribution: X _R (i), X _G (i), X _B
Once (i), X _N (i) (i = 1 to 16) is obtained, it becomes a problem to select an appropriate gradation curve according to these patterns as follows. There are various ways to do this,
As an example, a method using a "discriminant function" will be described. Four kinds of functions shown in FIG. 3: f ₁ (i), f
_{2 (i), f 3 (} i), referred to as the discriminant function f ₄ a (i).
Among these discriminant functions, f ₁ (i) is the gradation curve 2-2,
f ₂ (i) is the gradation curve 2-3, and f ₃ (i) and f ₄ (i)
The combination of and corresponds to the gradation curve 2-1. Therefore, if the density distribution pattern resembles the discriminant function: f ₁ (i), the gradation curve 2-2 is selected, and the information on the original document (concentrated in the highlight portion is concentrated. That is, it is possible to excellently reproduce the gradation. Then, the problem is how to judge which discriminant function the pattern of the density distribution for each color component is similar to, but here it is as follows. For example, the pattern of density distribution: X _R (i)
Then, the operation of the sum of the right side of Y _Rj = Σ {f _j (i) −X _R (i)} ² is executed for j = 1 to 4 (the sum is 1 for argument: i To 16). The smaller the value of Y _Rj , the more similar X _R (i) is to f _j (i). By executing the calculation, four quantities: Y _R1 , Y _R2 ,
Since Y _R3 and Y _R4 are obtained, the magnitude relationship between these is investigated,
Find the smallest one. For example, if Y _R4 is the minimum, the pattern X _R (i) of the density distribution is the discriminant function: f
_{4 The} tone curve that most closely resembles (i) is 2-
Select 1. The same applies to the density distribution patterns for images of other color components: X _G (i), X _B (i), and X _N (i). In the case of the apparatus example shown in FIG. 1, the pattern of the density distribution for each color component in the color image is detected as described above based on the result of reading the original 0 to obtain one color image. For each of the plurality of visible images necessary for the above, the gradation curve that best reproduces each pattern of the density distribution is selected by the above-mentioned discriminant function. After that, the conditions for image formation are controlled by the microcomputer so that the selected gradation curve is realized. For example, it is assumed that the gradation curves 2-2, 2-2, 2-3 and 2-1 are selected for the red, green and blue color separation images and the neutral portion in the above-described example. Then, the microcomputer creates a gradation curve 2- when forming a color-separated latent image in red on the photoconductor 10.
The exposure condition, that is, the exposure amount of the lamp 140 is set so that the second condition is realized. Under this condition, after the color separation latent image of red color is formed, the development with the cyan toner and the transfer of the cyan visible image are performed. The cyan visible image transferred to the transfer sheet S follows the gradation curve 2-2. Under the same exposure condition, a green color separation latent image is formed, development with magenta toner and transfer of a magenta visible image are performed. The magenta visible image transferred to the transfer sheet S follows the gradation curve 2-2. Next, the microcomputer sets exposure conditions so that the gradation curve 2-3 is realized, and after forming a color separation latent image in blue under these conditions, development with yellow toner and a visible image in yellow color are performed. Is transcribed. The yellow visible image transferred to the transfer paper S follows the gradation curve 2-3. Finally, the microcomputer sets the exposure conditions so that the gradation curve 2-1 is realized, and the electrostatic latent image corresponding to the neutral portion is formed, the development is performed with the black toner, and the black visible image is formed. Transfer is performed. The black visible image transferred to the transfer paper S follows the gradation curve 2-1. After that, when the color visible image is fixed, a color image is obtained. As described above, according to the present invention, a novel gradation correction method can be provided. Since the present invention is configured as described above, since the gradation curve most suitable for each color component of the input color image is used, it is possible to form the image by appropriately correcting the gradation of the color image. Although three kinds of gradation curves are prepared in the above description, two kinds of gradation curves may be prepared, or four kinds or more may be prepared.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an example of a color electronic copying machine which is an image forming apparatus to which the present invention can be applied. FIG. 2 is a diagram showing an example in which a gradation curve and a pattern of density distribution are represented as a histogram. FIG. 3 is a diagram for explaining a discriminant function for selecting a gradation curve according to a density distribution pattern. [Explanation of symbols] 2-1, 2-2, 2-3 gradation curve

Continuation of front page    (72) Inventor Yoshihiro Sakai               1-3-6 Nakamagome, Ota-ku, Tokyo               Ricoh Company (72) Inventor Goo Ikeda               1-3-6 Nakamagome, Ota-ku, Tokyo               Ricoh Company (72) Inventor Kazuo Sakai               1-3-6 Nakamagome, Ota-ku, Tokyo               Ricoh Company (72) Inventor Tsukasa Adachi               1-3-6 Nakamagome, Ota-ku, Tokyo               Ricoh Company                (56) References JP-A-59-163967 (JP, A)                 JP-A-58-39168 (JP, A)                 JP-A-59-15264 (JP, A)

Claims (1)

(57) [Claims] In a color image forming apparatus that develops with toner of a plurality of colors in accordance with color components of an input color image, the color image forming apparatus changes in accordance with image forming conditions, and the correspondence between the density of the input color image and the image forming density is Prepare a plurality of gradation curves to be determined in advance, and detect the pattern of each density distribution from the low density area to the high density area for each color component in the input color image based on the color component of the input color image. , For each of the plurality of visible images required to obtain one color image, according to the pattern of each density distribution above, each density distribution
So as to form a visible image that best reproduce the pattern
A gradation correction method, characterized by selecting and setting gradation curves respectively .