JP2005349808A - Color image forming apparatus, its control method and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color image forming apparatus which can implement a sufficient gradation expression even in a low density area and a gradation expression capability necessary for a high photograph quality, and can execute an image process calculation for implementing the gradation expression without reduction in an image forming speed, and to provide a control method for the color image forming apparatus and a program. <P>SOLUTION: First image data for forming an image with a high density recording material and second image forming data for forming an image with a low density recording material in recording materials having a similar hue but a different density, are created from input color images. The first image data are converted into the first gradation data of the number of a first gradation. The second image data are converted into the second gradation data of the number of a second gradation having the number of gradation more than the number of the first gradation. Based on the first and the second gradation data thus obtained, the images are formed on a recording medium using the recording material corresponding to the respective data. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a color image forming apparatus that forms a color image using a plurality of recording materials, a control method therefor, and a program.

  In recent years, printers have been colored and used for various purposes. In particular, color laser beam printers using an electrophotographic method are attracting attention due to the high recording quality of text and illustrations.

  The full-color laser beam printer expresses a full-color image by superimposing three recording material images with different hue angles, cyan, magenta, and yellow, and adds a black recording material to improve the recording quality of text and other materials. Yes. That is, a total of four color recording materials (particularly, in the case of a laser beam printer, toner is used as the recording material).

  In such a color laser beam printer, when expressing halftone density, since the coloring power of the toner itself is always constant, pseudo gradation processing is used.

  Here, the pseudo gradation processing will be described.

  Pseudo gradation processing is an image forming apparatus having a high resolution whose basic pixel area is equal to or less than the human identification limit. This is a method of expressing halftone density.

  For example, when the area ratio of the colored part is 100%, the density is the highest density that can be expressed by the recording material, and when the area ratio of the colored part is 50% and the area ratio of the non-colored part is 50%, the density can be expressed by the recording material. It is detected by the user as half the density.

  Therefore, in the pseudo gradation processing, the density adjustment level from the highest density to the lowest density is determined by the number of adjustment levels of the area ratio between the colored portion and the non-colored portion. A large number can be taken, that is, a large gradation can be provided.

  Among such pseudo gradation processes, representative examples include area gradation processes represented by the dither matrix method and multi-value gradation processes represented by the light beam pulse width modulation method. This pseudo gradation processing is mainly used for area gradation processing in an ink jet printer and mainly for area gradation processing in a laser beam printer, or a combination of area gradation processing and multi-value gradation processing.

  In these pseudo gradation processes, in order to improve the number of density gradation levels, the resolution of the image forming apparatus is increased, that is, the minimum area that can be colored is reduced, and the areas of the colored and non-colored parts are reduced. It is necessary to have a large number of ratio levels.

As a method for improving the gradation expression, in addition to recording materials (ink, toner, etc.) of basic colors such as cyan, magenta, yellow, etc., light cyan and light, which have different hues at the same hue angle as cyan and magenta, respectively. There is a method of supplementing the gradation expression by the basic color by adding a recording material such as magenta and improving the gradation property on the low density side, and has recently been put into practical use in an ink jet printer (for example, Patent Document 1). .
JP-A-5-16367

  However, the above method has the following problems.

  In general, in order to achieve a high picture quality equivalent to that of a silver salt photograph, it is said that a gradation expression ability of about 256 gradations is necessary. In particular, the human eye is insensitive to gradation changes in a high density area and sensitive to gradation changes in a low density area typified by a bright skin color. For this reason, in order to achieve a high image quality similar to that of a silver halide photograph using a digital image forming apparatus, it is essential to enhance the gradation expression in a low density region. As a specific example, when the optical density value is about 0.7 or less (measured by Macbeth X-RITE), and has an expressive power of 100 gradations or more, the photo quality is almost the same as that of a silver salt photograph. The result is achieved.

  However, the conventional techniques mentioned above cannot sufficiently improve the gradation in the low density region. Hereinafter, each technique and its reason are explained in full detail.

  When pseudo gradation processing is performed using recording materials of four basic colors of cyan, magenta, yellow, and black, in order to improve gradation, the resolution of the image forming apparatus is increased, and the basic pixels of the colored portion It is necessary to make (dots) small.

  However, in the case of an electrophotographic image forming apparatus, an isolated pixel (dot) having a smaller area is more difficult to develop and the reproducibility deteriorates. For this reason, when the density is lower than a certain density, the density is drastically decreased, which is detected by the user as a tone jump (gradation changes rapidly), or the color reproducibility is deteriorated.

  In addition, in the case of an ink jet printer, the finer the ink droplet, the greater the effect of air resistance, making it difficult to land on a recording medium accurately. In addition, it is difficult to design a nozzle for emitting fine ink droplets.

  For the above reasons, when using pseudo gradation processing with four basic colors with high coloring power, such as cyan, magenta, yellow, and black, it is difficult to improve the gradation in the low density area to the high image quality level of the photograph. is there.

  On the other hand, the method of supplementing the low density gradation expression using the toner with low coloring power is necessary to express the same density as compared with the case of using the basic color because the coloring power of the toner itself is lowered. The area of the colored portion becomes large. As a result, the expressive power of the low density region has been improved, and the low density region that has been difficult to express can be expressed with good reproducibility.

  However, with this method alone, it is possible to express 20 to 50 gradations in an area where the optical density value is 0.7 or less, and it is not possible to express 100 gradations or more, which is comparable to silver salt photography. Image quality printing cannot be realized.

  To deal with these issues, combining the two methods of increasing the resolution and using a light-colored recording material, the color space information obtained from the image information is converted into the basic colors (yellow, magenta, cyan, black). ) And a total of six colors of light colors (light magenta and light cyan) need to be converted into density information. In order to perform this conversion, the number of bits for expressing density information of each color increases, and the amount of data processing and the amount of calculation become enormous. For this reason, there is a problem that the image processing calculation cannot be executed in accordance with the recording speed of the printer and the throughput is lowered.

  The present invention has been made in view of the above problems, and can realize sufficient gradation expression even in a low density region and realize gradation expression necessary for high image quality of a photograph, thereby realizing this gradation expression. An object of the present invention is to provide a color image forming apparatus, a control method thereof, and a program capable of executing an image processing calculation for reducing the image forming speed without reducing the image forming speed.

In order to achieve the above object, a color image forming apparatus according to the present invention comprises the following arrangement. That is,
A color image forming apparatus for forming a color image using a recording material of a plurality of colors,
An input means for inputting a color image;
From the color image input by the input means, first image data for forming an image with a recording material having a high density among recording materials having the same hue and different densities and an image with a recording material having a low density are formed. Generating means for generating second image data for
First gradation conversion means for converting the first image data into first gradation data having a first gradation number;
Second gradation conversion means for converting the second image data into second gradation data having a second gradation number larger than the first gradation number;
Forming means for forming an image on a recording medium using a recording material corresponding to each of the first and second gradation data obtained by the first and second gradation converting means. A color image forming apparatus.

In order to achieve the above object, a color image forming apparatus control method according to the present invention comprises the following arrangement. That is,
A control method of a color image forming apparatus for forming a color image using a recording material of a plurality of colors,
An input process for inputting a color image;
From the color image input in the input step, first image data for forming an image with a recording material having a high density among recording materials having the same hue and different densities and an image with a recording material having a low density are formed. Generating a second image data for
A first gradation conversion step of converting the first image data into first gradation data having a first gradation number;
A second gradation conversion step of converting the second image data into second gradation data having a second gradation number greater than the first gradation number;
And a forming step of forming an image on a recording medium using recording materials corresponding to the first and second gradation data obtained in the first and second gradation conversion steps.

  According to the present invention, sufficient gradation expression even in a low density region and gradation expression necessary for high image quality can be realized, and image processing calculation for realizing this gradation expression can be performed at an image forming speed. It is possible to provide a color image forming apparatus that can be executed without being lowered, a control method therefor, and a program.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Embodiment 1>
FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to a first embodiment of the present invention.

  In the first embodiment, an electrophotographic color image forming apparatus will be described as an example of the image forming apparatus.

  As a schematic configuration, 1 is an image forming apparatus and 2 is a host computer.

  The image forming apparatus 1 includes an image processing unit 3 and each color image optical system 33C (for cyan), 33M (for magenta), 33Y (for yellow), 33K (for black), 33LC (for light cyan), 33LM (for light magenta). ) And the image forming unit 4.

  As the most characteristic part, the present image forming apparatus 1 has the same hue angle as cyan in addition to cyan (C), magenta (M), yellow (Y), and black (K), which are the four basic colors, and the density. A full-color image is formed by superimposing a total of six colors of light cyan (LC) having different hues and light magenta (LM) having a hue angle equal to that of magenta and light magenta (LM) having different densities.

  The four basic colors having a higher density than the light color have a resolution of 1200 dpi × 600 dpi (1 dot: 21.3 μm × 42.7 μm). On the other hand, the two light colors having lower densities than the four basic colors have a resolution of 2400 dpi × 1200 dpi (1 dot 10.7 μm × 21.3 μm) higher than the resolution of the four basic colors, and halftone expression is performed by pseudo gradation processing. It can be carried out.

  The host computer 2 generates and outputs image forming data for the image forming apparatus 1. The host computer 2 has standard components (for example, a CPU, a RAM, a ROM, a hard disk, an external storage device, a network interface, a display, a keyboard, a mouse, etc.) mounted on a general-purpose computer.

  Further, the host computer 2 and the image forming apparatus 1 are connected by a general-purpose communication interface such as a USB interface, an IEEE 1394 interface, a Centronics interface, a wireless LAN interface, a network interface, etc., and execute data transmission / reception between the apparatuses. can do.

  Next, the image processing unit 3 will be described.

  The image processing unit 3 includes a printer controller 31 and a laser driver 32. Here, the detailed configuration of the printer controller 31 will be described with reference to FIG.

  FIG. 2 is a diagram showing a detailed configuration of the printer controller according to the first embodiment of the present invention.

  The printer controller 31 mainly includes a CPU 321 that controls the printer 321 and a ROM 322 that stores a control program, and a DRAM 323 that temporarily stores various data.

  Hereinafter, image processing executed in the printer controller 31 will be described.

  First, the CPU 321 in the printer controller 31 receives an image (R, G, B multi-value image data) created by an arbitrary application from the host computer 2, and the brightness density conversion unit 351 performs brightness density conversion processing as follows. R, G, B multi-value image data is converted into C, M, Y image data.

  Next, the bitmap development unit 352 performs bitmap development processing on the bitmap area in the DRAM 323 using the C, M, and Y image data. Here, the bitmap-developed image data is stored in a form including density information in each pixel having a resolution of 600 dpi.

  For the density information of each color, K, LC, and LM are further extracted from the CMY image data, and the density information is distributed according to the capacity of each color.

  Specifically, for each pixel, C, M, Y, and K are assigned to density data of 5 bits and 32 gradations, and light cyan (LC) and light magenta (LM) are assigned to density data of 7 bits and 128 gradations. The distributed density data is subjected to pseudo gradation processing (binarization processing) using a dither matrix by the pseudo gradation processing unit 353.

  Specifically, for multi-value halftone area data of each color, C, M, Y, and K are 8 × 4 matrix shown in FIG. 3, and LC and LM are 16 × 8 matrix dither processing shown in FIG. And binarized. In this case, when recording A3 size data, the amount of buffer memory required for C, M, Y, and K is 2M bytes each, LC and LM are each 8M bytes, and the total of all colors is 24M bytes. It becomes.

  Here, if C, M, Y, K, LC, and LM all have the same resolution and gradation, it is necessary to make all colors 2400 dpi × 1200 dpi in order to achieve high picture quality. The amount of buffer memory is 8 Mbytes for each color, for a total of 48 Mbytes. This greatly increases the cost of the apparatus, and the processing capacity of the control unit (CPU 321) that handles data cannot keep up with the recording speed, thus reducing the throughput. This causes a problem.

  Therefore, in the first embodiment, the dither matrix used in the pseudo gradation processing is made different in size according to the type of color used for the recording material, thereby reducing the amount of data necessary for image formation. In addition, in the first embodiment, in order to realize sufficient gradation expression even in a low density region and to realize gradation expression necessary for high image quality, a recording material as described below is used. The idea is to select the density of the color to be used and the processing contents of the pseudo gradation processing.

  Returning to the description of FIG.

  The binarized image signal is sent to the laser driver 32 of the image forming unit 4. Thereby, the laser driver 32 blinks the semiconductor lasers in the respective color image optical systems 33C, 33M, 33Y, 33K, 33LC, and 33LM in accordance with the laser drive signal. Then, each of the color image optical systems 33C, 33M, 33Y, 33K, 33LC, and 33LM emits laser light onto the photosensitive drum of the image forming unit 4, and the image forming unit 4 forms a latent image, which is the first stage of the image forming operation. Is executed.

  Note that all or part of the luminance density conversion unit 351, the bitmap development unit 352, and the pseudo gradation processing unit 353 in FIG. 2 may be realized by a hardware circuit, or image processing executed by the CPU 321. It may be realized by software as a program.

  Next, detailed configurations of the color image optical systems 33C, 33M, 33Y, 33K, 33LC, and 33LM will be described with reference to FIG.

  FIG. 5 is a diagram showing a detailed configuration of the image optical system according to the first embodiment of the present invention.

  In FIG. 5, for example, the detailed configuration of the cyan image optical system 33C will be described, but the image optical systems of other colors also have the same configuration.

  The image optical system 34C includes a laser diode 341, a collimator lens 342, and a polygon scanner 343 and an imaging lens 344 that constitute a scanning image optical system. The spot diameter of the laser beam is determined by the characteristics of the laser chip and the scanning image optical system.

  The laser beam emitted from the laser diode 341 passes through the collimator lens 342 to become parallel light and enters the polygon scanner 343. The polygon scanner 343 is composed of six scanner surfaces and a motor (not shown) for driving the scanner surface. The polygon scanner 343 rotates in the direction of the arrow to scan the beam in the main scanning direction (arrow S). . The scanned beam is incident on an imaging lens 344 serving as an f-θ lens and a focusing lens, and forms an image on the photoreceptor 45C.

  The above is the structure of the cyan image optical system 34C. In the image forming apparatus according to the first embodiment, since the main configurations of the color image optical systems other than the laser spot diameter are the same, the image optical systems 34M, 34Y, 34K, 34LC, and 34LM of colors other than those for cyan are used. The description regarding is omitted.

  Here, a supplementary explanation of the above-described FIGS. 3 and 4 will be given.

  FIG. 3 shows the beam spot and dither matrix formed on the cyan (C), magenta (M), yellow (Y), and black (K) photoreceptor, and the beam scanning direction. In the figure, S is the beam scanning direction (or main scanning direction). Further, the beam spot formed on the cyan (C), magenta (M), yellow (Y), and black (K) photoconductor indicated by B1 in the drawing has a width of 30 μm in the main scanning direction × sub scanning direction. It is approximately oval with a width of 50 μm.

  The resolution of an image drawn by the beam spot B1 is 1200 dots / inch (hereinafter abbreviated as dpi) × 600 dpi, that is, a resolution of 21.3 μm × 42.7 μm per dot, and is a dither which is a basic unit of gradation conversion. The matrix is 8x4.

  On the other hand, FIG. 4 shows a beam spot and a dither matrix formed on a light cyan (LC) and light magenta (LM) photoreceptor, and a beam scanning direction. In the figure, S is the beam scanning direction (or main scanning direction). The beam spot formed on the light cyan (LC) and light magenta (LM) photoconductor indicated by B2 in the drawing is a road ellipse having a width of 15 μm in the main scanning direction and a width of 30 μm in the sub-scanning direction.

  The resolution of the image drawn by this beam spot B2 is 2400 dpi × 1200 dpi, that is, 1 dot 10.7 μm × 21.3 μm, and the dither matrix, which is the basic unit of gradation conversion, is 16 × 8.

  Next, a detailed configuration of the image forming unit 4 will be described with reference to FIG.

  FIG. 6 is a diagram illustrating a detailed configuration of the image forming unit according to the first exemplary embodiment of the present invention.

  The image forming unit 4 includes an image forming unit 41C that forms a cyan image, an image forming unit 41M that forms a magenta image, an image forming unit 41Y that forms a yellow image, and an image forming unit that forms a black image. There are six image forming units including a unit 41K, an image forming unit 41LC that forms a light cyan image, and an image forming unit 41LM that forms a light magenta image. They are arranged in a row at intervals.

  Each image forming unit will be described in detail.

  The laser light 41L from the cyan image optical system 33C is irradiated onto the photosensitive drum 44C in the image forming unit 41C to expose the surface of the photosensitive member, thereby forming an electrostatic latent image corresponding to the image information. Around the photosensitive drum 44C, a charging roller 45C, a developing device 46C, a primary transfer roller 47C, and a drum cleaner 48C are installed.

  Similarly, for the image optical systems 41M, 41Y, 41K, 41LC, and 41LM for colors other than cyan, the charging rollers 45M, 45Y, and 45L around the corresponding photosensitive drums 44M, 44Y, 44K, 44LC, and 44LM, respectively. 45K, 45LC, 45LM, developing devices 46M, 46Y, 46K, 46LC, 46LM, primary transfer rollers 47M, 47Y, 47K, 47LC, 47LM, drum cleaners 48M, 48Y, 48K, 48LC, 48LM are installed.

  The photosensitive drum 44C is rotationally driven in a direction indicated by an arrow by a driving device (not shown), and the charging roller 45C has a predetermined potential applied to the surface of the photosensitive drum 44C by a bias applied from a charging bias power source (not shown). Are uniformly charged. The developing device 46C contains a cyan developer (mixture of toner and carrier), and generates an electrostatic latent image formed on the photosensitive drum 44C by a developing bias applied from a developing bias power source (not shown). Develop with toner attached.

  A primary transfer roller 47C as a transfer unit transfers a toner image onto the intermediate transfer belt 57 by a transfer bias having a polarity opposite to that of toner applied from a transfer bias power source (not shown), and the above operation is performed by C and M. , Y, K, LC, and LM are sequentially performed to superimpose toner images of respective colors on the intermediate transfer belt 57 to form a full-color toner image. In this way, the toner image transfer from the photosensitive member to the intermediate transfer belt 57 is hereinafter referred to as primary transfer. In the primary transfer, transfer residual toner remaining on the photosensitive drum 44C is removed and collected by the drum cleaner 48C.

  Then, the full-color toner image on the intermediate transfer belt 57 is transferred to the transported transfer material (recording medium) P. In this way, the transfer from the intermediate transfer belt 57 to the transfer material P is hereinafter referred to as secondary transfer. In the secondary transfer, a transfer bias having a polarity opposite to that of the toner is applied from a bias power source (not shown) to the secondary transfer roller 53 and transferred in a batch.

  The intermediate transfer belt 57 is conveyed in a state where a constant tension is maintained by the intermediate transfer belt driving roller 49, the intermediate transfer belt stretching roller A (50), and the intermediate transfer belt stretching roller B (51). .

  Then, the transfer material P onto which the toner image has been transferred is conveyed to the fixing device 54, heated and pressed between the fixing roller 54a and the pressure roller 54b, and thermally fixed, and a series of image forming operations is completed. The residual toner remaining on the surface of the intermediate transfer belt 57 after the secondary transfer is removed by the belt cleaner 52 and collected.

  The above is the description of the overall image forming operation of the image forming unit 4.

  Next, details of a method of performing gradation expression using cyan toner will be described.

  FIG. 7 is a diagram showing changes in optical density values with respect to input data values (optical density values) when cyan toner is used for development in the dither matrix of FIG.

  In the image forming apparatus 1 of the first embodiment, the coloring power of the basic color cyan is set to the maximum optical density value 1.4. The optical density is measured by using, for example, a “Macbeth reflection densitometer” (X-RITE manufactured by Macbeth Co., Ltd.) to measure a relative density with respect to a printout image of a white background portion having a document density of 0.00.

  In the figure, the horizontal axis represents the dither matrix occupation density of the image data, and the vertical axis represents the optical density value. The dither matrix occupation density refers to how many dots of the electrostatic latent image on the drum are used when 32 dots are taken as one unit in the dither matrix of FIG. 3 (that is, 100% if all 32 dots are occupied). ).

  Here, the optical density value and the dither matrix occupation density should ideally keep the linear relationship of the straight line A. However, an ideal linear relationship cannot be obtained and a gamma curve C is drawn because of fluctuations in various conditions of the image forming apparatus 1 and the difficulty in developing isolated area dots.

  The relationship between the dither matrix occupation density (area) and the optical density value is stored as, for example, a gamma correction table in the pseudo gradation processing unit 353 in the printer controller 31. When halftone control is actually performed, gamma correction is performed on the gamma curve C with reference to the gamma correction table to correct the linear relationship shown in FIG. That is, when cyan toner is used, it is possible to express about 32 gradations from a density of 0 to 1.4.

  Similarly, such gamma correction is executed for the basic colors M, Y, and K other than cyan.

  Next, the gradation property obtained when gradation expression is performed using light cyan toner will be described.

  FIG. 9 shows the relationship between the dither matrix occupation density and the optical density value when a light cyan toner is used. FIG. 10 shows the relationship between the data value after gamma correction and the optical density difference.

  In FIG. 9, the horizontal axis represents the dither matrix occupation density of the image data, and the vertical axis represents the optical density value. In FIG. 10, the horizontal axis represents the data value after gamma correction, and the vertical axis represents the optical density value.

  Here, light cyan has a different coloring power from cyan, which is a basic color, and the density of the toner itself is light and is designed to reach a maximum density of 0.7 at an occupation density of 100%. Furthermore, since the dither matrix (16 × 8 matrix) of FIG. 4 is used, it is possible to express approximately 128 gradations from a density of 0 to 0.7. That is, in order to improve the image quality, it is possible to achieve a target of 100 gradations or more, particularly at an important density of 0 to 0.7.

  Furthermore, when it is desired to improve the gradation in the low density range (for example, 0 to 0.5), the toner coloring power is low, and it is desired to improve the gradation in the high density range (for example, 0 to 0.9). In some cases, the coloring power of the toner may be set high.

  In the first embodiment, the method of performing halftone control using cyan and light cyan has been described. However, the description regarding magenta and light magenta is completely the same in principle and method, and thus description thereof is omitted.

  In the image forming apparatus 1 according to the first exemplary embodiment, light cyan toner is used to express the low density portion of cyan. As a result, gradation expression with a density of 0.1 or less, which has been difficult to express in the past, can be significantly improved. The reason for this will be described below.

  As described above, in an electrophotographic image forming apparatus, an isolated pixel (dot) having a smaller area is more difficult to develop.

  FIG. 11 is an enlarged view of the low density portion (dither matrix occupation density of 7% or less) with respect to the change in the dither matrix occupation density and the optical density value of the image shown in FIG.

  In the figure, a straight line A is an ideal gamma curve, and a curve C in the figure is an actually obtained gamma curve. In the case of the image forming apparatus 1 of the first embodiment, it is not possible to develop an isolated dot having a dither matrix occupation density of 9% or less (shaded portion in the figure). In this case, the minimum expressible density is 0.06. Since the density jump of 0 to 0.06 is a level that can be identified by the user, the gradation expression must be further improved.

  Next, FIG. 12 shows a low density portion (dither matrix occupancy density of 7% or less) with respect to the change in the dither matrix occupancy density when a toner having low coloring power is used, and FIG. Show.

  Here, the developability is not related to the toner coloring power, and an isolated dot having a dither matrix occupation density of 5% or less cannot be developed. In this case, the minimum density that can be expressed is 0.01, but the density jump of 0 to 0.01 cannot be identified by the user. Smooth gradation expression is possible even in a low density region of 0.1 or less.

  FIG. 14 shows the final gradation expressing power obtained by using cyan toner and light cyan toner. The image forming apparatus according to the first embodiment suppresses the density jump from the density 0 to the lowest expressible density to 0.01, and further realizes about 128 gradations in the low density area of 0.7 or less. The same configuration and control is performed for magenta.

  More generally, among recording materials having substantially the same hue angle (similar colors), the highest density that can be colored by a recording material having a low density is the highest density that can be colored by a recording material having a high density. On the other hand, it is preferably less than half.

  In FIG. 14, in order to simplify the explanation, in the low density portion, a certain optical density is designed to be expressed using either cyan or light cyan. However, the present invention is not limited to this. Rather, for each desired optical density value, a ratio of using cyan and light cyan is determined in advance, and cyan and light cyan are mixed according to the ratio to obtain various desired optical density values. It can also be expressed.

  In the first embodiment, in order to efficiently improve the image quality at a low cost, the gradation is improved most with respect to magenta and cyan which are most frequently used in the skin color and natural color of a photographic image and require high gradation. I am trying. Other methods are not limited to the configuration of the first embodiment, and it is also effective to use gray or yellow light color to improve the gradation of black.

  As described above, in the first exemplary embodiment, at approximately two or more hue angles, the ratio of the area between the colored portion and the non-colored portion that has at least two toners with the same amount of adhesion and different concentrations and is colored by the high concentration toner. The number of adjustment levels of the area ratio between the colored portion and the non-colored portion that are colored by toner having a low density is increased.

  In other words, among the recording materials having the same hue and different densities, the second gradation number of the image formed with the recording material having a low density is larger than the first gradation number of the image formed with the recording material having a high density. It is configured to take.

  Note that this configuration is not limited to an electrophotographic color image forming apparatus, but gradation conversion is performed using pseudo gradation processing on an original image having a halftone, such as an ink jet printer or a thermal transfer printer, to generate a gradation. This is effective in a color image forming apparatus that expresses using a pixel of the basic unit of conversion n × m and expresses halftone density by the occupation ratio of the colored portion area in the pixel.

  As described above, according to the first embodiment, the gradation in the low density range that is sensitive to the human eye while minimizing the necessary buffer memory, preventing an increase in calculation amount and an increase in cost. Sexually can be enhanced effectively.

  Also, 100 gradations or more can be achieved in the target density range of 0 to 0.7. Further, the density jump on the low density side caused by the developability can be suppressed below the visual recognition limit.

<Embodiment 2>
In the second embodiment, a method of increasing the number of components by taking a large numerical value (number of basic pixels) of the basic unit n × m for performing gradation conversion in a low density toner will be described.

  Specifically, the dither matrix method is used for area gradation processing, and the value of n × m of the dither matrix used for the low density toner is set larger than the value of n × m of the dither matrix used for the high density toner. .

  Note that the main configuration and image forming operation of the image forming apparatus that realizes the second embodiment are the same as those of the first embodiment, and a detailed description thereof will be omitted.

  Hereinafter, only the characteristic configuration of the second embodiment will be described with reference to the drawings.

  In the second embodiment, the laser spot diameter irradiated onto the photoconductors 44C, 44M, 44Y, 44K, 44LC, and 44LM by the image optical systems 34C, 34M, 34Y, 34K, 34LC, and 34LM is all the width in the main scanning direction. 30 μm × width in the sub-scanning direction is 50 μm, and has the same resolution as 1200 dpi × 600 dpi.

  By adopting this configuration, the parts of the image optical system can be shared, and the cost can be reduced compared to the configuration of the first embodiment.

  In the second embodiment, all of the image optical systems have the same configuration and resolution, but the dither matrix has a different configuration depending on the type of color used as the recording material.

  Hereinafter, the configuration of the dither matrix for each color will be described.

  FIG. 15 shows beam spots and dither matrices formed on cyan (C), magenta (M), yellow (Y), and black (K) photoreceptors, and beam scanning directions. In the figure, S is the beam scanning direction (or main scanning direction). The beam spot formed on the cyan (C), magenta (M), yellow (Y), and black (K) photoconductor indicated by B3 in the figure has a width of 30 μm in the main scanning direction × width of 50 μm in the sub-scanning direction. It is a substantially oval shape.

  The resolution of the image drawn by the beam spot B3 is 1200 dpi × 600 dpi, that is, the resolution of 21.3 μm × 42.7 μm per dot, and the dither matrix that is the basic unit of gradation conversion is 8 × 4.

  Further, FIG. 16 shows a beam spot and dither matrix formed on a light cyan (LC) and light magenta (LM) photoconductor, and a beam scanning direction. In the figure, S is the beam scanning direction (or main scanning direction). The beam spot imaged on the light cyan (LC) and light magenta (LM) photoconductor indicated by B4 in the figure has a substantially elliptical shape with a width of 30 μm in the main scanning direction and a width of 50 μm in the sub-scanning direction.

  The resolution of the image drawn by the beam spot B4 is 1200 dpi × 600 dpi, that is, a resolution of 21.3 μm × 42.7 μm per dot, and the dither matrix, which is the basic unit of gradation conversion, is 16 × 8.

  Therefore, the number of adjustment levels from the lowest density to the highest density is 32 for cyan, magenta, yellow and black, and 128 for light cyan (LC) and light magenta (LM).

  As described above, in the image forming apparatus according to the second embodiment, in the image forming apparatus that performs halftone expression using the pseudo gradation processing method, the numerical value of the basic unit n × m that performs gradation conversion with a recording material having high density is obtained. On the other hand, since the numerical value of the basic unit n × m for performing gradation conversion with a recording material having a low density is large, recording with a low density is performed with respect to the number of adjustment levels from the minimum density to the maximum density that can be expressed by a recording material with a high density. The number of adjustment levels from the lowest density to the highest density that can be expressed by the material is increased.

  As described above, according to the second embodiment, by sharing the components of the image optical system of each color used as a recording material, while minimizing the cost increase of the unit that realizes the image optical system, An effect equivalent to that of the first embodiment can be obtained.

<Embodiment 3>
In the third embodiment, a method of increasing the number of adjustment levels of basic pixels colored by a recording material having a low density with respect to the number of adjustment levels of basic pixels colored by a high-density recording material will be described.

  Specifically, an electrostatic latent image is formed by scanning and exposing a light beam to a photoreceptor, represented by an electrophotographic image forming apparatus, and toner is attached to the electrostatic latent image. In the image forming apparatus visualized in this manner, one dot is divided in the main scanning direction using a pulse width modulation method.

  In this method, which is different from the first and second embodiments, the adjustment level of the area ratio between the colored portion and the non-colored portion can be increased by electrical control of the laser. Increasing the number of basic pixel adjustment levels is almost equivalent to improving the resolution in the main scanning direction.

  Note that the main configuration and image forming operation of the image forming apparatus that realizes the third embodiment are the same as those of the first embodiment, and a detailed description thereof will be omitted.

  Hereinafter, only the configuration characteristic of the third embodiment of the present invention will be described with reference to the drawings.

  In the third embodiment, the laser spot diameters irradiated on the photoconductors 44C, 44M, 44Y, and 44K by the semiconductor lasers 34C, 34M, 34Y, and 34K are all 30 × 50 μm in diameter, and the resolution is 1200 dpi × 600 dpi.

  On the other hand, the semiconductor lasers 34LC and 34LM are different in the number of adjustment levels of the laser emission time, and have four times the adjustment level compared to 34C, 34M, 34Y, and 34K. That is, the basic pixel can be further divided into four stages. As a result, the resolution in the main scanning direction is increased, the diameter of the laser spot irradiated onto the photoconductors 44LC and 44LM is 8 × 50 μm, and the resolution is 4800 dpi × 600 dpi.

  By adopting this configuration, the components of the image optical system can be shared, and the basic pixels for LC and LM can be adjusted in four stages in the main scanning direction. As compared with the configuration of the second embodiment, a high-definition and high-gradation image can be obtained.

  In the third embodiment, all the image optical systems have the same configuration, but the dither matrix has a different configuration according to the type of color used as the recording material.

  Hereinafter, the configuration of the dither matrix for each color will be described.

  FIG. 17 shows the beam spot and dither matrix formed on the cyan (C), magenta (M), yellow (Y), and black (K) photoreceptor, and the beam scanning direction. In the figure, S is the beam scanning direction (or main scanning direction). The beam spot formed on the cyan (C), magenta (M), yellow (Y), and black (K) photoconductor indicated by B5 in the figure has a width of 30 μm in the main scanning direction × width of 50 μm in the sub-scanning direction. It is a substantially oval shape.

  The resolution of an image drawn by the beam spot B5 is 1200 dpi × 600 dpi, that is, a resolution of 21.3 μm × 42.7 μm per dot, the number of adjustment levels of basic pixels is 1, and a dither matrix that is a basic unit of gradation conversion Is 8 × 4.

  Further, FIG. 18 shows a beam spot and dither matrix formed on a light cyan (LC) and light magenta (LM) photoconductor, and a beam scanning direction. In the figure, S is the beam scanning direction (or main scanning direction). The beam spot imaged on the light cyan (LC) and light magenta (LM) photoconductor indicated by B6 in the figure is a substantially oval shape having a width of 8 μm in the main scanning direction and a width of 50 μm in the sub-scanning direction. Scanned in the middle S direction.

  The resolution is 4800 dpi × 600 dpi, that is, a resolution of 5.3 μm × 42.7 μm per dot, and the dither matrix that is a basic unit of gradation conversion is 32 × 4. That is, the number of adjustment levels of the basic pixel is increased to 4 compared to cyan, magenta, yellow, and black.

  Accordingly, the number of adjustment levels from the lowest density to the highest density is 32 for cyan, magenta, yellow and black, 128 for light cyan (LC), and 128 for light magenta (LM).

  As described above, in the image forming apparatus according to the third embodiment, the density of light cyan (LC) and light magenta (LM) is changed by changing the number of emission time adjustment levels of the semiconductor laser while sharing the image optical system components. The number of adjustment levels is increased.

  In other words, in an image forming apparatus that forms an electrostatic latent image by scanning and exposing a light beam to a photoconductor, and visualizes it by attaching toner to the electrostatic latent image, coloring is performed by a recording material having a high density. The number of adjustment levels from the lowest density to the highest density that can be expressed by high-density toner by increasing the number of adjustment levels of the basic pixels that are colored by the recording material with low density compared to the number of adjustment levels of the basic pixels The number of adjustment levels from the lowest density to the highest density that can be expressed by a low density toner is increased.

  As described above, according to the third embodiment, the components of the image optical system are shared, and the cost of the unit that realizes the image optical system is minimized, and the cost is lower than those of the first and second embodiments. The same or better effect can be obtained.

  As another method, in a configuration in which the number of adjustment levels is given to the basic pixel, the basic unit of gradation conversion can be set to 1 × 1. In this case, the number of halftone density levels can be increased only by adjusting the laser emission time, which is further advantageous in terms of cost.

  Further, by using a combination of the configurations of Embodiments 1 to 3, higher effects can be obtained. Moreover, you may implement | achieve the structure which combined the structure of Embodiment 1-3 arbitrarily according to a use or the objective.

  Finally, an outline of processing executed in each embodiment in the image forming apparatus 1 of the present invention will be described with reference to FIG.

  FIG. 19 is a flowchart showing an outline of processing executed in each embodiment of the image forming apparatus of the present invention.

  First, in step S <b> 101, the image forming apparatus 1 inputs RGB multivalued image data from the host computer 2.

  In step S102, the luminance density conversion unit 351 converts RGB multivalued image data into CMY image data.

  In step S103, the bitmap developing unit 352 further extracts K, LC, and LM from the CMY image data, and distributes the density information according to the capacity of each color. Thereby, C, M, Y and K, LC and LM concentration data are generated.

  These density data are generated by a lookup table (LUT) prepared in advance in accordance with the processing contents executed in each embodiment.

  For example, in the first embodiment, as described above, a look-up table for distributing C, M, Y, and K into density data of 5 bits and 32 gradations and LC and LM as 7 bits and 128 gradations is used for CMY image data. It will be.

  In step S104, the pseudo gradation processing unit 353 uses the first matrix (for example, FIG. 3 in the first embodiment) for the C, M, Y, and K density data to perform pseudo gradation (binarization). ) Execute the process. That is, the C, M, Y, and K density data are converted into the first gradation data having the first gradation number.

  In step S105, the pseudo gradation processing unit 353 performs pseudo gradation (binarization) processing on the LC and LM density data using the second matrix (for example, FIG. 4 in the first embodiment). To do. That is, the LC and LM density data are converted into second gradation data having a second gradation number larger than the first gradation number.

  For convenience, step S104 and step S105 are shown as separate steps, but it goes without saying that step S104 and step S105 may be executed simultaneously by the pseudo gradation processing unit 353.

  In step S106, the gradation data (binarized data) generated by the pseudo gradation processing unit 353 is output to the image forming unit 4. As a result, an image is formed according to the binarized data obtained by the processing described in the above embodiments.

  As described above, according to the present invention, it is possible to drastically improve the gradation expressing power in the low density range that is sensitive to the human eye while preventing an increase in calculation amount and cost increase. Smooth gradation expression comparable to silver halide photography can be achieved.

  Although the embodiments have been described in detail above, the present invention can take an embodiment as, for example, a system, an apparatus, a method, a program, or a storage medium, and specifically includes a plurality of devices. The present invention may be applied to a system that is configured, or may be applied to an apparatus that includes a single device.

  In the present invention, a software program (in the embodiment, a program corresponding to the flowchart shown in the figure) that realizes the functions of the above-described embodiment is directly or remotely supplied to the system or apparatus, and the computer of the system or apparatus Is also achieved by reading and executing the supplied program code.

  Accordingly, since the functions of the present invention are implemented by computer, the program code installed in the computer also implements the present invention. In other words, the present invention includes a computer program itself for realizing the functional processing of the present invention.

  In that case, as long as it has the function of a program, it may be in the form of object code, a program executed by an interpreter, script data supplied to the OS, or the like.

  As a recording medium for supplying the program, for example, floppy (registered trademark) disk, hard disk, optical disk, magneto-optical disk, MO, CD-ROM, CD-R, CD-RW, magnetic tape, nonvolatile memory card ROM, DVD (DVD-ROM, DVD-R) and the like.

  As another program supply method, a client computer browser is used to connect to an Internet homepage, and the computer program of the present invention itself or a compressed file including an automatic installation function is downloaded from the homepage to a recording medium such as a hard disk. Can also be supplied. It can also be realized by dividing the program code constituting the program of the present invention into a plurality of files and downloading each file from a different homepage. That is, a WWW server that allows a plurality of users to download a program file for realizing the functional processing of the present invention on a computer is also included in the present invention.

  In addition, the program of the present invention is encrypted, stored in a storage medium such as a CD-ROM, distributed to users, and key information for decryption is downloaded from a homepage via the Internet to users who have cleared predetermined conditions. It is also possible to execute the encrypted program by using the key information and install the program on a computer.

  In addition to the functions of the above-described embodiments being realized by the computer executing the read program, the OS running on the computer based on the instruction of the program is a part of the actual processing. Alternatively, the functions of the above-described embodiment can be realized by performing all of them and performing the processing.

  Furthermore, after the program read from the recording medium is written in a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion board or The CPU or the like provided in the function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing.

1 is a schematic configuration diagram illustrating an image forming apparatus according to a first exemplary embodiment of the present invention. FIG. 2 is a diagram illustrating a detailed configuration of a printer controller according to the first embodiment of the present invention. It is a figure which shows an example of the 8x4 dither matrix used for the gradation expression of cyan of Embodiment 1 of this invention. It is a figure which shows an example of the 16 * 8 dither matrix used for the gradation expression of the light cyan of Embodiment 1 of this invention. It is a figure which shows the detailed structure of the image optical system of Embodiment 1 of this invention. It is a figure which shows the detailed structure of the image formation part of Embodiment 1 of this invention. FIG. 4 is a diagram illustrating a change in optical density value with respect to a dither matrix occupation density when cyan toner and the dither matrix of FIG. 3 are used in Embodiment 1 of the present invention. FIG. 4 is a diagram illustrating a gamma curve after tone correction when cyan toner and the dither matrix of FIG. 3 are used in Embodiment 1 of the present invention. FIG. 5 is a diagram illustrating a change in optical density value with respect to a dither matrix occupation density when a light cyan toner and the dither matrix of FIG. 4 are used in Embodiment 1 of the present invention. FIG. 5 is a diagram illustrating a gamma curve after tone correction in a case where a light cyan toner and the dither matrix of FIG. 4 are used in Embodiment 1 of the present invention. FIG. 5 is a diagram showing a change in optical density value with respect to the dither matrix occupation density (enlarged view of a low density portion) when cyan toner and the dither matrix of FIG. 3 are used in Embodiment 1 of the present invention. FIG. 5 is a diagram showing a change in optical density value with respect to the dither matrix occupation density (enlarged view of a low density portion) when a light cyan toner and the dither matrix of FIG. 4 are used in Embodiment 1 of the present invention. FIG. 6 is a diagram illustrating a gamma curve (enlarged view of a low density portion) after tone correction when using light cyan toner and the dither matrix of FIG. 4 in Embodiment 1 of the present invention. It is a figure which shows the gamma curve after the gradation correction | amendment of the image forming apparatus in Embodiment 1 of this invention. It is a figure which shows an example of the 8x4 dither matrix used for the gradation expression of cyan of Embodiment 2 of this invention. It is a figure which shows an example of the 16x8 dither matrix used for the light cyan gradation expression of Embodiment 2 of this invention. It is a figure which shows an example of the 8x4 dither matrix used for the gradation expression of cyan of Embodiment 3 of this invention. It is a figure which shows an example of the 32 * 4 dither matrix used for the gradation expression of the light cyan of Embodiment 3 of this invention. 5 is a flowchart illustrating an outline of processing executed in each embodiment of the image forming apparatus of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Host computer 3 Image processing part 4 Image forming part 31 Printer controller 32 Laser driver 33C, 33M, 33Y, 33K, 33LC, 33LM Image optical system of each color 331 Laser diode 332 Collimator lens 333 Polygon scanner 334 Imaging lens 41C, 41M, 41Y, 41K, 41LC, 41LM Each color image forming unit 44C, 44M, 44Y, 44K, 44LC, 44LM Each color photosensitive drum 45C, 45M, 45Y, 45K, 45LC, 45LM Each color charging roller 46C, 46M, 46Y, 46K, 46LC, 46LM Each color developer 47C, 47M, 47Y, 47K, 47LC, 47LM Each color primary transfer roller 48C, 48M, 48Y, 48K, 48LC, 48LM Each color drum Over Na 49 intermediate transfer belt drive roller 50 the intermediate transfer belt tension roller A
51 Intermediate transfer belt stretcher roller B
52 Cleaner 53 Secondary Transfer Roller 54 Fixing Device 54a Fixing Roller 54b Pressure Roller 57 Intermediate Transfer Belt 321 CPU
322 ROM
323 DRAM
P transfer material

Claims (9)

A color image forming apparatus for forming a color image using a recording material of a plurality of colors,
An input means for inputting a color image;
From the color image input by the input means, first image data for forming an image with a recording material with a high density among recording materials with the same hue and different density, and an image with a recording material with a low density are formed. Generating means for generating second image data for
First gradation conversion means for converting the first image data into first gradation data having a first gradation number;
Second gradation conversion means for converting the second image data into second gradation data having a second gradation number larger than the first gradation number;
Forming means for forming an image on a recording medium using a recording material corresponding to each of the first and second gradation data obtained by the first and second gradation converting means. A color image forming apparatus. The forming means includes a first forming means for forming an image using a recording material having a high density among the recording materials having the same hue and different densities, and a second forming means for forming an image using a recording material having a low density. The color image forming apparatus according to claim 1, further comprising: The color image forming apparatus according to claim 1, wherein the second resolution of the image formed by the second forming unit is higher than the first resolution of the image formed by the first forming unit. The color image forming apparatus according to claim 1, wherein the first resolution of the image formed by the first forming unit and the second resolution of the image formed by the second forming unit are the same. A second basic pixel that expresses a basic unit that performs gradation conversion by the second gradation conversion unit, with respect to the number of first basic pixels that express a basic unit that performs gradation conversion by the first gradation conversion unit. The color image forming apparatus according to claim 1, wherein the number of components is large. The forming unit forms an electrostatic latent image by exposing the photosensitive member with a light beam having a variable exposure amount, and forms an image by attaching a recording material to the electrostatic latent image. Means,
The exposure amount of an image formed using a recording material having a low density with respect to the first variable level number of the exposure amount for the image formed using a recording material having a high density among the recording materials having the same hue and different densities. The color image forming apparatus according to claim 1, wherein the number of second variable levels is large. The maximum density that can be recorded by a recording material having a low density among the recording materials having the same hue and different density is less than half of the maximum density that can be recorded by a recording material having a high density. The color image forming apparatus according to any one of 1 to 4. A control method of a color image forming apparatus for forming a color image using a recording material of a plurality of colors,
An input process for inputting a color image;
From the color image input in the input step, first image data for forming an image with a recording material having a high density among recording materials having the same hue and different densities and an image with a recording material having a low density are formed. Generating a second image data for
A first gradation conversion step of converting the first image data into first gradation data having a first gradation number;
A second gradation conversion step of converting the second image data into second gradation data having a second gradation number greater than the first gradation number;
A forming step of forming an image on a recording medium using recording materials corresponding to the first and second gradation data obtained in the first and second gradation conversion steps, respectively. A method for controlling a color image forming apparatus. A program that realizes control of a color image forming apparatus that forms a color image using recording materials of a plurality of colors,
A program code of an input process for inputting a color image;
From the color image input in the input step, first image data for forming an image with a recording material having a high density among recording materials having the same hue and different densities and an image with a recording material having a low density are formed. A program code of a generation process for generating second image data for
A program code of a first gradation conversion step for converting the first image data into first gradation data of a first gradation number;
A program code of a second gradation conversion step for converting the second image data into second gradation data having a second gradation number larger than the first gradation number;
And a program code of a forming process for forming an image on a recording medium using recording materials corresponding to the first and second gradation data obtained in the first and second gradation conversion processes, respectively. A program characterized by that.

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