JP4023095B2 - Electrophotographic apparatus and image processing program - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrophotographic apparatus, an image processing method thereof, and an image processing program, and more particularly to an electrophotographic apparatus capable of improving image quality by smoothing a change in gradation of a low density image, and an image processing method thereof. The present invention relates to an image processing program.
[0002]
[Prior art]
A color electrophotographic apparatus widely used in a color printer, a color copy, or the like, uses a latent image formed by exposing a photoconductor as cyan (C), magenta (M), yellow (Y), and black (K). Then, the toner image is transferred onto a support such as paper, and a color image is reproduced as a final image. A laser beam printer uses a laser beam to form a latent image on a photoconductor, and for each pixel region arranged along the main scanning direction in which the laser beam is scanned and the sub-scanning direction in which the support is sent, A latent image is formed by irradiating a laser beam. Among them, the type that modulates the width of the pulse that drives the laser beam can change the laser beam irradiation area in the pixel area, and even if the number of pixels per unit area is small, A color image with high resolution and higher gradation can be reproduced.
[0003]
In such a pulse width modulation type laser beam printer, there is a halftone method of halftone using a multi-level dithering method as a method of gradation reproduction of grayscale images. According to this multi-value dither method, a conversion table called a look-up table in which image reproduction information for determining a virtual dot size and the like is described for gradation data for each color included in an input signal is referred to Then, the size (including the position may be included) of the virtual dot in each pixel area is determined. By setting a plurality of levels between 0 and the maximum size as the size of this virtual dot, the output at each pixel is “multi-valued”.
[0004]
The “virtual dot” here is defined by the area scanned by the laser beam driven within the pixel area, and the size in the main scanning direction is the time during which the laser beam is driven and the scanning speed of the beam. The size in the sub-scanning direction is equal to the size of the pixel area in the sub-scanning direction. For the following reason, the virtual dot has a different shape from the “dot image” on the final image. A laser beam is driven in the virtual dot of each pixel area, and an irradiation area of the laser beam is formed on the photosensitive member. This irradiation region has a shape extending around the virtual dot because of the size of the laser beam and the rising and falling characteristics during driving. The irradiation area of the laser beam becomes a latent image area on the photoconductor, which is developed with toner, transferred onto a support such as paper, and forms a dot image on the final image. Even during this development process, the shape of the dot image is further changed from the virtual dot because of toner scattering. In this way, the dot image is changed from the virtual dot, but since this change depends on the electrophotographic process, the final dot image is controlled by controlling the virtual dot driven by the laser beam. be able to.
[0005]
FIG. 1 is a diagram showing a developing process of a general electrophotographic process. By irradiating the virtual dot area 2 on the surface of the photosensitive drum 1 having a negative voltage (−1000 V) with a laser beam, a 0 V latent image area 3 is formed. By supplying toner 5 charged to a negative voltage (−500 V) on the developing drum 4 to the photosensitive drum on which the latent image is formed, the latent image region 3 of 0 V is charged to a negative voltage. Toner 5 adheres. The adhered toner is then transferred to a support such as a printing paper to form a dot image.
[0006]
With halftone halftone method, a dot image in a single pixel, or a dot dot consisting of a cluster of dot images over a plurality of adjacent pixels, is formed, and the gradation of the grayscale image is reproduced according to the size of the halftone dot. . In other words, as the grayscale value of each pixel becomes darker, a virtual dot is generated, a growth nucleus of a halftone dot on the final image is generated, and when the grayscale value of the grayscale data becomes darker, the virtual dot is generated. As the number and area increase, the size of the halftone dots gradually increases. Therefore, the method of growing halftone dots corresponding to the increase in the gray value of the input gradation data is such that the area of the virtual dot grows rapidly in the pixel at the center of the halftone dot (near the growth nucleus), and the pixels around the halftone dot. In the (pixel away from the growth nucleus), the growth of the virtual dot area is slow.
[0007]
FIG. 2 is a diagram illustrating an example of halftone dot growth. The example of the halftone dot 10 in FIG. 2 is a cross-shaped shape composed of five pixels PX1 to PX5. The central pixel PX1 serves as a growth nucleus that grows first when the gradation of the gradation data increases. . That is, as shown in FIG. 2 (2), when the density increases, the size of the virtual dots 12 in the center pixel PX1 increases sequentially, and when the density further increases, the pixel PX2 as shown in FIG. 2 (3). The size of the inner virtual dot 12 is increased, and thereafter, the size of the inner virtual dot is increased in the order of the pixels PX3, 4, 5 as shown in FIGS. 2 (4) (5) (6). Finally, virtual dots increase in all pixels, and the halftone dot size is maximized.
[0008]
Thus, a plurality of look-up tables are used to vary the area growth characteristics of virtual dots among a plurality of pixels constituting one halftone dot. In the example of FIG. 2, a lookup table is assigned to each of the pixels PX1 to PX5. Then, a two-dimensional array having these lookup tables as elements of the array is constructed, and this two-dimensional array is repeatedly applied to the input image data while shifting the position so that tiles are laid out. Play with halftone dots. This two-dimensional array is referred to herein as a lookup table matrix (LUT matrix).
[0009]
In an electrophotographic apparatus, a latent image is formed by irradiating a virtual dot area in a pixel with a laser beam, and a dot image is formed by attaching toner to the latent image area. Then, a halftone dot composed of a dot image of a plurality of pixels is grown. In order to increase the resolution of the dot image, which is the final image, it is desirable to increase the number of screen lines connecting halftone dots as much as possible.
[0010]
However, it has been proposed to reduce the number of screen lines by keeping the density of growing dots low in order to stabilize the output characteristics of the electrophotographic process in the region where the gray level is low. This is because if the number of halftone dots to grow is large, the virtual dot area in each growth nucleus becomes too small, dot missing occurs, and the gray level may not be reproduced properly in the final image. When the virtual dot area is very small, whether or not a dot image is formed varies depending on the use environment of the engine in the apparatus. Due to such fluctuation, the above-mentioned dot dropout occurs. On the other hand, in the region where the lightness is comparatively dark, all halftone dots are grown at the same time, and the number of screen lines is increased to realize a high-resolution and sharp final image.
[0011]
[Problems to be solved by the invention]
However, the number of screen lines is reduced in the above-mentioned region where the shading is low, and halftone dots are grown. As the shading increases, the number of screen lines is increased, that is, the number of growth nuclei of the halftone dots is increased. It has been found that when the method of growing is used, the intensity of the final image due to the finally developed dot image does not increase appropriately. That is, the present inventor has found that the gradation of the final image after development increases in a stepped manner corresponding to the increase in the gradation of the input image data, and that the phenomenon of reversal of the gradation occurs in the worst case. It was.
[0012]
According to a detailed study by the inventor, the intensity of the final image is unnaturally increased or decreased at the boundary point where the number of screen lines increases or decreases as the intensity of the input data increases or decreases. I found something.
[0013]
SUMMARY OF THE INVENTION An object of the present invention is to provide an electrophotographic apparatus in which the density of a final image after development is normally increased or decreased with respect to the increase or decrease of the density of input image data, and an image processing program thereof.
[0014]
Furthermore, another object of the present invention is to form a final image with a stable gray level in an image with a low gray level, and to form a sharp final image with a high resolution in an image with a high gray level. An electrophotographic apparatus and an image processing program for the same are provided.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, according to a first aspect of the present invention, there is provided an image reproduction engine that forms a dot image by attaching toner to a virtual dot region in a pixel. In an electrophotographic apparatus that reproduces an image by expressing the shade by the formed halftone dots,
In the first area where the density of the input image data is low, the first group of dots is grown in accordance with the increase in the intensity, and in the second area where the intensity is higher than the first area, An image processing apparatus for generating image reproduction data for growing halftone dots of a second group different from the first group,
Further, in the first area, image processing for generating image reproduction data for simultaneously generating a minute virtual dot area that does not form a dot image by the toner in a halftone area other than the first group of halftone dots. It has the apparatus.
[0016]
The minute virtual dot area to the extent that the dot image is not formed has the following action. In the electrophotographic apparatus, an edge effect exists between the latent image area formed on the photosensitive drum and the toner density adhering thereto. As shown in FIG. 1, when the photosensitive drum on which the latent image is formed and the developing roller that supplies the charged toner are not in contact with each other, the edge effect is caused by excessive toner at the peripheral portion of the latent image region 3. It is a phenomenon that adheres. This seems to be caused by excessive supply of charged toner. Furthermore, due to the edge effect, even if the latent image region 3 is very small and its potential is not sufficiently increased, a sufficient toner concentration may adhere due to excessive supply of toner.
[0017]
However, this edge effect appears prominently when the density of virtual dots is low. When the density of virtual dots increases, the excessive supply state of toner is alleviated and the edge effect is reduced.
[0018]
Therefore, in the first area where the density of the input image data is low, the first group of dots is grown in accordance with the increase in the density, and in the second area where the intensity is higher than the first area. In the case of growing the second group of halftone dots different from the first group of halftone dots, the virtual dot density is low and the edge effect is large in the first region, and the latent image region corresponding to the virtual dot region is sufficient. However, in the second area, the virtual dot density is increased and the edge effect is reduced, and sufficient toner does not adhere to the latent image area. As a result, at the boundary point between the first area and the second area, the increase or decrease in the intensity of the final image made up of dot images has a stepped or worst reversal with respect to the increase or decrease in the intensity of the input image data. To do.
[0019]
Therefore, in the first region, a small virtual dot region is generated at the same time in a halftone dot region other than the originally grown first group of halftone dots so that a dot image is not formed by the toner. As a result, in the first region where the density of the input image data is low, the density of the virtual dots is higher than the density of the first group of dots that originally grow, and the edge effect in the first region is alleviated. The Therefore, at the boundary point between the first area and the second area, the change in the density of the virtual dots can be suppressed to a small extent, and the change in the edge effect can be reduced. It is prevented from becoming unnatural. That is, according to the present invention, the increase / decrease in the density of the final image becomes smoother than the increase / decrease in the intensity of the input image data.
[0020]
In a preferred embodiment of the first aspect of the invention, the halftone dot region other than the first group of halftone dots is the second group halftone dot region. Therefore, in this embodiment, in the first area of the gray level, the virtual dot area increases in accordance with the increase of the gray level in the first group of halftone dots, and minute virtual dots are generated in the second group of halftone dots. Therefore, there is no change in the density of the virtual dot region at the boundary between the first region and the second region, and there is no change in the edge effect before and after the boundary. As a result, the increase / decrease in the intensity of the final image is reproduced smoothly.
[0021]
Furthermore, in another preferred embodiment, the image processing unit has a third group of nets different from the first group and the second group of halftone dots in a third region having a higher shade than the second region. Generating image reproduction data for growing dots, and a halftone dot region other than the first group halftone dot in the first region is a region of the second group and third group halftone dots; The image processing unit is characterized in that, in the second region, the image reproduction data for simultaneously generating the minute virtual dot region in the third group of dot regions is generated.
[0022]
In this embodiment, in the first area, the virtual dot area is increased in accordance with the increase in shading in the first group of halftone dots, and the minute virtual dot area is generated in the second group and the third group of halftone dots. Furthermore, in the second region, a virtual dot region that has grown is generated at the first group of halftone dots, a virtual dot region that is grown at the second group of halftone dots, and a minute virtual dot region is generated at the halftone dot of the third group. Therefore, the density of the virtual dot areas in the first, second and third areas is kept the same.
[0023]
Furthermore, in another preferred embodiment, the image processing unit has a third group of nets different from the first group and the second group of halftone dots in a third region having a higher shade than the second region. Generating image reproduction data for growing dots, and a halftone dot area other than the first group halftone dot in the first area is the second group or third group halftone dot area; In the second area, the image processing unit generates the image reproduction data for simultaneously generating the minute virtual dot area in the third group of dot areas.
[0024]
In this embodiment, in the first area, the virtual dot area is increased at the first group of halftone dots, the minute virtual dot area is generated at the second group or the third group of halftone dots, and in the second area, In the first group halftone dot, a grown virtual dot area is generated, in the second group halftone dot, a grown virtual dot area is generated, and in the third group halftone dot, a minute virtual dot area is generated. Variations in the edge effect between the second regions are suppressed to a small extent.
[0025]
According to the second aspect of the present invention, there is provided an image reproduction engine for forming a dot image by attaching toner to a virtual dot region in a pixel, and the shading is performed by halftone dots formed by the dot images in a plurality of pixels. In an electrophotographic apparatus that reproduces an image by expressing a degree, it has an image processing unit that converts the gray level of input image data into image reproduction data of the virtual dot area,
The image processing unit grows halftone dots of the first density in accordance with the increase in the density in the first area where the density of the input image data is low, and the intensity is higher than that in the first area. In the high second region, image reproduction data for growing a halftone dot having a second density higher than the first density is generated.
Further, in the first area, the image processing unit simultaneously generates a minute virtual dot area that does not form a dot image with the toner in a halftone dot area other than the growing halftone dot. Is generated.
[0026]
According to the second aspect described above, in the first region with low shading, the density obtained by adding the minute virtual dot region to the growing virtual dot region is at least higher than the first density (or the second density). Therefore, the density difference between the virtual dot areas in the first area and the second area is small (or equivalent), and the change in the edge effect between them is small.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. However, this embodiment does not limit the technical scope of the present invention.
[0028]
FIG. 3 is a diagram showing the configuration of the electrophotographic apparatus according to this embodiment. In this example, the host computer 50 generates image data 56 composed of RGB gradation data (for example, 8 bits each for a total of 24 bits), and provides the image data 56 to an electrophotographic apparatus 60 such as a page printer. The electrophotographic apparatus 60 such as a page printer reproduces a color image based on the supplied input image data 56. The electrophotographic apparatus 60 includes a controller 62 that performs image processing and supplies laser drive data 69 to the engine, and an engine 70 that reproduces an image according to the drive data 69.
[0029]
In the host computer 50, character data, graphic data, bitmap data 53, and the like are generated by an application program 52 such as a word processor or a graphic tool. Each data 53 generated by the application program 52 is rasterized by the rasterizing function 54 of the driver 55 for the electrophotographic apparatus installed in the host computer 50, and the gradation of each RGB color for each pixel or dot. It is converted into input image data 56 consisting of data.
[0030]
The electrophotographic apparatus 60 also includes a microprocessor (not shown), and a controller (image processing unit) 62 including a color conversion unit 64, a halftone processing unit 66, a pulse width modulation unit 68, and the like is configured. In the engine 70, for example, the laser driver 72 drives the laser diode 74 for image drawing based on the drive data 69. The engine 70 includes a photosensitive drum, a transfer belt, and the like and a driving unit thereof, which are omitted in FIG.
[0031]
A color conversion unit 64 in the controller 62 converts RGB gradation data 56 for each pixel, which is input image data, into CMYK gradation data 65, which is a toner color. The CMYK gradation data 65 is, for example, gradation data of 8 bits for each color for each color plane of the CMYK color plane, and has 256 gradations at the maximum. The halftone processing unit 66 is supplied with gradation data 65 of shade corresponding to the pixel for each color plane.
[0032]
The halftone processing unit 66 refers to a conversion table having correspondence between input gradation data and image reproduction data created in advance for the gradation data 65 for each pixel, and generates image reproduction data 67 for each pixel. Generate. The halftone processing unit 66 uses, for example, a multi-value dither method to generate image reproduction data 67 that specifies the size of a virtual dot region that represents an intermediate gradation.
[0033]
The pulse width modulator 68 converts the laser drive pulse data 69 corresponding to the virtual dot region specified by the image reproduction data 67 and supplies it to the engine 70 in real time. The engine 70 drives the laser beam according to the drive pulse data 69, irradiates the specified virtual dot area with the laser beam, and forms a latent image on the photosensitive drum. Toner is attached to the latent image area and transferred to a support such as recording paper to form a final image.
[0034]
Therefore, the growth of halftone dots formed according to the gradation of the gradation data 65 obtained by color conversion of the input image data 56 is specified by the image reproduction data generated by the halftone processing unit 6 in the controller 62. Can be understood by paying attention to the virtual dot area.
[0035]
4, 5, and 6 are diagrams for explaining the growth of halftone dots in a region with low shading. 4 to 6 show a plurality of pixels PX and a plurality of halftone dot regions 10A, 10B, and 10C. Each halftone dot area is formed by arranging the center growth nucleus pixel and the surrounding four pixels in a cross in the same manner as the halftone dot area 10 shown in FIG. The halftone dots in the screen are composed of three halftone dot groups. Three halftone dot groups are indicated by the first group halftone dot indicated by “A” in FIG. 4, the second halftone dot indicated by “B” in FIG. 5, and “C” in FIG. 6. This is the third group of dots. In this example, the first group of halftone dots A is arranged at the first density, the second group of halftone dots B is also arranged at the same first density, and the third group of halftone dots C is the first density. They are arranged at a second density that is twice the density.
[0036]
As described with reference to FIG. 2, the growth of each halftone dot grows with the central pixel as a growth nucleus in a low-concentration region (light region). Therefore, in the first area where the gray level of the input image data is low, not only the growth nucleus pixels of all the halftone dots but only the growth nucleus pixels of the first group of halftone dots A, the virtual image is increased according to the increase in the gray level. Image reproduction data is generated so that the dot area grows. That is, a virtual dot region grows in the growth nucleus pixel region at the center of the first group of dot regions 10A in FIG.
[0037]
Therefore, in the first region with low shading, the halftone dots that grow as the shading increases are limited to the first group of halftone dots A, and therefore the pitch of the screen lines connecting the halftone dots made up of dot images. Pa becomes wider as shown. In other words, the number of screen lines per inch is small.
[0038]
In this way, by growing the halftone dots limited to the first group of halftone areas smaller than the actual halftone area, even in the first area with low shading, each of the growth nucleus pixels has the same density. The area of the virtual dot area can be increased, and the process characteristics of unstable electrophotography that changes from the virtual dot area, the latent image area, the toner adhesion area, and the dot image area in the engine can be suppressed. In other words, it suppresses unstable output characteristics when the size of the virtual dot area is small, eliminates missing dots in the final image, and reduces the change in the intensity of the final image in areas with low intensity that are sensitive to human eyes. Can be reproduced appropriately.
[0039]
In the first region having the lowest intensity, the virtual dot grows to the maximum in the growth nucleus pixel region of the first group of halftone dots A. Then, in the second area having the second highest density than the first area, as shown in FIG. 5, the second group of nets is obtained with the growth nucleus pixels of the first group of halftone dots A having been grown. Within the area of point B, the virtual dot area grows corresponding to the increase in the shade. That is, in addition to the first group of dots A, the second group of dots B grows. Therefore, in the second region, virtual dots are formed in the halftone dot regions of the first group and the second group. As a result, the pitch of the screen lines connecting the halftone dots A and B of the first group and the second group becomes a pitch Pb narrower than the pitch Pa as shown in FIG. That is, the number of screen lines per inch is larger than that in the first area.
[0040]
Further, in the second region, when the virtual dot grows to the maximum at the growth nucleus pixel of the second group of halftone dots B, as shown in FIG. In a state where the growth nucleus pixels of the first and second groups of halftone dots A and B have been grown, a virtual dot region grows in the region of the third group of halftone dots C corresponding to the increase in the shade. Therefore, in the third region, the halftone processing unit generates image reproduction data having virtual dots in all halftone dot regions. As a result, the screen line pitch is narrower than the pitch Pb, as shown in FIG. In other words, it becomes the maximum number of screen lines.
[0041]
Further, in a region with a high degree of lightness (dark region), image reproduction data is generated so that virtual dot regions grow sequentially in the pixel region corresponding to the increase in lightness in all halftone dots A, B, and C. Is done. The growth of the virtual dot region is as described in FIG.
[0042]
FIGS. 7 to 14 are specific diagrams for explaining the process of growing halftone dots with increasing shade. The process of growing halftone dots in this embodiment consists of steps S1 to S9 corresponding to the change in the lightness from zero to the maximum. Further, FIG. 15 is a diagram showing a lookup table in the conversion table in the halftone processing unit in the electrophotographic apparatus.
[0043]
Step S1 in FIG. 7 explains the growth of halftone dots in the lowest density area, and the virtual dot area increases in the first pixel area, which is the growth nucleus pixel of the first group of halftone areas A. To do. That is, it is the same as FIG. As shown by S1 in FIG. 15, in the lookup table, the input image data 65 having grayscale gradation data is compared with the first pixel area A-1 which is the growth nucleus pixel of the halftone dot area A. Output data indicating the size of the virtual dot area rises. Accordingly, FIG. 7 shows a state in which the virtual dot region is maximized in the first pixel region of the pixel region A.
[0044]
Step S2 in FIG. 8 shows the growth of halftone dots in the next lower density region, and the virtual dot region increases in the first pixel region which is the growth nucleus pixel of the second group of halftone dot regions B. It is the same as FIG. 5 described above, and it can be understood from FIG. 8 that the number of screen lines has increased. Then, as indicated by S2 in FIG. 15, in the lookup table, the output indicating the size of the virtual dot region in the first pixel region B-1 that is the growth nucleus pixel of the halftone dot region B is input to the input image data. Data rises. Since the density of the second group halftone dot area B is the same as that of the first group halftone dot area A, the slope of the lookup table is also the same.
[0045]
Step S3 in FIG. 9 shows the growth of halftone dots in the next lower density area, and the virtual dot area grows within the growth nucleus pixel of the third group of halftone areas C. It is the same as FIG. 6 described above, and the number of screen lines is further increased. In the look-up table, as indicated by S3 in FIG. 15, the output indicating the virtual dot area size in the first pixel area C-1 of the growth nucleus pixel in the halftone dot area C corresponding to the increase of the input image data. Data rises. However, since the density of the halftone dot region C of the third group is twice the density of the halftone dot region of the first group or the second group, the slope of the lookup table is ½ of them. .
[0046]
In steps S4, 5, and 6 of FIGS. 10, 11, and 12, in all halftone dot areas A, B, and C, the second pixel area (PX2 in FIG. 2), the third pixel area (PX3 in FIG. 2), the first In the four pixel region (PX4 in FIG. 2), virtual dot regions grow sequentially. When the density increases to some extent, it is no longer necessary to attach toner to a single small virtual dot area. Therefore, the instability of the electrophotographic process in the engine is reduced. For this reason, once the growth nucleus pixels have grown in all of the halftone dot groups, the final image can be sharpened with high resolution by growing the virtual dot region in all of the halftone dot regions.
[0047]
Therefore, in the look-up table in step S4, as shown in S4 in FIG. 15, the second pixel areas A-2, B- of the three groups of halftone dot areas are inputted with respect to the input which is the gradation data of the gradation. 2 and C-2, the virtual dot area of the output increases, and the gradient is 4 times that of steps S1 and S2 and 2 times that of step S3. It has doubled. Similarly in steps 5 and 6, as shown in S5 and S6 in FIG. 15, the third pixel areas A-3, B-3, C-3 of the three groups of dot areas, the fourth pixel area A- In 4, B-4, and C-4, the output virtual dot area increases sequentially.
[0048]
In step S7 in FIG. 13, the area is high in the shade, and the area in which the shade is low and the black and white are reversed. For this reason, the electrophotographic process in the engine becomes unstable for the same reason. Therefore, it is preferable to grow the halftone dot so as to avoid the area other than the virtual dot area (the white area in FIG. 13) from becoming smaller as much as possible. Therefore, in step S7 in FIG. 13, as in step S3, the virtual dot region is grown in the fifth pixel region (PX5 in FIG. 2) of the third group of halftone dot regions C in response to the increase in shading. . Accordingly, FIG. 13 has a negative / positive relationship with step S2 of FIG. That is, in the state where step S7 is completed, the number of lines connecting the white areas is reduced.
[0049]
The look-up table in step S7 is such that the virtual dot region in the fifth pixel C-5 of the halftone dot region C grows as shown in S7 of FIG. That is, the inclination is the same as in step S3.
[0050]
In step S8 of FIG. 14, the next region having the high gray level is a virtual dot region corresponding to the increase in the gray level within the fifth pixel region (PX5 in FIG. 2) of the second group of dot regions B. Will grow. The look-up table at that time is as shown in S8 of FIG. 15, and its inclination is the same as that in step S2.
[0051]
Although not shown, the final step S9 is the highest density region, and the virtual dot region grows in the fifth pixel region of the first group of dot regions. When the growth is completed, all the halftone dots become the maximum size, and a virtual dot area is formed on the entire surface. The look-up table at that time is as shown in S9 of FIG. 15, and its inclination is the same as that in step S1.
[0052]
This is the halftone dot growth process that suppresses instability in the electrophotographic process. In particular, in low-lightness areas that are sensitive to the human eye, the density of growing halftone dots is reduced, and the growth of halftone dots is accelerated in response to the increase in lightness. It is suppressed. That is, the slope of the lookup table is steep.
[0053]
As a modification of halftone dot growth, in the above steps S7, S8, and S9, a virtual dot area in the fifth pixel area may be grown simultaneously in three halftone dot groups.
[0054]
FIG. 16 is a diagram for explaining the edge effect. As described with reference to FIG. 1, the edge effect is a phenomenon in which a large amount of toner adheres to the peripheral portion of the latent image area when the charged toner is excessively supplied to the latent image area. In FIG. 16, the horizontal axis represents space, distance, and the like, and the vertical axis represents the latent image potential (solid line) and the toner density (broken line) attached.
[0055]
In the relatively thick latent image region 20 in FIG. 16A, the toner density 21 to be attached becomes higher at the peripheral portion. This tendency becomes the same toner density 23 even in the latent image area 22 having a certain width. In addition, when the virtual dot area becomes thin, the potential thereof tends to decrease as in the latent image areas 24 and 26. However, the toner concentrations 25 and 27 attached due to the edge effect tend to be high as in the case of the thick latent image areas 20 and 22, as shown.
[0056]
FIG. 16B shows that the edge effect is suppressed by the density of the virtual dot region. When the thin virtual dot region (latent image region) is present alone, the pattern 28 has a relatively high toner density 29 due to the edge effect as in the latent image 24 and the like, and is more reliably reproduced as a dot image. The However, when the virtual dot area (latent image area) is close and high in density as compared with the pattern 28 as in the pattern 30, the edge effect is suppressed and the toner density 31 adhering to each of the virtual dot areas is lowered. , The probability of being reproduced as a dot image decreases.
[0057]
As described above, depending on the density of the virtual dot area (latent image area), the edge effect is strengthened or suppressed, so that in the above-described halftone dot growth process, the density of the final image is low in the low density area. The phenomenon that the degree is not reproduced properly occurs.
[0058]
The upper graph in FIG. 15 shows the lightness of the final image with respect to the input, which is the lightness gradation data. As described above, in the region having the lowest lightness, a virtual dot region grows in the growth nucleus pixel A-1 in the halftone dot region A in the first group as in step S1, and in the next lightness region, a step is performed. As shown in S2, a virtual dot region grows at the growth nucleus pixel B-1 in the second group of dot regions B. At this boundary, the density of generated halftone dots is doubled. Therefore, as described in FIG. 16 (2), the edge effect is suppressed at the boundary from step S1 to S2 where the density of the virtual dot area forming the halftone dot is doubled, and the density of the final image is As shown by the broken-line circle in FIG. Or, in the worst case, a reverse phenomenon occurs in the intensity of the final image. Such a phenomenon causes a reduction in the quality of the final image. A similar phenomenon occurs at the boundary from step S2 to S3.
[0059]
FIG. 16 (3) illustrates a method for eliminating the dependency of the dot density on the edge effect. In the pattern 32, a thin latent image area 33 exists alone as in the pattern 28, but a minute virtual dot area that is not finally developed is generated in the vicinity thereof, and the minute latent image areas 34 and 35 are formed. . In the pattern 38, the thin latent image areas 39 and 40 are close to each other as in the pattern 30, but a minute virtual dot area is generated in the vicinity thereof to form a minute latent image area 41.
[0060]
The patterns 32 and 38 have different dot densities, but the virtual dot area and the latent image area have the same density. Therefore, the edge effect on both patterns 32 and 38 is similarly suppressed, and the density of the adhered toner is the same.
[0061]
By utilizing the above principle, it is possible to suppress an unnatural change in the intensity of the final image accompanying a change in the dot density at the boundary between steps S1, S2, and S3 of the dot growth process. 17 and 18 are diagrams showing a first halftone dot growth process in the present embodiment. FIG. 17 shows step S1, and FIG. 18 shows step S2. FIG. 19 is a diagram showing a look-up table for realizing the first halftone dot growth process.
[0062]
In the first halftone dot growth method, as shown in FIG. 17, in step S1 in the lowest shaded area, the increase in shade is accommodated in the growth nucleus pixel A-1 of the first group of halftone dots A. In addition to growing the virtual dot area, ultra-fine virtual dots that are not actually developed are formed in the growth nucleus pixels B-1 and C-1 of the second and third group halftone dot areas B and C. generate. In other words, in the dot areas other than the first group of dot areas A, extremely fine virtual dots 42 are generated to increase the density of the virtual dot areas and suppress the edge effect.
[0063]
Similarly, as shown in FIG. 18, in step S2 in the next lower density region, a virtual dot region is grown in the growth nucleus pixel B-1 in the second group of dot regions B, and the third group. An ultrafine virtual dot 42 is generated in the growth nucleus pixel C-1 in the halftone dot region C. Thereby, the virtual dot area density in step S2 becomes the same as that in step S1, and the edge effect is suppressed to the same extent.
[0064]
In steps S1 and S2, extremely fine virtual dot areas are generated. However, since these areas are extremely fine, the potential of the latent image area is not sufficient, and the final image is not developed as a dot image. Therefore, even when an extremely fine virtual dot region is generated, the density of the final image corresponds to the density of the input image data. Further, the number of screen lines in steps S1 and S2 is not changed even if an extremely fine virtual dot region 42 is generated.
[0065]
In step S3, a virtual dot region is grown in the growth nucleus pixel C-1 of the third group of dot regions C, as described with reference to FIG. Accordingly, the virtual dot area density at this time is also the same as in steps S1 and S2.
[0066]
Referring to the look-up table in FIG. 19, the method of generating the virtual dot area in steps S1 and S2 can be understood more clearly. In step S1, for the input data, the first group of halftone pixels A-1 grows in response to the increase in shading, and the second and third group of halftone pixels B-1, C- In 1, it is converted into output image reproduction data that generates a very fine virtual dot area. In step S2, the virtual dot region grows in the second group of halftone pixels B-1 with the growth of the first group of halftone pixels A-1 completed. The pixel C-1 is converted into output image reproduction data that generates a very fine virtual dot area. Further, in step S3, in the state where the growth of the virtual dot region at the pixels A-1 and B-1 of the first and second groups of halftone dots has been completed, The virtual dot area grows.
[0067]
Since the density of the virtual dot area in steps S1, S2, and S3 is kept constant, the edge effect does not change at the boundary between them. Therefore, the change in the intensity of the final image becomes smooth corresponding to the change in the intensity of the input data, and the change in the intensity of the final image as shown in FIG. 15 is stepped or reversed. Is avoided.
[0068]
There is no change in the edge effect in the region where the intensity is close to the maximum. Accordingly, as shown in FIG. 19, steps S7, S8, S9 are the same lookup table as FIG.
[0069]
FIG. 20 is a diagram showing a second halftone dot growth process in the present embodiment. FIG. 21 is a diagram showing a look-up table for realizing the second halftone dot growth process. FIG. 20 shows halftone dot growth in step S1. In the second halftone dot growth method, the virtual dot area is grown in the growth nucleus pixel A-1 of the first group of halftone areas A in step S1 of the lowest density area, and the second group of halftone dots. In the growth nucleus pixel B-1 in the region B, an extremely fine virtual dot region 42 that is not finally developed is generated. Therefore, as shown in the lookup table of FIG. 21, in step S1, virtual dot areas are generated in the first group of dot areas A and the second group of dot areas B, and the edge effect is suppressed.
[0070]
In the second halftone dot growth method, step S2 is the same as the first halftone dot growth method. That is, in the first group of dot areas A, the virtual dot area is maintained at the maximum size in the growth nucleus pixel, and in the second group of dot areas B, the virtual dot area is grown in the growth nucleus pixel B-1. Further, in the third group halftone dot area C, an extremely fine virtual dot area is generated. Again, the density of the virtual dot area is increased and the edge effect is suppressed.
[0071]
According to the second halftone dot growth method, the number of ultrafine virtual dot regions 42 in step S1 in the first halftone dot growth method is reduced. Therefore, even if the ultra-fine virtual dot regions 42 are finally developed due to variations in the characteristics of the electrophotographic process in the engine, the number of ultra-fine virtual dot regions 42 is small. This can be suppressed as compared with the first method.
[0072]
Also in the second halftone dot growth method, since the density of the virtual dot area is the same in steps S2 and S3, it is suppressed that the change in the intensity of the final image due to the fluctuation of the edge effect becomes unnatural. The
[0073]
FIG. 22 is a diagram showing a lookup table for further realizing the third halftone dot growth method. The third halftone dot growth method is a modification of the second halftone dot growth method. In step S1, a virtual dot region is grown on the growth nucleus pixel A-1 in the first group of halftone dot regions A, and the first halftone dot growth method is performed. In the growth nucleus pixel B-1 in the two groups of halftone dot regions B, an extremely fine virtual dot region that is not actually developed is gradually grown from zero. However, the extremely fine virtual dot area is of a size that is not developed at most. Therefore, the density of the virtual dot region is not changed at least at the boundary between steps S1 and S2 and the boundary between steps S2 and S3, and the change in edge effect is also suppressed. Further, similarly to the second halftone dot growth method, the number of ultrafine virtual dot regions in step S1 can be suppressed, and fluctuations in the density of the final image due to development due to process fluctuations can be suppressed.
[0074]
FIG. 23 is a diagram illustrating an example of a lookup table. This example corresponds to the first halftone dot growth method. The halftone processing unit 66 shown in FIG. 3 is supplied with input data 65 obtained by color-converting the input image data 56. This input image data corresponds to the pixel as a result of the rasterization process as shown in FIG. The conversion table in the halftone processing unit is composed of the LUT matrix in FIG. 23 (2) and the lookup table group in (3). The LUT matrix is an index table that associates the pixels of the input image data with the lookup table to be referred to, and the index A-1 in the LUT matrix is referred to for the pixel P shown in FIG. Therefore, the look-up table A1 for the first pixel A-1 in the halftone dot area A of the first group is referred to. The LUT matrix is associated with the pixels of the input image data while shifting so that tiles are laid out.
[0075]
The lookup table group includes a table of first pixels A-1, B-1, C-1, second pixels A-2, B-2, C-2, and fifth pixel A-5 of each halftone dot region. Is illustratively shown. The input grayscale data of 0 to i-1 corresponds to the area of step S1, in which the virtual dot area is represented by the lookup table of the first pixel A-1 of the first group of halftone dots. The size output value rises from 0 to 255 (input / output data is 8 bits). At the same time, in the first pixels B-1 and C-1 of the second and third group halftone dots, the output value is “5”, and an extremely fine virtual dot region is generated.
[0076]
Further, the region of input i to j-1 corresponds to the region of step S2, and the output value of the virtual dot size is 5 to 255 in the lookup table of the first pixel B-1 of the second group of halftone dots. Standing up. At that time, in the first pixel C-1 of the third group of halftone dots, the output value is “5”, and an extremely fine virtual dot region is generated.
[0077]
The region where the input is j to k-1 corresponds to the region of step S3, and the output value of the virtual dot size rises in the lookup table of the first pixel C-1 of the third group of halftone dots. The inclination is half that of Tables A-1 and B-1. Further, the area where the input is k or more corresponds to the area of step S4 ~, and in the lookup table for the second pixels A-2, B-2, C-2 of the first, second, and third group halftone dots, At the same time, the output value of the virtual dot size is rising. However, the slope of the rise is ¼ that of tables A-1 and B-1.
[0078]
In this manner, the growth of the virtual dot area of the output data corresponding to the increase in the gray level of the input image data can be freely changed by the lookup table.
[0079]
FIG. 24 is a configuration diagram of another example of the electrophotographic system. In the electrophotographic system shown in FIG. 3, the controller 62 in the electrophotographic apparatus 60 such as a printer has a halftone processing unit 66. The halftone processing unit 66 refers to the look-up table, converts the input image data having shading to image reproduction data having the size of the virtual dot area, and outputs it. On the other hand, in the example of FIG. 24, the above halftone processing function is realized by the driver program 80 installed in the host computer 50. Accordingly, the driver 80 includes a rasterizing module 54, a color conversion module 64, and a halftone processing module 66.
[0080]
The halftone processing module 66 has an LUT matrix and a look-up table as the conversion table, and by referring to them, the input image data having the gray level is reproduced as the image having the size of the virtual dot area. Convert to data. Therefore, image processing for performing halftone processing is realized by a computer program.
[0081]
This image processing program converts input image data having gray level into output image reproduction data having virtual dot area data. In the conversion process, the look-up table in the conversion table described above is referred to. The The look-up table defines how halftone dots are grown in response to increasing shades. Therefore, the image processing program is composed of, for example, a conversion procedure and a conversion table.
[0082]
24 includes a controller 62 having a pulse width modulation unit 68 that generates a laser drive pulse signal 69 from converted image reproduction data 67, and an engine 70 having a laser driver 72, a laser diode 74, and the like. Composed.
[0083]
In the embodiment described above, the halftone dot region is divided into three groups and the halftone dots are sequentially grown. However, the present invention is not limited to three groups, and is divided into two or more groups. This is applied to the case where the halftone dots are successively grown. Further, in the above embodiment, the plurality of halftone dots are sequentially grown even in a region with high shading, but the present invention does not have to be such a growth method. Further, as the growth of the halftone dots, after the growth nucleus pixel grows, the adjacent pixels may not grow sequentially, but the virtual dot region may grow simultaneously with a plurality of adjacent pixels.
[0084]
The present invention can be applied to a case where image reproduction data is generated in which the density of halftone dots changes in a stepped manner in response to an increase in density in an area where the density is low. Accordingly, not only when the virtual dot area grows sequentially in a plurality of halftone dot areas as in the embodiment, but also in the first area, halftone dots are generated at the first density in response to the increase in shading. However, the present invention can also be applied to a case where halftone dots are generated at a higher second density in a higher second region.
[0085]
【The invention's effect】
As described above, according to the present invention, since the gradation of the final image is appropriately reproduced, the image quality can be improved.
[Brief description of the drawings]
FIG. 1 is a diagram showing a developing step of a general electrophotographic process.
FIG. 2 is a diagram illustrating an example of growth of halftone dots.
FIG. 3 is a diagram illustrating a configuration of an electrophotographic apparatus according to an embodiment of the present invention.
FIG. 4 is a diagram for explaining the growth of halftone dots in a region with low shading.
FIG. 5 is a diagram for explaining the growth of halftone dots in a region with low shading.
FIG. 6 is a diagram for explaining the growth of halftone dots in a region with low shading.
FIG. 7 is a specific diagram for explaining the process of growing halftone dots as the lightness increases.
FIG. 8 is a specific diagram for explaining the process of growing halftone dots as the lightness increases.
FIG. 9 is a specific diagram for explaining the process of growing halftone dots as the lightness increases.
FIG. 10 is a specific diagram for explaining a process of growing halftone dots with increasing density.
FIG. 11 is a specific diagram for explaining the process of growing halftone dots as the lightness increases.
FIG. 12 is a specific diagram for explaining the process of growing halftone dots as the lightness increases.
FIG. 13 is a specific diagram for explaining a process of growing halftone dots as the lightness increases.
FIG. 14 is a specific diagram for explaining the process of growing halftone dots as the lightness increases.
FIG. 15 is a diagram illustrating a conversion table (lookup table) in a halftone processing unit in an electrophotographic apparatus.
FIG. 16 is a diagram illustrating an edge effect.
FIG. 17 is a diagram showing a first halftone dot growth process in the embodiment.
FIG. 18 is a diagram showing a first halftone dot growth process in the embodiment.
FIG. 19 is a diagram showing a look-up table for realizing the first halftone dot growth method.
FIG. 20 is a diagram showing a second halftone dot growth process in the embodiment.
FIG. 21 is a diagram showing a look-up table for realizing the second halftone dot growth method.
FIG. 22 is a diagram showing a look-up table for realizing a third halftone dot growth method.
FIG. 23 is a diagram illustrating an example of a lookup table.
FIG. 24 is a configuration diagram of another example of an electrophotographic system.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Halftone dot area | region 10A First halftone dot area | region 10B Second halftone dot area | region 10C Third halftone dot area | region 12 Virtual dot area | region 60 Electrophotographic apparatus 62 Controller 66 Halftone process part (image formation part)
70 engine
PX1 ~ 5 pixels

Claims (16)

An electronic photograph that has an image reproduction engine that forms a dot image by attaching toner to a virtual dot area in a pixel, and reproduces the image by expressing the shade by halftone dots formed by dot images in a plurality of pixels In the device,
An image processing unit for converting the gray level of the input image data into image reproduction data of the virtual dot region ;
An engine that forms a latent image area on the photosensitive drum in accordance with the image reproduction data and attaches toner to the latent image area to generate a dot image ;
The engine has an edge effect in which the density of the dot image when the latent image area is the first density is higher than the density of the dot image when the latent image area is the second density higher than the first density;
The image processing unit grows a first group of halftone dots in accordance with the increase in the lightness in the first lightness area where the lightness of the input image data is low, and more than in the first lightness area. in high gray level second gray level area, the image reproduction data to grow the dot of the second group different from the first group in response to the increase in the concentration Awad together to form a halftone dot of the first group Generate
Further, the image processing unit simultaneously generates a small virtual dot area in the first gray level area, in a halftone dot area other than the first group of halftone dots so that a dot image is not formed by the toner. Generating the image reproduction data ;
The image processing unit has a look-up table group for converting the input image data into image reproduction data, and the look-up table group grows first and second halftone dots, respectively. and two groups lookup table, the input electrophotographic apparatus according to claim Rukoto consists of an index table for associating the image of the first and second group to be referred to as pixel data look-up table. 2. The second group according to claim 1, wherein halftone dot regions other than the first group of halftone dots that generate the minute virtual dot region in the first lightness region grow in the second lightness region. An electrophotographic apparatus characterized by being an area of halftone dots. According to claim 1, wherein the image processing unit, wherein the second gray level is higher gray level than the region third gray level area, the first, the said concentration Awad together to form a halftone dot of the second group the first group and the dot of the second group halftone different third group to generate image reproduction data to be grown, Oite the fine virtual dot region in the first gray level area with an increase The halftone dot regions other than the first group halftone dot to be generated are the second and third group halftone dot regions that grow in the second and third grayscale regions, respectively , and the image The processing unit generates the image reproduction data for simultaneously generating the minute virtual dot area in the third group of dot areas in the second density area. According to claim 1, wherein the image processing unit, wherein the second gray level is higher gray level than the region third gray level area, the first, the said concentration Awad together to form a halftone dot of the second group the first group and the dot of the second group halftone different third group to generate image reproduction data to be grown, Oite the fine virtual dot region in the first gray level area with an increase The halftone dot region other than the first group halftone dot to be generated is the region of the second or third group halftone dot that grows in the second and third grayscale regions, respectively , and further the image processing unit, the in the second gray level area, the third group of an electrophotographic apparatus and generates the image reproduction data generating simultaneously the fine virtual dot area halftone dot region. According to claim 4, wherein the image and the image processing unit, in the second gray level area, where the fine virtual dot area in the region of the dot of the third group grows from zero with an increase of gray level An electrophotographic apparatus that generates data. 3. The first group and the second group according to claim 1 or 2, wherein the image processing unit has a third density region having a higher density than the second density area in accordance with the increase in the density. An electrophotographic apparatus characterized by simultaneously growing halftone dots. According to claim 3, 4 or 5, wherein the image processing unit, wherein in the third higher gray level than gray level area fourth gray level area, the first group in response to the increase in the gray level, the An electrophotographic apparatus, wherein the halftone dots of the second group and the third group are grown simultaneously. An electronic photograph that has an image reproduction engine that forms a dot image by attaching toner to a virtual dot area in a pixel, and reproduces the image by expressing the shade by halftone dots formed by dot images in a plurality of pixels In the device,
An image processing unit for converting the gray level of the input image data into image reproduction data of the virtual dot region ;
An engine that forms a latent image area on the photosensitive drum in accordance with the image reproduction data and attaches toner to the latent image area to generate a dot image ;
The engine has an edge effect in which the density of the dot image when the latent image area is the first density is higher than the density of the dot image when the latent image area is the second density higher than the first density;
Wherein the image processing unit, wherein the first gray level area is low gray level of the input image data, to grow halftone dots of the first density in accordance with the increase in the gray level, than the first gray level area also in the second gray level area is high gray level, and generates image reproduction data to grow the dot of the higher than first density second density in accordance with the increase in the concentration Awad,
Further, the image processing unit simultaneously generates a small virtual dot region in the first gray level region so that a dot image by the toner is not formed in a halftone dot region other than the growing halftone dot. Generate playback data ,
The image processing unit includes a look-up table group for converting the input image data into image reproduction data, and the look-up table group grows the first density halftone dots and the second density halftone dots. electrophotographic apparatus, wherein the first and second look-up table, the Rukoto is composed of an index table that associates said input image said first and second reference should be the pixel data look-up table to . 9. The density according to claim 8, wherein a density obtained by adding the minute virtual dots to the virtual dots of the growing halftone dots is higher than the first density and higher than the second density. An electrophotographic apparatus characterized by being low. 9. The electrophotography according to claim 8, wherein, in the first density region, a density obtained by adding the minute virtual dots to the virtual dots of the growing halftone dots is the same as the second density. apparatus. Image reproduction data that reproduces an image by reproducing the image with a halftone dot formed by dot images in a plurality of pixels is displayed on an image reproduction engine that forms a dot image by attaching toner to a virtual dot area in a pixel. In an image processing program for causing a computer to execute image processing to be generated,
The engine forms a latent image area on the photosensitive drum in accordance with the image reproduction data, attaches toner to the latent image area to generate a dot image, and further generates a dot image when the latent image area has the first density. An edge effect in which the density of the image is higher than the density of the dot image in the case of a second density higher than the first density;
The image processing includes processing for converting the gray level of the input image data into image reproduction data of the virtual dot region,
Wherein the image reproduction data, the input in the first gray level area is low gray level of the image data, to grow a halftone dot of the first group in response to the increase in the gray level, than the first gray level area in high gray level second gray level area, is grown halftone dots of the second group different from the first group in response to the increase in the concentration Awad together to form a halftone dot of the first group, further, the In the first lightness area, a minute virtual dot area that does not form a dot image with the toner is generated simultaneously in a halftone dot area other than the first group of halftone dots .
In the image processing, a look-up table group for converting the input image data into image reproduction data is referred to, and the look-up table group grows first and second halftone dots of the first and second groups, respectively. and the group of look-up tables, the electrophotographic apparatus image processing program characterized by Rukoto consists of an index table for associating the look-up table of the input image and the first and second group to be referred to as pixel data . 12. The second group according to claim 11, wherein a halftone dot region other than the first group of halftone dots that generates the minute virtual dot region in the first lightness region grows in the second lightness region. An image processing program for an electrophotographic apparatus, wherein the image processing program is a halftone dot area. Image reproduction data that reproduces an image by reproducing the image with a halftone dot formed by dot images in a plurality of pixels is displayed on an image reproduction engine that forms a dot image by attaching toner to a virtual dot area in a pixel. In an image processing program for causing a computer to execute image processing to be generated,
The engine forms a latent image area on the photosensitive drum in accordance with the image reproduction data, attaches toner to the latent image area to generate a dot image, and further generates a dot image when the latent image area has the first density. An edge effect in which the density of the image is higher than the density of the dot image in the case of a second density higher than the first density;
The image processing includes processing for converting the gray level of the input image data into image reproduction data of the virtual dot region,
Wherein the image reproduction data, the first shade level area is low gray level of the input image data, to grow halftone dots of the first density in accordance with the increase in the gray level, than the first gray level area in even a high gray level second gray level area, is grown halftone dots of the second density higher than the first density in accordance with the increase in the concentration Awad, further, in the first gray level region, At the same time, a minute virtual dot region is generated in a halftone dot region other than the growing halftone dot so that a dot image is not formed by the toner ,
In the image processing, a lookup table group for converting the input image data into image reproduction data is referred to, and the lookup table group grows the first density halftone dots and the second density halftone dots. a first and second look-up table is composed of an index table that associates said input image said first and second reference should be a pixel of the data look-up table of the electrophotographic apparatus according to claim Rukoto Image processing program. 14. The density according to claim 13, wherein a density obtained by adding the minute virtual dots to the virtual dots of the growing halftone dots is higher than the first density and higher than the second density. An image processing program for an electrophotographic apparatus characterized by being low. 14. The electrophotography according to claim 13, wherein in the first density region, a density obtained by adding the minute virtual dots to the virtual dots of the growing halftone dots is the same as the second density. Image processing program for the device. An electronic photograph that has an image reproduction engine that forms a dot image by attaching toner to a virtual dot area in a pixel, and reproduces the image by expressing the shade by halftone dots formed by dot images in a plurality of pixels In the device
Image processing unit for converting the intensity of input image data into image reproduction data of the virtual dot area When,
An engine that forms a latent image area on the photosensitive drum according to the image reproduction data and attaches toner to the latent image area to generate a dot image;
The engine has an edge effect in which the density of the dot image when the latent image area is the first density is higher than the density of the dot image when the latent image area is the second density higher than the first density;
The image processing unit grows a first group of halftone dots in accordance with the increase in the lightness in the first lightness area where the lightness of the input image data is low, and more than in the first lightness area. In the second lightness region having a high lightness, the image reproduction data for forming the first group of halftone dots and growing the second group of dots different from the first group in accordance with the increase of the lightness. Generate
Further, the image processing unit simultaneously generates a small virtual dot area in the first gray level area, in a halftone dot area other than the first group of halftone dots so that a dot image is not formed by the toner. An electrophotographic apparatus that generates the image reproduction data.

Images