JP3968217B2 - 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 examination 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 an aspect of the present invention, a photosensitive body on which a latent image is formed and a toner supply body are opposed to each other in a non-contact state, and the toner supply is applied to a virtual dot area in a pixel of the photosensitive body. In an electrophotographic apparatus that has an image reproduction engine that forms a dot image by attaching toner from a body, and reproduces an image by expressing the gray level by halftone dots formed by dot images in a plurality of pixels.
  An image processing unit for converting the gray level of the input image data into image reproduction data of the virtual dot region;
  The halftone dots have a first group of halftone dots in which the halftone dots are spaced apart, and a second halftone dot in which each halftone dot is arranged between the halftone dots of the first group,
  In the first region where the gray level of the input image data is low, the image processing unit grows virtual dots of pixels in a first group of halftone dots according to the increase in the gray level, and the first region In the second region where the intensity is higher than that of the first group, the virtual dots of the pixels in the halftone dots of the second group different from the first group are grown, and in the third area where the intensity is higher than that of the second area, Generating image reproduction data for simultaneously growing virtual dots of pixels within the halftone dots of the first group and the second group;
  Furthermore, the image processing unitIn order to increase the intensity of the dot image without reversing at the boundary between the first and second regions,In the first region, the virtual dots of the pixels in the second group of dots start to grow while the virtual dots of the pixels in the first group of dots start to grow in response to the increase in the shade. , Generating the image reproduction data in which the growth of the virtual dots of the pixels in the first group of dots stops while the virtual dots of the pixels in the second group of dots are growing.
[0016]
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 one aspect of the present invention, in the first region, the second group of dots starts growing while the first group of dots is growing in response to the increase in the shade, The image reproduction data in which the growth of the first group of halftone dots stops while the second group of halftone dots is growing is generated. Since the growth of the second group of dots is started during the growth of the first group of dots, the influence of the edge effect can be reduced in the vicinity of the shade where the growth of the second group of dots is added. The intensity of the image can be reproduced appropriately.
[0020]
In a preferred embodiment of the invention, the image processing unit further generates the image reproduction data in which the number of screen lines connecting the generated halftone dots gradually increases from the first area to the second area. It is characterized by that. By gradually increasing the number of screen lines in the low density area, it is possible to appropriately reproduce the density in the low density area and increase the resolution by increasing the number of screen lines in the high density area. Even if such a halftone dot growth method is adopted, in this embodiment, the influence of the edge effect can be suppressed and the gray level of the final image can be appropriately reproduced.
[0021]
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.
[0022]
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.
[0023]
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.
[0024]
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.
[0025]
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.
[0026]
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.
[0027]
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.
[0028]
Therefore, the growth of halftone dots formed according to the shades of the input image data 56 and 65 is made by paying attention to the virtual dot region specified by the image reproduction data generated by the halftone processing unit 6 in the controller 62. Understood.
[0029]
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.
[0030]
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.
[0031]
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.
[0032]
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.
[0033]
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. 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.
[0034]
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.
[0035]
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.
[0036]
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.
[0037]
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.
[0038]
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.
[0039]
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. .
[0040]
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.
[0041]
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.
[0042]
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.
[0043]
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.
[0044]
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. 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.
[0045]
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.
[0046]
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.
[0047]
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.
[0048]
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.
[0049]
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.
[0050]
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.
[0051]
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.
[0052]
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.
[0053]
FIG. 16 (3) illustrates a method for suppressing the influence of the edge effect. The pattern 32 is a latent image distribution in an area of lightness G1, a single latent image area 33 is generated, and a toner density 34 is generated in the latent image area. On the other hand, the pattern 33 is a latent image distribution in a region having a lightness G2 higher than the lightness G1, and has a growing latent image region 35 and a newly started latent image region 36. Therefore, the latent image area 35 is larger in size than the latent image area 33 of the pattern 32.
[0054]
When the growth of the pixel in the halftone dot area A is completed and the growth of the pixel in the halftone dot area B is started as shown in the boundary between steps S1 and S2 in FIG. 15, the density of the virtual dot area increases rapidly, Largely affected by fluctuations. On the other hand, as shown in FIG. 16 (3), the growth of the new latent image region 36 is started while growing from the latent image region 33 to 35, so that the density of the virtual dot region is increased. , Since the latent image area 35 has grown to a larger size, the influence of fluctuations in the edge effect can be suppressed. As a result, it is possible to reproduce the input image data so as to appropriately increase the density of the final image corresponding to the increase of the intensity of the input image data.
[0055]
FIG. 17 is a diagram showing a look-up table for halftone dot growth in the present embodiment using the above principle. 18, 19 and 20 are diagrams for explaining the halftone dot growth process. The halftone dot growth in this embodiment is obtained by replacing the above-described halftone dot growth steps S1, S2, and S3 with steps S1A, S2A, and S3A described below.
[0056]
In step S1A of the lowest intensity area, as shown in FIG. 18, first, a virtual dot area grows only in the growth nucleus pixel A-1 in the first group of halftone dot areas A. As a result, the probability of occurrence of a minute virtual dot region is lowered, it is less susceptible to the output fluctuation of the electrophotographic process, and an image having a light gray level can be reproduced appropriately. Further, as shown in FIG. 19, during the growth of the virtual dot area in the growth nucleus pixel of the first group of halftone dot areas A in the middle of step S1A, the growth nucleus pixel B- of the second group of halftone dot areas B- Start growing virtual dots in 1. That is, when the growth of the virtual dot region in the growth nucleus pixel B-1 is started, the growth of the virtual dot region in the growth nucleus pixel A-1 in the halftone dot region A is continued. By doing so, as the virtual dot region in the growth nucleus pixel B-1 starts to grow, the dot density increases in a stepped manner and the edge effect is suppressed. Since the virtual dot area continues to grow, it is possible to prevent the increase in the intensity of the final image from being inappropriately suppressed or reversed.
[0057]
Furthermore, even in step S2A of the next lowest intensity region, only the virtual dot region of the growth nucleus pixel B-1 grows in the second halftone dot region B in the first half, but in the second half, in addition to the growth nucleus pixel B-1 Thus, the virtual dot area of the growth nucleus pixel C-1 also starts growing in the halftone dot area C of the third group. That is, as shown in FIG. 20, in the second half of step S2A, the virtual dot region in the growth nucleus pixel grows in the halftone dot regions B and C.
[0058]
Also in this case, when the growth in the halftone dot region C newly starts, the growth in the halftone dot region B continues. Accordingly, it is possible to reduce the influence of the change in the edge effect when the density of halftone dots that grow in the same manner increases.
[0059]
Subsequent step S3A is the same as the previous step S3, and further, steps S4 to S9 in the region with high density are the same as the example of FIG.
[0060]
As described above, in this embodiment, unlike FIG. 15, when increasing the number of halftone dot regions that grow in the low density region, the growth of additional halftone dots is started while the halftone dots are growing. , Reduce the influence of changes in the edge effect. In the example of FIG. 15, since the growth of the additional halftone dot area B starts when the growth of the halftone dot area A is completed, the size of the virtual dot area of the halftone dot area A is changed at the boundary of the shade. Therefore, it is greatly affected by changes in the edge effect.
[0061]
FIG. 21 is a diagram illustrating an example of a lookup table. The halftone processing unit 66 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. 21 (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. For the pixel P shown in FIG. 21 (1), the index A1 in the LUT matrix is referred to, and accordingly. , The look-up table A-1 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.
[0062]
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 S1A. In this area, the virtual dot area is determined 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). In the middle, the virtual dot region starts to rise in the lookup table of the first pixel B-1 of the second group of halftone dots.
[0063]
Further, the region of input i to j-1 corresponds to the region of step S2A, and the output value of the virtual dot size rises to 255 in the lookup table of the first pixel B-1 of the second group of halftone dots. On the way, the virtual dot region starts to rise at the first pixel C-1 of the third group of halftone dots.
[0064]
The region where the input is j to k-1 corresponds to the region of step S3A, and the output value of the virtual dot size rises up to 255 at the lookup table of the first pixel C-1 of the third group of halftone dots. It has become. 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. Since there is no change in the density of halftone dots at the boundary between steps S3A and S4, there is no change in the edge effect.
[0065]
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.
[0066]
FIG. 22 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.
[0067]
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 generation for performing halftone processing is realized by a computer program.
[0068]
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.
[0069]
22 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.
[0070]
In the embodiment described above, the halftone dot area 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.
[0071]
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. Therefore, not only when the virtual dot region sequentially grows in a plurality of groups of halftone dot regions as in the embodiment, but in accordance with the increase in the lightness, the first lightness of the lightness in the lightness region has the first density. The present invention can also be applied to the case where dots are generated and halftone dots are generated at a higher second density in the second region where the intensity is higher.
[0072]
【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 look-up table for halftone dot growth in the present embodiment example.
FIG. 18 is a diagram for explaining a halftone dot growth process in the embodiment.
FIG. 19 is a diagram for explaining a halftone dot growth process in the embodiment.
FIG. 20 is a diagram for explaining a halftone dot growth process in the embodiment.
FIG. 21 is a diagram illustrating an example of a lookup table.
FIG. 22 is a configuration diagram of another example of an electrophotographic system.
[Explanation of symbols]
10 Halftone dot area
10A First group of halftone dots
10B Halftone dot region of the second group
10C Third group dot area
12 Virtual dot area
60 Electrophotographic equipment
62 controller
66 Halftone processing unit (image forming unit)
70 engine
PX1 ~ 5 pixels

Claims (4)

An image reproduction engine for forming a dot image by causing a photosensitive member on which a latent image is formed and a toner supply member to face each other in a non-contact state and attaching toner from the toner supply member to a virtual dot region in a pixel of the photosensitive member In an electrophotographic apparatus that reproduces an image by expressing the shade by a halftone dot formed by dot images in a plurality of pixels,
An image processing unit for converting the gray level of the input image data into image reproduction data of the virtual dot region;
The halftone dots have a first group of halftone dots in which the halftone dots are spaced apart, and a second halftone dot in which each halftone dot is arranged between the halftone dots of the first group,
In the first region where the gray level of the input image data is low, the image processing unit grows virtual dots of pixels in a first group of halftone dots according to the increase in the gray level, and the first region In the second region where the intensity is higher than that of the first group, the virtual dots of the pixels in the halftone dots of the second group different from the first group are grown, and in the third area where the intensity is higher than that of the second area, Generating image reproduction data for simultaneously growing virtual dots of pixels in the halftone dots of the first group and the second group;
Furthermore, the image processing unit responds to the increase in the lightness in the first region so that the lightness of the dot image increases without reversing at the boundary between the first and second regions. While the virtual dots of the pixels in the first group of dots are growing, the virtual dots of the pixels in the second group of dots start to grow, and the virtual dots of the pixels in the second group of dots are An electrophotographic apparatus that generates the image reproduction data in which the growth of virtual dots of pixels in the first group of halftone dots stops during the growth. In claim 1,
Furthermore, the image processing unit generates the image reproduction data in which the number of screen lines connecting the generated halftone dots gradually increases in the first region to the second region. . In claim 1,
The image processing unit performs the growth of virtual dots of pixels in the first group of halftone dots in the first region by increasing the virtual dot region in the growth nucleus pixel region of the halftone dots, The growth of virtual dots of pixels in the second group of halftone dots in the first and second regions is performed by increasing the virtual dot region in the growth nucleus pixel region of the halftone dots , and the third so that conducted by the growth of the virtual dot in the pixel in the halftone dot of the first and second groups in the region of the virtual dot areas in the pixel region around the growth nucleus pixel of the halftone dot increases And generating the image reproduction data. An image reproduction engine for forming a dot image by causing a photosensitive member on which a latent image is formed and a toner supply member to face each other in a non-contact state and attaching toner from the toner supply member to a virtual dot region in a pixel of the photosensitive member Reproduction of image reproduction data for an electrophotographic apparatus having image data for generating image reproduction data that reproduces an image by expressing the shade by halftone dots formed by dot images in a plurality of pixels In the image processing program to be
The halftone dots have a first group of halftone dots in which the halftone dots are spaced apart, and a second halftone dot in which each halftone dot is arranged between the halftone dots of the first group,
The image processing
Converting the gray level of the input image data into image reproduction data of the virtual dot region,
In the image reproduction data, in the first area where the density of the input image data is low, virtual dots of pixels in the first group of halftone dots are grown according to the increase in the intensity, and the first area In the second region where the intensity is higher than that of the first group, the virtual dots of the pixels in the halftone dots of the second group different from the first group are grown, and in the third area where the intensity is higher than that of the second area, Simultaneously growing virtual dots of pixels in the halftone dots of the first and second groups;
Further, in the first region, the first group of nets is increased according to the increase in the lightness so that the lightness of the dot image increases without reversing at the boundary between the first and second regions. The virtual dot of the pixel in the second group of dots is started to grow while the virtual dot of the pixel in the point is growing, and the first of the virtual dot of the pixel in the second group of dots is grown. An image processing program for an electrophotographic apparatus, which stops the growth of virtual dots of pixels in a group halftone dot.

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