Electrophotographic image forming apparatus and image forming program product thereof

[0001] The present invention relates to an electrophotographic image forming apparatus for performing halftone processing by halftone dots composed of a plurality of dot images and an image forming program product thereof, and is particularly desired in a pixel region. The present invention relates to a novel image forming apparatus capable of improving image quality by forming virtual dots having a desired area at a position, and an image forming program product thereof.

[0002] A color electrophotographic apparatus widely used in a color printer, a color copier, or the like produces a latent image formed by exposing a photoconductor in cyan (C), magenta (M), or yellow (. It is developed with Y) and black (K) toner, the toner image is transferred onto a support such as paper, and the color image is reproduced as the final image. A laser beam printer that uses a laser beam to form a latent image on a photoconductor has a pixel region that is arranged along the main scanning direction in which the laser beam is scanned and the sub-scanning direction in which the support is sent. The drive of the laser beam is controlled to form a latent image. Among them, in particular, in the type that modulates the width of the pulse that drives the laser beam, the irradiation area of the laser beam can be variously changed within the pixel area, and even when the number of pixels per unit area is small, It makes it possible to reproduce a color image with higher resolution and higher gradation.

[0003] In such a pulse width modulation type laser beam printer, there is a halftone dot half toning method using a multi-level dithering method as a method for reproducing gradation of a grayscale image. According to this multi-value dither method, it is called a look-up table in which image reproduction information that determines the size and position of virtual dots in the pixel area is described for the gradation data for each color that is an input signal. The position and size of virtual dots in each pixel area are determined by referring to the conversion table. By setting multiple levels between 0 and the maximum size for this size, the output at each pixel is "multi-valued".

[0004] The "virtual dot" referred to here is defined as a region in which a laser beam is driven and scanned in order to form a dot image with toner on the final image, and the magnitude in the main scanning direction thereof is a laser. It is the product of the time the beam is driven and the scanning speed of the beam, and the magnitude in the sub-scanning direction is equal to the magnitude in the sub-scanning direction of the pixel region. For the reasons shown below, the virtual dots have a different shape from the "dot image" on the final image, so they are described separately here. A laser beam is driven in the virtual dots of each pixel region, and an irradiation region of the laser beam is formed on the photoconductor. Due to the size of the laser beam and the rising and falling characteristics during driving, this irradiation region has a shape that bleeds and spreads from the virtual dots. The irradiation region of the laser beam becomes a latent image region on the photoconductor, is developed by toner, and is transferred onto a support such as paper to form a dot image on the final image. Even in these processes, the shape of the dot image is further changed from the virtual dots due to the scattering of toner and the like. In this way, the dot image is a change from the virtual dot, but since this change is determined by the electrophotographic process, the dot image can be controlled by controlling the virtual dot.

[0005] In the halftone dot halftoning method, a dot image in a single pixel or a halftone dot consisting of a block of dot images extending over a plurality of adjacent pixels is formed, and a halftone dot image is displayed depending on the size of the halftone dot image. Reproduce the gradation. That is, as the gradation data of each pixel becomes darker, virtual dots are generated to generate halftone dot growth nuclei on the final image, and when the gradation data becomes darker, the virtual dots are generated. As the number and area of the dots increase, the size of the halftone dots gradually increases. Therefore, the halftone dot growth method corresponding to the increase in the grayscale value of the input gradation data is such that the area of the virtual dot grows quickly in the pixels in the center of the halftone dots (near the growth nucleus), and the pixels around the halftone dots. The growth of the virtual dot area is slow at (pixels away from the growth nucleus).

[0006] By the way, in order to improve the image quality of an image formed by an electrophotographic apparatus, it is required to optimize the growth method of a dot image in a pixel region. For example, in many electrophotographic devices, low gradation levels increase the area of each isolated halftone dot, and intermediate gradation levels connect adjacent halftone dots to form a line along the screen angular direction. It is known that a growth method that increases the area leads to high image quality. One of the reasons is that, as described above, in an electrophotographic apparatus, a latent image is formed by using light such as a laser beam, so that the latent image is blurred around the halftone dot latent image, and the ambient temperature and humidity are there. Since development is performed by adhering toner with charging characteristics that depend on the halftone dots, the reproducibility of development around the halftone dots is poor. Therefore, by making the line connecting the halftone dots as much as possible, the peripheral length of the halftone dots Is shortened, and the reproducibility of development can be improved. Therefore, at a low gradation level that cannot be made into a line, the area of the separated halftone dots is increased, and at an intermediate gradation level or higher, the halftone dots arranged in the screen angle direction are connected to form a line. The area of is increased.

[0007] When such a halftone dot growth method is adopted, the virtual dots formed in the pixel region not only change their area according to the gradation level, but also change their positions (development) according to the gradation level. It is also necessary to change the position).

[0008] However, in the conventional image forming method, only changing the laser drive pulse width corresponding to the virtual dot area in the pixel area is considered, and changing the virtual dot position in the pixel area is considered. It has not been. Therefore, the position of the virtual dot in the pixel region is not changed according to the input gradation level as described above.

[0009] Therefore, an object of the present invention is to provide an electrophotographic image forming method and an image forming apparatus thereof capable of forming an image of higher image quality by controlling the positions of virtual dots in a pixel region. is there.

[0010] Another object of the present invention is to flexibly control the position of the virtual dot in the pixel region without increasing the capacity of the conversion table from the input gradation level to the image reproduction information for determining the virtual dot. It is an object of the present invention to provide a method for forming an electrophotographic image and an image forming apparatus thereof.

[Means for Solving the Problems] In order to achieve the above object, the first aspect of the present invention is formed by halftone dots composed of a set of a plurality of dot images formed in each pixel region. In an electrophotographic image forming apparatus that expresses the gradation of light and shade and reproduces an image, the image is reproduced for each pixel by referring to a conversion table having a correspondence between the gradation data of the light and shade of the image and the image reproduction data. It has a halftone processing unit that generates data. The conversion table includes a look-up table group having a correspondence between gradation data and image reproduction data, and a pattern matrix indicating the look-up table to be referred to by being associated with a plurality of pixels in a predetermined region of the image. However, the image reproduction data of the look-up table is characterized by having area data and position data of virtual dots corresponding to the dot image.

[0012] In a more preferred embodiment, the image reproduction data of at least one look-up table is characterized in that the position data differs depending on the gradation level. In a more preferred embodiment, the virtual dot position data indicates information on whether the virtual dot is on the left side or the right side of the pixel region depending on the gradation level. Alternatively, the position data of the virtual dot indicates information on which position (not only the left and right but also an intermediate position) the virtual dot is in the pixel region according to the gradation level.

[0013] According to the invention according to the first aspect, since the position of the virtual dot can be changed to a desired position according to the gradation level of shading, a high-quality image can be generated.

[0014] In the second aspect of the present invention, an electrophotographic image forming apparatus that reproduces an image by expressing gradation of shading by halftone dots composed of a set of a plurality of dot images formed in each pixel region. In the above, the halftone processing unit for generating the image reproduction data for each pixel is provided with reference to the conversion table having the correspondence between the gradation data of the shade of the image and the image reproduction data. Then, the conversion table includes a first look-up table group having a correspondence between the gradation data and the first image reproduction data including the area data of the virtual dots corresponding to the dot image, the gradation data, and the gradation data. The second look-up table group having a correspondence with the second image reproduction data including the position data of the virtual dots, the first look-up table to be referred to by being associated with a plurality of pixels in a predetermined area of the image, and the above. It has a pattern matrix showing a second look-up table. The number of the first and second look-up tables is smaller than the number of elements in the pattern matrix.

[0015] In the invention according to the second aspect described above, a plurality of tables having similar virtual dot area data are collectively required from the first look-up table group corresponding to all the elements of the pattern matrix. The number of tables is less than the number of tables to be used, and similarly, from the second lookup table group, a plurality of tables having similar virtual dot position data are grouped together to reduce the number of tables to the number of tables originally required. Therefore, the total amount of data in the conversion table can be reduced. Therefore, it is possible to have the position data of the virtual dots that change according to the gradation level in the conversion table without increasing the memory capacity constituting the conversion table, and it is possible to generate a high-quality image.

[0016] According to the first and second aspects described above, the growth of halftone dots composed of a block of dot images is grown by separating the halftone dots in a region where the gradation level is low, and in a region where the gradation level is high. It can be easily grown into a line connecting halftone dots in the angle direction of the screen.

[0017] In the third aspect of the present invention, an electrophotographic image forming apparatus that reproduces an image by expressing gradation of shading by halftone dots composed of a set of a plurality of dot images formed in each pixel region. In the above, the halftone processing unit for generating the image reproduction data for each pixel is provided with reference to the conversion table having the correspondence between the gradation data of the shade of the image and the image reproduction data. The conversion table includes a look-up table group having a correspondence between gradation data and image reproduction data, and a pattern matrix showing the look-up table to be referred to by being associated with a plurality of pixels in a predetermined region of an image. However, the pattern matrix is characterized by having virtual dot position data corresponding to the dot image in addition to the lookup table to be referred to.

[0018] According to the third aspect, since the pattern matrix has the lookup table to be referred to and the position data of the virtual dots, the position information for each pixel can be included in the conversion table of the halftone processing unit. High-quality images can be played back.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, examples of embodiments of the present invention will be described with reference to the drawings. However, such embodiments do not limit the technical scope of the invention.

[0020] FIG. 1 is a diagram showing an example of halftone dots in the present embodiment. In this example, for example, by forming virtual dots shown by black areas in the figure in pixel areas D1 to D6 arranged at a pitch of 600 dpi, one halftone dot SP consisting of a block of dot images is generated. .. The formation of the halftone dots SP will be described in the case of a laser beam printer of a type that uses a laser beam whose pulse width is modulated based on image reproduction data.

[0021] In the pixel region D1, virtual dots exist in a region of about 1/4 on the right side of the region, and a laser beam is irradiated in the region corresponding to the virtual dots. The laser beam has, for example, a diameter in the vertical direction of the pixel region, and irradiates a desired region while scanning in the horizontal direction (main scanning direction) in FIG. Therefore, in the case of the pixel region D1, the drive pulse for driving the laser beam has the timing (position) and width corresponding to the region of about 1/4 on the right side of the pixel region. The position of the virtual dot can be controlled by controlling the timing, and the area of the virtual dot can be controlled by controlling the pulse width.

[0022] The pixel area D2 is adjacent to the pixel area D1, and there are virtual dots in an area of about 1/10 on the left side of the pixel area. As a result, the virtual dots having a predetermined width are realized by combining with the virtual dots in the adjacent pixel area D1. Further, in the pixel area D3, there are virtual dots in the entire pixel area. Further, in the pixel area D4 adjacent to the pixel area D4, there are virtual dots in an area of about 2/3 on the left side. As a result, the virtual dots in the pixel areas D3 and D4 become thick virtual dots that are combined. Further, similarly, the pixel area D5 has about half of the virtual dots on the right side, and the pixel area D6 has about 1/4 of the virtual dots on the left side.

A laser beam is irradiated based on such virtual dots, and a dot image is formed through development and transfer. In the case of the mesh point SP shown in FIG. 1, the dot images formed in the pixel areas D1 and D2 are thinner than the dot images formed in the pixel areas D5 and D6, and the adjacent pixel areas D1, D2 and pixels are formed. The dot images formed in the regions D3 and D4 and the pixel regions D5 and D6 are all shifted to the left in each region. As a result, the position of the center of gravity (center of the halftone dots) of the halftone dots SP formed in the pixel areas D1 to D6 is slightly upper left than the center position of the pixel areas D3 to D6 as shown by X in the figure. .. Further, by locating the virtual dots in the pixel area D2 on the left side and locating the virtual dots in the pixel area D5 on the right side, the halftone dots SP on the final image are the halftone dots generated above and below the screen angle direction. Are separated. Therefore, by shifting the positions of the virtual dots in the pixel areas D2 and D5 to, for example, the center or the left side, they are connected to the halftone dots generated above and below, and a line in the screen angular direction is formed.

[0024] As described above, the shape of the dot image does not completely match the virtual dots shown in black in the figure, and therefore, the shape of the halftone dot SP composed of these blocks is a broken line in the figure. It has the shape shown in. Alternatively, as described above, by shifting the positions of the virtual dots within the pixel area, a line-like shape connected to adjacent halftone dots is obtained. Therefore, by changing the position of the virtual dots according to the gradation level of shading, it is possible to freely form halftone dots separated from each other or line-shaped halftone dots connected to each other.

[0025] FIG. 2 is a diagram showing an example of a halftone dot growth method to which the present embodiment is applied. In Fig. 2, halftone dot shapes ((1) in the figure) and corresponding virtual dots ((2) in the figure) are shown in the horizontal direction, and (A) low gradation level and (B) middle floor in the vertical direction. The tonality level and (C) high gradation level are shown.

[0026] As described above, in the region (A) where the gradation level of shading is low, virtual dots are formed in the pixel region of the central region of the halftone dots. As a result, relatively small circular halftone dots SP1 and SP2 are formed apart from each other. In that case, the virtual dot corresponding to the halftone dot SP2 on the right side becomes an area of about 1/3 of the entire pixel area on the right side in the pixel area Da. In addition, virtual dots do not occur in the pixel areas Db and Dc.

(B) In the region where the gradation level of the light and shade is intermediate, the halftone dots SP1 and SP2 generated at the low gradation level have an elliptical shape extending in an oblique direction along the screen angle while increasing the area. Grow to halftone dots SP3 and SP4. Therefore, in the pixel area Da, the virtual dots are formed on the right side at the low gradation level, whereas the positions are changed to the left side at the middle gradation level. As a result, the halftone dot SP4 can be made into an oblique elliptical shape.

[0028] Further, in the region where the gradation level of (C) shading is high, the halftone dots SP3 and SP4, which had an elliptical shape at the middle gradation level, are connected in the screen angle direction while increasing the area to form a line shape. To grow into. Therefore, the entire pixel area Dc is a virtual dot area, and in the pixel area Db, virtual dots are formed on the left side at the medium gradation level, whereas the position is changed to the right side at the high gradation level. There is. This makes it possible to connect the two halftone dots SP3 and SP4.

[0029] As shown in FIG. 2, by forming virtual dots whose positions differ depending on the gradation level, halftone dots are grown apart at a low gradation level, and the halftone dots are grown at a medium gradation level. From to high gradation level, it can be connected and grown in a line shape. Therefore, it is possible to stabilize the development accompanied by toner adhesion and form a high-quality image.

[0030] FIG. 3 is a schematic configuration diagram of an electrophotographic printing system having an image forming apparatus of the present embodiment. In this example, in the host computer 50, image data 56 composed of RGB gradation data (8 bits each for a total of 24 bits) is generated and given to an electrophotographic apparatus 60 such as a page printer. An electrophotographic apparatus 60 such as a page printer reproduces a color image based on the supplied image data 56. The electrophotographic apparatus 60 includes a controller 62 that performs image processing and supplies a laser drive pulse 69 to the engine, and an engine 70 that reproduces an image according to the drive pulse 69.

[0031] In the host computer 50, character data, graphic data, bitmap data, and the like are generated by an application program 52 such as a word processor and a graphic tool. Each of the data generated by these application programs 52 is rasterized by the rasterization function 54 of the driver 80 for the electrophotographic apparatus installed in the host computer 50, and consists of gradation data of each RGB color for each pixel. Converted to image data 56. In this example, the image data 56 is composed of RGB 8-bit and 256-gradation data, respectively, and has a total of 24-bit data.

A microprocessor (not shown) is also built in the electrophotographic apparatus 60, and includes a color conversion unit 64, a halftone processing unit 66, a pulse width modulation unit 68, and the like depending on the microprocessor and the installed control program. The controller 62 is configured. Further, 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 its drive unit, but they are omitted in FIG.

[0033] The color conversion unit 64 in the controller 62 converts the supplied RGB gradation data 56 for each pixel into CMYK gradation data 10 which is the color of the toner. The CMYK gradation data 10 is gradation data of 8 bits for each color for each pixel for each color plane of CMYK, and has a maximum of 256 gradations. The halftone processing unit 66 is supplied with gradation data 10 corresponding to pixels for each plane of each color.

[0034] The halftone processing unit 66 refers to a conversion table having a correspondence between the gradation data created in advance and the image reproduction information for the gradation data 10 for each pixel, and refers to the image reproduction data for each pixel. Generate 30. The halftone processing unit 66 is an image forming apparatus that generates image reproduction data 30 expressing intermediate gradations by using the above-mentioned multi-value dither method. The conversion table (not shown) is as follows.

[0035] FIG. 4 is a diagram showing an example of a conversion table of the image forming apparatus in the first embodiment. As described above, this conversion table is stored in the memory in the halftone processing unit 66 provided in the controller in the electrophotographic apparatus. The image data of FIG. 4A has gradation data of shades of each color for each pixel.

[0036] The pattern matrix of FIG. 4 (B) and the conversion table having the lookup table group of FIG. 4 (C) are associated with the image data. The pattern matrix is composed of a 12 × 12 matrix in this example, and the lookup table for the reference number i (i = 1 to 144) of the pattern matrix is stored in the lookup table group of FIG. 4 (C). Then, the pattern matrix is associated with a predetermined pixel of the image data. For example, if the reference number of the pattern matrix for the pixel P of the image data is 27, the image reproduction information for the gradation data of the pixel P is determined by the lookup table corresponding to the reference number 27 in the lookup table group. To. That is, with reference to the lookup table corresponding to the reference number 27, the image reproduction information which is the output value for the gradation data of the image data which is the input level is read out.

[0037] Each look-up table in the look-up table group (C) has an area data PW and a position data PP of virtual dots formed in the pixel area as image reproduction information for input levels 0 to 255. Has. Area data PW is indicated by 0 to 255 in the lookup table, and position data is indicated by PP in the lookup table, omitted in the figure. The position data PP is, for example, data indicating from which position in the dot a virtual dot is formed along the main scanning direction. Specifically, the laser drive pulse is moved to the right or left in the pixel region. It is data indicating whether to move. The area data PW is, for example, data indicating the width of a virtual dot, and specifically, is data obtained by normalizing the ratio of the pulse width of the laser drive pulse to the pixel region width to 8 bits (0 to 255).

[0038] Therefore, the halftone processing unit 66, which is an image processing device, outputs image reproduction information (image reproduction data) 30 including area data and position data corresponding to the input gradation data with reference to the lookup table. .. According to the image reproduction data 30, the pulse width modulation unit 68 generates a drive pulse 69 corresponding to the virtual dots in the pixel region, and outputs the drive pulse 69 to the engine 70 in synchronization with the timing of the main scanning of the laser beam.

[0039] According to the conversion table shown in FIG. 4, the position data PP and the area data PW are prepared for each level of the input gradation data 0 to 255, so that the virtual data formed in each pixel area is formed. The position and area of the dots can be freely set according to the input gradation level. Therefore, as shown in FIG. 2, the virtual dots can be formed on the right side, on the left side, or at a desired position according to the input gradation level.

FIG. 5 is an example of a characteristic graph showing the relationship between the input gradation level and the area data PW of the output image reproduction data. This characteristic graph is a plot of an output consisting of area data of how much area the pixel region should be irradiated with a laser beam for an input level of 0 to 255 of the input gradation data. Output 0 corresponds to a drive pulse width of 0, and output 255 corresponds to a drive pulse width corresponding to the entire pixel area width. In the figure, the characteristic graph j is an example of area data that takes a large value when the input level is low. The area data of the look-up table referenced by the elements that grow for relatively low gradation levels in the pattern matrix has such characteristics. The characteristic graph m is an example having an output level that is almost proportional to the input level, and corresponds to a look-up table referenced by an element that grows for a relatively intermediate gradation level in the pattern matrix. Then, the characteristic graph p corresponds to the look-up table referred to by the element that does not grow at the stage where the input level is low and grows only when the input level becomes a relatively high gradation.

FIG. 6 is an example of a characteristic graph of a look-up table showing the correspondence between the input gradation level and the position data PP of the output image reproduction data. When the position data PP is 1 bit, the position data PP of the lookup table is on the left side of the pixel area (data "1") with respect to the input gradation level or in the pixel area as shown in FIG. 6 (A). The data indicates either the right side (data "0"). Further, when the position data PP has a plurality of bits, for example, 6 bits, the position data PP of the lookup table has 64 types of positions in the pixel area with respect to the input gradation level as shown in FIG. 6 (B). It becomes the data which shows. Therefore, the positions of the virtual dots in the pixel region can be sequentially changed as shown in the figure.

[0042] Then, in the example of FIG. 4, since 144 types of look-up tables are associated with the pattern matrix of 12 rows × 12 columns, the characteristics of the area data and the position data shown in FIGS. There are 144 types of each.

[0043] FIG. 7 is a diagram showing an example of an index-type conversion table in the second embodiment. In the first embodiment shown in FIG. 4, the conversion table has 144 types of look-up tables for a 12 × 12 pattern matrix. However, in order to form an image with higher image quality, it is required to use a pattern matrix having a larger number of elements. This is because various merits such as being able to set the screen angle to an arbitrary angle can be expected by increasing the pattern matrix.

[0044] In that case, if a one-to-one lookup table is set for all the elements of the pattern matrix, it is necessary to increase the memory capacity of the conversion table of the halftone processing unit 66. Usually, a high-speed semiconductor memory capable of high-speed reference, for example, static RAM (SRAM) is used as the conversion table. Such a large-capacity high-speed semiconductor memory has an adverse effect on cost reduction.

Therefore, in the second embodiment, it is possible to set different area data and position data for each input gradation while reducing the amount of data in the conversion table. Therefore, as shown in FIG. 7, in the second embodiment, an index type conversion table is used.

[0046] The index-type conversion table uses the look-up table group as the area lookup table group (first), which has a correspondence between the gradation data and the area data of the virtual dots, as shown in FIG. 7 (C-1). It is divided into a look-up table group) and a position lookup table group (second look-up table group) having a correspondence between gradation data and virtual dot position data as shown in FIG. 7 (C-2). Then, the conversion table further includes an area and position index table (D) indicating an area lookup table and a position lookup table to be referred to, in addition to the pattern matrix (B) associated with a plurality of pixels in a predetermined area of the reproduced image. It has -1) and (D-2) respectively.

Then, the number of look-up tables in the area lookup table group (C1) is summarized to be smaller than the number of elements 144 of the pattern matrix. That is, among the area lookup table groups having 144 types of characteristics shown in FIG. 5, lookup tables with similar characteristics are grouped together to reduce the total number of tables. Similarly, the number of position lookup tables is also grouped into fewer than 144 elements in the pattern matrix. That is, among the look-up tables of the position data having the characteristics shown in FIG. 6, the lookup tables having similar characteristics are put together to reduce the total number of tables. In the example of FIG. 7, there are 36 types of area lookup tables and 15 types of position lookup tables.

Thus, for example, the lookup table for elements 27 and 36 in the pattern matrix (B) shows the same area lookup table 17 with reference to the area index table (D-1). Therefore, the areas of virtual dots are set for the elements 27 and 36 with the same characteristics. On the other hand, elements 27 and 36 show different position lookup tables 15 and 7 with reference to the position index table (D-2). However, for elements 1 and 2, reference to the area index table (D-1) shows a separate area lookup table, while reference to the position index table (D-2) shows the same position lookup. Table 1 is shown.

[0049] As described above, in the second embodiment, the lookup table is divided into an area lookup table and a position lookup table, and a pattern matrix and an index table are used to refer to each pixel. The up table is a combination of the number of area lookup tables and the number of position lookup tables. That is, the pixels corresponding to the elements 27 and 36 use different position lookup tables even if they have the same area lookup table. Further, in the pixels corresponding to the elements 1 and 2, different area lookup tables are used even if the same position lookup table is used. Therefore, a substantially usable look-up table is composed of a combination of position and area look-up tables, the type of which can be a multiplication value of the number of each table at the maximum.

[0050] In FIG. 7, the area index table (D-1) and the position index table (D-2) are combined, and the area and position lookup tables are used for the elements 1 to 144 of the pattern matrix (B). The combination may be shown.

FIG. 8 is a diagram showing another example of the index method conversion table in the second embodiment. In this example, the pattern matrix of FIG. 7 and the index table are combined and displayed. That is, the area pattern matrix (B-1) has a reference number of which table in the area lookup table group (C-1) should be referred to for 144 elements. Further, the position pattern matrix (B-2) also has a reference number of which table in the position lookup table group (C-2) should be referred to for 144 elements. In the example of FIG. 8, similarly to FIG. 7, different position lookup tables 15 and 7 are referred to for two elements that refer to the same area lookup table 27. Also, different area lookup tables 1 and 2 are referenced for two elements that reference the same position lookup table 1.

[0052] FIG. 9 is a diagram showing a specific example of an index type pattern matrix. This example has a lookup table number to reference for the elements of the 12x12 matrix. That is, it corresponds to the example of the area pattern matrix or the position pattern matrix of FIG. As shown in FIG. 9, the lookup table 1 is associated with a plurality of elements D1,1, D1,4, D2,7, D3,10, D7,12, and D10,11 among the 144 elements.

FIG. 10 is a diagram showing an example of grown halftone dots. This is an example of halftone dots when the pattern matrix in FIG. 9 is assumed to be an area pattern matrix. For pixels corresponding to elements D1,1, D1,4, D2,7, D3,10, D7,12, and D10,11. Since the same area lookup table 1 is associated, it is shown that virtual dots (black parts) having the same area are formed.

[0054] FIG. 11 is a diagram showing an example of a conversion table in the third embodiment. In this example, the corresponding area lookup table number and the virtual dot position data PP are stored in the pattern matrix (B). Therefore, as a look-up table ,. Only the area lookup table group (C) consisting of a plurality of area lookup tables is provided. Therefore, in the case of the conversion table of the third embodiment, the elements D1 and 1 refer to the area lookup table 1, but the position data only uses the fixed data PP in the pattern matrix. is there. The position data PP is set independently for each element of the pattern matrix.

FIG. 12 is another configuration diagram of the electrophotographic printing system. This system configuration example is a modification of the system configuration example shown in FIG. In the system of FIG. 12, the driver 80 installed on the host computer 50 has a rasterization function 54, a color conversion function 64, and a halftone processing function 66. Each of these functions 64 and 66 has the same function as the processing unit with the same reference number shown in FIG. Then, the image reproduction data (pulse width data and pulse position data) 30 for each color generated by the halftone processing function is transmitted to the pulse width modulation unit 68 of the controller 62 provided in the electrophotographic apparatus 60 such as a page printer. It is supplied, converted into the desired drive data (or drive pulse) 69, and given to the engine 70.

In the system example of FIG. 12, color conversion processing and halftone processing are performed by the driver 80 installed on the host computer side. In the example of FIG. 3, the color conversion processing and the halftone processing are performed by the controller in the electrophotographic apparatus, but in the example of FIG. 12, they are performed on the host computer 50 side. When the price of the electrophotographic apparatus 60 is required to be reduced, it is required to reduce the capacity of the controller 62 to reduce the price. In that case, it is effective to realize the color conversion processing and halftone processing, which are part of the functions performed by the controller in Fig. 3, by the driver program installed on the host computer instead. When halftone processing is realized by the driver 80, a storage medium containing a program for causing the computer to execute the above-mentioned halftone processing procedure is built in the host computer 50.

[0057] In the above embodiment, the conversion table of the halftone processing unit includes a look-up table of pulse width data corresponding to the area of virtual dots and a look-up table of pulse position data corresponding to the positions of virtual dots. By having each of the above, it is possible to form a display dot having an arbitrary area at an arbitrary position according to the gradation data. Therefore, by using it, halftone dots formed by a block of dot images can be formed at an arbitrary shape and position according to the gradation level of shading.

[0058] For example, as shown in FIG. 2, round halftone dots can be grown at a gradation level with low shading, and can be grown in a line shape at a gradation level with medium to high shading, resulting in high image quality. The image can be played back. Alternatively, by setting the center position of the halftone dots to an arbitrary position according to the position and area of the virtual dots, the screen angle defined by the positions of a plurality of halftone dots can be set to an arbitrary angle, and the height with less moire pattern. It is also possible to play back high-quality images.

[0059] The present invention has been described above by taking a color electrophotographic image forming apparatus using a laser beam as an example. However, the modulation direction of the pulse width is set as the sub-scanning direction, and the positions such as "right" and "left" are images. By reading "upper" and "lower" in the region, the present invention can be directly applied to an electrophotographic image forming apparatus using an LED line head. The present invention can also be applied to a monochrome electrophotographic apparatus, and has high gradation reproduction capability and resolution while reducing the effects of drive system feed unevenness, environmental fluctuations, manufacturing variations, etc. related to image formation. It goes without saying that it is possible to obtain high-quality image output that has both capabilities.

[0060] As described above, the scope of protection of the present invention is not limited to the above-described examples of embodiments, but extends to the inventions described in the claims and their equivalents.

[Effects of the Invention] As described above, according to the image forming apparatus and method of the present invention, virtual dots having an arbitrary area can be formed in a pixel region at an arbitrary position according to a gradation level of shading. It is possible to generate possible image reproduction data.

BRIEF DESCRIPTION OF THE DRAWINGS [FIG. 1] FIG. 1 is a diagram showing an example of halftone dots in an example of the present embodiment.

FIG. 2 is a diagram showing an example of a halftone dot growth method to which the present embodiment is applied.

FIG. 3 is a schematic configuration diagram of an electrophotographic printing system having an image forming apparatus of the present embodiment.

FIG. 4 is a diagram showing an example of a conversion table of an image forming apparatus in the first embodiment.

FIG. 5 is an example of a look-up table showing the relationship between the input gradation level and the area data of the output image reproduction data.

FIG. 6 is an example of a look-up table showing the relationship between the input gradation level and the position data of the output image reproduction data.

FIG. 7 is a diagram showing an example of an index-type conversion table in the second embodiment.

FIG. 8 is a diagram showing another example of an index-type conversion table in the second embodiment.

FIG. 9 is a diagram showing a specific example of an index type pattern matrix.

FIG. 10 is a diagram showing an example of grown halftone dots.

FIG. 11 is a diagram showing an example of a conversion table in the third embodiment.

FIG. 12 is another configuration diagram of an electrophotographic printing system.

[Code description] 60 Electrophotographic device 62 Controller 66 Halftone processing unit, image forming device D Pixel or pattern matrix element SP halftone dot