JP2007295589A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form an image with an enhanced quality by adding a function of position control of virtual dots in an image region according to gradation levels, while preventing increase of capacity of a conversion table. <P>SOLUTION: A conversion table of a half-tone processor of electrophotography for representing gray level gradation by a dot matrix includes a pattern matrix B corresponding to a plurality of picture elements in a predetermined region of a reproduced image, an area lookup table group C-1 in which gradations data correspond to area data of virtual dots, a position lookup table group C-2 in which gradation data correspond to position data of virtual dots, and area and position index tables D-1 and D-2 which indicate corresponding portions of the lookup table groups C-1 and C-2 to be referred to for the elements of the matrix B. Each of the area and lookup table groups collects the respective data of similar virtual dots, so that the number of tables can be made smaller than that of the elements of the pattern matrix. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus that performs halftone processing using halftone dots composed of a plurality of dot images and an image forming program product thereof, and more particularly, to form a virtual dot having a desired area at a desired position in a pixel region. The present invention relates to a novel image forming apparatus and an image forming program product that can improve image quality.

  A color electrophotographic apparatus widely used in a color printer, a color copy, and 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 that uses a laser beam to form a latent image on a photoconductor, 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 controlling the driving of the laser beam. Among them, in particular, in the type that modulates the width of the pulse for driving the laser beam, the irradiation region of the laser beam can be variously changed in the pixel region, and even when the number of pixels per unit area is small, A color image with higher resolution and higher gradation can be reproduced.

  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 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 a pixel area is described for gradation data for each color as an input signal. The position and size of the virtual dot in each pixel area are determined with reference to the conversion table. By setting a plurality of levels between 0 and the maximum size as this size, the output at each pixel is “multi-valued”.

  The “virtual dot” here is defined by an area scanned with a laser beam to form a dot image with toner on the final image, and its size in the main scanning direction is driven by the laser beam. The size of the pixel region in the sub-scanning direction is equal to the size of the pixel region 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 that spreads more than the virtual dots 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 these processes, the toner image is scattered, so that the shape of the dot image is further changed from the virtual dot. As described above, the dot image is changed 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.

  In the halftone method of halftone dots, a dot image in a single pixel or a halftone dot consisting of a cluster of dot images over a plurality of adjacent pixels is formed, and the gradation of the grayscale image is adjusted according to the size of the halftone dot. Reproduce. That is, as the gray value of the gradation data of each pixel becomes darker, virtual dots are generated, a growth nucleus of the halftone dot on the final image is generated, and when the gray value of the gradation data becomes darker, the virtual dots As the number and area increase, the size of the halftone dots gradually increases. Therefore, the halftone dot growth method corresponding to the increase in the gray value of the input gradation data has a fast growth of the area of the virtual dot 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.

  Patent Document 1 below refers to an index table corresponding to a pixel, a gamma table that is a reference destination of the index table, and a gamma conversion cell that is a reference destination of the gamma table, and image gradation data of each pixel. Describes an image processing apparatus for converting the signal into pulse width data.

Japanese Patent Application Laid-Open No. 2004-259561 discloses an image processing apparatus that varies the center position of a halftone dot by changing a gamma table to be referred to, thereby suppressing feed unevenness extending in the main scanning direction and unevenness extending in the complex Usa direction. Are listed.
Japanese Patent Laid-Open No. 2000-22529 JP 2000-94752 A

  By the way, in order to improve the quality of an image formed by an electrophotographic apparatus, it is required to optimize the dot image growth method in the pixel region. For example, in many electrophotographic apparatuses, the area of each isolated halftone dot is increased at a low gradation level, and adjacent halftone dots are connected at an intermediate gradation level to form a line along the screen angle direction. It is known that a growth method that increases the area leads to high image quality. One reason for this is that, as described above, since a latent image is formed using light such as a laser beam in an electrophotographic apparatus, the latent image is blurred around the halftone dot latent image, and the ambient temperature and humidity are there. Since the development is performed by attaching toner having charging characteristics that depend on the density, the reproducibility of development around the halftone dots is poor. Therefore, the perimeter of the halftone dots is obtained by connecting the halftone dots as much as possible. This is because the reproducibility of development can be improved. Therefore, the area of each separated halftone dot is increased at a low gradation level that cannot be formed into a line, and at an intermediate gradation level or higher, halftone dots arranged in the screen angle direction are connected to form a line. The area is increased.

  When such a halftone dot growth method is adopted, the virtual dots formed in the pixel region not only change the area according to the gradation level but also change the position (development position) according to the gradation level. You also need to change.

  However, the conventional image forming method only considers changing the laser driving pulse width corresponding to the virtual dot area in the pixel region, and does not consider changing the virtual dot position in the pixel region. . Therefore, the position of the virtual dot in the pixel area is not changed according to the input gradation level as described above.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an electrophotographic image forming method and an image forming apparatus thereof capable of forming a higher quality image by controlling the position of virtual dots in a pixel region.

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

  In order to achieve the above object, according to a first aspect of the present invention, an image is reproduced by expressing shades of gray with halftone dots each consisting of a set of a plurality of dot images formed in each pixel region. The electrophotographic image forming apparatus includes a halftone processing unit that generates image reproduction data for each pixel with reference to a conversion table having correspondence between grayscale data of the image and image reproduction data. The conversion table includes a look-up table group having correspondence between gradation data and image reproduction data, and a pattern matrix indicating the look-up table to be referred to in association with a plurality of pixels in a predetermined area of the image. The image reproduction data of the lookup table includes virtual dot area data and position data corresponding to the dot image.

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

  According to the first aspect of the invention, the position of the virtual dot can be changed to a desired position according to the light and shade gradation level, so that a high-quality image can be generated.

  According to a second aspect of the present invention, in an electrophotographic image forming apparatus that reproduces an image by expressing gradations of light and shade by a halftone dot composed of a set of a plurality of dot images formed in each pixel region, A halftone processing unit that generates image reproduction data for each pixel with reference to a conversion table having correspondence between grayscale data of the image and image reproduction data is provided. The conversion table includes a first look-up table group having correspondence between gradation data and first image reproduction data including area data of virtual dots corresponding to the dot image, gradation data, A second look-up table group having correspondence with second image reproduction data including virtual dot position data, the first look-up table to be referred to in association with a plurality of pixels in a predetermined region of the image, and A pattern matrix indicating a second lookup table. The number of the first and second look-up tables is smaller than the number of elements of the pattern matrix.

  In the invention according to the second aspect, a plurality of tables having similar virtual dot area data are collected from the first lookup table group corresponding to all elements of the pattern matrix, and the number of tables originally required Similarly, it is possible to combine a plurality of tables having similar virtual dot position data from the second look-up table group so that the number of tables is smaller than the number of tables originally required. Therefore, the total data amount of the conversion table can be reduced. Therefore, without increasing the memory capacity constituting the conversion table, the position data of the virtual dots that change according to the gradation level can be provided in the conversion table, and a high-quality image can be generated.

  According to the first and second aspects described above, the growth of halftone dots composed of a cluster of dot images is caused to grow separately in the area where the gradation level is low, and in the screen angle direction in the area where the gradation is higher. It is easy to grow in the shape of a line connecting the halftone dots.

  According to a third aspect of the present invention, in an electrophotographic image forming apparatus that reproduces an image by expressing gradation of light and shade by a halftone dot comprising a set of a plurality of dot images each formed in a pixel region, A halftone processing unit that generates image reproduction data for each pixel with reference to a conversion table having correspondence between grayscale data of the image and image reproduction data is provided. The conversion table includes a look-up table group having correspondence between gradation data and image reproduction data, and a pattern matrix indicating the look-up table to be referred to in association with a plurality of pixels in a predetermined area of the image. The pattern matrix includes virtual dot position data corresponding to the dot image in addition to a lookup table to be referred to.

  According to the third aspect, since the look-up table to be referred to in the pattern matrix and the position data of the virtual dots are included, the position information for each pixel can be included in the conversion table of the halftone processing unit. Images can be played back.

  As described above, according to the image forming apparatus and method of the present invention, it is possible to generate image reproduction data that can form a virtual dot having an arbitrary area in an arbitrary position in a pixel area in accordance with a gradation level of lightness and darkness. Can do.

  Embodiments of the present invention will be described below with reference to the drawings. However, such an embodiment does not limit the technical scope of the present invention.

  FIG. 1 is a diagram illustrating an example of halftone dots in the present embodiment. In this example, for example, virtual dots indicated by black areas in the drawing are formed in the pixel areas D1 to D6 arranged at a pitch of 600 dpi, thereby generating one halftone dot SP consisting of a cluster of dot images. . The formation of the halftone dots SP will be described in the case of a laser beam printer of the type using a laser beam that has been pulse width modulated based on image reproduction data.

  In the pixel region D1, a virtual dot exists in the approximately ¼ region on the right side of the region, and a laser beam is irradiated in the region corresponding to the virtual dot. The laser beam has a diameter in the vertical direction of the pixel region, for example, and is applied to 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 a timing (position) and a width corresponding to about a quarter region 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.

  The pixel area D2 is adjacent to the pixel area D1, and there is a virtual dot in about 1/10 of the left side of the pixel area. Thus, a virtual dot having a predetermined width is realized by combining with the virtual dot of the adjacent pixel region D1. Further, in the pixel region D3, there are virtual dots throughout the pixel region. Further, in the pixel region D4 adjacent thereto, there is a virtual dot in the region about 2/3 on the left side. Thereby, the virtual dots of the pixel regions D3 and D4 become a combined thick virtual dot. Similarly, the pixel area D5 has about half of the virtual dots on the right side, and the pixel area D6 has the virtual dots on about ¼ of 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 halftone dot SP shown in FIG. 1, the dot images formed in the pixel areas D1 and D2 are narrower than the dot images formed in the pixel areas D5 and D6, and are adjacent to the adjacent pixel areas D1, D2, and pixels. 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 barycentric position (the center of the halftone dot) of the halftone dots SP formed in the pixel areas D1 to D6 is slightly higher than the center position of the pixel areas D3 to D6, as indicated by X in the figure. . Further, by placing the virtual dots in the pixel region D2 on the left side and the virtual dots in the pixel region D5 on the right side, the halftone dots SP on the final image are the halftone dots generated above and below in the screen angle direction. Are separated. Therefore, by shifting the positions of the virtual dots in the pixel regions D2 and D5 to, for example, the center or the left side, the dots are connected to the vertically generated halftone dots, and lines in the screen angle direction are formed.

  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 dots SP formed by these lumps is indicated by a broken line in the figure. Become a shape. Alternatively, as described above, by shifting the position of the virtual dot within the pixel region, a line shape connected to the adjacent halftone dot is obtained. Therefore, by changing the position of the virtual dot according to the gradation level of light and shade, it is possible to freely form halftone dots that are separated from each other and line-like halftone dots that are connected to each other.

  FIG. 2 is a diagram showing an example of a halftone dot growing method to which the present embodiment is applied. In FIG. 2, a halftone dot shape ((1) in the figure) and a corresponding virtual dot ((2) in the figure) are shown in the horizontal direction, (A) low gradation level in the vertical direction, and (B) middle floor. A tone level and (C) a high tone level are shown.

  As described above, (A) in a region where the gradation level of shading is low, virtual dots are formed in the pixel region in 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 this case, the virtual dot corresponding to the right halftone dot SP2 is an area of about 1/3 of the entire pixel area on the right side in the pixel area Da. Further, virtual dots are not generated in the pixel areas Db and Dc.

  (B) In a region where the gradation level is light and dark, the halftone dots SP1 and SP2 generated at the low gradation level are increased in area and are elliptical halftone dots SP3 extending obliquely along the screen angle. , Grow to SP4. Therefore, in the pixel area Da, a virtual dot is formed on the right side at the low gradation level, whereas the position is changed on the left side in the middle gradation level. Thereby, the shape of halftone dot SP4 can be made into the slant ellipse shape.

  Further, (C) in the region where the gradation level of light and shade is high, the halftone dots SP3 and SP4, which are elliptical at the intermediate gradation level, are grown in a line shape by increasing the area and connecting them in the screen angle direction. . For this reason, 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 middle gradation level, whereas the position is changed on the right side at the high gradation level. Yes. Thereby, two halftone dots SP3 and SP4 can be connected.

  As shown in FIG. 2, by forming virtual dots whose positions differ according to the gradation level, the halftone dots are grown separately at the low gradation level, and from the intermediate gradation level to the high gradation. At the level, they can be joined together and grown into a line. Therefore, development with toner adhesion can be stabilized and high-quality images can be formed.

  FIG. 3 is a schematic configuration diagram of an electrophotographic printing system having the image forming apparatus according to the present embodiment. In this example, the host computer 50 generates image data 56 composed of RGB gradation data (8 bits for each of 24 bits in total), and supplies 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 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 in accordance with the drive pulse 69.

  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 or a graphic tool. Each data generated by the application program 52 is rasterized by the rasterizing function 54 of the driver 80 for the electrophotographic apparatus installed in the host computer 50, and is composed of gradation data of RGB colors for each pixel. It is converted into image data 56. In this example, the image data 56 is composed of RGB data of 8 bits and 256 gradations, for a total of 24 bits.

  The electrophotographic apparatus 60 also includes a microprocessor (not shown), and a controller 62 including a color conversion unit 64, a halftone processing unit 66, a pulse width modulation unit 68, and the like according to the microprocessor and a control program installed therein. Composed. 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.

  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 a toner color. The CMYK gradation data 10 is gradation data of 8 bits for each color for each pixel for each CMYK color plane, and has 256 gradations at the maximum. The halftone processing unit 66 is supplied with gradation data 10 corresponding to a pixel for each color plane.

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

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

  Such image data is associated with a conversion table having the pattern matrix in FIG. 4B and the lookup table group in FIG. The pattern matrix is a 12 × 12 matrix in this example, and a lookup table for the reference number i (i = 1 to 144) of the pattern matrix is stored in the lookup table group in FIG. Then, the pattern matrix is associated with predetermined pixels of the image data. For example, when the reference number 27 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. The That is, with reference to the look-up table corresponding to the reference number 27, image reproduction information that is an output value for gradation data of image data that is an input level is read.

  Each lookup table in the lookup table group (C) has area data PW and position data PP of virtual dots formed in the pixel region as image reproduction information for input levels 0 to 255. The area data PW is indicated by 0 to 255 in the lookup table, and the position data is indicated by PP in the lookup table, omitted in the drawing. The position data PP is, for example, data indicating from which position in the dot the virtual dot is formed along the main scanning direction. Specifically, the laser drive pulse is shifted to the right or left in the pixel area. It is data indicating whether or not to send. The area data PW is, for example, data indicating the width of the virtual dot, and specifically, 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).

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

  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 the positions of the virtual dots formed in each pixel area And the area can be freely set according to the input gradation level. Therefore, as shown in FIG. 2, it is possible to form virtual dots 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 output composed of area data indicating how much area should be irradiated with a laser beam with respect to an input level of 0 to 255 of input gradation data. The output 0 corresponds to the drive pulse width 0, and the output 255 corresponds to the full width of the pixel area. In the figure, a 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 referred to by elements that grow for a relatively low gradation level in the pattern matrix has such characteristics. The characteristic graph m is an example having an output level substantially proportional to the input level, and corresponds to a look-up table referred to by an element that grows for a relatively intermediate gradation level in the pattern matrix. The characteristic graph p does not grow at a stage where the input level is low, and corresponds to a look-up table referred to by an element that 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 look-up table is the left side of the pixel area (data “1”) of the pixel area with respect to the input gradation level as shown in FIG. The data indicates either the right side (data “0”). In addition, when the position data PP is a plurality of bits, for example, 6 bits, the position data PP in the look-up table has 64 types of positions in the pixel area with respect to the input gradation level as shown in FIG. It becomes the data which shows. Therefore, the position of the virtual dot in the pixel area can be sequentially changed as shown in the figure.

  In the example of FIG. 4, 144 types of lookup tables are associated with the pattern matrix of 12 rows × 12 columns, so that the characteristics of the area data and the position data shown in FIGS. is there.

  FIG. 7 is a diagram illustrating 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 lookup tables for a 12 × 12 pattern matrix. However, in order to form a higher quality image, it is required to use a pattern matrix having a larger number of elements. This is because by enlarging the pattern matrix, various merits such as being able to set the screen angle to an arbitrary angle are expected.

  In that case, if the look-up table is set on a one-to-one basis for all the elements of the pattern matrix, the memory capacity of the conversion table included in the halftone processing unit 66 needs to be enormous. Usually, a high-speed semiconductor memory capable of high-speed reference, for example, a static RAM (SRAM) is used as the conversion table. Such a large-capacity high-speed semiconductor memory is an adverse effect of cost reduction.

  Therefore, in the second embodiment, while reducing the data amount of the conversion table, different area data and position data can be set for each input gradation. For this purpose, as shown in FIG. 7, the second embodiment uses an index conversion table.

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

  The number of look-up tables in the area look-up table group (C1) is grouped into a smaller number than the number 144 of pattern matrix elements. In other words, among the area look-up table groups having 144 types of characteristics shown in FIG. 5, look-up tables having similar characteristics are collected to reduce the total number of tables. Similarly, the number of position look-up tables is also reduced to a number smaller than the number 144 of pattern matrix elements. That is, among the look-up tables for position data having the characteristics shown in FIG. 6, look-up tables with similar characteristics are collected 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.

  Therefore, for example, the lookup table for the 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 elements 27 and 36 set the virtual dot area with the same characteristics. On the other hand, elements 27 and 36 show different position look-up tables 15 and 7 with reference to the position index table (D-2). However, for elements 1 and 2, separate area lookup tables are shown when the area index table (D-1) is referenced, but the same position lookup is seen when the position index table (D-2) is referenced. Table 1 is shown.

  As described above, in the second embodiment, the lookup table is divided into the area lookup table and the position lookup table, and the lookup table to be referred to for each pixel is obtained using the pattern matrix and the index table. The number of area lookup tables and the number of position lookup tables are combined. That is, for the pixels corresponding to the elements 27 and 36, different position look-up tables are used even with the same area look-up table. Further, in the pixels corresponding to the elements 1 and 2, different area lookup tables are used even if they are the same position lookup table. Therefore, the substantially usable look-up table is constituted by a combination of the position and area look-up tables, and the type can be a multiplication value of each table number at the maximum.

  In FIG. 7, the area index table (D-1) and the position index table (D-2) are combined to show the combinations of area and position lookup tables for the elements 1 to 144 of the pattern matrix (B). You may do it.

  FIG. 8 is a diagram illustrating another example of the index-based 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 indicating 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 indicating which table in the position lookup table group (C-2) should be referred to for 144 elements. Also in the example of FIG. 8, as in FIG. 7, different position lookup tables 15 and 7 are referred to for two elements that refer to the same area lookup table 27. Different area lookup tables 1 and 2 are referred to for two elements that refer to the same position lookup table 1.

  FIG. 9 is a diagram illustrating a specific example of an index pattern matrix. This example has the number of the lookup table to be referenced for the elements of the 12x12 matrix. That is, it corresponds to an 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, D10,11 among 144 elements.

  FIG. 10 is a diagram illustrating an example of a grown halftone dot. FIG. 9 is an example of halftone dots when the pattern matrix in FIG. 9 is assumed to be an area pattern matrix, and in pixels corresponding to elements D1,1, D1,4, D2,7, D3,10, D7,12, D10,11. Since the same area look-up table 1 is associated, it is indicated that virtual dots (black portions) having the same area are formed.

  FIG. 11 is a diagram illustrating 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 lookup table: Only an area lookup table group (C) composed of a plurality of area lookup tables is provided. Therefore, in the case of the conversion table of the third embodiment, the element D1,1 refers to the area lookup table 1, except that 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 in 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 a function equivalent to the processing unit having the same reference number shown in FIG. Then, image reproduction data (pulse width data and pulse position data) 30 for each color generated by the halftone processing function is sent to a pulse width modulation section 68 of a controller 62 provided in an electrophotographic apparatus 60 such as a page printer. This is supplied, converted into desired drive data (or drive pulse) 69, and supplied to the engine 70.

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

  In the above embodiment, the conversion table of the halftone processing unit includes a pulse width data lookup table corresponding to the virtual dot area and a pulse position data lookup table corresponding to the virtual dot position, respectively. By having it, display dots of an arbitrary area can be formed at an arbitrary position according to the gradation data. Therefore, by utilizing this, halftone dots formed by a cluster of dot images can be formed in an arbitrary shape and position according to the grayscale level.

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

  As described above, the color electrophotographic image forming apparatus using a laser beam has been described as an example of the present invention. The pulse width modulation direction is the sub-scanning direction, and positions such as “right” and “left” are within the image area. In other words, 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, which can reduce the influence of drive system unevenness, environmental fluctuations, manufacturing variations, etc., related to image formation, while also providing high gradation reproduction capability and resolution. Needless to say, it is possible to obtain a high-quality image output with compatible capabilities.

  As described above, the protection scope of the present invention is not limited to the above-described embodiment, but extends to the invention described in the claims and equivalents thereof.

It is a figure which shows the example of the halftone dot in this Example. It is a figure which shows an example of the growth method of the halftone dot to which this example of an embodiment is applied. 1 is a schematic configuration diagram of an electrophotographic printing system having an image forming apparatus according to an embodiment of the present invention. 6 is a diagram illustrating an example of a conversion table of the image forming apparatus in the first embodiment. FIG. 6 is an example of a look-up table showing a relationship between an input gradation level and area data of output image reproduction data. 6 is an example of a look-up table showing a relationship between an input gradation level and position data of output image reproduction data. It is a figure which shows the example of the conversion table of the index system in a 2nd embodiment. It is a figure which shows another example of the conversion system of the index system in a 2nd embodiment. It is a figure which shows the specific example of the pattern matrix of an index system. It is a figure which shows the example of the halftone dot which grew. It is a figure which shows the example of a conversion table in the example of 3rd Embodiment. It is another block diagram of an electrophotographic printing system.

Explanation of symbols

60 electrophotographic device 62 controller 66 halftone processing unit, image forming device D pixel or pattern matrix element SP halftone dot

Claims (12)

In an electrophotographic image forming apparatus that reproduces an image by expressing gradation of light and shade by a halftone dot consisting of a set of a plurality of dot images formed in each pixel,
A halftone processing unit that generates image reproduction data for each pixel with reference to a conversion table having correspondence between grayscale data of the image and image reproduction data;
The conversion table includes a plurality of lookup tables having a correspondence between the gradation data and the image reproduction data, and a pattern matrix indicating the lookup table to be referenced in association with a plurality of pixels in a predetermined area of the image. And the image reproduction data of the lookup table includes image area data and position data of virtual dots corresponding to the dot image. In claim 1,
The image forming apparatus according to claim 1, wherein the position data in at least one of the image reproduction data of the lookup table is different depending on the gradation level. In claim 1,
The image forming apparatus according to claim 1, wherein the position data includes at least information on whether the virtual dot is on the right side or the left side in the pixel region, and includes any information according to the gradation level. In claim 1,
The position data includes position information indicating in which position in the pixel area the virtual dot is located, and at least one lookup table has a position of the position information that changes according to the gradation level. An image forming apparatus. In an electrophotographic image forming apparatus that reproduces an image by expressing gradations of light and shade by a halftone dot consisting of a set of a plurality of dot images formed in each pixel,
A halftone processing unit that generates image reproduction data for each pixel with reference to a conversion table having correspondence between grayscale data of the image and image reproduction data;
The conversion table includes a plurality of first look-up tables having correspondence between the gradation data and first image reproduction data including area data of virtual dots corresponding to the dot image, the gradation data, A plurality of second look-up tables having a correspondence with the second image reproduction data including the virtual dot position data, and the first look to be associated with a plurality of pixels in the predetermined region of the image. An up table and a pattern matrix indicating the second look-up table;
The number of said 1st and 2nd look-up tables is less than the number of elements of the said pattern matrix, The image forming apparatus characterized by the above-mentioned. In claim 5,
The image forming apparatus according to claim 1, wherein the position data in at least one of the image reproduction data of the lookup table is different depending on the gradation level. In claim 5 or 6,
The image forming apparatus, wherein the pattern matrix includes first and second elements associated with the same first lookup table and associated with different second lookup tables. In claim 5 or 6,
The image forming apparatus, wherein the pattern matrix includes first and second elements associated with different first lookup tables and associated with the same second lookup table. In claim 1 or 5,
In the reproduced image, when the gray level is at the first level, the halftone dots grow apart from each other, and the second gray level is higher than the first level. In the image forming apparatus, the halftone dots are connected to each other and grow in a line shape. In an electrophotographic image forming apparatus that reproduces an image by expressing gradations of light and shade by a halftone dot consisting of a set of a plurality of dot images formed in each pixel,
A halftone processing unit that generates image reproduction data for each pixel with reference to a conversion table having correspondence between grayscale data of the image and image reproduction data;
The conversion table includes a plurality of lookup tables having a correspondence between the gradation data and the image reproduction data, and a pattern matrix indicating the lookup table to be referenced in association with a plurality of pixels in a predetermined area of the image. And an image forming apparatus characterized in that the pattern matrix includes, in addition to a lookup table to be referred to, virtual dot position data corresponding to the dot image. In an electrophotographic image forming program product that reproduces an image by expressing gradation of light and shade by a halftone dot consisting of a set of a plurality of dot images formed in each pixel,
With reference to a conversion table having correspondence between grayscale data of the image and image reproduction data, the computer is caused to execute halftone processing for generating image reproduction data for each pixel,
The conversion table includes a plurality of lookup tables having a correspondence between the gradation data and the image reproduction data, and a pattern matrix indicating the lookup table to be referenced in association with a plurality of pixels in a predetermined area of the image. And the image reproduction data of the look-up table includes area data and position data of virtual dots corresponding to the dot image. In an electrophotographic image forming program product that reproduces an image by expressing gradation of light and shade by a halftone dot consisting of a set of a plurality of dot images formed in each pixel,
With reference to a conversion table having correspondence between grayscale data of the image and image reproduction data, the computer is caused to execute halftone processing for generating image reproduction data for each pixel,
The conversion table includes a plurality of first look-up tables having correspondence between the gradation data and first image reproduction data including area data of virtual dots corresponding to the dot image, the gradation data, A plurality of second look-up tables having a correspondence with the second image reproduction data including the virtual dot position data, and the first look to be associated with a plurality of pixels in the predetermined region of the image. An up table and a pattern matrix indicating the second look-up table;
An image forming program product, wherein the number of the first and second look-up tables is smaller than the number of elements of the pattern matrix.

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