JP4337670B2 - Image processing apparatus, image processing method, and program - Google Patents

Description

  The present invention relates to halftone processing of gradation image data in an image processing apparatus such as a laser printer. Specifically, the dot (or halftone dot) corresponding to the input tone value in the cell is generated at the center of gravity of the fixed cell frame, and the difference in inclination of the cell (or dot or halftone dot) between the two colors is approximately 30. This is related to halftone processing.

  2. Description of the Related Art Conventionally, an image processing apparatus such as a printer converts gradation data having a multi-value gradation value for each pixel into a binary value representing the presence / absence of a dot and performs printing on printing paper. Has been made. In general, a process of converting a multi-value gradation value into a binary value is referred to as a halftone process.

  As such halftone processing, there is known halftone processing by so-called halftone processing in which dots are generated by comparing an input gradation value and a threshold matrix (conversion table) having a predetermined size. In the halftone processing, the halftone dot size changes according to the input grayscale value, and the halftone of the input image is reproduced by the halftone dot size.

  In general, in commercial printing apparatuses such as newspapers and magazines, halftone dots of CMYK (cyan, magenta, yellow, and black) are superimposed by halftone processing to reproduce a color image. Depending on the angle of the halftone dot (screen angle) of each color, so-called moire fringes are generated due to the periodicity of the halftone dot, so that it can be minimized by tilting it at an angle of about 30 ° between the two colors. It realizes a visually comfortable overlay called a rosette pattern. Conventionally, halftone dots are set such that Y is 0 °, K is 45 °, and C or M is 15 ° or 75 °.

  However, in an image processing apparatus such as a laser printer, since the dot arrangement is fixed in the main scanning direction in which the laser beam is scanned and the sub-scanning direction in which the paper is fed, the screen angle can be set arbitrarily as in a commercial printing apparatus. The angle cannot be set.

  Therefore, conventionally, there is a technique that realizes a screen angle of substantially 30 ° between two colors by shifting the center of gravity of the halftone dot from the center position of the dot (for example, Patent Document 1 below). .

In addition, there is a technique in which a parameter value representing a halftone dot angle or the like is determined from a moire fringe interval in an actual output image to obtain a high-quality color image (for example, Patent Documents 2 and 3 below). ).
JP 2000-228728 A JP 2002-101297 A JP-A-5-176170

  However, all of the above-mentioned patent documents do not change in that an output tone value indicating the presence / absence of a dot is obtained by comparing the input tone value with a threshold value. In such halftone processing, the halftone dot position is fixed. Therefore, the resolution is deteriorated to the halftone dot period, and there are problems such as jaggy jagged characters and difficult to read thin characters. This is because the input gradation value and the output gradation value are changed such that the input gradation value is cut off due to the threshold setting, etc., and the thin line is not reproduced, or vice versa, the input gradation value is converted to a thick line with a low input gradation value. The discrepancy is considered as one cause.

  Therefore, an object of the present invention is to provide an image processing apparatus, an image processing method, and a program that can obtain an output image with excellent resolution while keeping moiré fringes to a minimum.

  Another object of the present invention is to provide an image processing apparatus compatible with a commercial printing apparatus in which the screen angle between two colors is set to approximately 30 °.

  In order to achieve the above object, the present invention is based on a predetermined pixel group in an image processing apparatus for converting input image data of a plurality of colors into output image data having gradation values of two or more gradations. Centroid position determining means for determining the centroid position for each color component from the input image data of each pixel included in the pixel group, and providing means for assigning gradation values to the pixels based on the centroid position determined by the centroid position determining means; The difference in inclination of the pixel group between any two of the plurality of colors is approximately 30 °. Thereby, for example, dots and halftone dots are generated at the center of gravity of each cell, and an output image with higher resolution can be obtained as compared with the conventional threshold processing. Further, since the inclination of the pixel group is approximately 30 ° between the two colors, an output image with the minimum moire fringes can be obtained.

  Further, the present invention is characterized in that the image processing apparatus further comprises pulse width modulation means for generating a K-level (K is a positive integer) pulse width for the gradation value given by the giving means. Yes. Thereby, for example, it is possible to realize halftone processing that enables many gradation expressions.

  Furthermore, in the image processing apparatus according to the present invention, the center-of-gravity position determining unit calculates a product of a pixel position of each pixel and a gradation value of input image data of each pixel for all pixels included in the pixel group. The value obtained by dividing the sum by the total value of the gradation values of the input image data of each pixel included in the pixel group is determined as the barycentric position. Thereby, for example, the position of the center of gravity including the input gradation value of each pixel can be easily calculated.

  Furthermore, the present invention is characterized in that, in the image processing apparatus, the pixel based on the barycentric position is a pixel closest to the barycentric position. Thereby, for example, since dots and halftone dots faithful to the input gradation value can be obtained, an output image faithful to the input image can be obtained.

  Furthermore, in the image processing apparatus according to the aspect of the invention, when the assigning unit assigns a gradation value to the pixel closest to the barycentric position, the assigning unit adds the gradation given from the total input gradation value of each pixel included in the pixel group. If the remainder is obtained by subtracting the tone value, the remaining gradation value is applied to a pixel other than the pixel to which the surplus gradation value is applied and closest to the center of gravity position. Thereby, for example, dots and halftone dots grow from the position of the center of gravity, and the remaining gradation value is also taken into consideration, so that an output image faithful to the input gradation value can be obtained.

  Furthermore, the present invention is characterized in that, in the image processing apparatus, the assigning means assigns a sum of input image data of each pixel included in the pixel group to a pixel including at least a barycentric position. Thereby, for example, it is possible to obtain dots or the like in which the input gradation value is stored in the cell, and it is possible to obtain an output image with high resolution.

  In order to achieve the above object, the present invention converts an input image data of a plurality of colors into a gradation value of two gradations or more, and obtains output image data by a halftone dot corresponding to the gradation value. In the processing device, based on a predetermined pixel group, a centroid position determining unit that determines a centroid position for each color component from input image data of each pixel included in the pixel group, and a centroid position determined by the centroid position determining unit Providing means for assigning a gradation value to the pixel based on it, and the difference in the gradient of the halftone dot between any two of the plurality of colors is approximately 30 °. Thereby, for example, it is possible to obtain an output image with high resolution while minimizing moire fringes.

  Furthermore, in order to achieve the above object, the present invention converts an input image data of a plurality of colors into a gradation value of two or more gradations and obtains output image data with dots corresponding to the gradation values. In the apparatus, based on a predetermined pixel group, a centroid position determining unit that determines a centroid position for each color component from input image data of each pixel included in the pixel group, and based on the centroid position determined by the centroid position determining unit Providing means for assigning a gradation value to the pixel, and the difference in dot inclination between any two of the plurality of colors is approximately 30 °. Thereby, for example, it is possible to obtain an output image with high resolution while minimizing moire fringes.

  Furthermore, in order to achieve the above object, the present invention provides a predetermined pixel group in an image processing method for converting input image data of a plurality of colors into output image data having gradation values of two or more gradations. Centroid position determination step for determining the centroid position for each color component from the input image data of each pixel included in the pixel group based on the above, and provision for assigning gradation values to the pixels based on the centroid position determined in the centroid position determination step A difference in inclination of the pixel group between any two colors of the plurality of colors is approximately 30 °. Thereby, for example, an output image with high resolution can be obtained while keeping the moiré fringes to a minimum.

  Furthermore, in order to achieve the above object, the present invention converts an input image data of a plurality of colors into a gradation value of two or more gradations, and obtains output image data by a halftone dot corresponding to the gradation value. In the processing method, a centroid position determining step for determining a centroid position for each color component from input image data of each pixel included in the pixel group based on a predetermined pixel group, and a centroid position determined in the centroid position determining step And a step of applying a gradation value to a pixel based thereon, wherein a difference in inclination of the halftone dot between any two of a plurality of colors is approximately 30 °. Thereby, for example, it is possible to obtain an output image with high resolution by minimizing moire fringes.

  Furthermore, in order to achieve the above object, the present invention converts an input image data of a plurality of colors into a gradation value of two or more gradations and obtains output image data with dots corresponding to the gradation values. In the method, a center-of-gravity position determining step for determining a center-of-gravity position for each color component from input image data of each pixel included in the pixel group based on a predetermined pixel group, and a center-of-gravity position determined in the center-of-gravity position determining step Providing a gradation value to the pixel, and a difference in dot inclination between any two colors of the plurality of colors is approximately 30 °. Thereby, for example, it is possible to obtain an output image with high resolution while minimizing moire fringes.

  Furthermore, in order to achieve the above object, the present invention converts the input image data of a plurality of colors stored in the storage means into gradation values of two or more gradations and stores them in the storage means. In a program for causing a computer to execute image processing for reading out image data to obtain output image data, input image data of each pixel included in the pixel group is read from the storage unit based on a predetermined pixel group, and color components are input from the input image data. A center-of-gravity position determination process for determining the center-of-gravity position for each of the data, and an adding process for storing a gradation value in a storage unit so as to give a gradation value to a pixel based on the center-of-gravity position determined by the center-of-gravity position determination unit. The difference in the inclination of the pixel group between any two of the colors is approximately 30 °. Thereby, for example, an output image with high resolution can be obtained while keeping the moiré fringes to a minimum.

  Furthermore, in order to achieve the above object, the present invention converts the input image data of a plurality of colors stored in the storage means into gradation values of two or more gradations and stores them in the storage means. Storage unit for storing input image data of each pixel included in a pixel group based on a predetermined pixel group in a program for causing a computer to execute image processing for reading image data and obtaining output image data from a halftone dot corresponding to a gradation value Centroid position determination processing for determining the centroid position for each color component from the input image data, and storing the gradation value in the storage means so as to give the gradation value to the pixel based on the centroid position determined by the centroid position determination means And a difference between the gradients of the halftone dots between any two colors of the plurality of colors is approximately 30 °. As a result, it is possible to obtain an output image with high moiré fringes and high resolution.

  Furthermore, in order to achieve the above object, the present invention converts the input image data of a plurality of colors stored in the storage means into gradation values of two or more gradations and stores them in the storage means. In a program for causing a computer to execute image processing for reading output data and obtaining output image data with dots corresponding to gradation values, input image data of each pixel included in the pixel group is stored from a storage unit based on a predetermined pixel group. Read out and store the gradation value in the storage means so as to give the gradation value to the pixel based on the centroid position determined in the centroid position determination step, and the centroid position determination process for determining the centroid position for each color component from the input image data A difference between the inclinations of the dots between any two of the plurality of colors is approximately 30 °. Thereby, it is possible to obtain an output image with high resolution while keeping the moiré fringes to a minimum.

  The best mode for carrying out the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the configuration of the entire system to which the present invention is applied. The host computer 10 and the image processing apparatus 20 are configured as a whole.

  The host computer 10 includes an application unit 11 and a rasterizing unit 12. The application unit 11 generates data to be printed such as character data, graphic data, and bitmap data. For example, character data, graphic data, and the like are generated by operating the keyboard or the like using an application program such as a word processor or a graphic tool in the host computer 10. These generated data are output to the rasterizing unit 12.

  The rasterizing unit 12 converts the print target data output from the application unit 11 into gradation data of 8 bits for each color for each pixel. Therefore, each pixel has gradation values from 0 to 255. The generation of gradation data in the rasterizing unit 12 is actually processed by a driver installed in the host computer 10. The gradation data output from the rasterizing unit 12 is output to the image processing device 20. In this embodiment, the gradation data is RGB (red, green, blue) color data. Therefore, 24-bit gradation data is output from the host computer 10.

  The image processing apparatus 20 includes an image processing unit 21 and a print engine 22 as a whole. The image processing unit 21 includes a color conversion processing unit 211, a halftone processing unit 212, and a pulse width modulation unit 213.

  The color conversion processing unit 211 receives RGB gradation image data from the host computer 10, converts it into CMYK gradation image data, and outputs it. The halftone processing unit 212 receives the CMYK gradation image data from the color conversion processing unit 211, converts it into a multi-value value of two or more values, and outputs quantized data. The pulse width modulation unit 213 receives the quantized data output from the halftone processing unit 212 and generates drive data indicating the presence or absence of a laser drive pulse for the quantized data. The generated drive data is output to the print engine 22.

  The print engine 22 includes a laser driver 221 and a laser diode (LD) 222. The laser driver 221 receives drive data from the pulse width modulation unit 213, generates control data indicating the presence or absence of a drive pulse based on the drive data, and outputs the control data to the laser diode 222. The laser diode 222 is driven based on the control data output from the laser driver 221. Further, a photosensitive drum and a transfer belt (not shown) are driven, and input image data from the host computer 10 is actually recorded on a recording medium such as printing paper. Are printed as output image data having gradation values of two or more gradations.

  Next, a specific configuration of the image processing apparatus 20 will be described with reference to FIG. Here, in the image processing apparatus 20 of FIG. 1, the color conversion processing unit 211, the halftone processing unit 212, and the pulse width modulation unit 213 correspond to the CPU 24, the ROM 25, and the RAM 26 in FIG.

  The image processing apparatus 20 includes an input interface (I / F) 23, a CPU 24, a ROM 25, a RAM 26, and a print engine 22, which are connected to each other via a bus 27.

  The input I / F 23 serves as an interface between the host computer 10 and the image processing apparatus 20. The input I / F 23 receives gradation data from the host computer 10 transmitted by a predetermined transmission method, and is converted into data that can be processed by the image processing apparatus 20 under the control of the CPU 24. The input RGB gradation data is temporarily stored in the RAM 26.

  The CPU 24 appropriately reads out the program stored in the ROM 25 and performs various processes such as a color conversion process and a halftone process according to the present invention. Details thereof will be described later. The RAM 26 serves as a working memory for each process executed by the CPU 24 and stores various data during and after the process.

  The print engine 22 has the same configuration as the print engine of FIG. 1, and the drive data stored in the RAM 26 is input under the control of the CPU 24, and the above-described print processing is performed.

  Next, details of the halftone processing according to the present invention will be described with reference to the drawings. First, an outline of the present invention will be briefly described with reference to FIGS. First, as shown in FIGS. 3 to 4, four types of cell blocks 251 to 254 are prepared for gradation data of a certain color component in CMYK. That is, the upper left cell block 251 shown in FIG. 3A, the upper cell block 252 shown in FIG. 3B, the left cell block 253 shown in FIG. 4A, and FIG. The normal area cell block 254 shown in FIG.

  In each cell block 251 to 254, there are a plurality of pixel groups (hereinafter referred to as “cells”, indicated by bold lines in the figure). There are a plurality of input pixels in each cell of the cell blocks 251 to 254, and the center of gravity position is obtained in each cell and the process for dot generation proceeds. The numbers in each cell indicate the order in which the processing in the cell block proceeds. In the drawing, the right direction is the main scanning direction, and the lower direction is the sub-scanning direction.

These four cell blocks 251 to 254 are applied to the input image 100 for one page (or one frame) as shown in FIG. That is, the upper left end cell block 251 is applied to the upper left end block Bk ₁₁ of the input image 100, and the upper end cell block 252 is applied to the upper end blocks Bk _{21 to} Bkm ₁ (m ≧ 2) of the input image 100. The The left end cell block 253 is applied to the left end blocks Bk _{12 to} Bk _1n (n ≧ 2) of the input image 100, and the other blocks Bk _xy (2 ≦ x ≦ m, 2 ≦ y ≦) of the input image 100 are applied. The normal area cell block 254 is applied to n).

  Then, the centroid position is calculated in each cell in each cell block 251 to 254, and an output gradation value indicating halftone dot (or dot) generation is assigned to the pixel existing at the centroid position. Since the input gradation values in each cell are concentrated on the pixel at the center of gravity, a stable dot having a certain size is generated. In addition, since each cell frame is determined in advance, it is not necessary to perform an operation for configuring the cell every time, so that the processing can be made faster.

  Here, when focusing on each cell, if the main scanning direction is 0 ° as shown in FIG. 6, the cell 261 is applied with an inclination of approximately −18 °. Then, the cell blocks 255 to 258 shown in FIGS. 7 to 8 are applied to planes of different colors in CMYK. Also in this case, the upper left cell block 255 (see FIG. 7A), the upper cell block 256 (see FIG. 7B), and the left end cell block 257 (see FIG. 8A) with respect to the input image 100. ), Four types of normal cell block 258 ((B) in the figure) are applied. Then, an output gradation value indicating dot generation is assigned to the pixel closest to the center of gravity position in each cell. In this case, as shown in FIG. 9, each cell 200 has an inclination of approximately + 18 °.

  The difference in cell inclination is approximately 30 ° between the cell block in which the inclination between cells is set to approximately −18 ° and the cell block in which the inclination between cells is set to approximately + 18 °. Thereby, moire fringes can be minimized. Further, four cell blocks 259 to 262 (see FIGS. 10 and 11) in which the inclination between cells is set to approximately + 45 ° are applied to the third color of CMYK. Even in the cell block set to approximately + 18 ° and the cell block set to approximately + 45 °, the difference in the inclination of each cell is approximately 30 °, so that an output image in which so-called moire fringes are minimized can be obtained. . For the last color of CMYK, any one of −18 °, + 18 °, and 45 ° cell blocks may be applied, and there is an inclination between these angles. Alternatively, a cell block (for example, 0 °) may be applied.

  Compared with conventional halftone processing, the present invention generates halftone dots (or dots) that have the same gradation value as the sum of the gradation values contained in the cells, and also generates dots at the center of gravity of each cell. Will be. Therefore, reproducibility of thin lines and edges is improved, jaggies do not occur, small thin characters are easy to read, and a high-resolution halftone image can be generated. An output image faithful to the input tone value can be obtained.

  Returning to FIG. 3, in the upper left cell block 251, the cell adjacent to the lower right of the third cell is not the fourth cell but the seventh cell. This is to search for the unprocessed pixel at the upper left and proceed with the process. Therefore, the fourth to sixth cells are assigned numbers as shown in FIG. The same applies to other cell blocks 252 and the like.

  Further, in this cell block 251, the right half of the sixth cell is included in the right cell block 252. Therefore, the left side of the first cell at the upper left of the upper end cell block 252 is assigned a cell number. No processing is performed in the upper end cell block 252. This is because if a cell is divided into two and processed separately in different cell blocks, dots will be shot separately, leading to the generation of isolated dots and image quality degradation.

  The outline of the present invention has been briefly described above. Next, the details of the processing according to the present invention will be described with reference to FIGS. Among these, FIG. 12 to FIG. 14 are flowcharts showing the operation of the halftone processing according to the present invention.

  As shown in FIG. 12, the CPU 24 reads the program for executing this processing from the ROM 25, and the processing is started (S10). Next, the CPU 24 performs processing for reading the input image 100 after color conversion into the RAM 26 (S11). For example, each gradation value of the color input image 100 in the input buffer area 265 provided in a predetermined area of the RAM 26 is stored. An example of this is shown in FIG. This example is an example of an image in which a thin fine line exists at the upper end of the input image 100. Although the input buffer area 265 is shown as an example of 5 rows and 5 columns, this is for ease of explanation, and the configuration may be arbitrary rows and columns. The other three colors are also stored in the buffer area in the RAM 26 in the same manner. However, for ease of explanation, the following description will be given focusing on this one color.

Returning to FIG. 12, the CPU 24 then sets the cell block at the upper left of the input image 100, and substitutes “1” for N (S12). That is, the upper left end cell block is applied to the upper left end block Bk ₁₁ of the input image 100, and “1” is substituted for N indicating the cell number in the cell block. This is because the processing is started from the area of the upper left end portion of the upper left end block Bk ₁₁ . Here, description will be made assuming that cell blocks 251 to 254 in which each cell is inclined at −18 ° are applied. In the example of FIG. 15A, the number 1 cell (hereinafter cell (1)) of the upper left cell block 251 is applied (see FIG. 15B).

  Next, the CPU 24 refers to all the pixels included in the Nth cell in the block (S13). In the example of FIG. 15B, all the pixels included in the cell (1) are referred to.

  Next, the CPU 24 calculates the sum of the gradation values of the reference pixels in the cell (S14). This is because it is necessary for subsequent processing of the center of gravity position calculation. In the example of FIG. 15B, the sum of the gradation values in the cell (1) is “175”. The total value is stored in a predetermined area of the RAM 26 by the CPU 24, for example.

  Next, the CPU 24 calculates the center of gravity of the reference pixel (S15). This is because a process of generating halftone dots (or dots) based on the position of the center of gravity is performed. The following formula is used for the calculation of the center of gravity position.

This [Equation 1] is stored, for example, in the ROM 25, and the CPU 24 reads it from the ROM 25 and performs the calculation during this process. In the example of FIG. 15B, (X _centroid , Y _centroid ) ≈ (0.53, 1.86) (see centroid position 263 in FIG. 16A). The calculated center-of-gravity position is stored in the RAM 26, for example.

  Returning to FIG. 12, the CPU 24 then performs a dot generation process (S16). In other words, processing is performed as to which pixel position a dot (or halftone dot) is to be generated from the calculated barycentric position. Details of the dot generation processing are shown in FIG. When the process proceeds to the dot generation process (S16), the CPU 24 first searches for a dot non-output pixel closest to the center of gravity position (S161). FIG. 16B shows an example of the output buffer area 266 provided in a predetermined area of the RAM 26. In the above example, since the coordinates of the center of gravity position are (0.53, 1.86), the pixel of (1, 2) located at the center of gravity is searched.

  Returning to FIG. 14, the CPU 24 then determines whether or not the searched pixel is a non-output pixel (S162). In the example of FIG. 16B, since the searched pixel (1, 2) is a dot non-output pixel, “YES” is determined in this step, and the process proceeds to S163. On the other hand, when the searched pixel is a dot output pixel (“NO” in S162), since the searched pixel cannot be output as a dot, the dot generation process ends and the process proceeds to S17 in FIG.

  In S163, the CPU 24 determines whether or not the remaining gradation value is larger than “255”. In the example of FIG. 16B, the total value of the gradation values is “175”, which is smaller than “255”, so “NO” is selected, and the process proceeds to S167.

  In S167, the CPU 24 outputs the remaining gradation value to the searched pixel. In the example of FIG. 16B, the remaining gradation value “175” is output to the pixel (1, 2) located at the searched centroid. Then, the dot generation process ends, and the process proceeds to S17 in FIG.

  On the other hand, when the remaining gradation value is larger than “255” (“YES” in S163), “255” is output to the searched pixel (S164), and “255” is subtracted from the remaining gradation value (S165). If there is a remaining gradation value, the dot non-output pixel closest to the center of gravity is searched again (S161). Then, “255” (S164) or all of the remaining gradation values are output to the searched pixel (S167). This process is repeated until there are no remaining gradation values (until the total value of gradation values in the cell becomes “0”). By repeating this process, an output gradation value is assigned to the pixel located at the center of gravity first, the pixel closest to the center of gravity next, and the center of gravity sequentially.

  Therefore, when the total tone value in the cell is large by this dot generation process, the number of pixels that output “255” increases accordingly, and the generation of dots of a certain color increases. On the contrary, when the total value is low, dot generation is reduced. Therefore, it is possible to generate dots corresponding to the size of the input gradation value in the cell.

When the dot generation process (S16) ends, the process returns to FIG. 12 again, and the processes after S17 are performed. These processes are processes for moving the cell blocks 251 to 254. That is, it is determined whether or not the cell number N is the maximum value N _{MAX of the} number of cells included in each of the cell blocks 251 to 254 (S17). This is to determine whether or not the processing for all the cells in each cell block 251 to 254 has been completed.

When the cell number N is not equal to N _MAX (“NO” in S17), the CPU 24 then adds 1 to the cell number N (S18), and repeats the above processing in each cell until N becomes equal to N _MAX. (Transition to S13).

On the other hand, when the cell number N is equal to N _MAX (“YES” in S17), that is, when the above processing is completed for all the cells in one cell block 251 to 254, unprocessed from the right end of the block to the right. It is determined whether there is a pixel (S19 in FIG. 13). This is to determine whether the process can proceed in the main scanning direction.

  If there is an unprocessed pixel on the right side of the right end of the block (“YES” in S19), the cell blocks 251 to 254 are moved to the right by one and the cell number N is set to “1” to perform the above-described processing on that block. Is substituted (S20). Then, the above process is repeated in each cell in the block (shift to S13).

  On the other hand, when there is no unprocessed pixel to the right of the right end of the block (“NO” in S19), that is, when the processing proceeds to the right end of the input image 100, the CPU 24 determines whether there is an unprocessed pixel below the bottom end of the block. Judgment is made (step S21). This is because it is determined whether or not processing can proceed in the sub-scanning direction. If there is an unprocessed pixel below the lower end of the block (“YES” in S21), the CPU 24 then moves the cell blocks 251 to 254 to the leftmost position one step below, and performs cell number N to perform processing in that block. "1" is substituted for (S22), and the process proceeds to S13.

  On the other hand, when there is no unprocessed pixel below the lower end of the block (“NO” in S21), the processing is completed for all the pixels in the main scanning direction and the sub-scanning direction, so that the series of processing ends. (S23). Since the above processing is processing for a certain color component of CMYK, a series of processing is similarly applied to another color component to generate each dot of CMYK.

  Then, an output gradation value indicating the presence or absence of dot generation for each color component stored in the output buffer area 266 is output to the pulse width modulation unit 213 as quantized data. Thereafter, a pulse width based on this value is generated, and a print image is formed on a print sheet or the like.

  FIG. 17A shows an example of a dot of a certain color formed on the printing paper 300 by the cell (1) described above. In the pixel (1, 2), a dot (175/255) corresponding to the output gradation value “175” is formed. Since the output gradation value is “0” in the other pixels in the cell (1), no dot is generated. Further, FIG. 17B shows the dots for the cell (2), and FIG. 18A shows the dots in the cell (9) located in the downward direction of the cell (1). As a result, dots as shown in FIG. 18B are formed in this region.

  In this embodiment, the pulse width modulation unit 213 generates 256 steps of pulses per pixel. That is, each output gradation value stored in the output buffer area 266 represents not only the presence / absence of dot generation but also the dot size. As a result, many gradation expressions (brightness expressions) are possible. Of course, the pulse width modulation unit 213 may generate pulses of “256” steps or more, or may generate pulses of any number of steps below this.

  On the other hand, an input image having the same input gradation value in the same region is compared with an output image obtained by conventional halftone processing as shown in FIG. The size of the pattern matrix is the same as that of the cell (1) of this embodiment. For example, when the matrix number referred to in the pixel (0, 2) is “6” and the γ table referred to is FIG. 19C, the gradation value “80” of the pixel may be the output level depending on the γ table. It is possible that the tone value is “0”, and no halftone dot is generated on the printing paper 300 as shown in FIG. However, in the present invention, dots (or halftone dots) corresponding to the total value of the input gradation values are always generated in each cell, and dots faithful to the input image are generated. In addition, since the difference in the inclination of each cell in the cell block applied for each color component is approximately 30 ° between the two colors, an output image with the minimum moire fringes can be obtained.

  However, when generating dots at the center of gravity position in the fixed cell frame, for example, in an area where the input tone value changes in the input image such as an edge area, each cell has an inclination of approximately −18 ° or the like. Despite this, the generated dots may not be generated at this angle. That is, if a dot is generated at the center position in one cell (position considering only the coordinate position), each cell has an inclination of approximately −18 °, etc., so the dot is also generated with this inclination. In some cases, no dot is generated at the position. Therefore, the inclination of the dot between the two colors cannot be maintained at approximately 30 °, and moire fringes may occur in the output image.

  However, the moire fringes are easily perceived by the human eye in the flat image portion of the output image with no change in gradation. A certain large area is necessary to recognize that moire fringes are generated in the human eye. Therefore, even if a dot (or halftone dot) between two colors does not have an inclination of approximately 30 ° in a narrow region where the input tone value changes such as an edge portion, the moiré fringe is observed in the human eye. Is not recognized as occurring.

  In addition, since the dot is generated at the center of gravity position in the cell as in the present invention, the output gradation value is arranged at the optimum position corresponding to the change even if the input gradation value changes, so that the edge Can be reproduced faithfully. That is, an output image with high resolution can be obtained.

  In other words, in the edge portion, it is better to generate the dot at the center of gravity than to set the inclination of the dot to approximately 30 ° in order to suppress the moire fringes, and the resolution is excellent and visually comfortable and faithful to the input image. The output can be obtained.

  On the other hand, in a region where the input tone value does not change, the center position of each cell becomes the center of gravity position, and the probability that a dot is generated at that position increases. Therefore, if the cell has an inclination of approximately 30 ° between the two colors, the generated dots are also formed with the inclination. That is, a visually comfortable output in which moire fringes are not generated in a flat region where the input tone value does not change can be obtained.

  For this reason, by setting the cell slope to approximately 30 ° between the two colors and generating dots in consideration of the position of the center of gravity, the dots are naturally generated in the area where the input gradation value where the moire is conspicuous does not change. Or, in a region where the halftone dot is inclined by approximately 30 ° between two colors to suppress the generation of moiré, while the input tone value is less noticeable, the resolution is given priority and edge reproducibility is improved. Excellent output can be obtained. That is, according to the present invention, an optimum output according to the characteristics of each region can be obtained.

  As described above, in the present invention, dots are generated at the center of gravity within a predetermined fixed cell frame, and the difference in cell inclination between any two colors is approximately 30 °, or any two colors. Since the difference in the inclination of the dots (or halftone dots) is approximately 30 °, an output image with excellent resolution can be obtained while keeping the moire fringes to a minimum.

  In the above-described example, cells having inclinations of approximately −18 ° for a certain color component of CMYK, approximately + 18 ° for the second color component, and approximately + 45 ° for the third color are configured. This is an example used to explain that the inclination of a cell (or dot or halftone dot) between two colors is approximately 30 °. Therefore, if the angle between the two colors is approximately 30 °, the inclination of each cell may be formed at an arbitrary angle. However, ideally, as shown in FIG. 20, a cell (or dot or halftone dot) having a slope of C or M of 15 ° or 75 °, Y of 0 °, and K of 45 ° is formed. You can do it. This is because compatibility can be maintained by applying the present invention to a commercial printing apparatus in which the difference in screen angle between two colors is 30 °.

  In order to realize this 15 °, for example, a cell as shown in FIG. 21 may be configured. However, since this figure is approximately −15 °, it is possible to realize approximately + 15 ° by configuring the cells symmetrically. Furthermore, it is easy for those skilled in the art to construct a cell having an inclination of about 75 ° from this cell.

  In the above example, the halftone process according to the present invention is performed in the image processing apparatus 20, but the process may be performed on the host computer 10 side as shown in FIG. That is, the quantized data after halftone processing is output from the host computer 10 and the image processing apparatus 20 performs pulse width modulation. Even in this case, an output image with excellent resolution can be obtained while keeping the moiré fringes to a minimum.

  Further, in the above example, the pulse width modulation is performed on the quantized data after the halftone process. However, the present invention can be applied even when there is no such a modulation process, and similar effects can be obtained. As shown in FIG. 23, the quantized data after the halftone process is directly output to the print engine 22 for printing. In this case, the halftone processing unit 212 may set a threshold value for the output gradation value to control dot on / off. For this reason, the present invention is not necessarily applied only to the laser printer, but can be applied to various printers such as an ink jet printer and a bubble jet printer (Bubble Jet is a registered trademark), and has the same effects. be able to.

  Further, as an example of the image processing apparatus 20, in addition to the above-described printer, an information portable terminal such as a facsimile, a copier, a multifunction peripheral, a display, a digital camera, or a mobile phone may be used. Even in this case, the same effect as the above-described example is obtained.

It is a figure which shows the structure of the whole system to which this invention is applied. It is a figure which shows the specific structure of an image processing apparatus. It is a figure which shows the example of a cell block. It is a figure which shows the example of a cell block. It is a figure for demonstrating the kind of cell block applied with respect to an input image. It is a figure for demonstrating the cell applied with respect to an input image. It is a figure which shows the example of a cell block. It is a figure which shows the example of a cell block. It is a figure for demonstrating the cell applied with respect to an input image. It is a figure which shows the example of a cell block. It is a figure for demonstrating the cell applied with respect to an input image. It is a flowchart which shows operation | movement of the process which concerns on this invention. It is a flowchart which shows operation | movement of the process which concerns on this invention. It is a flowchart which shows operation | movement of a dot production | generation process. 5 is a diagram illustrating an example of an input buffer area 265. FIG. 5 is a diagram illustrating an example of an input buffer area 265 and an output buffer area 266. FIG. 5 is a diagram illustrating an example of dots generated on the printing paper 300. FIG. 5 is a diagram illustrating an example of dots generated on the printing paper 300. FIG. It is a figure which shows the example of a halftone process. It is a figure which shows the angle of an ideal cell (or dot or halftone dot). It is a figure which shows the example of a cell block. It is a figure which shows the structure of the other whole system with which this invention is applied. It is a figure which shows the structure of the other whole system with which this invention is applied.

Explanation of symbols

10 host computer, 20 image processing device, 21 image processing unit, 212 halftone processing unit, 213 pulse width modulation unit, 22 print engine, 24 CPU, 25 ROM, 251 to 254 of approximately −18 ° with respect to the main scanning direction Cell block composed of cells having an inclination, 255 to 258 Cell block composed of cells having an inclination of about + 18 ° with respect to the main scanning direction, 259 to 262 Consisting of cells having an inclination of about + 45 ° to the main scanning direction Cell block, 26 RAM, 265 input buffer area, 266 output buffer area

Claims (14)

In an image processing apparatus for converting input image data of a plurality of colors into output image data having gradation values of two or more gradations,
For the pixel group of a predetermined fixed cell frame for each color component, a center-of-gravity position determining means for determining the center of gravity position for each of the color components from the input image data of each pixel included in the pixel group,
An assigning means for assigning the gradation value to a pixel based on the centroid position determined by the centroid position determining means;
And the difference in inclination of the pixel group between two sets of two colors among the three colors included in the plurality of colors is approximately 30 °. The image processing apparatus according to claim 1.
Further, pulse width modulation means for generating a K-stage (K is a positive integer) pulse width for the gradation value given by the giving means,
An image processing apparatus comprising: The image processing apparatus according to claim 1.
The center-of-gravity position determining means calculates the product of the pixel position of each pixel and the gradation value of the input image data of each pixel for all the pixels included in the pixel group, and the sum is calculated in the pixel group A value obtained by dividing by a total value of gradation values of the input image data of each pixel included in each pixel is determined as a centroid position. The image processing apparatus according to claim 1.
The pixel based on the gravity center position is a pixel closest to the gravity center position. The image processing apparatus according to claim 4.
When the gradation value is imparted to the pixel closest to the barycentric position, the provision means subtracts the imparted gradation value from the total input gradation value of each pixel included in the pixel group. If there is, the image processing apparatus characterized in that the surplus gradation value is assigned to a pixel other than the assigned pixel and closest to the center of gravity position. The image processing apparatus according to claim 1.
The image processing apparatus according to claim 1, wherein the adding unit applies a sum of input image data of each pixel included in the pixel group to a pixel including at least the barycentric position. In an image processing apparatus that converts input image data of a plurality of colors into gradation values of two or more gradations for each color component, and obtains output image data with halftone dots corresponding to the gradation values.
For the pixel group of a predetermined fixed cell frame for each color component, a center-of-gravity position determining means for determining the center of gravity position for each of the color components from the input image data of each pixel included in the pixel group,
An assigning means for assigning the gradation value to a pixel based on the centroid position determined by the centroid position determining means;
And the difference in inclination of the halftone dots between two sets of two colors among the three colors included in the plurality of colors is approximately 30 °. In an image processing apparatus that converts input image data of a plurality of colors into gradation values of two or more gradations and obtains output image data with dots according to the gradation values.
For the pixel group of a predetermined fixed cell frame for each color component, a center-of-gravity position determining means for determining the center of gravity position for each of the color components from the input image data of each pixel included in the pixel group,
An assigning means for assigning the gradation value to a pixel based on the centroid position determined by the centroid position determining means;
An image processing apparatus, wherein a difference in inclination of the dot between two sets of two colors among the three colors included in the plurality of colors is approximately 30 °. In an image processing method for converting input image data of a plurality of colors into output image data having gradation values of two or more gradations,
For the pixel group of a predetermined fixed cell frame for each color component, and the position of the center of gravity determination step of determining the center of gravity position for each of the color components from the input image data of each pixel included in the pixel group,
An assigning step of assigning the gradation value to a pixel based on the centroid position determined in the centroid position determining step;
And the difference in inclination of the pixel group between two sets of two colors among the three colors included in the plurality of colors is approximately 30 °. In an image processing method for converting input image data of a plurality of colors into a gradation value of 2 gradations or more and obtaining output image data with a halftone dot corresponding to the gradation value,
For the pixel group of a predetermined fixed cell frame for each color component, and the position of the center of gravity determination step of determining the center of gravity position for each of the color components from the input image data of each pixel included in the pixel group,
An assigning step of assigning the gradation value to a pixel based on the centroid position determined in the centroid position determining step;
An image processing method characterized in that a difference in inclination of the halftone dot between two sets of two colors among the three colors included in the plurality of colors is approximately 30 °. In an image processing method for converting input image data of a plurality of colors into gradation values of two or more gradations and obtaining output image data with dots corresponding to the gradation values,
For the pixel group of a predetermined fixed cell frame for each color component, and the position of the center of gravity determination step of determining the center of gravity position for each of the color components from the input image data of each pixel included in the pixel group,
An assigning step of assigning the gradation value to a pixel based on the centroid position determined in the centroid position determining step;
An image processing method characterized in that a difference in inclination of the dot between two sets of two colors among the three colors included in the plurality of colors is approximately 30 °. Image processing for converting the input image data of a plurality of colors stored in the storage means into gradation values of two or more gradations and storing them in the storage means, and reading the gradation values to obtain output image data In a program to be executed by a computer,
For the pixel group of a predetermined fixed cell frame for each color component, reading the input image data of each pixel included in the pixel group from said storage means, the center of gravity position for each of the color components from the input image data Centroid position determination processing to determine
An applying process for storing the gradation value in the storage means so as to give the gradation value to a pixel based on the gravity center position determined by the gravity center position determining means;
And a difference in inclination of the pixel group between two sets of two colors among the three colors included in the plurality of colors is approximately 30 °. A plurality of colors of input image data stored in the storage means are converted into gradation values of two or more gradations, stored in the storage means, the gradation values are read out, and a network corresponding to the gradation values is read. In a program for causing a computer to execute image processing to obtain output image data by points,
For the pixel group of a predetermined fixed cell frame for each color component, reading the input image data of each pixel included in the pixel group from said storage means, the center of gravity position for each of the color components from the input image data Centroid position determination processing to determine
An applying process for storing the gradation value in the storage means so as to give the gradation value to a pixel based on the gravity center position determined by the gravity center position determining means;
And a difference in inclination of the halftone dot between two sets of two colors among the three colors included in the plurality of colors is approximately 30 °. A plurality of color input image data stored in the storage means is converted into gradation values of two or more gradations and stored in the storage means, and the gradation values are read out and dots corresponding to the gradation values are read. In a program for causing a computer to execute image processing to obtain output image data by:
For the pixel group of a predetermined fixed cell frame for each color component, the input image data of each pixel included in the pixel group is read out from said storage means, the center of gravity for each of the color components from the input image data Centroid position determination processing to determine the position;
An assigning process for storing the gradation value in the storage means so as to assign the gradation value to a pixel based on the gravity center position determined in the gravity center position determining step;
And the difference in inclination of the dots between two sets of two colors among the three colors included in the plurality of colors is approximately 30 °.

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