JP2006033643A - Image processing apparatus, image forming apparatus, image processing method, and computer program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processing apparatus, an image forming apparatus, an image processing method, and a computer program which can apply an appropriate gradation reproduction processing to image data corresponding to density of each pixel data of the image data. <P>SOLUTION: A density region discrimination section 161 of a gradation reproduction processing section 16 discriminates to which of a low density region, a medium density region and a high density region each pixel value of the received image data is included. The density region discrimination section 161 transfers pixel data discriminated to be included in the medium density region to an error spread processing section 162 and transfers pixel data discriminated to be included in the low density region or the high density region to an FM screen processing section 164. The error spread processing section 162 executes the error spread processing resulting from referencing a quantization error of peripheral pixels by a peripheral reference section 163 for the acquired pixel data, and the FM screen processing section 164 executes FM screen processing utilizing a threshold screen table 165 for the acquired pixel data. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image processing apparatus and an image processing method for performing gradation reproduction processing on input image data to generate pseudo-halftone output image data, an image forming apparatus provided with the image processing apparatus, and a computer that uses the image processing apparatus. The present invention relates to a computer program for realizing the above.

  Conventionally, as a method for outputting an image to a recording medium such as paper, various recording methods such as a thermal transfer method, an electrophotographic method, and an ink jet method have been adopted. Some image forming apparatuses using these recording methods are configured to be able to form respective dot patterns in a plurality of density regions (see, for example, Patent Document 1).

For example, it is possible to artificially form a gradation image including a halftone using a binary output image forming apparatus. In this case, the number of gradations is converted into two gradations in consideration of gradation reproducibility. It is necessary to perform binarization processing. As a binarization processing method, various methods such as a method of comparing with a threshold value, a dither method, or an error diffusion method are conventionally used.
JP 2001-257880 A Kakutani Shigeaki, “Threshold Operation Method in Error Diffusion”, Journal of Electrophotographic Society, 1998, Vol. 37, No. 2, p. 186-192

  In the following, the tone reproduction process performed according to the conventional general error diffusion method will be described. In the error diffusion method, an error (hereinafter referred to as a quantization error) generated when binarizing the density (hereinafter referred to as a pixel value) of each pixel of an original image is converted into a peripheral pixel that has not been binarized. This is a method of binarizing while distributing. If the pixel to be binarized is the target pixel, the quantization error of the target pixel is weighted according to the relative position from the target pixel and then before the binarization process positioned around the target pixel. Is added to each pixel value.

FIG. 11 shows an example of a weighting coefficient matrix used in the conventional error diffusion method. In the example of FIG. 11, a 2 × 3 weighting coefficient matrix including the pixel of interest (I _X , I _Y ) with the horizontal direction (processing direction) as the X direction and the vertical direction as the Y direction is shown. The weighting coefficient matrix indicates the weighting coefficient of each relative position (lower left neighbor, lower neighbor, lower right neighbor, right neighbor) with reference to the target pixel (I _X , I _Y ). For example the target pixel (I _X, I _Y) is compared with the pixel value threshold, the target pixel (I _X, I _Y) if greater than a threshold ON, the pixel of interest is smaller (I _X, I _Y ) Off (quantized). Next, the difference (quantization error) between the quantized pixel value and the pixel value of the target pixel (I _X , I _Y ) is distributed to the surrounding pixels before the binarization process based on the weighting coefficient matrix. To do. However, the target pixel (I _X, I _Y) left adjacent pixels of _{(I X-1, I Y} ) is the pixel of interest (I _X, I _Y) because it is already quantized before the quantization error Is not allocated.

Here, if the quantization error is Err, the pixel (I _{X + 1} , I _Y ) on the right side of the pixel of interest (I _X , I _Y ), the pixel on the lower right (I _{X + 1} , I _{Y +) 1} ), the lower adjacent pixel (I _X , I _{Y + 1} ) and the lower left adjacent pixel (I _X-1 , I _{Y + 1} ) have Err × (7/16) and Err × (1/16), respectively. ), Err × (5/16), Err × (3/16).

  Such an error diffusion method distributes quantization errors to surrounding unprocessed pixels based on a weighting coefficient matrix, so that local fluctuations in dot density do not occur in the binarized image, and moire is not caused. It has the advantage that the image quality is superior to other methods such as dithering, such as difficulty in producing patterns.

  However, since the error diffusion method diffuses errors based on the same matrix for each pixel, there is a problem that a worm (portion in which dots are partially connected) occurs in a highlight portion or the like. It is necessary to take measures such as adding noise to the threshold value when performing the conversion (see Non-Patent Document 1, for example).

  On the other hand, Patent Document 1 proposes a method for determining dot on / off with reference to the arrangement of surrounding dots. In such a method, since the dots are dispersed particularly in the highlight portion, it is necessary to increase the range of the dots to be referred to. When the dot reference range is increased, the number of accesses to the memory increases, which increases the processing load. Therefore, when image processing is performed using a computer, it does not matter so much. However, in a dedicated device such as a digital copying machine, a digital multi-function peripheral, or a printer, not only an increase in memory capacity to be mounted but also calculation is required. There is a problem that processing time and calculation cost increase due to an increase in the amount.

  The present invention has been made in view of such circumstances, and an image processing apparatus capable of performing appropriate gradation reproduction processing based on the density of each pixel data in input image data without increasing the amount of calculation, and An object is to provide an image processing method, an image forming apparatus including the image processing apparatus, and a computer program for realizing the image processing apparatus by a computer.

  An image processing apparatus according to the present invention includes a gradation reproduction processing unit that performs gradation reproduction processing on input image data to generate pseudo-halftone output image data, wherein the gradation reproduction processing unit includes: Determining means for determining whether the density of each pixel data in the input image data is included in the first density area or the second density area divided in advance, and when the determining means is included in the first density area A first processing unit that executes a first process based on the density of surrounding pixel data for the determined pixel data, and each pixel for the pixel data that the determination unit determines to be included in the second density region And a second processing means for executing a second processing based on the density of the data.

  In the image processing apparatus according to the present invention, the first density area is an area including an intermediate density between the maximum density and the minimum density related to pixel data, and the second density area is other than the first density area. It is the area | region containing the density | concentration of this.

  In the image processing apparatus according to the present invention, the first processing unit determines that the pixel data determined to be included in the second density region by the determination unit is included in the first density region by the determination unit. The first processing is performed on pixel data at a position within a predetermined range from the position of the pixel data that has been determined, and the second processing means determines that the pixel is included in the second density region The second process is performed on the pixel data at a position within the predetermined range in the data after the first process is performed by the first processing unit.

  In the image processing apparatus according to the present invention, the first processing unit determines that the pixel data determined to be included in the second density region by the determination unit is included in the first density region by the determination unit. The first processing is performed on pixel data having a density within a predetermined range with respect to the density to be processed, and the second processing means determines that the pixel is included in the second density area The second process is performed on the pixel data having the density within the predetermined range in the data after the first process is performed by the first processing unit.

  An image forming apparatus according to the present invention includes any one of the above-described image processing apparatuses and a unit that forms an image according to output image data generated by the image processing apparatus on a sheet. .

  An image processing method according to the present invention is an image processing method for generating pseudo-halftone output image data by performing gradation reproduction processing on input image data, and the density of each pixel data in the input image data is divided in advance. A step of determining whether the pixel is included in the first density region or the second density region, and a first process based on the density of surrounding pixel data for the pixel data determined to be included in the first density region And a step of executing a second process based on the density of each pixel data for the pixel data determined to be included in the second density region.

  In the image processing method according to the present invention, the pixel data determined to be included in the second density region is located at a position within a predetermined range from the position of the pixel data determined to be included in the first density region. Performing the first process on the pixel data; and applying the first process to pixel data at a position within the predetermined range of the pixel data determined to be included in the second density region. And a step of executing the second process after execution.

  The image processing method according to the present invention has a density within a predetermined range with respect to a density determined to be included in the first density area among pixel data determined to be included in the second density area. Executing the first process on pixel data; and the first process on pixel data having a density within the predetermined range of pixel data determined to be included in the second density region. And a step of executing the second process after the execution of.

  A computer program according to the present invention is a computer program for causing a computer to perform gradation reproduction processing on input image data to generate pseudo-halftone output image data. Determining whether the density is included in the first density area or the second density area that is divided in advance, and for the pixel data determined to be included in the first density area in the computer, Executing a first process based on the density of peripheral pixel data; and executing a second process based on the density of each pixel data for the pixel data determined to be included in the second density area in the computer. And a step of causing

  According to the present invention, when gradation output processing is performed on input image data to generate pseudo-halftone output image data, the density of each pixel data in the input image data is a first density region that is divided in advance. Alternatively, it is determined in which of the second density regions. Then, for pixel data determined to be included in the first density region, first processing based on the density of surrounding pixel data is executed, and for pixel data determined to be included in the second density region Thus, the second process based on the density of each pixel data is executed. Therefore, by performing different processing according to the density of each pixel data, it is possible to perform processing for each pixel data while preventing deterioration of image quality suitable for each density.

  According to the present invention, an area including an intermediate density between the maximum density and the minimum density that can be taken by each pixel data in the input image data is defined as a first density area, and an area including a density other than the first density area is defined. By using the second density region, it is possible to perform different processing on the intermediate density pixel data and the other density pixel data.

  According to the present invention, when the first process is performed on the pixel data determined to be included in the first density region, the first of the pixel data determined to be included in the second density region. The first process is executed including pixel data at a position within a predetermined range from the position of the pixel data determined to be included in one density region. Therefore, when the first process is performed, even if the pixel data is determined to be included in the second density region, the pixel data around the pixel data determined to be included in the first density region is also considered. Thus, the first process based on the density of the surrounding pixel data can be executed with high accuracy. Further, the second process is performed by executing the second process on the pixel data included in the second density area located around the pixel data included in the first density area after the first process. The second process can be executed with high accuracy even for the pixel data.

  According to the present invention, when the first process is performed on the pixel data determined to be included in the first density region, the first of the pixel data determined to be included in the second density region. The first process is executed including pixel data having a density within a predetermined range with respect to the density of one density region. Therefore, when the first process is performed, even pixel data determined to be included in the second density region is considered by taking into account pixel data having a density within a predetermined range from the density of the first density region. It is not necessary to detect a boundary portion between the pixel data determined to be the first density region and the pixel data determined to be the second density region, the calculation cost can be reduced, and The first process based on the density of the pixel data can be executed with high accuracy. Further, the second process is performed by executing the second process on the pixel data included in the second density area having a density within a predetermined range from the density of the first density area, after executing the first process. The second process can be executed with high accuracy even for the pixel data.

  According to the present invention, it is possible to obtain high-quality output image data by providing an image processing device that performs different gradation reproduction processes that prevent deterioration in image quality according to the density of each pixel data in the input image data. Thus, it is possible to obtain an image in which this output image data is formed on a recording sheet, for example.

  According to the present invention, it is possible to cause a computer to execute an operation for performing different gradation reproduction processing for each pixel data in the input image data in accordance with the respective densities, preventing deterioration of image quality. It is possible to use a general image processing method for general purposes.

  In the present invention, when the input image data is subjected to gradation reproduction processing to generate pseudo-halftone output image data, the density of each pixel data in the input image data is included in the first density area that is preliminarily classified. Is determined, the first process based on the density of surrounding pixel data is performed on the pixel data. On the other hand, when it is determined that the density of each pixel data in the input image data is included in the second density area divided in advance, the second process based on the own density is performed on the pixel data. In this way, by performing different processing depending on the density of each pixel data, it is possible to perform processing for each pixel data while preventing deterioration of image quality suitable for each density.

  For example, processing is performed by an error diffusion method that considers pixel values of surrounding pixels for a target pixel to be processed, such as an intermediate density between the maximum density and the minimum density that each pixel data in the input image data can take. In this case, processing based on the error diffusion method can be performed on the density region where a good image in which fluctuation of the formed dot density is prevented can be obtained. Also, depending on the error diffusion method, processing based on a method that does not consider the pixel values of surrounding pixels for low density regions (highlight areas) or high density regions that may cause image quality degradation due to the occurrence of worms or the like. Can be done. As described above, by performing processing suitable for the density of each pixel data, it is possible to prevent deterioration in image quality and increase in the amount of calculation, thereby realizing high image quality and low cost. .

  In the present invention, when the first process is performed on the pixel data determined to be included in the first density region, the first density region among the pixel data determined to be included in the second density region. The first processing is executed including the pixel data around the pixel data determined to be included in. Thus, for example, the first process based on the density of the surrounding pixel data like the error diffusion method can be executed with high accuracy, and the image based on the pixel data of the first density area on which the first process has been performed, The boundary portion with the image based on the pixel data of the second density area that has been subjected to the two processes becomes smooth, and a good image can be formed in which deterioration in image quality is prevented.

  In the present invention, when the first process is performed on the pixel data determined to be included in the first density region, the first density region among the pixel data determined to be included in the second density region. The first process is executed including pixel data having a density within a predetermined range with respect to the density of the first. Accordingly, the second density region around the first density region is detected without detecting a boundary portion between the pixel data determined to be the first density region and the pixel data determined to be the second density region. The first process including the pixel data included can be executed, and the calculation amount can be reduced, so that the calculation cost can be reduced.

  According to the present invention, it is possible to obtain high-quality output image data by providing an image processing device that performs different gradation reproduction processing that prevents deterioration in image quality according to the density of each pixel data in the force image data. For example, an image in which the output image data is formed on a recording sheet can be obtained.

  In the present invention, it is possible to cause the computer to execute an operation for performing different gradation reproduction processing for each pixel data in the input image data in accordance with the respective densities, preventing deterioration of the image quality. The processing method can be general purpose.

(Embodiment 1)
Hereinafter, an image forming apparatus according to the present invention will be described in detail with reference to the drawings showing a printer as one embodiment thereof. FIG. 1 is a block diagram illustrating an internal configuration example of a printer according to the first embodiment. In FIG. 1, reference numeral 100 denotes the printer of the first embodiment. The printer 100 forms an image on a sheet based on an image input unit 10, a color image processing apparatus 1 that processes image data input via the image input unit 10, and image data processed by the color image processing apparatus 1. The digital color printer includes an image output unit 17 that operates and an operation panel 20 on which the operator operates the printer 100. Although not shown, the printer 100 includes a CPU (Central Processing Unit) that controls each unit described above.

  The image input unit 10 has an interface conforming to, for example, a USB (Universal Serial Bus) standard or an IEEE (Institute of Electrical and Electronic Engineers) 1394 standard. Therefore, the image input unit 10 is connected to a digital camera or a computer through such an interface, acquires image data captured by the connected digital camera or image data edited by causing the computer to execute software, The acquired image data is sent to the color image processing apparatus 1.

  The color image processing apparatus 1 includes a decompression processing unit 11, a spatial filter processing unit 12, a color information processing unit 13, a black generation and under color removal unit 14, an output tone correction unit 15, and a tone reproduction processing unit 16. The color image processing apparatus 1 converts the image data received from the image input unit 10 into a decompression processing unit 11, a spatial filter processing unit 12, a color information processing unit 13, a black generation and under color removal unit 14, an output tone correction unit 15, and CMYK (C: cyan, M: magenta, Y: yellow, K: black) digital image data is generated by transferring the data in the order of the gradation reproduction processing unit 16 and performing predetermined processing. Further, the image data generated by the color image processing apparatus 1 is temporarily stored in a storage unit (not shown) and output to the image output unit 17 at a predetermined timing.

  When the image data acquired from the image input unit 10 is compressed, the decompression processing unit 11 performs expansion processing (decompression processing) corresponding to the compression method of the acquired image data. For example, when JPEG (Joint Photographic Experts Group) format compressed image data is input from the image input unit 10, YCbCr (Y: luminance component, CbCr: color difference component) digital image data is input to the color image processing apparatus 1. The Therefore, the decompression processing unit 11 performs predetermined decompression processing on the acquired YCbCr image data and decodes it, and further performs inverse quantization processing, inverse orthogonal transformation processing, and the like to convert the pixel data into RGB (R: red, G: Green, B: Blue) Digital image data is generated. The RGB digital image data generated by the decompression processing unit 11 is output to the spatial filter processing unit 12.

  The spatial filter processing unit 12 performs a spatial filter process using a digital filter on the RGB digital image data input from the decompression processing unit 11 and corrects the spatial frequency characteristics of the image, thereby outputting the image from the image output unit 17. Processing for improving image blurring or graininess deterioration is performed. The color information processing unit 13 converts the RGB digital image data input from the spatial filter processing unit 12 into CMY image data, and also includes CMY including unnecessary absorption components in order to realize faithful color reproduction by the image output unit 17. Processing to remove color turbidity based on the spectral characteristics of the color material and color matching processing between the original and the copy (output image) are performed, and the corrected CMY image data is output to the black generation and under color removal unit 14. .

  The black generation and under color removal unit 14 generates a black signal (K signal) from the three color signals (C signal, M signal, and Y signal) constituting the color-corrected CMY image data input from the color information processing unit 13. A black generation process for generating is performed. Further, the black generation and under color removal unit 14 performs processing to generate new CMY image data by subtracting the K signal obtained by the black generation processing from the original CMY image data, and generates the generated CMYK four-color signal (CMYK image data). ) To the output tone correction unit 15.

As a general black generation process, there is a method of generating black using skeleton black. In this method, the input / output characteristic of the skeleton curve is y = f (x), the input data is C, M, Y, the output data is C ′, M ′, Y ′, K ′, UCR (Under Color When the removal rate is α (0 <α <1), the specific black generation and under color removal processing is expressed by the following equation.
K ′ = f {min (C, M, Y)}
C ′ = C−αK ′
M ′ = M−αK ′
Y ′ = Y−αK ′

  The output tone correction unit 15 performs an output tone correction process according to tone reproduction characteristics by the image output unit 17 on the CMYK image data input from the black generation and under color removal unit 14, thereby performing a tone reproduction processing unit. 16 output. The gradation reproduction processing unit 16 takes CMYK image data that has undergone output gradation correction processing input from the output gradation correction unit 15, for example, binary or multivalued (for example, considering the recording density characteristics of the image output unit 17). The data is converted into (quaternary) data, and finally, gradation reproduction processing (halftone generation) is performed so that the gradation of each pixel can be reproduced by being separated into pixels. The detailed configuration of the gradation reproduction processing unit 16 will be described later with reference to FIG.

  The CMYK image data binarized or multi-valued by the gradation reproduction processing unit 16 is temporarily stored in a storage unit (not shown) and is read out to the image output unit 17 at a predetermined timing. The image output unit 17 includes an electrophotographic printing unit, an inkjet printing unit, or the like. Based on the CMYK image data input from the color image processing apparatus 1, an image is output on a sheet such as paper or an OHP film. It functions as a means for forming.

  The operation panel 20 is an input means for an operator to input instructions by operating keys. An operator instruction is output as a control signal from the operation panel 20 to the color image processing apparatus 1 and the image output unit 17. For example, the printer 100 receives image data from an external computer via the image input unit 10, and after the signal processing by the color image processing apparatus 1, an image is formed on a sheet by the image output unit 17, thereby enabling a digital color printer. Function as. The above processing is controlled by a CPU (not shown).

  Next, the configuration of the gradation reproduction processing unit 16 will be described. FIG. 2 is a block diagram showing an example of the internal configuration of the gradation reproduction processing unit 16. The tone reproduction processing unit 16 determines a density region to which a data value (density) of each pixel data in the image data (hereinafter referred to as input image data) input from the output tone correction unit 15 belongs. (Determination means) 161, an error diffusion processing section (first processing means) 162 that performs processing based on the error diffusion method (first processing), and a peripheral reference section 163 that assists the error diffusion processing performed by the error diffusion processing section 162 And an FM screen processing unit (second processing means) 164 that performs processing (second processing) based on an FM (Frequency Modulation) screen method, and a threshold screen table 165 for data conversion used by the FM screen processing unit 164. ing.

  The gradation reproduction processing unit 16 according to the first embodiment is a device that generates binary output image data in which the number of gradations (for example, 256 gradations) of input image data is reduced to two gradations. Therefore, each of the error diffusion processing unit 162 and the FM screen processing unit 164 executes gradation reproduction processing for converting the data value of each pixel data (hereinafter referred to as pixel value) in the input image data into binary data. When the image output unit 17 forms a four-value (four gradation) image, the gradation reproduction processing unit 16 may generate four-value image data.

  In the gradation reproduction processing unit 16 having the above-described configuration, the density region determination unit 161 determines which of the three density regions divided in advance includes each pixel value (density) of the input image data. In this embodiment, each pixel value of the input image data is expressed by 256 gradations. For example, a pixel value of 0 to 9 is set as a low density region (second density region), and a pixel value of 10 to 229 is set as a middle value. A density area (first density area) is set, and pixel values of 245 to 255 are set as high density areas (second density areas). Note that such density area divisions are merely examples, and are not limited to these values.

  The gradation reproduction processing unit 16 inputs pixel data of pixel values determined to be included in the low density region or the high density region by the density region determination unit 161 to the FM screen processing unit 164 and performs processing based on the FM screen method. To do. On the other hand, the gradation reproduction processing unit 16 inputs pixel data of pixel values determined to be included in the medium density region by the density region determination unit 161 to the error diffusion processing unit 162 and performs processing based on the error diffusion method. .

  The error diffusion processing unit 162 refers to each pixel value of the peripheral pixel as a quantization error by the peripheral reference unit 163 for each pixel data determined to be included in the medium density region by the density region determination unit 161, An error diffusion process is executed using the quantization error. The peripheral reference unit 163 refers to these pieces of information when the error diffusion processing unit 162 performs gradation reproduction processing for forming a cluster by connecting dots and the number and arrangement of peripheral dots, for example. In addition, when the information on the peripheral dots appears as an error, the peripheral reference unit 163 refers to the information on the peripheral dots. Note that the error diffusion method used by the error diffusion processing unit 162 may be a method using the weighting coefficient matrix shown in FIG.

  The FM screen processing unit 164 executes processing based on the FM screen method on each pixel data determined to be included in the low density region or the high density region by the density region determination unit 161. Below, the process based on the FM screen method in this embodiment is demonstrated.

  In the gradation reproduction processing unit 16 of the present embodiment, the threshold screen table 165 used by the FM screen processing unit 164 is a low density region table 165a composed of threshold values having a matrix size of 128 × 128 as shown in FIG. And a high density region table 165b having a matrix size threshold value of 128 × 128 as shown in FIG. The low density area table 165a and the high density area table 165b are respectively a table for C (cyan), a table for M (magenta), a table for Y (yellow), and a table for K (black). In FIG. 3A and FIG. 3B, for example, a table for C is shown as a representative.

  The FM screen processing unit 164 selects the low-density region table 165a or the high-density region table 165b from the threshold screen 165 as described above according to the density region corresponding to each pixel data determined by the density region determination unit 161. Is read. Further, each pixel value input from the density area determination unit 161 is compared with the threshold value corresponding to each pixel in the threshold screen table 165 (the low density area table 165a or the high density area table 165b), and the dot value is determined. ON / OFF (1/0) is determined.

  More specifically, the pixel data of the pixel values included in the low density area or the high density area input from the density area determination unit 161 is the X pixel and the Y direction (shown in the right direction in FIG. 3). 3 (downward direction 3), when an area in which Y pixels are arranged in a two-dimensional matrix (hereinafter also referred to as an FM processing area) is configured, the upper left pixel of the FM processing area is represented by coordinates (1, 1) and coordinates (2, 1), (3, 1)... (X, 1), (1, 2), (2, 2) (3, 2)... (X, 2) in the order of processing direction. , (1, 3)... (X, Y).

  The FM screen processing unit 164 then outputs the pixel value V (1,1) of the pixel (1,1) in the FM processing area and the corresponding coordinates (1,1) in the threshold screen table 165. The threshold TH (1, 1) is compared. Therefore, when the pixel value V (1,1) is larger than the threshold value TH (1,1), the pixel (1,1) is turned on (1), and when it is smaller, the pixel (1,1) is turned off (0). ).

  Subsequently, the FM screen processing unit 164 outputs the pixel value V (2, 1) of the pixel (2, 1) in the FM processing area and the threshold value of the coordinate (2, 1) in the threshold screen table 165 corresponding thereto. Comparison with TH (2,1) determines on / off of pixel (2,1). The FM screen processing unit 164 sequentially repeats the comparison process between each pixel value V (x, y) in the FM processing area and each threshold value TH (x, y) in the threshold screen table 165 to obtain a pixel. When ON / OFF of (128, 1) is determined, the pixel value V (129, 1) of the pixel (129, 1) in the FM processing area and the coordinate (1, 1) in the threshold screen table 165 The on / off state of the pixel (129, 1) is determined by comparing with the threshold value TH (1, 1).

  Similarly, the FM screen processing unit 164 converts the pixel value V (130,1) to on or off by comparing the pixel value V (130,1) and the threshold value TH (2,1), and Repeat until the conversion process of the value V (X, 1) is completed. On the other hand, when the conversion processing of the pixel value V (X, 1) is completed, the FM screen processing unit 164 compares the pixel value V (1,2) with the threshold value TH (1,2) to compare the pixel (1 , 2) is turned on or off. Output image data output by the image output unit 17 is generated by the FM screen processing unit 164 performing the above-described processing on all pixel data in the FM processing region.

  In this way, by performing error diffusion processing only on the pixel data in the middle density region where the best image can be obtained by processing based on the error diffusion method, the present invention prevents fluctuations in dot density and prevents image fluctuations. High image quality can be realized. On the other hand, with respect to pixel data of low density regions and high density regions where it is not expected to obtain a good image by the error diffusion method, highlighting in which dots are dispersed sparsely by performing FM screen processing that does not refer to surrounding pixels. In the pixel data of the portion (low density region), not only can the dots be dispersed properly, but also the image quality deterioration (moire) caused by the worm and dot periodicity that occurs when error diffusion processing is performed. A good prevented image can be formed.

  Note that it is not necessary to refer to neighboring pixels for the same reason for an image in a high density region, and therefore, a good image can be obtained by performing FM screen processing. Therefore, in the present invention, a good image can be obtained by performing gradation reproduction processing corresponding to each pixel value (density) on input image data having each pixel value from the low density region to the high density region. it can.

  Hereinafter, the procedure of the gradation reproduction process performed by the gradation reproduction processing unit 16 having the above-described configuration will be described. FIG. 4 is a flowchart showing the procedure of gradation reproduction processing by the gradation reproduction processing unit 16. The following processing is executed by a CPU (not shown) in accordance with a control program stored in a storage unit such as a ROM (not shown) in the printer 100.

  The tone reproduction processing unit 16 determines the density region of each pixel data in the input image data input from the output tone correction unit 15 by the density region determination unit 161 (S1). Specifically, the density area determination unit 161 determines whether each pixel data is included in the low density area, the medium density area, or the high density area according to the above-described density classification. It is determined whether it is determined, or whether it is determined to be included in either the low density region or the high density region (S2).

  If the density area determination unit 161 determines that it is included in the medium density area (S2: YES), it transfers the pixel data to the error diffusion processing unit 162 (S3). The error diffusion processing unit 162 performs the error diffusion processing with reference to the quantization error of the peripheral pixels by the peripheral reference unit 163 as described above on the transferred pixel data (S4). On the other hand, if the density area determination unit 161 determines that the medium area is not included (S2: NO), that is, pixel data determined to be included in either the low density area or the high density area is subjected to FM screen processing. The data is transferred to the unit 164 (S5). The FM screen processing unit 164 executes the FM screen processing as described above based on the threshold screen table 165 corresponding to either the low density region or the high density region determined by the density region determination unit 161 (S6).

  The density region determination unit 161 determines whether or not the processing for all the pixel data in the input image data from the output tone correction unit 15 has been completed (S7), and determines that the processing has not been completed (S7). : NO), the process is returned to step S1, and the process for the next pixel data is executed. If it is determined that the processing for all pixel data has been completed (S7: YES), the tone reproduction processing unit 16 ends the tone reproduction processing.

  In the first embodiment described above, the configuration in which the density of each pixel data of the input image data is divided into three density areas has been described. For example, two density areas, a low density area, a medium density area, and a high density area, are provided. The FM screen process may be executed only for the low density area, and the error diffusion process may be executed for the medium density area and the high density area. Further, the above-described gradation reproduction processing unit 16 is exemplified as a configuration that performs gradation reproduction processing on binary data, but there is no problem in gradation reproduction processing on multi-value data as well as binary data. Can be applied. In this case, binary data can be generated by FM screen processing, multi-value data can be generated by error diffusion processing, and multi-value data can be generated by both processes. If a good image can be obtained, There is no particular limitation.

  The threshold screen table 165 used by the FM screen processing unit 164 has a 128 × 128 matrix size, but is not limited to this as long as a good image can be obtained. Furthermore, in the first embodiment, the configuration in which the threshold screen table 165 used for the gradation reproduction process is provided in the printer 100 (image forming apparatus) has been described. If not, it can be stored in an external device (not shown) and transferred to the printer 100 together with the image data every time the image forming process (printing) is performed.

  Examples of the external device include a computer connected to the printer 100 via a cable, a fax receiving unit, and the like. Since these are relatively less subject to storage capacity limitations, they are effective for obtaining high image quality. In such a configuration, an operation such as appropriately updating the threshold screen table 165 becomes more effective in order to form a higher quality image. In this way, setting necessary information outside the main body is desirable in order to avoid a reduction in performance of the printer 100 without imposing a burden on the CPU of the printer 100. Further, when the driver of the computer as an external device does not have the above-described function, the threshold screen table 165 and software update for executing the gradation reproduction process via an electronic data storage medium such as a flexible disk or a CD-ROM are updated. Is possible.

(Modification of Embodiment 1)
When an image is formed on recording paper based on the output image data obtained by performing the gradation reproduction process as described above, the boundary portion between the low density region and the middle density region, and the middle density region and the high density region There may be a case where a region where dots are locally sparse occurs at the boundary portion of the, or the formed dot pattern is uniform. FIG. 5 is an explanatory diagram for explaining a dot pattern having a region where dots are sparse. FIG. 5A schematically shows a part of an image formed based on input image data. Only a pixel having a pixel value of 10 is formed on the right side of the broken line in the drawing, and a pixel value is illustrated on the left side. Only 9 pixels are formed.

  As described above, the FM screen processing by the FM screen processing unit 164 is performed on the pixel data of the pixel value 9 determined to be the low density region for such an image, and the pixel value determined to be the medium density region. When error diffusion processing by the error diffusion processing unit 162 is performed on the 10 pixel data, like the region surrounded by a circle in FIG. 5B, the right side of the boundary portion between the low density region and the medium density region, that is, Dots are less likely to occur at the boundary between the pixel value 10 area and the pixel value 9 area, and the human eye may perceive something like a white line. This is because when the error diffusion process is performed on the boundary portion, a sufficient error cannot be obtained at the boundary portion, so that the dot on delays and the on dot does not appear at an appropriate pixel position. .

  Therefore, in the modification of the first embodiment, the following processing is performed on the boundary portion between the low concentration region and the medium concentration region and the boundary portion between the medium concentration region and the high concentration region. FIG. 6 is an explanatory diagram for explaining a modification of the gradation reproduction processing by the gradation reproduction processing unit 16. In FIG. 6, each cell represents a pixel, and the numbers in each cell represent the respective pixel values.

  In the gradation reproduction processing unit 16 according to this modification, the error diffusion processing unit 162 is predetermined from the position of the pixel determined to be included in the medium density region among the pixels determined to be included in the low density region. Error diffusion processing is performed on each pixel data in a region including pixels at positions within the range (here, as an example, two pixels). Further, the FM screen processing unit 164 is a pixel determined to be included in the low density region. As described above, the error diffusion processing unit 162 applies the pixels within the range of two pixels from the middle density region. After the error diffusion process is completed, FM screen processing is executed.

  Accordingly, the gradation reproduction processing unit 16 sequentially executes FM screen processing by the FM screen processing unit 164 on the pixel having the pixel value 9 (low density region) for the input image data shown in FIG. First, error diffusion processing by the error diffusion processing unit 162 is performed on pixels corresponding to two pixels (hatched portion in FIG. 6A) at the boundary with the pixel value 10 (medium density region) among the pixels with the pixel value 9. Execute. Subsequently, the gradation reproduction processing unit 16 performs error diffusion processing by the error diffusion processing unit 162 on the pixel having the pixel value of 10. As a result, for each pixel in the middle density area, the error of the pixel in the area shown by hatching in FIG. Can be generated.

  In addition, the tone reproduction processing unit 16 performs FM screen processing by the FM screen processing unit 164 on the pixel having the pixel value 9 in the area shown by hatching in FIG. By executing, the ON / OFF of the dot of this area | region is determined. In this way, by performing the above-described processing on the boundary portion between the low density region and the medium density region, as shown in FIG. An image having an arrangement can be obtained.

  Note that processing for determining the final dot on / off by FM screen processing after calculating an appropriate error once by error diffusion processing in this way is not limited to pixel data in the low density region. In the case of having a boundary portion between the medium density region and the high density region, it can be appropriately applied to pixel data in the high density region. In the above description, it has been shown that FM screen processing is performed after error diffusion processing is performed on pixel data having a width of 2 pixels from the medium density region. If a pattern can be obtained, it may be 1 pixel wide, 3 pixels wide, or even larger.

  As described above, in this modification, both error diffusion processing and FM screen processing are performed on the pixel data indicated by hatching in FIG. There is a concern that it will be long. However, since FM screen processing is very simple processing compared to error diffusion processing, the amount of calculation can be reduced sufficiently compared to when error diffusion processing is performed on all pixel data of input image data. it can.

  Hereinafter, a processing procedure of a modified example of the above-described gradation reproduction processing will be described. FIG. 7 is a flowchart showing the procedure of gradation reproduction processing by the gradation reproduction processing unit 16. The following processing is executed by a CPU (not shown) in accordance with a control program stored in a storage unit such as a ROM (not shown) in the printer 100.

  The gradation reproduction processing unit 16 initializes pixel position data for specifying a pixel to be processed by the FM screen processing unit 164 after the processing by the error diffusion processing unit 162 to 0 and stores the data in a memory (not shown). S11). The tone reproduction processing unit 16 determines whether each pixel data in the input image data input from the output tone correction unit 15 is included in a low density region, a medium density region, or a high density region. (S12), it is determined whether it is determined to be included in the medium density region or whether it is determined to be included in either the low density region or the high density region (S13).

  When the density region determination unit 161 determines that the pixel is included in the medium density region (S13: YES), the pixel data is transferred to the error diffusion processing unit 162 (S14), and the error diffusion processing unit 162 uses the peripheral reference unit 163 to perform the peripheral processing. Error diffusion processing with reference to the quantization error of the pixel is executed (S15). On the other hand, when the density area determination unit 161 determines that the pixel data is not included in the medium density area (S13: NO), that is, when it is determined that the pixel data is included in either the low density area or the high density area, the pixel data is medium. It is determined whether or not it is located near the boundary of the density region (S16).

  When it is determined that the density area determination unit 161 is near the boundary with the middle density area (S16: YES), specifically, when the pixel data is within two pixel widths from the peripheral pixels of the middle density area, the pixel concerned The pixel position data indicating the data position is stored in the memory (S17), the pixel data is transferred to the error diffusion processing unit 162 (S14), and the error diffusion processing by the error diffusion processing unit 162 is executed (S15).

  If it is determined in step S16 that it is not near the boundary with the medium density region (S16: NO), the density region determination unit 161 transfers the pixel data to the FM screen processing unit 164 (S18), and the FM screen processing unit 164 To execute FM screen processing based on the threshold screen table 165 (S19). The density area determination unit 161 determines whether or not the processing for all the pixel data in the input image data from the output tone correction unit 15 has been completed (S20), and determines that the processing has not been completed (S20). : NO), the process is returned to step S12, and the process for the next pixel data is executed.

  If it is determined that the processing for all the pixel data has been completed (S20: YES), the gradation reproduction processing unit 16 determines whether the pixel position data stored in the memory is not 0 (S21). If it is determined that there is (S21: NO), the tone reproduction process is terminated as it is, and if it is not 0 (S21: YES), the pixel data corresponding to the pixel position data stored in the memory is displayed on the FM screen. The image is transferred to the processing unit 164 (S22), and FM screen processing based on the threshold screen table 165 is executed by the FM screen processing unit 164 (S23), and then the gradation reproduction processing ends.

  As described above, the error diffusion process is performed by executing the error diffusion process on the boundary portion between the low density region and the medium density region, specifically, the pixel data in the area indicated by hatching in FIG. In the middle density region that is the execution target of the above, an image having a smooth boundary portion with the low density region can be generated.

  However, when FM screen processing is performed after error diffusion processing is performed on pixel data in the area indicated by hatching in FIG. 6A, it is necessary to detect a boundary portion where the pixel value changes, It is not desirable from the viewpoint of calculation time. Accordingly, pixel data having a data value (specifically, 9,230) whose difference from a pixel value (here, 10 to 229) determined to be included in the intermediate density region is within a predetermined range (for example, 1). It is also possible to execute error diffusion processing on the image and further execute FM screen processing after the error diffusion processing.

  In this case, the gradation reproduction processing unit 16 performs error diffusion processing and FM screen processing on the pixel having the pixel value 9 in the low density region and the pixel having the pixel value 230 in the high density region. Specifically, for the input image data shown in FIG. 6B, the gradation reproduction processing unit 16 sequentially executes FM screen processing by the FM screen processing unit 164 for pixels having a pixel value of 8 (low density region). . Next, the gradation reproduction processing unit 16 first performs error diffusion processing by the error diffusion processing unit 162 on the pixel having the pixel value 9 (hatched portion in FIG. 6B), and subsequently performs error diffusion on the pixel having the pixel value 10. Error diffusion processing by the processing unit 162 is executed.

  As a result, it is possible to appropriately refer to the error of each pixel in the boundary portion with the low density region for each pixel in the middle density region without detecting the boundary portion between the low density region and the middle density region. Thus, output image data having a good dot arrangement can be generated. In addition, the tone reproduction processing unit 16 performs FM screen processing by the FM screen processing unit 164 on the pixel having the pixel value 9 in the area shown by hatching in FIG. By executing, the ON / OFF of the dot of this area | region is determined.

  In this method, as described above, both error diffusion processing and FM screen processing are performed on pixels larger than the number of pixels in the area shown by hatching in FIG. 6A. Compared with the diffusion process, the calculation amount is far less, so that there is no substantial increase in calculation cost. This method also eliminates the need to detect the boundary portion, so that the calculation cost can be further reduced, and is more desirable from the viewpoint of speeding up and cost reduction.

(Embodiment 2)
In the first embodiment described above, the configuration in which the image processing apparatus of the present invention is applied to a printer has been described. In the following second embodiment, a configuration in which the image processing apparatus of the present invention is applied to a digital copying machine will be described. FIG. 8 is a block diagram illustrating an internal configuration example of the digital copying machine according to the second embodiment. In FIG. 8, reference numeral 101 denotes a digital copying machine according to the second embodiment. The digital copying machine 101 includes a color image input device 21, a color image processing device 3 that processes image data input via the color image input device 21, and an image based on the image data processed by the color image processing device 3. A color image output device 22 for forming the image on a sheet, and an operation panel 20 for an operator to operate the digital copying machine 101. Although not shown, the digital copying machine 101 includes a CPU that controls each unit described above. Further, the same components as those of the printer 100 shown in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  The color image input device 21 is configured by a scanner unit including, for example, a CCD (Charge Coupled Device), and a reflected light image from an original is read by the CCD, and RGB (R: red, G: green, B: blue). ) Analog signal is generated. The generated RGB analog signal is sent to the color image processing apparatus 3. The color image processing apparatus 3 includes an A / D (analog / digital) conversion unit 31, a shading correction unit 32, an input tone correction unit 33, a region separation processing unit 34, a color information processing unit 13, and a black generation and under color removal unit 14. A spatial filter processing unit 35, an output tone correction unit 15, and a tone reproduction processing unit 16.

  The color image processing device 3 converts an analog signal read by the color image input device 21 into an A / D conversion unit 31, a shading correction unit 32, an input tone correction unit 33, a region separation processing unit 34, and a color information processing unit 13. Then, the CMYK digital image data is generated by transferring them in the order of the black generation and under color removal unit 14, the spatial filter processing unit 35, the output tone correction unit 15, and the tone reproduction processing unit 16, and performing predetermined processes respectively. Further, the image data generated by the color image processing device 3 is temporarily stored in a storage unit (not shown), and is output to the color image output device 22 at a predetermined timing.

  The A / D conversion unit 31 receives the RGB analog signal from the color image input device 21, converts the received RGB analog signal into an RGB digital signal, and sends the RGB digital signal to the shading correction unit 312. The shading correction unit 32 performs a process of removing various distortions generated in the illumination system, the imaging system, and the imaging system of the color image input device 21 on the RGB digital signal received from the A / D conversion unit 31. This is sent to the input tone correction unit 33. The input tone correction unit 33 adjusts the color balance for the RGB digital signal (RGB reflectance signal) received from the shading correction unit 32 and is processed by the image processing system employed in the color image processing apparatus 3. After performing processing for conversion to a density signal that is easy to perform, the signal is sent to the region separation processing unit 34.

  The region separation processing unit 34 separates each pixel in the image of the RGB digital signal received from the input tone correction unit 33 into one of a character region, a halftone dot region, and a photographic region, and based on the separation result, the pixel Is output to the black generation and under color removal unit 14, the spatial filter processing unit 35, and the gradation reproduction processing unit 16. Further, the RGB digital signal received by the region separation processing unit 34 from the input tone correction unit 33 is sent to the color information processing unit 13 as it is.

  The color information processing unit 13 and the black generation and under color removal unit 14 have the same configurations as those described in the first embodiment, and the RGB information signal input from the region separation processing unit 34 is received by the color information processing unit 13 as CMY. The signal is converted into a signal, converted into a CMYK signal by the black generation and under color removal unit 14 and sent to the spatial filter processing unit 35. The spatial filter processing unit 35 performs a spatial filter process using a digital filter on the image of the CMYK signal received from the black generation and under color removal unit 14 based on the region identification signal, corrects the spatial frequency characteristics, Processing for improving the deterioration of graininess is performed, and the result is sent to the output tone correction unit 15.

  The output tone correction unit 15 has the same configuration as that of the first embodiment described above, and outputs the CMYK signals input from the spatial filter processing unit 35 according to the tone reproduction characteristics of the color image output device 22. A gradation correction process is performed and sent to the gradation reproduction processing unit 16. The gradation reproduction processing unit 16 performs predetermined processing on the image data of the CMYK signal from the output gradation correction unit 15 based on the region identification signal from the region separation processing unit 34.

  Specifically, the region separated as the character region by the region separation processing unit 34 is subjected to sharp enhancement processing included in the spatial filter processing performed by the spatial filter processing unit 35 in order to improve the reproducibility of black characters or color characters. Increase the amount of high frequency enhancement. In addition, the gradation reproduction processing unit 16 performs high-resolution binarization or multi-level processing suitable for high frequency reproduction.

  Further, with respect to the region separated as the halftone dot region by the region separation processing unit 34, the spatial filter processing unit 35 performs low-pass filter processing for removing the input halftone dot component. The output tone correction unit 15 performs output tone correction processing for converting a signal such as a density signal into a halftone dot area ratio that is a characteristic value of the color image output device 32. Then, gradation reproduction processing (halftone generation) is performed to binarize or multi-value so that the image is finally separated into pixels and each gradation can be reproduced. Further, with respect to the region separated as the photographic region by the region separation processing unit 34, the gradation reproduction processing unit 16 performs binarization or multi-value processing with an emphasis on gradation reproducibility.

  The CMYK signal (image data) binarized or multi-valued by the gradation reproduction processing unit 16 is sent to the color image output device 22. The color image output device 22 is a device that forms an image on a recording medium such as paper based on the CMYK signal received from the color image processing device 3. For example, an electrophotographic or inkjet color image output device can be used.

(Embodiment 3)
In the first embodiment, the configuration in which the image processing apparatus of the present invention is applied to a printer and in the second embodiment the image processing apparatus of the present invention is applied to a digital copying machine has been described. However, a printer driver is provided in a computer. Also good. FIG. 9 is a block diagram showing a configuration example of such an image processing system. The image processing system according to the third embodiment includes a computer 4, a color image input device 21 such as a scanner or a digital camera, and a color image output device 22 such as a printer. The computer 4 includes a printer driver 40. Installed.

  The printer driver 40 is a printer language translator that translates image data into a printer language (PJL), such as the color information processing unit 13, the output tone correction unit 15, and the tone reproduction processing unit 16 as described above. Part 41. The image data input from the color image input device 21 to the computer 4 can be processed and edited by executing various application programs.

  The computer 4 performs color information processing, output gradation correction processing, and image processing on the image data acquired from the color image input device 21 by the color information processing unit 13, the output gradation correction unit 15, the gradation reproduction processing unit 16, and the like. Gradation reproduction processing or the like is performed as application software, and the processed image data is input to a color image output device 22 such as a printer. The color information processing by the color information processing unit 13 includes a black generation and under color removal process.

  The processed image data output via the color information processing unit 13, the output gradation correction unit 15, the gradation reproduction processing unit 16, and the like is converted into a printer language by the printer language translation unit 41, and the communication port driver 42. The image is output to the color image output device 22 via the communication port (RS232C, LAN, etc.) 43, and an image corresponding to the image data is recorded.

(Embodiment 4)
A computer program for causing a computer to execute the above-described image processing method of the present invention may be recorded on a computer-readable recording medium. In this case, a recording medium on which a computer program for performing the image processing method of the present invention is recorded can be provided in a portable manner.

  FIG. 10 is a schematic diagram showing a configuration example of an image processing system constructed by installing a computer program according to the present invention. In the figure, reference numeral 52 denotes an information processing apparatus such as a personal computer or a workstation. The information processing apparatus 52 includes an image input apparatus 51 such as a flatbed scanner, a film scanner, and a digital camera, and an image forming apparatus such as an ink jet printer and a laser printer provided with means for forming an image on a sheet such as paper and an OHP film. 53, peripheral devices such as an image display device 54 such as a CRT display and a liquid crystal display are connected.

  The information processing device 52 includes a reading device such as an FD drive or a CD-ROM drive for reading the computer program from a recording medium 55 such as an FD or CD-ROM in which the computer program according to the present invention is recorded. The computer program read by the reading device is stored in a predetermined storage area in the information processing device 52. The CPU (not shown) of the information processing apparatus 52 loads the computer program from this storage area and executes it, thereby realizing the image processing method (tone reproduction processing) as described above.

  With such a configuration, the image processing method of the present invention can be used according to the user's preference. For example, when the image processing method of the present invention is executed in this image processing system, it becomes easy to arbitrarily change the threshold setting, and it is set according to the result (preview) displayed on the image display device 54. Processing according to the user's preference, such as redoing, can be performed. The threshold value can be changed by directly inputting a numerical value using the keyboard 56 or dragging a symbol representing the threshold value with the mouse 57.

  The recording medium 55 for recording the computer program may be any storage medium that can store the program and can be read directly or indirectly by the computer. For example, in addition to the FD and CD-ROM described above, optical recording media such as MO, MD, and DVD, magnetic recording media such as hard disks, card-type recording media such as IC cards, memory cards, and optical cards, mask ROMs, EPROMs ( A semiconductor memory such as an Erasable Programmable Read Only Memory (EEPROM), an EEPROM (Electrically Erasable Programmable Read Only Memory), or a flash ROM can also be used.

  When the information processing device 52 includes a modem or the like as a communication means for connecting to a server device or the like via a communication network, the computer program of the present invention is stored in the server device in advance. The computer program may be downloaded from the server device and installed in the information processing device 52.

  In each of the embodiments described above, FM screen processing is used as one of the gradation reproduction processes. However, the present invention is not limited to this as long as the gradation reproduction process uses a threshold array. The image processing apparatus of the present invention can be applied not only to an ink jet type or electrophotographic type image forming apparatus, but also to an image forming apparatus using, for example, a toner jet type or a thermal transfer type. The image processing apparatus of the present invention can also be applied to a monitor that is connected to a computer and used, a television monitor that requires gradation reproduction processing, a liquid crystal device, or electronic paper that uses a twist ball.

FIG. 2 is a block diagram illustrating an internal configuration example of a printer according to the first embodiment. It is a block diagram which shows the example of an internal structure of a gradation reproduction process part. It is a schematic diagram which shows the structural example of a threshold value screen table. It is a flowchart which shows the procedure of the gradation reproduction process by a gradation reproduction process part. It is explanatory drawing for demonstrating a dot pattern. It is explanatory drawing for demonstrating the modification of a gradation reproduction process. It is a flowchart which shows the procedure of the gradation reproduction process by a gradation reproduction process part. FIG. 6 is a block diagram illustrating an internal configuration example of a digital copying machine according to a second embodiment. FIG. 10 is a block diagram illustrating a configuration example of an image processing system according to a third embodiment. FIG. 10 is a schematic diagram illustrating a configuration example of an image processing system according to a fourth embodiment. It is a schematic diagram which shows the example of the weighting coefficient matrix used for the conventional error diffusion method.

Explanation of symbols

100 printer (image forming apparatus)
1 Color image processing device (image processing device)
DESCRIPTION OF SYMBOLS 10 Image input part 16 Gradation reproduction process part 161 Density area | region determination part (determination means)
162 Error diffusion processing unit (first processing means)
164 FM screen processing unit (second processing means)
165 Threshold screen table 17 Image output unit 101 Digital copier (image forming apparatus)
3 Color image processing device (image processing device)
21 Color Image Input Device 22 Color Image Output Device 55 Recording Medium

Claims (9)

In an image processing apparatus including a gradation reproduction processing unit that performs gradation reproduction processing on input image data to generate pseudo-halftone output image data,
The gradation reproduction processing unit
Determination means for determining whether the density of each pixel data in the input image data is included in the first density area or the second density area that is divided in advance;
First processing means for executing a first process based on the density of surrounding pixel data for the pixel data determined by the determining means to be included in the first density area;
An image processing apparatus comprising: second processing means for executing second processing based on the density of each pixel data for the pixel data determined by the determination means to be included in the second density region. The first density region is a region including an intermediate density between the maximum density and the minimum density related to pixel data,
The image processing apparatus according to claim 1, wherein the second density region is a region including a density other than the first density region. The first processing means has a predetermined range from the position of the pixel data determined to be included in the first density area by the determination means among the pixel data determined to be included in the second density area by the determination means. The first process is performed on pixel data at a position in
The second processing means performs the first processing by the first processing means on the pixel data at a position within the predetermined range among the pixel data determined to be included in the second density region. The image processing apparatus according to claim 1, wherein the second process is executed. The first processing means is within a predetermined range with respect to the density determined to be included in the first density area by the determination means among the pixel data determined to be included in the second density area by the determination means. The first process is performed on pixel data having a density of
The second processing means performs after the first processing by the first processing means on pixel data having a density within the predetermined range of the pixel data determined to be included in the second density area. The image processing apparatus according to claim 1, wherein the second process is executed.   An image forming apparatus comprising: the image processing apparatus according to claim 1; and a unit that forms an image according to output image data generated by the image processing apparatus on a sheet. . In an image processing method for generating pseudo-halftone output image data by performing gradation reproduction processing on input image data,
Determining whether the density of each pixel data in the input image data is included in the first density area or the second density area that is divided in advance;
Performing a first process based on the density of surrounding pixel data for pixel data determined to be included in the first density area;
And a step of executing a second process based on the density of each pixel data for the pixel data determined to be included in the second density area. Of the pixel data determined to be included in the second density region, the first processing is performed on pixel data at a position within a predetermined range from the position of the pixel data determined to be included in the first density region. A step of performing
Performing the second process on the pixel data at a position within the predetermined range of the pixel data determined to be included in the second density region after the first process is performed. The image processing method according to claim 6. Of the pixel data determined to be included in the second density region, the first processing is performed on pixel data having a density within a predetermined range with respect to the density determined to be included in the first density region. A step of performing
Performing the second process on the pixel data having the density within the predetermined range among the pixel data determined to be included in the second density area after the first process is performed. The image processing method according to claim 6. In a computer program for causing a computer to generate gradation halftone output image data by performing gradation reproduction processing on input image data,
Causing the computer to determine whether the density of each pixel data in the input image data is included in a first density area or a second density area that is divided in advance;
Causing the computer to execute a first process based on the density of surrounding pixel data for pixel data determined to be included in the first density area;
A computer program comprising: causing the computer to execute a second process based on the density of each pixel data for pixel data determined to be included in the second density region.

Images