JP2014011544A - Image processing apparatus and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress fog or drop of images due to error accumulation in an error diffusion processing.SOLUTION: Image processing apparatus (10) includes: a correction amount acquisition means (50) acquiring a gradation correction amount of a multilevel image generated based on an input binary image; an error addition acquisition means (61) acquiring error addition information calculated based on gradation correction amount of a target pixel and error information; a determination means (62) determining a pixel value of the target pixel in an output binary image; a binarization error acquisition means (65) acquiring binarization error information calculated based on the pixel value of the target pixel in the output binary image, the pixel value of the target pixel in the input binary image, and the error addition information; a change means (66) changing the binarization error information when a numerical value indicated by the binarization error information is not within a predetermined range; and a supply means (68) supplying error information used in the next target pixel to the error addition acquisition means (61) based on the binarization error information.

Description

  The present invention relates to an image processing apparatus and a program.

  In Patent Document 1, color correction processing is performed on a multi-value image obtained by descreening a binary image, error diffusion processing is performed based on the color correction amount in the color correction processing, and the result of the error diffusion processing is obtained. Accordingly, a technique for changing the pixel value of the edge portion of the halftone dot in the binary image is described.

JP 2005-027270 A

  An object of the present invention is to suppress image fogging or omission due to error accumulation in error diffusion processing.

  In order to achieve the above object, the image processing apparatus according to claim 1 includes gradation correction amount acquisition means for acquiring a gradation correction amount of a multi-valued image generated based on an input binary image for each pixel. , Based on the error addition information, error addition acquisition means for acquiring error addition information calculated based on the tone correction amount of the target pixel in the input binary image and error information based on error diffusion processing, Determination means for determining a pixel value of the target pixel in the output binary image, a pixel value of the target pixel in the output binary image, a pixel value of the target pixel in the input binary image, and the error addition information And binarization error acquisition means for acquiring binarization error information calculated based on, and if the numerical value indicated by the binarization error information is not within a predetermined range, the binarization error information is changed. Change means to do and before Based on the changed binary-coding error information by changing means, characterized in that it comprises a supply means for supplying the error information to the error addition acquiring means used in the next pixel of interest.

  In the image processing apparatus according to claim 2, when the absolute value of the numerical value indicated by the binarization error information is greater than or equal to a reference value, the changing unit is configured so that the absolute value is less than the reference value. The binarization error information is changed.

  In the image processing apparatus according to claim 3, the changing unit may be configured such that the numerical value indicated by the binarization error information is greater than or equal to a first reference value, or the numerical value indicated by the binarization error information is The binarization error information is changed so that the numerical value indicated by the binarization error information is less than the first reference value and greater than or equal to the second reference value when it is less than the second reference value smaller than one reference value. It is characterized by.

  The image processing apparatus according to claim 4, wherein the determining unit determines a pixel value of a pixel corresponding to an edge portion of the input binary image among the pixels based on the error addition information. It is characterized by.

  The image processing apparatus according to claim 5, a unit that acquires an evaluation value of each pixel based on a distance between each pixel and an edge portion of the input binary image, and the determination unit that determines the target pixel. Means for determining a pixel value of the surrounding pixel based on a determination result of the pixel value and the evaluation value of the surrounding pixel of the target pixel;

  The image processing apparatus according to claim 6, a unit that acquires an evaluation value of each pixel based on a pixel value of the input binary image, and a determination result of the pixel value of the target pixel by the determination unit, And means for determining a pixel value of the surrounding pixel based on the evaluation value of the surrounding pixel of the target pixel.

  The image processing apparatus according to claim 7, a unit that acquires an evaluation value of each pixel based on a pixel value of the output binary image, and a determination result of the pixel value of the target pixel by the determination unit And means for determining a pixel value of the surrounding pixel based on the evaluation value of the surrounding pixel of the target pixel.

  The program according to claim 8 is a gradation correction amount acquisition unit that acquires, for each pixel, a gradation correction amount of a multi-valued image generated based on an input binary image, and a target pixel in the input binary image. Error addition acquisition means for acquiring error addition information calculated based on the gradation correction amount and error information based on error diffusion processing, and the pixel of the target pixel in the output binary image based on the error addition information Determining means for determining a value, binarization calculated based on a pixel value of the target pixel in the output binary image, a pixel value of the target pixel in the input binary image, and the error addition information Binarization error acquisition means for acquiring error information, if the numerical value indicated by the binarization error information is not in a predetermined range, changing means for changing the binarization error information, and the changing means changed by the changing means two Reduction based on the error information, the next target pixel supply means for supplying the error information to the error addition acquiring means used by the computer to function as a.

  According to the first and eighth aspects of the invention, image fogging or omission due to error accumulation in error diffusion processing is suppressed.

  According to the second aspect of the present invention, the absolute value of error accumulation in the error diffusion process is kept below the reference value.

  According to the third aspect of the present invention, error accumulation in the error diffusion process is limited to a predetermined range.

  According to the invention described in claim 4, the pixel value of the pixel corresponding to the edge portion is determined.

  According to the fifth, sixth, and seventh aspects of the invention, the pixel value of the surrounding pixel is determined in consideration of the evaluation value of the surrounding pixel.

It is a figure which shows the hardware constitutions of the image processing apparatus which concerns on this invention. It is a functional block diagram of an image processing apparatus. It is a figure which shows an example of an input binary image. It is a figure which shows an example of the conversion table used for a gradation conversion process. It is a flowchart which shows the process which an image processing apparatus performs. It is a figure which shows an example of the error diffusion process performed in S5. It is a figure which shows the example of a part of input binary image. It is a functional block diagram of modification (2). It is a figure for demonstrating the evaluation value calculated in a modification (2). It is a figure for demonstrating the evaluation value calculated in a modification (3). It is a figure for demonstrating the evaluation value calculated in a modification (3).

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments (hereinafter referred to as embodiments) for carrying out the invention will be described with reference to the drawings.

[1. Hardware configuration of image processing apparatus]
FIG. 1 is a diagram showing a hardware configuration of an image processing apparatus according to the present invention. As shown in FIG. 1, the image processing apparatus 10 includes a control unit 12, a storage unit 14, and a communication unit 16. The image processing apparatus 10 may be realized by a computer such as a personal computer. The image processing apparatus 10 may include an operation unit, a display unit, and the like in addition to the configuration illustrated in FIG.

  The control unit 12 includes a CPU. The control unit 12 controls each unit of the image processing apparatus 10 by executing a program stored in the storage unit 14.

  The storage unit 14 includes various memories such as a hard disk and a RAM. The storage unit 14 is a storage medium that stores a program executed by the control unit 12, for example. In addition, for example, the storage unit 14 is also used as a work memory of the control unit 12. Various memories can be applied as the storage unit 14. For example, the storage unit 14 may be realized by a storage device such as a semiconductor memory.

  The communication unit 16 is for performing data communication with an external device connected to the image processing apparatus 10. The program and various data described as being stored in the storage unit 14 in the present embodiment may be acquired from an external device via the communication unit 16.

  In the present embodiment, the image processing apparatus 10 is connected to the image forming apparatus 20 via the communication unit 16 so that data communication is possible. The image forming apparatus 20 includes a color printer or the like. The image forming apparatus 20 forms an image on a recording medium (for example, paper or film) based on image data received from an external apparatus. In the present embodiment, the image forming apparatus 20 receives binary image data from the image processing apparatus 10 and forms an image on a recording medium. That is, the image forming apparatus 20 performs proof printing before a printing plate for printing is created by a digital plate making apparatus (not shown).

[2. Functions realized in image processing apparatus]
FIG. 2 is a functional block diagram of the image processing apparatus 10. As shown in FIG. 2, the image processing apparatus 10 includes a binary image acquisition unit 30, a smoothing unit 40, a gradation correction amount acquisition unit 50, an error diffusion unit 60, a binary image output unit 70, including. Each function illustrated in FIG. 2 is realized by the control unit 12 executing a program stored in the storage unit 14.

  In the present embodiment, among the functions shown in FIG. 2, the binary image acquisition unit 30, the smoothing unit 40, and the gradation correction amount acquisition unit 50 are realized mainly by the control unit 12, and the error diffusion unit 60 is The control unit 12 and the storage unit 14 are realized mainly, and the binary image output unit 70 is realized mainly by the control unit 12 and the communication unit 16.

[2-1. Binary image acquisition unit]
The binary image acquisition unit 30 acquires an input binary image. The binary image acquisition unit 30 receives an input binary from a unit (for example, the storage unit 14) that stores image data of the input binary image or a unit (for example, the communication unit 16) that inputs image data of the input binary image. Get the image data of the image. Hereinafter, the image data of the input binary image is referred to as input binary image data.

  FIG. 3 is a diagram illustrating an example of the input binary image. As shown in FIG. 3, the input binary image is a binary image having a halftone dot component, and is described in, for example, a page description language (a computer language for instructing the image forming apparatus 20 to perform drawing). This is an image obtained by converting original data by RIP (Raster Image Processor) processing.

  A halftone dot is a dot used for expressing color shading on a recording medium such as paper, and for example, means one or a plurality of pixels constituting the dot. The color density is expressed on the recording medium by the interval between the pixels constituting the halftone dots and the arrangement pattern of the pixels constituting the halftone dots.

  The input binary image data includes information indicating whether each pixel is a dot (that is, information indicating whether each pixel is a black pixel or a white pixel), and each pixel of the input binary image The pixel value is represented by 1 bit. For example, the value of the black pixel is “1”, and the value of the white pixel is “0”. A dot is represented by a pixel having a pixel value “1” among the pixels of the binary image.

  In the present embodiment, as shown in FIG. 3, a two-dimensional coordinate (Xs-Ys coordinate system) is set with the upper left corner of the input binary image as the origin. The number of pixels arranged in the Xs axis direction (horizontal direction) is Xmax, and the number of pixels arranged in the Ys axis direction (vertical direction) is Ymax. That is, the input binary image is composed of Xmax × Ymax pixels. Hereinafter, each pixel is described as a pixel P [x, y] (where 1 ≦ x ≦ Xmax, 1 ≦ y ≦ Ymax).

[2-2. Smoothing section]
The smoothing unit 40 acquires a multi-value image based on the input binary image. A multi-value image is an image in which the pixel value of each pixel P [x, y] is represented by a plurality of bits (for example, 8 bits), and has an intermediate color (that is, gray) between black and white. It can be said.

  The smoothing unit 40 generates image data of a multi-value image (hereinafter referred to as multi-value image data) based on the input binary image data. The smoothing unit 40 performs a conversion process so that the pixel value of each pixel P [x, y] represented by 1 bit in the input binary image is represented by 8 bits in the multi-valued image, Input binary image data is converted into multi-value image data.

Specifically, first, the smoothing unit 40 1-bit pixel value (that is, “0” or “1”) of each pixel P [x, y] of the input binary image indicated by the input binary image data. Are generated as image data of a simple multivalued image expressed by a plurality of bits (hereinafter referred to as simple multivalued image data). When the multi-valued image is expressed by n bits (n is a natural number of 2 or more; for example, n = 8), the smoothing unit 40 uses the pixel P [x, pixel value of the input binary image whose pixel value is “0”. For y], the pixel value of the pixel P [x, y] of the multi-valued image is “0”, and the pixel value of the input binary image is “1”. By setting the pixel value of the pixel P [x, y] of the value image to “2 ⁿ −1” (for example, 255), simple multi-value image data is generated.

  The smoothing unit 40 performs halftone dot removal processing (smoothing processing) on the simple multi-value image indicated by the generated simple multi-value image data. For example, the smoothing unit 40 performs a smoothing filter process on the simple multi-valued image, so that the multi-value image indicating the multi-valued image in which the halftone dot component of the simple multi-valued image is removed from the simple multi-valued image. Get the data.

The smoothing filter corrects the pixel value of the target pixel based on the pixel value of surrounding pixels (pixels determined by the target pixel and a given filter size), and by performing the smoothing filter process, A pixel value between “0” and “2 ⁿ −1” is calculated, and an intermediate color between white and black is expressed. As the smoothing filter process used by the smoothing unit 40, various known smoothing filter processes can be applied. For example, a simple average filter (moving average filter) may be used as the smoothing filter process. In addition, for example, a Gaussian filter that uses a Gaussian distribution may be used in which the weight for calculating the average value increases as the pixel is closer to the target pixel, and the weight decreases as the distance increases.

[2-3. Gradation correction amount acquisition unit]
The gradation correction amount acquisition unit 50 acquires the gradation correction amount of the multi-valued image generated based on the input binary image for each pixel P [x, y]. The gradation correction amount acquisition unit 50 performs gradation conversion processing on each pixel P [x, y] of the multi-value image indicated by the multi-value image data based on the multi-value image data generated by the smoothing unit 40. Data indicating the gradation correction amount when applied is acquired for each pixel P [x, y].

  The gradation correction amount is information indicating how much the pixel value (density) of the image is increased or decreased, and is a change amount of the pixel value before and after the gradation conversion process. The gradation conversion process is executed based on, for example, the association between the pixel value before gradation conversion and the pixel value after gradation conversion. The association may be a mathematical expression format or a table format. Data indicating the association is stored in the storage unit 14. In this association, the pixel values before and after the gradation conversion are set so that the color of the printed matter printed by the image forming apparatus 20 and the printed matter printed by the printing press produced by the digital plate making apparatus are the same. Associated.

  FIG. 4 is a diagram illustrating an example of a conversion table used for gradation conversion processing. As shown in FIG. 4, the pixel value before gradation conversion and the pixel value after gradation conversion are associated with each other in the conversion table. The gradation correction amount acquisition unit 50 associates the pixel value of the pixel P [x, y] with the pixel value for each pixel P [x, y] of the multi-value image data generated by the smoothing unit 40. A gradation correction amount is acquired by calculating a difference between the obtained pixel value after gradation conversion.

  Here, the case where the pixel value before gradation conversion and the pixel value after gradation conversion are associated has been described here, but the pixel value before gradation conversion and the gradation correction amount are associated. You may do it. In this case, the gradation correction amount acquisition unit 50 associates each pixel P [x, y] of the multi-value image data generated by the smoothing unit 40 with the pixel value of the pixel P [x, y]. The gradation correction amount is acquired.

[2-4. Error diffusion section]
The error diffusion unit 60 performs error diffusion processing based on the gradation correction amount of each pixel P [x, y] acquired by the gradation correction amount acquisition unit 50, and the input binary image indicated by the input binary image data. It is determined whether or not to change the pixel value of each pixel P [x, y]. The error diffusion unit 60 supplies image data of an output binary image (hereinafter referred to as output binary image data) generated based on the determination result to the binary image output unit 70.

  In the present embodiment, the error diffusion unit 60 includes an error addition acquisition unit 61, an on / off determination unit 62, a pixel value change unit 63, a fluctuation acquisition unit 64, a binarization error acquisition unit 65, and a binarization. An error change unit 66, a binarization error storage unit 67, and an error supply unit 68 are included. Among the functions of the error diffusion unit 60, the binarized error storage unit 67 is realized mainly by the storage unit 14, and the other functions are realized mainly by the control unit 12.

[2-4-1. Error addition acquisition unit]
The error addition acquisition unit 61 calculates error addition information calculated based on the tone correction amount of the pixel of interest P [x, y] and error information supplied by the error supply unit 68 (error information based on error diffusion processing). To get. The error addition acquisition unit 61 adds the gradation correction amount of the target pixel P [x, y] acquired by the gradation correction amount acquisition unit 50 and the error information supplied to the error addition acquisition unit 61. Thus, error addition information is generated for each pixel of interest P [x, y].

  The target pixel P [x, y] is a pixel P [x, y] that is the target of error diffusion processing among the pixels P [x, y], and the pixel value determination process of the output binary image is performed. It can also be said that the target pixel is being processed. The error information is a numerical value related to an error of the pixel value generated in the pixel P [x, y] for which the pixel value of the output binary image has been determined in the past, and is supplied to the error addition acquisition unit 61 by the error supply unit 68 described later. The The numerical value indicated by the error information being positive indicates that the pixel value of the target pixel P [x, y] should be “1”, and the numerical value indicated by the error information is negative indicates that the target pixel P [x, y] is negative. This indicates that the pixel value of x, y] should be “0”.

[2-4-2. ON / OFF decision section]
The on / off determination unit 62 determines the pixel value of the pixel of interest P [x, y] in the output binary image based on the error addition information acquired by the error addition acquisition unit 61. The on / off determination unit 62 sets the pixel value of the target pixel P [x, y] of the output binary image to error addition information corresponding to the target pixel P [x, y] (that is, the target pixel P [x, y]. ] Is determined based on the numerical value of the error addition information calculated based on the tone correction amount.

  The on / off determination unit 62 determines whether or not the numerical value indicated by the error addition information acquired by the error addition acquisition unit 61 is within a predetermined range. The on / off determination unit 62 determines the pixel value of the pixel of interest P [x, y] in the output binary image based on the determination result. When the numerical value of the error addition information corresponding to the pixel of interest P [x, y] is within a predetermined range (for example, when the value is equal to or greater than the first threshold), the on / off determination unit 62 performs the attention of the output binary image. When the pixel value of the pixel P [x, y] is “1” and the numerical value of the error addition information corresponding to the target pixel P [x, y] is not in a predetermined range (for example, a second value smaller than the first threshold value). When the value is equal to or less than the threshold value), the pixel value of the target pixel P [x, y] of the output binary image is set to “0”.

  For example, when the numerical value indicated by the error addition information is 1 or more, the on / off determination unit 62 determines that the pixel value of the pixel of interest P [x, y] in the output binary image is “1”, and the error addition information is When the numerical value shown is −1 or less, the pixel value of the pixel of interest P [x, y] in the output binary image is determined to be “0”. The on / off determination unit 62 may determine not to change the pixel value of the input binary image when the numerical value indicated by the error addition information is “0”.

[2-4-3. Pixel value change unit]
The pixel value changing unit 63 changes the pixel value of the input binary image based on the determination result of the on / off determining unit 62. The pixel value changing unit 63 generates output binary image data by using the pixel value determined by the on / off determining unit 62 as the pixel value of the output binary image. For example, the pixel value changing unit 63 outputs the pixel value of each pixel P [x, y] of the input binary image indicated by the input binary image data by changing the pixel value to the pixel value determined by the on / off determining unit 62. Binary image data is generated.

Here, the pixel value changing unit 63 executes any one of the following four processes.
(1) The pixel value of the pixel P [x, y] whose pixel value is “1” is set to “0”. (2) The pixel value of the pixel P [x, y] whose pixel value is “0” is set to “1”. (3) The pixel value of the pixel P [x, y] whose pixel value is “1” remains “1”. (4) The pixel value of the pixel P [x, y] whose pixel value is “0” Leave "0"

[2-4-4. Fluctuation acquisition unit]
The variation acquisition unit 64 acquires the amount of change in pixel value (hereinafter referred to as variation information) by the pixel value change unit 63 for each pixel P [x, y]. The fluctuation acquisition unit 64 calculates the difference between the pixel value of each pixel P [x, y] of the input binary image and the pixel value of each pixel P [x, y] of the output binary image. The amount of change in pixel value is acquired.

Here, the fluctuation acquisition unit 64 acquires fluctuation information in any of the following four cases.
(1) When the pixel value of the pixel P [x, y] whose pixel value is “1” is changed to “0”, “1-2 ⁿ ” (for example, −255) is used as the variation information (2 ) When the pixel value of the pixel P [x, y] whose pixel value is “0” is changed to “1”, “2 ⁿ −1” (for example, +255) is used as variation information. (3) Pixel value When the pixel value of the pixel P [x, y] with “1” is “1” and is not changed, “0” is used as the variation information. (4) The pixel P [x with the pixel value “0” , Y] is changed to “0” when the pixel value of “, y] remains“ 0 ”and is not changed.

[2-4-5. Binarization error acquisition unit]
The binarization error acquisition unit 65 includes a pixel value of the target pixel P [x, y] in the output binary image, a pixel value of the target pixel P [x, y] in the input binary image, error addition information, The binarization error information calculated based on is acquired. The binarization error information is a numerical value indicating an error that occurs when the gradation correction amount of the target pixel P [x, y] is binarized.

  The binarization error acquisition unit 65 acquires binarization error information based on the variation information acquired by the variation acquisition unit 64 and the error addition information acquired by the error addition acquisition unit 61. Here, the binarization error acquisition unit 65 acquires a numerical value obtained by subtracting variation information from the error addition information acquired by the error addition acquisition unit 61 as binarization error information.

[2-4-6. Binarization error change unit]
The binarization error changing unit 66 changes the binarization error information when the numerical value indicated by the binarization error information acquired by the binarization error acquisition unit 65 is not in a predetermined range. When the numerical value indicated by the binarization error information is not within a predetermined range, the binarization error changing unit 66 causes the binarization error so that the numerical value indicated by the binarization error information is within the predetermined range. Increase or decrease the numerical value indicated by the information.

  In the present embodiment, when the absolute value of the numerical value indicated by the binarization error information is greater than or equal to a reference value (for example, 1000), the binarization error changing unit 66 makes the absolute value less than the reference value. , Change binarization error information. The binarization error changing unit 66 changes the numerical value indicated by the binarization error information so that the absolute value of the numerical value indicated by the binarization error information decreases (so that the absolute value approaches 0).

  For example, the binarization error changing unit 66, when the numerical value indicated by the binarization error information is a first reference value (for example, 1000) or more, or the numerical value indicated by the binarization error information is less than the first reference value. If it is less than the second reference value (for example, −1000), the binarization error information is changed so that the numerical value indicated by the binarization error information is less than the first reference value and greater than or equal to the second reference value.

  The binarization error changing unit 66 changes the numerical value indicated by the binarization error information so that the numerical value indicated by the binarization error information falls within a range less than the first reference value and greater than or equal to the second reference value. Note that the greater the difference (absolute amount) between the first reference value and the second reference value, the higher the priority of the tone conversion possible amount, and the smaller the difference, the higher priority is given to the prevention of image fogging and omission. Become.

[2-4-7. Binarization error storage unit]
The binarization error storage unit 67 temporarily stores the binarization error information changed by the binarization error change unit 66. For example, the binarization error storage unit 67 is calculated based on the information for identifying the target pixel P [x, y] and the result of the process of determining the pixel value of the target pixel P [x, y]. The value error information is stored in association with each other. The binarization error information stored in the binarization error storage unit 67 is used, for example, in error diffusion processing for the next pixel of interest P [x, y].

[2-4-8. Error supply unit]
The error supply unit 68 supplies error information used in the next pixel of interest P [x, y] to the error addition acquisition unit 61 based on the binarization error information. Based on the binarization error information generated by executing the error diffusion process for one target pixel P [x, y], the error supply unit 68 determines the error of the next target pixel P [x, y]. Error information used in the diffusion process is supplied to the error addition acquisition unit 61.

  The error supply unit 68 executes the product-sum operation process based on the binarization error information generated by executing the error diffusion process of the one pixel of interest P [x, y], thereby performing the next attention. Error information used in processing of the pixel P [x, y] is generated. The mathematical formula used in the product-sum operation may be a predetermined mathematical formula. For example, a numerical value obtained by multiplying the binarization error information corresponding to each of the peripheral pixels of the pixel of interest P [x, y] by a given coefficient is added to correspond to the pixel of interest P [x, y]. Error information to be calculated is calculated.

  Note that the order in which the target pixel is selected among all the pixels P [x, y] may be determined based on a given method. For example, the pixel P [1,1] is set as the first pixel of interest, and thereafter the pixel of interest is selected in the right direction like the pixel P [2,1], the pixel P [3,1]. When [Xmax, 1] is selected, the next row of pixels may be selected next, such as pixel P [1,2], pixel P [2,2]. In addition, for example, the target pixel P [x, y] may be selected in the Hilbert curve order or the Bearo curve order.

[2-5. Binary image output unit]
The binary image output unit 70 outputs the output binary image data to the image forming apparatus 20. The image forming apparatus 20 forms an image on a recording medium based on the output binary image data received from the binary image output unit 70.

[3. Processing executed in image processing apparatus]
FIG. 5 is a flowchart showing processing executed by the image processing apparatus 10. The control unit 12 executes the process shown in FIG. 5 according to the program stored in the storage unit 14. By executing the processing shown in FIG. 5, each function shown in FIG. 2 is realized in the image processing apparatus 10.

  As shown in FIG. 5, the control unit 12 first acquires input binary image data stored in the storage unit 14 or an external storage medium (S1). The input binary image data acquired in S1 includes the pixel value of each pixel P [x, y] of the input binary image.

  The control unit 12 converts the input binary image data acquired in S1 into simple multi-value image data (S2). In S2, the control unit 12 converts 1-bit input binary image data into simple multi-value image data having a plurality of bits.

  The control unit 12 performs a smoothing process on the simple multi-value image indicated by the simple multi-value image data, and generates multi-value image data of a multi-value image having an intermediate color (S3). In S3, the control unit 12 generates multi-value image data by changing the pixel value of each pixel P [x, y] of the simple multi-value image based on the pixel values of surrounding pixels.

  The control unit 12 refers to the conversion table (FIG. 4) stored in the storage unit 14, and the gradation correction amount of each pixel P [x, y] of the multi-value image indicated by the multi-value image data generated in S3 Is calculated (S4). In S4, the control unit 12 calculates the difference between the pixel value of each pixel P [x, y] of the multi-valued image and the pixel value after gradation processing associated with the pixel value. The gradation correction amount is calculated for each pixel P [x, y]. Data indicating the gradation correction amount of each pixel P [x, y] calculated in S4 is temporarily stored in the storage unit 14.

  Thereafter, the control unit 12 performs an error diffusion process for each pixel P [x, y] (S5). FIG. 6 is a diagram illustrating an example of the error diffusion process executed in S5. As illustrated in FIG. 6, the control unit 12 uses the error diffusion process corresponding to the target pixel P [x, y] based on the gradation correction amount and the error information of the target pixel P [x, y]. The error addition information to be calculated is calculated (S6). The error addition information calculated in S6 is temporarily stored in the storage unit 14. Note that when error diffusion processing corresponding to the first pixel of interest P [x, y] is executed, error diffusion processing has not been executed for other pixels P [x, y], so error information A given initial value (eg, 0) is used.

  The control unit 12 refers to the numerical value indicated by the error addition information calculated in S6 (S7). When the numerical value indicated by the error addition information is 1 or more (S7; error addition information ≧ 1), the control unit 12 determines to turn on the pixel value of the target pixel P [x, y] in the output binary image ( S8). In S8, the control unit 12 determines to set the pixel value of the target pixel P [x, y] to “1”.

  When the numerical value indicated by the error addition information is −1 or less (S6; error addition information ≦ −1), the control unit 12 determines to turn off the pixel value of the pixel of interest P [x, y] in the output binary image. (S9). In S9, the control unit 12 determines to set the pixel value of the target pixel P [x, y] to “0”.

  When the numerical value indicated by the error addition information is 0 (S6; error addition information = 0), the control unit 12 determines not to change the pixel value of the target pixel P [x, y] (S10). In S10, the control unit 12 directly uses the pixel value of the target pixel P [x, y] in the input binary image as the pixel value of the target pixel P [x, y] in the output binary image.

  The control unit 12 determines the pixel value of the target pixel P [x, y] of the output binary image based on the determination results of S8 to S10 (S11). Data indicating the pixel value of the target pixel P [x, y] determined in S <b> 11 is temporarily stored in the storage unit 14.

  The control unit 12 determines whether or not the error diffusion process has been executed for all the pixels P [x, y] (S12). In S12, the control unit 12 determines whether or not the processes in S6 to S11 have been executed for all the pixels P [x, y]. For example, the control unit 12 determines whether there is a pixel P [x, y] for which the pixel value of the output binary image has not yet been determined.

  When it is not determined that the error diffusion process has been executed for all the pixels P [x, y] (S12; N), the control unit 12 outputs the pixel value of the target pixel P [x, y] in the input binary image and the output. Based on the pixel value of the target pixel P [x, y] in the binary image, pixel value variation information is calculated (S13). In S13, the control unit 12 changes a numerical value obtained by subtracting the pixel value of the target pixel P [x, y] in the input binary image from the pixel value of the target pixel P [x, y] in the output binary image. Calculate as information.

  The control unit 12 calculates binarization error information based on the error addition information calculated in S6 and the fluctuation information calculated in S13 (S14). In S14, the control unit 12 calculates a numerical value obtained by subtracting the variation information from the error addition information as the binarization error information.

  The control unit 12 refers to the numerical value indicated by the binarization error information calculated in S14 (S15). When the numerical value indicated by the binarization error information is greater than or equal to a reference maximum value (for example, 1000) (S15; binarization error information ≧ reference maximum value), the control unit 12 sets the numerical value indicated by the binarization error information to A reference maximum value is substituted (S16). In S16, binarization error information indicating the reference maximum value is temporarily stored in the storage unit 14. The reference maximum value may be a predetermined numerical value, and may be an arbitrary positive integer value, for example.

  When the numerical value indicated by the binarization error information is less than a reference minimum value (for example, −1000) (S15; binarization error information <reference minimum value), the control unit 12 sets the numerical value indicated by the binarization error information. The reference minimum value is substituted (S17). In S17, the binarization error information indicating the reference minimum value is temporarily stored in the storage unit 14. The reference minimum value may be a predetermined numerical value, and may be any negative integer, for example.

  On the other hand, when the numerical value indicated by the binarization error information is less than the reference maximum value and greater than or equal to the reference minimum value (S14; reference maximum value> binarization error information ≧ reference minimum value), the processes of S16 and S17 are not executed. In this case, the binarization error information calculated in S14 is temporarily stored in the storage unit 14 and used in the error diffusion process for the next pixel of interest P [x, y].

  The control unit 12 calculates error information based on the binarization error information stored in the storage unit 14 (S18), and the process proceeds to S6. In S18, the control unit 12 calculates the error information by substituting the numerical value indicated by the binarization error information into a given mathematical formula and performing a product-sum operation. In subsequent S6, error addition information is calculated based on the calculated error information and the gradation correction amount of the next pixel of interest P [x, y]. Based on the calculated error addition information, an error diffusion process for the next pixel of interest P [x, y] is executed.

  On the other hand, when it is determined that the error diffusion process has been executed for all the pixels P [x, y] (S12; Y), the control unit 12 transmits the output binary image data to the image forming apparatus 20 (S19). The process ends. The image forming apparatus 20 forms an image on a sheet based on the received output binary image data.

  When the numerical value indicated by the binarization error information is not within a predetermined range, the image processing apparatus 10 described above changes the binarization error information so that the numerical value falls within the predetermined range. Error diffusion processing of the pixel of interest P [x, y] is performed to prevent the error from being accumulated beyond the range in which the pixel value can be changed, so that image fogging due to error accumulation in the error diffusion processing or Suppresses omission. Further, the image processing apparatus 10 keeps the absolute value of error accumulation in the error diffusion process below the reference value. Further, the image processing apparatus 10 keeps error accumulation in the error diffusion processing within a predetermined range.

[4. Modified example]
In addition, this invention is not limited to said embodiment. Modifications can be made as appropriate without departing from the spirit of the present invention.

  (1) For example, in the embodiment, the case where the pixel values are determined by the error diffusion unit 60 for all the pixels P [x, y] (that is, Xmax × Ymax pixels) has been described. The error diffusion unit 60 may determine the pixel values for only some of the pixels P [x, y] selected by the above.

  The on / off determination unit 62 of the modification (1) determines the pixel value of the pixel corresponding to the edge portion of the input binary image among the pixels P [x, y] based on the error addition information. The edge part is a boundary (contour) of halftone dots included in the input binary image, and the pixel value of the input binary image is “1” (on pixel) and “0” pixel (off pixel). It is a boundary.

  FIG. 7 is a diagram illustrating an example of a part of an input binary image. As shown in FIG. 7, a halftone dot L between a pixel P [x, y] having a pixel value “1” and a pixel P [x, y] having a pixel value “0” corresponds to an edge portion. To do. The halftone dot edge L is specified by comparing the pixel value of the target pixel P [x, y] with the pixel value of the surrounding pixel P [x, y].

  In this modification, among all the pixels P [x, y], a pixel P [x, y] (indicated by hatching in FIG. 7) whose distance from the halftone dot edge L is within the reference distance is an error diffusion unit. This is a pixel subject to pixel value change by 60. For example, the error diffusion unit 60 determines a pixel value for a pixel P [x, y] whose distance from the halftone dot edge L is within k pixels (k is a natural number, k = 1 in FIG. 7). Note that as the numerical value of k increases, the pixel value is changed from the pixel P [x, y] farther from the halftone dot, so that the shape of the halftone dot is maintained as the numerical value of k decreases.

  In the modification example (1), the same processing as in the embodiment (FIGS. 5 and 6) is executed, but the distance from the halftone dot edge L is within the reference distance among all the pixels P [x, y]. This is different from the embodiment in that the processing of S5 (that is, the processing of S6 to S18) is executed for the pixel P [x, y]. For example, before the process of S5 is executed, the control unit 12 specifies the halftone dot L based on the pixel value of each pixel P [x, y] of the input binary image indicated by the input binary image data. . Then, the control unit 12 specifies a pixel P [x, y] whose distance from the halftone dot edge L is within the reference distance. Only the identified pixel P [x, y] is the processing target of S5.

  As described above, the image processing apparatus 10 according to the modified example (1) determines the pixel value of the pixel corresponding to the edge portion.

  (2) Further, for example, whether or not to change the pixel value may be determined according to the evaluation value calculated for each pixel P [x, y] by a given method.

  FIG. 8 is a functional block diagram of Modification (2). As illustrated in FIG. 8, the image processing apparatus 10 according to the modified example (2) implements an evaluation value acquisition unit 80 in addition to the functions described in the embodiment. The evaluation value acquisition unit 80 is realized mainly by the control unit 12.

  The evaluation value acquisition unit 80 of the present modification evaluates each pixel P [x, y] based on the distance between each pixel P [x, y] and the edge portion (halftone edge L) of the input binary image. Get the value. The evaluation value acquisition unit 80 acquires the evaluation value of each pixel P [x, y] based on the association between the distance between each pixel P [x, y] and the halftone dot edge L and the evaluation value. The association may be a mathematical expression format or a table format. Data indicating the association is stored in the storage unit 14.

  FIG. 9 is a diagram for explaining an evaluation value calculated in the modification (2). The evaluation value acquisition unit 80 sets the evaluation value associated with the distance between each pixel P [x, y] and the edge portion as the evaluation value of the pixel P [x, y]. As shown in FIG. 9, for example, the evaluation value is set lower as the distance between each pixel P [x, y] and the halftone dot edge L becomes shorter. The evaluation value is set higher as the distance is longer. In the example illustrated in FIG. 9, the evaluation value is calculated as “32 × (k−1)” for the pixel P [x, y] whose distance from the halftone dot edge L is k pixels. That is, the lower the evaluation value, the closer to the halftone dot edge L.

  The pixel value changing unit 63 of the modified example (2) includes the determination result of the pixel value of the target pixel P [x, y] by the on / off determination unit 62 and the evaluation values of the surrounding pixels of the target pixel P [x, y]. , The pixel values of the surrounding pixels are determined. Here, the surrounding pixels are pixels whose distance from the target pixel P [x, y] is within a predetermined distance, for example, pixels surrounding the target pixel P [x, y].

  The pixel value changing unit 63 specifies surrounding pixels based on the position of the target pixel P [x, y]. When the on / off determination unit 62 determines the pixel value of the pixel of interest P [x, y], the pixel value changing unit 63 determines the on / off determination unit for pixels that satisfy the given evaluation value among the specified surrounding pixels. The pixel value determined by 62 is used.

  In the present modification, the lower the evaluation value, the closer to the halftone dot edge L, the pixel P [x, y]. For example, the pixel value changing unit 63 has the lowest evaluation value among the surrounding pixels. The pixel value determined by the on / off determination unit 62 is used for the pixel. When the evaluation value is calculated so that the evaluation value becomes higher as the distance from the halftone dot edge L is shorter (for example, when the evaluation value is “255−32 × (k−1)”), The pixel value changing unit 63 may set the pixel value determined by the on / off determining unit 62 for the pixel having the highest evaluation value among the surrounding pixels.

  As described above, the image processing apparatus 10 according to the modified example (2) performs the error diffusion process in consideration of the evaluation value determined based on the distance from the edge portion.

  In the above description, the pixel value determined by the on / off determination unit 62 is set to the pixel having the lowest evaluation value among the surrounding pixels. However, the on / off determination unit 62 is set to the surrounding pixel determined based on another method. The pixel value determined by the above may be used.

  In addition, for example, the pixel value changing unit 63 compares the evaluation value of each surrounding pixel with the reference evaluation value, and outputs the pixel value determined by the on / off determination unit 62 based on the comparison result to the output binary image. Alternatively, the pixel value of the surrounding pixels may be used. As the reference evaluation value, a predetermined value (for example, 96) may be used.

  For example, in the case where the lower the evaluation value indicates that the pixel P [x, y] is closer to the halftone dot edge L, the pixel value changing unit 63 sets the pixel values of surrounding pixels whose evaluation value is less than the reference evaluation value. Is the pixel value determined by the on / off determination unit 62. When the evaluation value is calculated so that the evaluation value increases as the distance from the halftone dot edge L becomes shorter (for example, when the evaluation value is “255−32 × (k−1)”), the pixel value The changing unit 63 sets the pixel value of the surrounding pixels whose evaluation value is equal to or higher than the reference evaluation value as the pixel value determined by the on / off determining unit 62.

  (3) The method for calculating the evaluation value of each pixel P [x, y] is not limited to the above example. For example, the evaluation value may be calculated based on the pixel value of the input binary image.

  The evaluation value acquisition unit 80 of the modification (3) acquires the evaluation value of each pixel P [x, y] based on the pixel value of the input binary image. The evaluation value acquisition unit 80 acquires the evaluation value of each pixel P [x, y] based on the association between the pixel value of the input binary image and the evaluation value. The association may be a mathematical expression format or a table format. Data indicating the association is stored in the storage unit 14.

  FIG. 10 is a diagram for explaining an evaluation value calculated in the modification (3). The evaluation value acquisition unit 80 sets the evaluation value associated with the pixel value of each pixel P [x, y] of the input binary image as the evaluation value of the pixel P [x, y]. In the example of FIG. 10, the evaluation value of the pixel P [x, y] whose pixel value is “1” is set higher than the evaluation value of the pixel P [x, y] whose pixel value is “0”. For example, the evaluation value of the pixel P [x, y] whose pixel value is “1” is “128”, and the evaluation value of the pixel P [x, y] whose pixel value is “0” is “0”.

  The processing content of the pixel value changing unit 63 is the same as that of the modification (2). In the case of the example shown in FIG. 10, the pixel values of the surrounding pixels whose pixel value is “0” are easily changed, so that gradation correction for increasing the pixel value is performed as a whole.

  In contrast to the above, gradation correction that lowers the pixel value may be easily performed. FIG. 11 is a diagram for explaining an evaluation value calculated in the modification (3). In the example of FIG. 11, the evaluation value of the pixel P [x, y] with the pixel value “0” is set higher than the evaluation value of the pixel P [x, y] with the pixel value “1”. For example, the evaluation value of the pixel P [x, y] whose pixel value is “1” is “0”, and the evaluation value of the pixel P [x, y] whose pixel value is “0” is “128”. In this case, since the pixel P [x, y] having the pixel value “1” is easily changed to “0”, gradation correction for reducing the pixel value as a whole is performed.

  As described above, the image processing apparatus 10 according to the modified example (3) determines the pixel value of the surrounding pixel in consideration of the evaluation value of the surrounding pixel.

  In the above description, the pixel value is changed for the surrounding pixels whose evaluation value is less than the reference value (for example, the pixel whose evaluation value is “0”). However, in the modification example (3), the modification example ( Similarly to 2), pixel values may be changed for surrounding pixels (for example, pixels having an evaluation value of “0”) whose evaluation value is greater than or equal to the reference value. Similarly, based on whether or not the pixel value of the surrounding pixel is in a predetermined range, the pixel value changing process of the surrounding pixel may be executed.

  (4) Further, for example, the evaluation value may be calculated based on the pixel value of the output binary image.

  The evaluation value acquisition unit 80 of the modification (4) acquires the evaluation value of each pixel P [x, y] based on the pixel value of the output binary image. That is, the evaluation value acquisition unit 80 of the modification (4) acquires evaluation values for the pixels whose pixel values of the output binary image have already been determined by the pixel value changing unit 63.

  The evaluation value calculation method based on the pixel value and the processing content of the pixel value changing unit 63 are the same as in the modification (3). That is, the evaluation value acquisition unit 80 acquires the evaluation value of each pixel P [x, y] based on the association between the pixel value of the output binary image and the evaluation value. The association may be a mathematical expression format or a table format. Data indicating the association is stored in the storage unit 14.

  The evaluation value acquisition unit 80 sets the evaluation value associated with the pixel value of each pixel P [x, y] of the output binary image as the evaluation value of the pixel P [x, y]. For example, the evaluation value of the pixel having the pixel value “1” is set high, and the evaluation value of the pixel having the pixel value “0” is set low. Conversely, the evaluation value of the pixel having the pixel value “0” may be set high, and the evaluation value of the pixel having the pixel value “1” may be set low.

  As described above, the image processing apparatus 10 according to the modified example (4) performs the error diffusion process in consideration of the evaluation value determined based on the pixel value of the output binary image.

  (5) Further, for example, the above modifications may be combined. For example, the modification (1) may be combined with any of the modifications (2) to (4). In this case, an evaluation value is calculated only for pixels close to the edge portion, and on / off of the pixel value is determined based on the evaluation value. Moreover, you may make it combine modification (2)-(4). That is, the evaluation value may be calculated based on both the distance from the edge portion and the pixel value of the input binary image or the output binary image.

  In addition, for example, in modification examples (2) to (4), the pixel value may not be changed for pixels in a range where the evaluation value is determined in advance. In other words, the change of the pixel value is limited for the surrounding pixels whose evaluation value is in the predetermined range, and the change of the pixel value is permitted for the surrounding pixels whose evaluation value is not the predetermined range. Also good.

  Further, in the embodiment, when the binarization error signal is not between the given reference maximum value and the reference minimum value, the case where the binarization error signal is set to the numerical value between these is described. Only one of the value and the reference minimum value may be determined. That is, only when the numerical value indicated by the binarization error signal is greater than or equal to the reference maximum value, when the binarization error signal is changed or the numerical value indicated by the binarization error signal is less than the reference minimum value. Only the binarization error signal may be changed.

  For example, when the image is always printed darker than the ideal state in the image forming apparatus 20, gradation correction for lowering the pixel value should be performed, so that only the reference minimum value needs to be set. . Similarly, when the image is always printed lighter than the ideal state in the image forming apparatus 20, gradation correction for increasing the pixel value should be performed, so that only the reference maximum value is set. Good.

  DESCRIPTION OF SYMBOLS 10 Image processing apparatus, 12 Control part, 14 Storage part, 16 Communication part, 20 Image forming apparatus, 30 Binary image acquisition part, 40 Smoothing part, 50 Gradation correction amount acquisition part, 60 Error diffusion part, 61 Error addition Acquisition unit, 62 On-off determination unit, 63 Pixel value change unit, 64 Fluctuation acquisition unit, 65 Binary error acquisition unit, 66 Binary error change unit, 67 Binary error storage unit, 68 Error supply unit, 70 Binary Value image output unit, 80 evaluation value acquisition unit.

Claims (8)

Gradation correction amount acquisition means for acquiring a gradation correction amount of a multi-valued image generated based on an input binary image for each pixel;
Error addition acquisition means for acquiring error addition information calculated based on the gradation correction amount of the pixel of interest in the input binary image and error information based on error diffusion processing;
Determining means for determining a pixel value of the target pixel in the output binary image based on the error addition information;
Binary to obtain binarization error information calculated based on the pixel value of the target pixel in the output binary image, the pixel value of the target pixel in the input binary image, and the error addition information Error error acquisition means;
When the numerical value indicated by the binarization error information is not within a predetermined range, a changing unit that changes the binarization error information;
Supply means for supplying the error information used in the next pixel of interest to the error addition acquisition means based on the binarization error information changed by the change means;
An image processing apparatus comprising: When the absolute value of the numerical value indicated by the binarization error information is greater than or equal to a reference value, the changing unit changes the binarization error information so that the absolute value is less than the reference value.
The image processing apparatus according to claim 1. In the case where the numerical value indicated by the binarization error information is greater than or equal to a first reference value, or the numerical value indicated by the binarization error information is less than a second reference value that is smaller than the first reference value. Changing the binarization error information so that the numerical value indicated by the binarization error information is less than the first reference value and greater than or equal to the second reference value.
The image processing apparatus according to claim 1, wherein the image processing apparatus is an image processing apparatus. The determining unit determines a pixel value of a pixel corresponding to an edge portion of the input binary image among the pixels based on the error addition information.
The image processing apparatus according to claim 1, wherein the image processing apparatus is an image processing apparatus. The image processing apparatus includes:
Means for obtaining an evaluation value of each pixel based on a distance between each pixel and an edge portion of the input binary image;
Means for determining the pixel value of the surrounding pixel based on the determination result of the pixel value of the pixel of interest by the determining means and the evaluation value of the surrounding pixel of the pixel of interest;
The image processing apparatus according to claim 1, further comprising: The image processing apparatus includes:
Means for obtaining an evaluation value of each pixel based on a pixel value of the input binary image;
Means for determining the pixel value of the surrounding pixel based on the determination result of the pixel value of the pixel of interest by the determining means and the evaluation value of the surrounding pixel of the pixel of interest;
The image processing apparatus according to claim 1, further comprising: The image processing apparatus includes:
Means for obtaining an evaluation value of each pixel based on a pixel value of the output binary image;
Means for determining the pixel value of the surrounding pixel based on the determination result of the pixel value of the pixel of interest by the determining means and the evaluation value of the surrounding pixel of the pixel of interest;
The image processing apparatus according to claim 1, further comprising: Gradation correction amount acquisition means for acquiring the gradation correction amount of the multi-valued image generated based on the input binary image for each pixel;
Error addition acquisition means for acquiring error addition information calculated based on the gradation correction amount of the target pixel in the input binary image and error information based on error diffusion processing;
Determining means for determining a pixel value of the target pixel in the output binary image based on the error addition information;
Binary to obtain binarization error information calculated based on the pixel value of the target pixel in the output binary image, the pixel value of the target pixel in the input binary image, and the error addition information Error acquisition means,
A changing means for changing the binarization error information when the numerical value indicated by the binarization error information is not in a predetermined range;
Supply means for supplying the error information used in the next pixel of interest to the error addition acquisition means based on the binarization error information changed by the changing means;
As a program to make the computer function as.

Images