Patent Publication Number: US-10762402-B2

Title: Image conversion device and image conversion method

Description:
The present application is based on, and claims priority from JP Application Serial Number 2018-215182, filed Nov. 16, 2018, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an image conversion device and an image conversion method. 
     2. Related Art 
     A configuration of an image data conversion unit, in which original image data is converted into gradation data for printing in accordance with a concentration of each pixel and the gradation data for printing is output after compression processing, which includes a standard mode conversion unit that converts the original image data into gradation data for printing with a predetermined number of gradations, a data compression mode conversion unit that converts the original image data into a gradation data for printing with a smaller number of gradations than the predetermined number of gradations, a mode switching unit that selects either data conversion using the standard mode conversion unit and data conversion using the data compression mode conversion unit, and data compression unit that performs compression processing on the gradation data for printing using data conversion in the mode selected by the mode switching unit, is known (see JP-A-2013-74509). 
     From the viewpoints of high-speed data transfer, reduction of a communication load required for data transfer, high-speed printing processing, and the like, a further increase in compression rate has been required in regard to compression processing of image data. However, it is not easy to modify or expand, for example, an existing compression function implemented on a device that executes compression processing and an existing decoding function implemented on a device, such as a printer, which receives transfer of image data after compression, in order to improve the compression rate of the image data. 
     SUMMARY 
     An image conversion device according to an aspect of the present disclosure performs conversion processing on image data before compression processing is performed, and includes: a pixel designation unit that successively designates, as a second pixel, pixels in a raster line that forms the image data in a direction away from a first pixel, which is a pixel included in the raster line, starting from a pixel adjacent to the first pixel in the raster line as a start point of the designation; an evaluation value calculation unit that calculates, as an evaluation value, an absolute value of a difference between pixel values of the first pixel and the second pixel or a square value of the difference; a determination unit that determines whether or not the evaluation value exceeds a predetermined threshold value; and a conversion unit that calculates a representative value of pixel values from the first pixel to a third pixel that is a pixel adjacent to the second pixel at a side of the first pixel at a timing at which the evaluation value is determined to exceed the threshold value and converts the pixel values from the first pixel to the third pixel into the representative value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an outline configuration of a system. 
         FIG. 2  is a diagram for simply explaining a flow of processing performed on image data. 
         FIG. 3  is a flowchart illustrating details of image conversion processing according to a first embodiment. 
         FIG. 4  is a diagram illustrating an example of a threshold value table. 
         FIG. 5  is a diagram for explaining a specific case of the image conversion processing. 
         FIG. 6  is a flowchart illustrating details of image conversion processing according to a second embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, embodiments of the disclosure will be described with reference to the respective drawings. Note that the respective drawings are just illustrative examples that explain the embodiments. 
     1. Description of System Outline 
       FIG. 1  simply illustrates a configuration of a system  1  according to an embodiment. The system  1  includes an image conversion device  10  and a printer  20 . The image conversion device  10  is realized by, for example, a personal computer, a server, a smartphone, a tablet terminal, or an information processing device that has a processing capability that is equivalent to these devices. The image conversion device  10  includes a control unit  11 , a display unit  13 , an operation receiving unit  14 , a communication interface  15 , and the like. The interface will be abbreviated as an “IF”. The control unit  11  includes one or more ICs that each have a CPU  11   a  that serves as a processor, a ROM  11   b , a RAM  11   c , and the like and other non-volatile memory and the like. 
     The control unit  11  controls the image conversion device  10  by the processor, that is, the CPU  11   a  executing arithmetic operations in accordance with programs saved in the ROM  11   b , another memory, or the like using the RAM  11   c  or the like as a work area. In the embodiment, the control unit  11  cooperates with the programs and serves as an image conversion unit  12 , a compression processing unit  16 , and the like. The image conversion unit  12  further has the respective functional units such as a pixel designation unit  12   a , an evaluation value calculation unit  12   b , a determination unit  12   c , and a conversion unit  12   d . Note that the processor is not limited to one CPU, and a configuration in which a plurality of CPUs or a hardware circuit such as an application specific integrated circuit (ASIC) performs processing may be employed, or a configuration in which the CPU and the hardware circuit cooperate to perform processing may also be employed. 
     The “image conversion device” is just a name of a device  10  that realizes at least functions of the image conversion unit  12 . In the example illustrated in  FIG. 1 , the device  10  may be referred to as an image processing device in consideration of the point that the device  10  also serves as the compression processing unit  16 , or the device  10  may be referred to as a print control device or the like from the viewpoint that the device  10  controls the printer  20 . 
     The display unit  13  is a mechanism for displaying visual information and includes, for example, a liquid crystal display, an organic EL display, or the like. The display unit  13  may have a configuration including a display and a drive circuit for driving the display. An operation receiving unit  14  is a mechanism for receiving user&#39;s operations and is realized by, for example, a physical button, a touch panel, a mouse, a keyboard, or the like. It is a matter of course that the touch panel may be realized as a function of the display unit  13 . The display unit  13  and the operation receiving unit  14  can be collectively referred to as an operation panel of the image conversion device  10 . 
     The display unit  13  and the operation receiving unit  14  may be a part of the configuration of the image conversion device  10  or may be peripheral devices that are externally attached to the image conversion device  10 . The communication IF  15  is a collective name of one or more IFs for the image conversion device  10  executing wired or wireless communication with the outside in accordance with a predetermined communication protocol including known communication standards. The control unit  11  communicates with the printer  20  via the communication IF  15 . 
     Generally, the printer  20  includes a print control unit  21 , a print mechanism  22 , and the like. The print control unit  21  is a controller at the side of the printer  20  that includes the ASIC, for example, and generates print data by performing, on image data transferred from the image conversion device  10 , processing required in accordance with a state and a format of the image data. The required processing described here means decoding processing performed on image data in a compressed state, so-called color conversion processing performed on image data after decoding, half-tone processing, and the like. The decoding processing will also be referred to as decompression processing. 
     The print control unit  21  prints an image expressed by the print data on a print medium by controlling the print mechanism  22  based on the print data generated as described above. The print mechanism  22  performs printing on the print medium by, for example, an ink jet scheme or an electrophotographic scheme. The print medium is typically a paper but may be a material other than paper, such as a film or a medium for textile printing, for example. 
     The image conversion device  10  and the printer  20  may be coupled to each other through a network, which is not illustrated in the drawing. The printer  20  may be a composite machine provided with a plurality of functions, such as a scanner function and a facsimile communication function, as well as the printing function. The image conversion device  10  may be realized not only by an independent information processing device but also by a plurality of information processing devices that are communicably connected to each other via a network. 
       FIG. 2  is a diagram for simply explaining a flow of processing that the control unit  11  executes on image data as a target of the processing. The image data as a target of the processing is any image data that a user selects by operating the operation receiving unit  14 , for example. Now, an example in which image data is bit map data having gradation values of the respective colors, namely, red, green, and blue (RGB) for each pixel will be described. Now the image data may be bit map data having gradation values of the respective colors in another color system, such as cyan, magenta, yellow, and black (CMYK), for each pixel. The gradation value of each color in the image data is expressed by 256 gradations from 0 to 255, for example. 
     The position of each pixel included in the image data is defined by an XY coordinate system of two axes (an X axis and a Y axis) that perpendicularly intersect with each other. In the image data, a pixel row of a plurality of pixels aligned in a specific direction (X-axis direction) is referred to as a “raster line”. Raster R 1 , raster G 1 , and raster B 1  illustrated in  FIG. 2  are the respective raster lines when one raster line is deconstructed for each color. That is, the raster R 1  is a raster line in which each pixel has only a gradation value of R, the raster G 1  is a raster line in which each pixel has only a gradation value of G, and the raster B 1  is a raster line in which each pixel has only a gradation value of B. 
     As illustrated in  FIG. 2 , the control unit  11  performs image conversion processing  100  on each of the raster R 1 , the raster G 1 , and the raster B 1  in a parallel manner. The image conversion processing  100  is processing performed by the image conversion unit  12 . The image conversion processing  100  will be described later in detail. Data obtained after the image conversion processing  100  is performed on the raster R 1  is raster R 2 , data obtained after the image conversion processing  100  is performed on the raster G 1  is raster G 2 , and data obtained after the image conversion processing  100  on the raster B 1  is raster B 2 . 
     The Control unit  11  performs compression processing  300  on each of the raster R 2 , the raster G 2 , and the raster B 2  in a parallel manner. The compression processing  300  is processing performed by the compression processing unit  16 . As the compression processing  300 , lossless compression such as a run-length coding method is executed. Data obtained after the compression processing  300  is performed on the raster R 2  is compression data R 3 , data obtained after the compression processing  300  is performed on the raster G 2  is compression data G 3 , and data obtained after the compression processing  300  is performed on the raster B 2  is compression data B 3 . 
     The control unit  11  transfers the compression data R 3 , the compression data G 3 , and the compression data B 3  to the printer  20  via the communication IF  15 . In this manner, the control unit  11  repeatedly executes the image conversion processing  100  and the compression processing  300  on the image data as a target of the processing in units of raster lines for the respective colors. Therefore, the printer  20  successively acquires the raster lines in which the respective pixels have gradation values of the respective colors by the print control unit  21  decoding the compression data R 3 , the compression data G 3 , and the compression data B 3  transferred from the image conversion device  10 . 
     2. First Embodiment 
     The image conversion processing  100  will be described.  FIG. 3  illustrates details of the image conversion processing  100  using a flowchart. The embodiment described with reference to  FIGS. 3 to 5  will also be referred to as a first embodiment. The image conversion processing  100  performed on each of the raster lines deconstructed from one raster line depending on the respective colors, for example, the raster R 1 , the raster G 1 , and the raster B 1  illustrated in  FIG. 2  has a same procedure. Therefore, the image conversion processing  100  performed on a raster line of any one color among the respective raster lines deconstructed depending on the respective colors will be described. 
     In Step S 110 , the pixel designation unit  12   a  designates a head pixel of the raster line as a head pixel of a conversion target section. The head pixel of the raster line is a pixel with the smallest X coordinate value among pixels included in the raster line. The “conversion target section” is a section in which pixels as targets of conversion in Step S 180  or the like, which will be described later, are continued. In  FIG. 3 , the conversion target section is simply illustrated as a “section”. The head pixel of the conversion target section will be referred to as a “section head pixel”. The section head pixel corresponds to a “first pixel”. That is, the head pixel in the raster line is designated as an initial section head pixel in the raster line in Step S 110 . 
     In Step S 120 , the pixel designation unit  12   a  sets a total pixel value Sum and a section pixel number i. The “total pixel value Sum” is a sum of pixel values of pixels included in the conversion target section. The pixel values are gradation values that the pixels in the raster line of each color as a target of the image conversion processing  100  have. If the raster line of each color as a target of the image conversion processing  100  is the raster R 1 , the gradation values of R correspond to the pixel values. The “section pixel number i” is the number of pixels included in the conversion target section. 
     In Step S 120 , the pixel designation unit  12   a  sets the total pixel value Sum=a pixel value P[0] of the section head pixel and sets the section pixel number i=1. The section pixel number i is a number indicating the position of an “evaluation pixel” starting from a pixel adjacent to the section head pixel (i=1) as a start point. The evaluation pixel is a pixel for which a difference between the pixel values thereof and of the section head pixel is evaluated and corresponds to a “second pixel”. Therefore, processing of setting the section pixel number i is also processing of designating the evaluation pixel from the pixels included in the raster line. The section pixel number i=1 means that the pixel adjacent to the section head pixel is designated as the evaluation pixel. 
     In Step S 130 , the evaluation value calculation unit  12   b  calculates the evaluation value V based on a difference between the pixel value P[0] of the section head pixel and the pixel value P[i] of the evaluation pixel. Specifically, the evaluation value calculation unit  12   b  evaluates, as the evaluation value V, an absolute value of a difference between the pixel value P[0] and the pixel value P[i]. Alternatively, the evaluation value calculation unit  12   b  regards, as the evaluation value V, a square value of the difference between the pixel value P[0] and the pixel value P[i]. Hereinafter, description will be continued on the assumption that the evaluation value V is the square value of the difference between the pixel value P[0] and the pixel value P[i]. 
     In Step S 140 , the determination unit  12   c  determines whether or not the evaluation value V calculated in Step S 130  exceeds a predetermined threshold value Th[i−1]. The determination unit  12   c  determines “Yes” and moves on to the processing in Step S 170  when the evaluation value V exceeds the threshold value Th[i−1]. Meanwhile, the determination unit  12   c  determines “No” and moves on to processing in Step S 150  when the evaluation value V does not exceed the threshold value Th[i−1]. 
       FIG. 4  illustrates an example of a threshold value table  17  for defining the threshold value Th[i−1]. The threshold value table  17  is information that the control unit  11  can refer to and is stored in advance in a predetermined memory in the control unit  11 , for example. In the threshold value table  17 , a correspondence between i−1 and the threshold value Th[i−1] is defined. Also, the threshold value Th[i−1] that becomes a smaller value as i−1 increases is defined in the threshold value table  17 . The threshold value Th[i−1] may linearly decrease, non-linearly decrease, or decrease in a stepwise manner as i−1 increases. 
     The determination unit  12   c  reads a threshold value Th[0]=1024 corresponding to i−1=0 from the threshold value table  17  and compares the threshold value Th[0]=1024 with the evaluation value V in Step S 140  when the section pixel number i=1, for example. In the example of the threshold value table  17 , it is assumed that all the threshold values Th[i−1] corresponding to i−1&gt;8 are 0. However, it is needless to say that the embodiment is not intended to be interpreted as being limited to the specific numerical values illustrated in the threshold value table  17 . 
     In Step S 150 , the pixel designation unit  12   a  adds the pixel value P[i] of the evaluation pixel to the total pixel value Sum and then adds 1 to the section pixel number i. That is, the pixel designation unit  12   a  increments the section pixel number i. The incrementing of the section pixel number i means designating, as the next evaluation pixel, a pixel adjacent to the evaluation pixel that has been designated until then on a further side from the section head pixel. 
     In Step S 160 , the conversion unit  12   d  determines whether or not the position of the evaluation pixel has exceeded the end pixel of the raster line. The end pixel of the raster line is a pixel with the largest X coordinate value from among the pixels included in the raster line. It is determined that the position of the evaluation pixel has exceeded the end pixel of the raster line when the position adjacent to the end pixel on the further side from the section head pixel, that is, the position at which no pixel is present is designated as the evaluation pixel as a result of incrementing the section pixel number i in Step S 150 . The conversion unit  12   d  returns to Step S 130  from Step S 160  when the position of the evaluation pixel is determined not to have exceeded the end pixel of the raster line. Meanwhile, when the position of the evaluation pixel is determined to have exceeded the end pixel of the raster line, the conversion unit  12   d  moves on to processing in Step S 200  from Step S 160 . 
     When such a cycle of Steps S 130 →S 140 →S 150 →S 160 →S 130  is repeated, the pixels in the raster line are successively designated one by one as an evaluation pixel in a direction away from the section head pixel, starting from the pixel adjacent to the section head pixel in the raster line as a start point of the designation. When the evaluation value V calculated in Step S 130  is determined not to exceed the threshold value Th[i−1] in Step S 140  in a state in which the end pixel of the raster line is designated as an evaluation pixel, the section pixel number i is incremented in Step S 150 , and the processing proceeds from Step S 160  to Step S 200 . 
     In Step S 170 , the conversion unit  12   d  calculates an “average value Ave” of the pixel values from the section head pixel to the pixel adjacent to the evaluation pixel at the side of the section head pixel. The pixel adjacent to the evaluation pixel at the side of the section head pixel will be referred to as an “immediately previous pixel” with a meaning that the pixel is immediately before the evaluation pixel. The immediately previous pixel corresponds to a “third pixel”. That is, the section from the section head pixel to the immediately previous pixel of the raster line is one conversion target section. The conversion unit  12   d  calculates the average value Ave by dividing the total pixel value Sum by the section pixel number i. 
     In Step S 180 , the conversion unit  12   d  converts the pixel values of the respective pixels in the conversion target section into the average value Ave calculated in Step S 170 . In this manner, the respective pixels from the section head pixel to the immediately previous pixel have a unified pixel value. 
     In Step S 190 , the pixel designation unit  12   a  updates the section head pixel. In this case, the pixel designation unit  12   a  designates the evaluation pixel as the next section head pixel. With such updating of the section head pixel, the pixel designation unit  12   a  deals the pixel value P[i] of the evaluation pixel as the pixel value P[0] of the next section head pixel. The pixel designation unit  12   a  returns to Step S 120  after Step S 190 . 
     Note that since the section head pixel and the immediately previous pixel are the same pixel in a situation in which the section pixel number i=1, the pixel included in the conversion target section is only the section head pixel. When “Yes” determination is made in Step S 140  in a situation in which the section pixel number i=1, the average value Ave calculated in Step S 170 =the pixel value P[0] of the section head pixel. Therefore, when “Yes” determination is made in Step S 140  in the situation in which the section pixel number i=1, the conversion in Step S 180  is not substantially performed, and the pixel value of the conversion target section, that is, the section head pixel is maintained. When “Yes” determination is made in Step S 140  in the situation in which the section pixel number i=1, a state in which the pixel values have suddenly varied between the section head pixel and the evaluation pixel adjacent to the section head pixel, that is, a state in which either the section head pixel or the evaluation pixel adjacent to the section head pixel corresponds to an edge in an image has been achieved. In the embodiment, since the pixel value of the section head pixel is maintained, and the edge is held without being blunted when “Yes” determination is made in Step S 140  in the situation in which the section pixel number i=1. In this manner, degradation of image quality due to a blunted edge is avoided in the image conversion processing  100 . 
     Steps S 200  and S 210  may be understood as the same processing as Steps S 170  and S 180 . However, the end pixel of the raster line is regarded as the immediately previous pixel in Steps S 200  and S 210 . That is, the average value Ave calculated in Step S 200  is an average value of the pixel values from the section head pixel to the end pixel, and in Step S 210 , the pixel values of the respective pixels from the section head pixel to the end pixel, which corresponds to the conversion target section, are converted into the average value Ave calculated in Step S 200 . After the execution of Step S 210 , the image conversion processing  100  on the raster line ends. 
       FIG. 5  is a diagram for explaining a specific case of the image conversion processing  100  and illustrates a part of the raster R 1  as an example of the raster line.  FIG. 5  illustrates a state in which the pixel values of the respective pixels in the raster R 1  are converted in the process of the image conversion processing  100 . In  FIG. 5 , the respective rectangles included in the raster R 1  correspond to the respective pixels, and the numerical values illustrated in the pixels are the pixel values. It is assumed that the raster line includes N pixels aligned in the X-axis direction. When the head pixel of the raster line is assumed to be the first pixel (the same applies to the following description), the N-th pixel P N  is the end pixel. 
     It is assumed that in Step S 190 , which is a specific timing in the image conversion processing  100 , the n-th pixel Pn is designated as the section head pixel as illustrated in the upper section in  FIG. 5 . n&lt;N. In  FIG. 5 , the pixel that serves as the section head pixel is illustrated with a gray color for easy understanding. In this case, the evaluation pixels are successively designated starting from the n+1-th pixel Pn+1 by the cycle of Steps S 130  to S 160  being repeated after Step S 120 . Here, it is assumed that the evaluation value V based on the difference between the pixel value “92” of the evaluation pixel at a timing at which the section pixel number i=6 illustrated in the upper section in  FIG. 5 , that is, the pixel n+6-th pixel Pn+6 and the pixel value “100” of the section head pixel (pixel Pn) is determined to have exceeded a threshold value Th[5] in Step S 140 . In this case, the pixel values from the section head pixel (pixel Pn) to the immediately previous pixel, that is the n+5-th pixel Pn+5 are converted into the average value Ave=109 of the pixel values from the section head pixel to the immediately previous pixel through the processing in Steps S 170  and S 180 . 
     Next, the n+6-th pixel Pn+6, which has been the evaluation pixel until then, is designated as the next section head pixel as illustrated in the middle section in  FIG. 5  in Step S 190 . Thereafter, the evaluation pixel is successively designated as the evaluation pixel starting from the n+7-th pixel Pn+7 by the cycle from Steps S 130  to S 160  being repeated after Step S 120 . It is assumed that the evaluation value V based on a difference between the pixel value “70” of the evaluation pixel at a timing at which the section pixel number i=4, that is, the n+10-th pixel Pn+10 and the pixel value “92” of the section head pixel (pixel Pn+6) is then determined to have exceeded a threshold value Th[3] in Step S 140  as illustrated in the middle section of  FIG. 5 . In this case, the pixel values from the section head pixel (Pn+6) to the immediately previous pixel, that is, the n+9-th pixel Pn+9 are converted into the average value Ave=85 of the pixel value from the section head pixel to the immediately previous pixel through the processing in Steps S 170  and S 180 . 
     Thereafter, the n+10-th pixel Pn+10, which has been the evaluation pixel until then, is designated as the next section head pixel as illustrated in the lower section of  FIG. 5 , and the processing is similarly repeated in Step S 190 . Then, data on which the image conversion processing  100  has been executed, that is, the raster R 2  is obtained by the conversion being executed on the conversion target section including at least the end pixel (pixel P N ) in Step S 210  and the image conversion processing  100  performed on the raster R 1  ending. 
     3. Second Embodiment 
     Next, a second embodiment will be described.  FIG. 6  illustrates details of an image conversion processing  100  according to the second embodiment using a flowchart. In regard to the second embodiment, different points from those of the first embodiment will be described on the assumption of the first embodiment. The flowchart in  FIG. 6  is generally different from the flowchart in  FIG. 3  in that  FIG. 6  includes Steps S 115 , S 125 , S 145 , S 165 , and S 195 . 
     After Step S 110 , the pixel designation unit  12   a  performs Step S 115 . Step S 115  is the same as Step S 120  in the first embodiment in that the total pixel value Sum and the section pixel number i are set. The pixel designation unit  12   a  further initializes the pixel value variation number CC and sets the pixel value variation number CC=0 in Step S 115 . The “pixel value variation number CC” is a value for counting the number of times the pixel values vary among adjacent pixels included in the conversion target section. 
     In Step S 125  after Step S 115 , the evaluation value calculation unit  12   b  determines whether or not the pixel value P[i] of the evaluation pixel conforms to the pixel value P[i−1] of the immediately previous pixel. The evaluation value calculation unit  12   b  determines “Yes” and moves on to Step S 150  when the pixel value P[i] conforms to the pixel value P[i−1] while the evaluation value calculation unit  12   b  determines “No” and moves on to Step S 130  when the pixel value P[i] does not conform to the pixel value P[i−1]. When the determination in Step S 125  is made in the situation where the section pixel number i=1, the immediately previous pixel is the section head pixel, and the evaluation value calculation unit  12   b  thus determines whether or not the pixel value P[i] of the evaluation pixel conforms to the pixel value P[0] of the section head pixel. 
     In Step S 140 , the determination unit  12   c  moves on to processing in Step S 165  when the evaluation value V exceeds the threshold value Th[i−1] while the determination unit  12   c  moves on to processing in Step S 145  when the evaluation value V does not exceed the threshold value Th[i−1]. 
     In Step S 145 , the pixel designation unit  12   a  adds 1 to the pixel value variation number CC. That is, the pixel value variation number CC is incremented. In the second embodiment, the pixel value variation number CC is incremented when the pixel value P[i] of the evaluation pixel differs from the pixel value P[i−1] of the immediately previous pixel and the evaluation value V based on the difference between the pixel value P[i] of the evaluation pixel and the pixel value P[0] of the section head pixel does not exceed the threshold value Th[i−1] in this manner. The pixel designation unit  12   a  executes Step S 150  after Step S 145 . 
     In Step S 160 , the conversion unit  12   d  returns to Step S 125  when the position of the evaluation pixel is determined not to exceed the end pixel of the raster line. Meanwhile, when the position of the evaluation pixel is determined to exceed the end pixel of the raster line, the conversion unit  12   d  moves on to the processing in Step S 195  from Step S 160 . 
     In Step S 165 , the conversion unit  12   d  determines whether or not a condition that the section pixel number i is greater than a threshold value ThA and that the pixel value variation number CC is smaller than a threshold value ThB is satisfied. This condition will be referred to as a “conversion avoidance condition”. Both the threshold values ThA and ThB are preset values. The threshold value ThA corresponds to a “first predetermined number”, and the threshold value ThB corresponds to a “second predetermined number”. In one example, the threshold value ThA=6, and the threshold value ThB=3. As can be understood from the above description, the section pixel number i is the number of pixels from the section head pixel to the immediately previous pixel. Also, the pixel value variation number CC is the number of times the pixel values change in a section from the section head pixel to the immediately previous pixel, that is, in the conversion target section. 
     In Step S 165 , the conversion unit  12   d  determines “Yes” and moves on to processing in Step S 190  when the conversion avoidance condition is satisfied. Meanwhile, when the conversion avoidance condition is not satisfied, the conversion unit  12   d  determines “No” and executes Step S 170 . In the second embodiment, the conversion unit  12   d  does not executes Steps S 170  or S 180  on a specific conversion target section when the conversion target section includes a relatively large number of pixels and the pixel value variation number CC in the conversion target section is relatively small in this manner. 
     Determination made in Step S 195  is the same as the determination made in Step S 165 . That is, the conversion unit  12   d  determines “Yes” and ends the image conversion processing  100  when the conversion avoidance condition is satisfied in Step S 195 . Meanwhile, when the conversion avoidance condition is not satisfied, the conversion unit  12   d  determines “No” and executes Step S 200 . 
     4. Conclusion 
     According to the embodiment, the image conversion device  10  performs conversion processing on image data before compression processing is performed in this manner. The image conversion device  10  includes: the pixel designation unit  12   a  that successively designates, as the second pixel, pixels in a raster line that forms image data in a direction away from a first pixel, which is a pixel included in the raster line, starting from a pixel adjacent to the first pixel in the raster line as a start point of the designation; the evaluation value calculation unit  12   b  that calculates, as the evaluation value V, an absolute value of a difference between the pixel values of the first pixel and the second pixel or a square value of the difference; the determination unit  12   c  that determines whether or not the evaluation value V exceeds a predetermined threshold value; and a conversion unit  12   d  that calculates a representative value of pixels from the first pixel to a third pixel that is a pixel adjacent to the second pixel at the side of the first pixel at a timing at which the evaluation value V is determined to exceed the threshold value, that is, at a timing at which “Yes” determination is made in Step S 140  and converts the pixel values from the first pixel to the third pixel into the representative value. 
     According to the aforementioned configuration, the pixel values of a section in which the pixels whose evaluation values V based on the difference between the pixel values thereof and the pixel value of the first pixel (section head pixel) in the raster line is equal to or less than the threshold value continue from the first pixel at the head are converted into the representative value of the pixels in this section. In this manner, it is possible to reduce the number of colors in units of raster lines that are included in the image data. The number of colors described herein is the number of colors when colors with different gradation values are regarded as one color. The data compression rate in the compression processing  300  that is executed on the raster line is improved by reducing the number of colors in units of raster lines. That is, the data compression rate in the compression processing  300  can be improved as a result by using existing hardware or programs for the compression processing  300  and decoding processing executed at the side of the printer  20  in accordance with the compression processing  300  and executing the image conversion processing  100  according to the embodiment before the compression processing  300 . In particular, it is possible to improve the data compression rate in the compression processing  300  by executing the image conversion processing  100  on an image with a large number of colors, such as a natural image, and reducing the number of colors. 
     High-speed data transfer from the image conversion device  10  to the printer  20  and reduction of a communication burden required for the data transfer are realized by improving the data compression rate in the compression processing  300 , and this contributes to high-speed print processing. 
     The representative values of the pixel values from the first pixel to the third pixel mean the average value Ave of the pixel values from the first pixel to the third pixel according to the above description. For preventing image quality from being degraded, the average value Ave is preferably a value after the conversion of the pixel values from the first pixel to the third pixel. However, the representative value may not be the average value in terms of reduction of the number of colors as described above. The representative value of the pixel values from the first pixel to the third pixel may be either a central value of the pixel values from the first pixel to the third pixel or the pixel values from the first pixel to the third value, for example. 
     The predetermined threshold value, that is, the threshold value Th[i−1] may be a specific value. However, sensitivity of human vision for identifying differences in grayscale is low with respect to a high-frequency region in which color change cycles are short while the sensitivity tends to be high with respect to a low-frequency region in which color change cycles are long. According to such properties of vision, it is difficult to visually recognize degradation of image quality even if the pixel values in a short pixel section are converted into the representative value. Meanwhile, it becomes easier to visually recognize degradation of image quality when the pixel values in the section are converted into the representative value as the pixel section becomes longer. 
     Thus, the image conversion unit  12  may reduce the threshold value Th[i−1] in accordance with an increase in distance between the first pixel and the second pixel. According to the threshold value table  17 , the threshold value Th[i−1] becomes a smaller value as the distance between the first pixel (section head pixel) and the second pixel (evaluation pixel), that is, the section pixel number i increases. According to the aforementioned configuration, it is possible to more strictly set a condition for extending the length of the section in which the pixel values are converted into the representative value as the section pixel number i increases, and it is thus possible to prevent degradation of image quality due to the decrease in the number of colors in accordance with the properties of vision. 
     According to the embodiment, the pixel designation unit  12   a  regards, as the next first pixel, the second pixel at a timing at which the evaluation value V is determined to exceed the threshold value and designates the second pixel starting from the pixel adjacent to the next first pixel in the raster line. That is, Step S 120  and the following steps or Step S 115  and the following steps are repeated after Step S 190  in the flowchart in  FIG. 3 or 6 . According to the configuration, the section from the first pixel (section head pixel) to the third pixel (immediately previous pixel) is repeatedly set in the raster line, and the number of colors in the raster line can be reduced. 
     According to the embodiment, the conversion unit  12   d  calculates the representative value of the pixel values from the first pixel to the end pixel and converts the pixel values from the first pixel to the end pixel into the representative value of the pixel values from the first pixel to the end pixel if the determination unit  12   c  determines that the evaluation value V calculated when the end pixel of the raster line is designated as the second pixel does not exceed the threshold value, that is, if “Yes” determination is made in Step S 160 . According to the aforementioned configuration, it is possible to avoid a situation in which the pixel values in a partial section including the end pixel in the raster line cannot be converted into the representative value. 
     Here, a so-called gradation pattern in which the pixel values change for a number of pixels gotten together to some extent in the raster line is assumed to be an image of a processing target. In the image conversion processing  100  according to the first embodiment, such a gradation pattern has a regular variation in the pixel values, a relatively long section in which a color variation is to be protected in the raster line is thus regarded as one conversion target section, and the pixel values are easily converted into the representative value. Therefore, further arrangement is required in terms of protection of image quality of the gradation pattern. 
     According to the second embodiment, the conversion unit  12   d  does not execute the conversion when the number of pixels from the first pixel to the third pixel is greater than the first predetermined number (threshold value ThA) and the number of times of variation in the pixel value (pixel value variation number CC) in the section from the first pixel to the third pixel is smaller than the second predetermined number (threshold value ThB) at the timing at which the evaluation value V is determined to exceed the threshold value. That is, according to the second embodiment, it is possible to exclude the section from the first pixel to the third pixel in the raster line that is a section in which the pixel value variation number CC is relatively small regardless of a large number of pixels from the processing of converting the pixel values in the section into the representative value. In this manner, it is possible to prevent degradation of image quality of the gradation pattern. 
     It is possible to state that the flowcharts in  FIGS. 3 and 6  disclose an image conversion method. Specifically, Step S 120  (or Step S 115 ) and S 150  correspond to a pixel designation step of designating the second pixel. Also, Step S 130  corresponds to an evaluation value calculation step of calculating the evaluation value V, and Step S 140  corresponds to a determination step of determining whether or not the evaluation value V exceeds the predetermined threshold value. In addition, Steps S 170  and S 180  correspond to a conversion step of calculating the representative value and converting the pixel values from the first pixel to the third pixel into the representative value at the timing at which the evaluation value is determined to exceed the threshold value. 
     In the flowchart in  FIG. 3 , the end pixel can be designated as the next section head pixel in Step S 190 . Since substantially no evaluation pixel is present in Step S 120  and the following steps that are executed after the end pixel is designated as the section head pixel, the evaluation value calculation unit  12   b  cannot calculate the evaluation value V in Step S 130 . Thus, the same determination as that in Step S 160  may be executed at a timing after Step S 120  and before Step S 130  in the flowchart in  FIG. 3 . That is, the image conversion unit  12  moves on to Step S 130  when the position of the evaluation pixel indicated by the section pixel number i is determined not to exceed the end pixel after Step S 120 . Meanwhile, the image conversion unit  12  may move on to the processing in Step S 200  or may end the image conversion processing  100  when the position of the evaluation pixel indicated by the section pixel number i is determined to exceed the end pixel after Step S 120 . 
     Similarly, the end pixel can be designated as the next section head pixel in Step S 190  in the flowchart in  FIG. 6 . Since substantially no evaluation pixel is present in Step S 115  and the following steps that are executed after the end pixel is designated as the section head pixel, the determination in Step S 125  and the calculation of the evaluation value V in Step S 130  cannot be performed. Thus, the same determination as that in Step S 160  may also be executed at a timing after Step S 115  and before Step S 125  in the flowchart in  FIG. 6 . That is, the image conversion unit  12  moves on to Step S 125  when the position of the evaluation pixel indicated by the section pixel number i is determined not to exceed the end pixel after Step S 115 . Meanwhile, the image conversion unit  12  may move on to the processing in Step S 195  or may end the image conversion processing  100  when the position of the evaluation pixel indicated by the section pixel number i is determined to have exceeded the end pixel after Step S 115 .