JP2010206599A - Image processing apparatus, printing system, image processing method, and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress an excessive decrease in image quality of a print image while suppressing compressibility of data. <P>SOLUTION: An image processing unit specifies, as specified pixel groups B1, B2, two sets having a predetermined upper-limit number of pixels or below which are arranged successively in a line direction in a two-dimensional array of pixels that object image data represent and in which all the pixel values are included in decision ranges R1, R2 of pixel values determined based upon the values of pixels included in the set of pixels. Values of all the pixels in the specified pixel groups B1, B2 are changed to one value included in the decision ranges R1, R2 to acquire processed image data. Consequently, compressibility of data at a following data compression unit can be improved. Further, the number of pixels in the specified pixel groups B1, B2 is limited to the upper-limit value or below to prevent line-directional variation in contrast of the processed image data from being excessively lost, thereby suppressing an excessive decrease in the image quality of the print image based upon the processed image data. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a technique for compressing image data used for variable printing, and a printing system that performs variable printing.

  Conventionally, variable printing for printing individual information on each printing paper (each portion corresponding to one page when the printing paper is roll paper) has been performed. In recent years, the size of image data corresponding to one printing paper has increased with the improvement of the recording resolution of various printing apparatuses, and image data is generated in conjunction with the speeding up of the printing operation in the printing apparatus. Data transfer from the image processing apparatus to the printing apparatus may be a rate-determining step of the printing speed in the entire printing system.

Specifically, in a printing apparatus of a printing system, ink is moved from a plurality of ejection openings arranged over the entire width of the roll paper while moving a continuous portion of the roll paper at a speed of 100 meters (m) per minute. In the case where variable printing is performed by ejecting a small liquid droplet onto a roll paper, the recording resolution in the printing apparatus is 360 dpi (dot per inch) × 360 dpi, and the width of the print area of the roll paper is 20 inches (about 500 mm). ), The number of pixels in the width direction of the roll paper is 7200, and the number of pixels in the movement direction per 100 m of roll paper is 1.44 × 10 ⁶ (mega). In this case, data indicating the value of about 10 × 10 ⁹ (giga) (= 7200 × 1.44 × 10 ⁶ ) pixels per minute (data indicating the value of 170 × 10 ⁶ pixels per second) ) Need to be constantly transmitted from the image processing apparatus to the printing apparatus. Further, when the recording resolution of the printing apparatus is improved and the printing operation is speeded up, the amount of data to be transmitted per unit time further increases. Of course, parallel transfer of the communication path between the image processing device and the printing device and improvement of the hardware performance related to the communication will increase the transfer speed to some extent (that is, increase the amount of data that can be transmitted per unit time). However, there is a certain technical limit to the improvement in hardware performance as the manufacturing cost of the printing system increases.

  Therefore, it is considered to improve the apparent transfer speed by compressing (encoding) the image data. For example, in Patent Document 1, a multi-value 1 pixel is expressed by a binary 4-pixel. When it is planned to binarize the value image, four threshold values corresponding to the pixel are selected from the pixel position of each pixel of the multi-value image in the binarization process to be finally performed, Among the five threshold ranges defined by these threshold values, a threshold range that includes the pixel value of the pixel is specified, and within the threshold range, the pixel value is convenient for subsequent image coding. A technique for improving coding efficiency by changing the above is disclosed. Also, in Patent Document 2, when image data is compressed by performing a reversible encoding process after filtering the image data, the target pixel based on the image data is used in the filtering process. The pixel value of the target pixel is compared with the predicted value, and if the difference is smaller than the default value, the predicted value is output. A method for outputting is disclosed.

JP 2003-158635 A JP 2004-186934 A

  By the way, when performing compression processing on image data by a run-length method such as the packbits method, the two-dimensional array of pixels indicated by the image data is continuously arranged in one arrangement direction and within a certain range. It is conceivable to improve the data compression rate in the compression processing by changing the values of all the pixels in the pixel group including all the pixel values to one value. However, in this case, the change in shading in the one arrangement direction is excessively lost, and the image quality of the printed image based on the compressed image data may be excessively lowered.

  The present invention has been made in view of the above problems, and an object of the present invention is to suppress an excessive decrease in image quality of a printed image while improving a data compression rate.

  The invention according to claim 1 is an image processing apparatus for compressing image data used for variable printing, and in a two-dimensional array of pixels indicated by the image data, a predetermined upper limit number aligned in one array direction. A set of the following pixels, the values of all the pixels of a pixel group including all pixel values in a pixel value determination range determined based on a value of at least one pixel of the set of pixels. A data processing unit that acquires processed image data by changing to one value included in the determination range, and a data compression unit that performs reversible compression on the processed image data to acquire compressed image data With.

  A second aspect of the present invention is the image processing apparatus according to the first aspect, wherein the data processing unit changes the upper limit number irregularly.

  A third aspect of the present invention is the image processing apparatus according to the second aspect, wherein the upper limit number is set for each of a plurality of pixel value ranges obtained by dividing the entire pixel value range of the image data. A range to be obtained is determined in advance, and the upper limit number corresponding to each pixel group is determined based on a pixel value range to which a value representative of the determination range of each pixel group belongs.

  According to a fourth aspect of the present invention, in the image processing apparatus according to the third aspect, the upper limit number can be taken in a pixel value range in which the maximum pixel value is a highlight side value in the plurality of pixel value ranges. The middle value of the range is small.

  A fifth aspect of the present invention is the image processing apparatus according to the second aspect, wherein a range that the upper limit number can take is predetermined for each of a plurality of spatial frequency ranges of the image data, The higher the maximum spatial frequency in the spatial frequency range, the smaller the central value of the range that the upper limit number can take, and the upper limit number corresponding to each pixel group includes at least a part of each pixel group. It is determined based on a spatial frequency range to which a representative spatial frequency of a spatial frequency spectrum obtained from values of a plurality of pixels continuous in the one arrangement direction belongs.

  A sixth aspect of the present invention is the image processing apparatus according to the first aspect, wherein the upper limit number is set in advance for each of a plurality of pixel value ranges obtained by dividing the entire pixel value range of the image data. The upper limit number corresponding to each pixel group is determined based on a pixel value range to which a value representative of the determination range of each pixel group belongs.

  The invention according to claim 7 is the image processing apparatus according to claim 6, wherein the upper limit number is smaller in a pixel value range in which the maximum pixel value is a highlight side value in the plurality of pixel value ranges. ing.

  The invention according to claim 8 is the image processing apparatus according to claim 1, wherein the upper limit number that is smaller as the maximum spatial frequency is higher is predetermined in a plurality of spatial frequency ranges of the image data, The upper limit number corresponding to each pixel group includes at least a part of each pixel group and a spatial frequency range to which a representative spatial frequency of a spatial frequency spectrum obtained from values of a plurality of pixels continuous in the one arrangement direction belongs To be determined.

  The invention according to claim 9 is the image processing apparatus according to any one of claims 1 to 8, wherein when a predetermined number of pixels having the same value are continuously arranged in the one arrangement direction, The data processing unit excludes the predetermined number or more of pixels from the pixel group.

  A tenth aspect of the present invention is a printing system that performs variable printing, comprising the image processing apparatus according to any one of the first to ninth aspects, and a printing apparatus that prints an image on a predetermined print medium. The image processing device further includes a data transmission unit that transmits the compressed image data to the printing device, and the printing device receives the compressed image data from the data transmission unit; A data decompression unit for decompressing the compressed image data.

  An invention according to an eleventh aspect is an image processing method for compressing image data used for variable printing, and a) in a two-dimensional array of pixels indicated by the image data, a predetermined array continuously arranged in one array direction A set of pixels equal to or less than the upper limit number of pixels in which all pixel values are included in a pixel value determination range determined based on a value of at least one pixel of the set of pixels. A step of obtaining processed image data by changing a value to one value included in the determination range; and b) a step of obtaining reversible compression on the processed image data to obtain compressed image data. With.

  A twelfth aspect of the present invention is the image processing method according to the eleventh aspect, wherein the upper limit number is irregularly changed in the step a).

  The invention according to claim 13 is a program used for compression of image data used for variable printing, and the computer executes the program in a) a two-dimensional array of pixels indicated by the image data. A set of pixels having a predetermined upper limit number or less that are continuously arranged in one arrangement direction, and the pixel value determination range determined based on the value of at least one pixel of the set of pixels A step of acquiring processed image data by changing the value of all the pixels of the pixel group including the value to one value included in the determination range; and b) reversible with respect to the processed image data. Performing compression and acquiring compressed image data.

  The invention according to claim 14 is the program according to claim 13, wherein the upper limit number is irregularly changed in the step a).

  ADVANTAGE OF THE INVENTION According to this invention, the excessive fall of the image quality of the printing image based on processed image data can be suppressed, improving the compression rate of data.

  According to the second, twelfth and fourteenth aspects of the present invention, it is possible to prevent the occurrence of streak unevenness in the printed image by preventing the positions where the pixel values change in one arrangement direction from becoming constant.

  According to the third to eighth aspects of the present invention, it is possible to further suppress the deterioration of the image quality of the printed image. In the ninth aspect of the present invention, it is possible to prevent the pixel value indicating the pattern for managing the print quality from being changed. can do.

1 is a diagram illustrating a configuration of a printing system. It is a block diagram which shows the function structure which a computer implement | achieves. It is a figure which shows the flow of the process which produces | generates and compresses target image data. It is a figure which shows a part of object image data. It is a figure which shows a part of processed image data. It is a figure which shows a part of compressed image data. It is a figure which shows the flow of the process which acquires processed image data. It is a figure which shows the flow of the process which acquires processed image data. It is a figure for demonstrating the process which acquires processed image data. It is a figure for demonstrating the process which acquires processed image data. It is a figure for demonstrating the process which acquires processed image data. It is a figure which shows the value of the pixel of object image data and processed image data. It is a figure which shows a target image. It is a figure which shows the processed image when making an upper limit number constant. It is a figure which shows the processed image in the case of changing an upper limit number irregularly. It is a figure which shows a part of object image data. It is a figure which shows a part of processed image data. It is a figure which shows a part of processed image data.

  FIG. 1 is a diagram showing a configuration of a printing system 1 according to an embodiment of the present invention. The printing system 1 includes a computer 11 and a printing device 12. The printing device 12 receives a signal from the computer 11 and prints an image on a printing medium such as printing paper such as roll paper or cut paper by an ink jet method. Note that the printing apparatus 12 may be a printing apparatus of another system such as an electrophotographic system.

  The computer 11 has a general computer system configuration in which a CPU 101 that performs various arithmetic processes, a ROM 102 that stores basic programs, and a RAM 103 that stores various information are connected to a bus line. The bus line further includes a fixed disk 105 for storing information, a display 106 for displaying various kinds of information, a keyboard 107a and a mouse 107b for receiving input from an operator, an optical disk, a magnetic disk, a magneto-optical disk and the like. A reading / writing device 108 that reads information from the recording medium 91 and writes information to the recording medium 91, and a communication unit 109 that communicates with the printing device 12 appropriately via an interface (I / F). Equally connected.

  The computer 11 reads the program 92 from the recording medium 91 in advance via the reading / writing device 108 and stores it in the fixed disk 105. When the program 92 is copied to the RAM 103 and the CPU 101 executes arithmetic processing according to the program in the RAM 103 (that is, when the computer executes the program), the computer 11 is used for variable printing in the printing apparatus 12. A function as an image processing apparatus that generates and compresses image data to be compressed is realized.

  FIG. 2 is a block diagram illustrating a functional configuration realized by the computer 11. In FIG. 2, the image processing unit 2 (image data generation unit 21, data processing unit 22, and data compression unit 23) shows functions realized by the CPU 101 and the like, and the data transmission unit 24 shows functions realized by the communication unit 109. . Note that these functions may be realized by a dedicated electrical circuit, or a dedicated electrical circuit may be partially used. For example, the functions of the data compression unit 23 and the data transmission unit 24 may be realized by a single electric circuit (the same applies to a data reception unit 121 and a data expansion unit 122 described later in the printing apparatus 12).

  The image data generation unit 21 in FIG. 2 is a so-called RIP (Raster Image Processor), which generates a raster image from various types of data prepared as a printing target in the printing system 1 to generate a plurality of color components (this embodiment). Is a multi-tone image of cyan (C), magenta (M), yellow (Y) and black (K)) (for example, an image including a natural image portion such as a photograph). .) Data. Further, as will be described later, the data processing unit 22 sets the values of all the pixels of a specific pixel group that are continuously arranged in one arrangement direction in the two-dimensional arrangement of pixels of each color component indicated by the target image data to one value. The processed image data is obtained by changing the image data, and the data compression unit 23 performs reversible compression (compression encoding) of, for example, packbits on the processed image data. get. Further, the data transmission unit 24 transmits the compressed image data to the printing apparatus 12.

  The printing apparatus 12 in FIG. 1 includes a data receiving unit 121 that receives compressed image data from the data transmission unit 24, a data expansion unit 122 that is an electrical circuit that expands compressed image data, and a data expansion unit 122. And a printing unit 123 that prints an image on a print medium based on the decompressed image data (that is, processed image data).

  Actually, the printing unit 123 performs a halftone process (shading process) on the image indicated by the processed image data to generate halftone image data, and a plurality of colors (C, M, Y and K) a head unit that ejects micro droplets of ink toward the print medium, a moving mechanism that continuously moves the print medium relative to the head unit, and ejection control of the head unit according to the halftone image data A control unit is provided. In the head unit, a plurality of ejection openings are arranged over the entire width of the print medium, and the printing apparatus 12 is a variable printing machine that completes printing only once the print medium passes under the head unit. Yes. Note that the function of the data decompression unit 122 may be realized by software as with the data compression unit 23.

  FIG. 3 is a diagram showing a flow of processing in which the image processing unit 2 generates and compresses target image data used for variable printing. In the following description, only one color component among C, M, Y, and K is focused, but the same processing is performed for the other color components. First, the image data generation unit 21 of the image processing unit 2 generates a raster image from various types of input data and generates target image data (step S11).

  FIG. 4 is a diagram showing a part of target image data. In FIG. 4, the value of each pixel is described inside a rectangle indicating the pixel (see FIGS. 5, 8 to 10 described later, and The same applies to FIGS. 15 to 17). In the data processing unit 22, the processed image data shown in FIG. 5 is acquired by changing the value of a specific pixel in the two-dimensional array of pixels indicated by the target image data (step S12). Details of the processing in the data processing unit 22 will be described later.

  When the processed image data is acquired, the data compression unit 23 determines that β pixels having the same value α in the array of pixels indicated by the processed image data are β and α. The processed image data is subjected to lossless compression, and compressed image data is acquired (step S13). For example, in the ten pixels in the processed image data in FIG. 5, from the leftmost to the right, two pixels having a value 2 are continuous, four pixels having a value 6 are continuous, and four pixels having a value 4 are continuous. Pixels are arranged in order, and as shown in FIG. 6, in the compressed image data, the 10 pixels of FIG. 5 are represented by a set of 6 values. In FIG. 6, the number of pixels β is represented by Roman numerals.

  Actually, the generation of the target image data in the image data generation unit 21 in FIG. 2, the generation of the processed image data in the data processing unit 22, and the generation of the compressed image data in the data compression unit 23 are performed in parallel. . That is, the image data generation unit 21 sequentially outputs each part of the target image data, the data processing unit 22 performs a process of changing the value of a specific pixel with respect to the part of the target image data, and the data compression unit 23 Encoding processing is performed on the processed image data portion. Then, each portion of the compressed image data is sequentially transmitted to the printing apparatus 12, and images are continuously printed on the print medium in parallel with the processing in the image processing unit 2. In the printing system 1, target image data is generated by an external RIP, stored in the fixed disk 105 of the computer 11, and each part of the target image data in which processing in the data processing unit 22 is sequentially read from the fixed disk 105. May be performed sequentially.

  FIG. A and FIG. B is a diagram showing a flow of processing in which the data processing unit 22 acquires processed image data. Note that FIG. The process of step S32 surrounded by a broken-line rectangle in B is performed in a process example described later, and is not performed in this process example.

  In the present embodiment, in the two-dimensional array of pixels indicated by the target image data, a plurality of pixels (hereinafter also referred to as “pixel lines”) arranged in one array direction (hereinafter also referred to as “line direction”). A number that is incremented by one from the pixel at one end to the pixel at the other end (hereinafter referred to as “pixel number”) is assigned, and the number next to the last pixel number of one pixel line is this number. This is the first pixel number of the pixel line adjacent to the pixel line (that is, the pixel number increases from one pixel line in the other arrangement direction to the other pixel line). As will be described below, the data processing unit 22 performs processing by changing the pixel of interest in the order of pixel numbers.

  When acquiring processed image data, first, the pixel number of a pixel of interest (hereinafter referred to as “target pixel number”) Pn is set to 0, and the reference value to be described later is the pixel number 0. In addition, the count number is set to 1 (step S21). Further, the start pixel number indicating the pixel number of the first pixel of the pixel group to be changed to the same value is set to the target pixel number 0 (step S22), and the target pixel number Pn is incremented to 1 (step S22). S23). Subsequently, when it is confirmed that the pixel of the target pixel number 1 exists in the target image data (step S24), the absolute value of the difference between the pixel value of the target pixel number 1 and the reference value is obtained as the evaluation value. .

  FIG. 8 is a diagram for explaining processing for obtaining processed image data. 8 shows a plurality of pixels included in one pixel line of the target image data. In the uppermost stage of FIG. 8, the pixels P0, P1, and P2 are assigned to the pixels with pixel numbers 0 to 2. . Further, in the second row from the top in FIG. 8, as a reference value, an average value of values from the pixel P0 having the start pixel number 0 to the pixel immediately below each pixel P0 and P1 (in the following description, the decimal part is rounded down). (The same applies to FIGS. 9 and 10 to be described later).

  As described above, since the current target pixel number Pn is 1 and the reference value is the value of the pixel with pixel number 0, here, the value 3 of the pixel P1 with pixel number 1 in the uppermost row in FIG. The absolute value 2 of the difference from the value 1 of the pixel P0 with the pixel number 0 is obtained as the evaluation value. In the third row from the top in FIG. 8, evaluation values corresponding to the uppermost pixels P1 and P2 in FIG. 8 are shown directly below the pixels P1 and P2.

  In the data processing unit 22, the threshold value is set to 2 in advance by the operator via the keyboard 107a or the like (the same applies to the upper limit number described later), and the evaluation value is equal to or less than the threshold value (that is, the pixel of the target pixel number Pn The value is Img [Pn], the reference value is Av, the threshold is Th, and the absolute value of the value M is expressed as abs (M) (abs (Img [Pn] −Av) ≦ Th)). If confirmed (step S25), the count is incremented to 2 (step S26). When the entire pixel value range of the target image data is 0 to 255 (256 gradations), the threshold value is 1 or more and 24 or less (a value corresponding to about 10% of the entire pixel value range). Is preferred.

  Subsequently, when it is confirmed that the count number 2 is equal to or less than the upper limit number 4 input in advance (that is, (Cnt ≦ Max) where the count number is Cnt and the upper limit number is Max) (step S27). The average value 2 of the pixel values from the start pixel number 0 to the target pixel number 1 is obtained (see the value immediately below the pixel P1 in the second row from the top in FIG. 8), and the reference value is updated to the average value. (Step S28).

  When the reference value is updated, the process returns to step S23, and the target pixel number Pn is incremented to 2. Subsequently, when it is confirmed that the pixel P2 of the target pixel number 2 exists in the target image data (step S24), the absolute difference between the value 5 of the pixel P2 of the target pixel number 2 and the current reference value 2 is determined. Value 3 is obtained as the evaluation value (see the value immediately below pixel P2 in the third row from the top in FIG. 8). Here, it is confirmed that the evaluation value 3 is larger than the threshold value 2 (step S25), and the pixel values from the start pixel number 0 to the pixel number (Pn-1) (that is, the pixel number 1) become the reference value 2. The value of these pixels in the modified image data is determined (step S29). 8 shows a plurality of pixels included in the processed image data. Similarly to the uppermost stage in FIG. 8, the value of each pixel is described inside the rectangle indicating the pixel, and the pixel number Reference numerals P0 and P1 are assigned to the 0 and 1 pixels (the same applies to FIGS. 9 and 10 described later). Then, the reference value is changed to the value 5 of the pixel P2 of the target pixel number 2, and the count number is changed to 1 (step S30).

  When the reference value and the count number are changed, the process returns to step S22, the start pixel number is changed to the target pixel number 2, and then the target pixel number Pn is incremented to 3 (step S23). Subsequently, when it is confirmed that the pixel of the target pixel number 3 exists in the target image data (step S24), the value 7 of the pixel P3 of the target pixel number 3 shown in the top row of FIG. The absolute value 2 of the difference from 5 is obtained as the evaluation value (see the value immediately below the pixel P3 in the third row from the top in FIG. 9). When it is confirmed that the evaluation value 2 is equal to or less than the threshold value 2 (step S25), the count number is incremented to 2 (step S26). When it is confirmed that the count number 2 is equal to or less than the upper limit number 4 (step S27), the average value 6 of the pixel values from the start pixel number 2 to the target pixel number 3 is obtained (from the top 2 in FIG. 9). In the step, the reference value is updated to the average value (see the value immediately below the pixel P3) (step S28).

  As described above, in the data processing unit 22, there is no pixel of the target pixel number Pn in the target image data (step S24), the evaluation value is larger than the threshold 2 (step S25), and the count number is Until any of the upper limit numbers 4 is confirmed (step S27), the pixel number Pn of interest is incremented (step S23), the count number is incremented (step S26), and the reference value is updated (step S28). Is repeated.

  In the uppermost example of FIG. 9, when the target pixel number Pn is 6, the count number is 5 (that is, the number of pixels from the start pixel number 2 to the current target pixel number 6 is 5). Thus, it is confirmed in the process of step S27 that the count number 5 is larger than the upper limit number 4, and as shown in the lowermost stage of FIG. 9, the pixel number (Pn−1) (that is, the pixel number) from the start pixel number 2 The values of the pixels P2 to P5 up to 5) are changed to the current reference value 6 (step S29). Then, the reference value is changed to the value 5 of the pixel P6 of the target pixel number 6, and the count number is changed to 1 (step S30).

  When the reference value and the count number are changed, the process returns to step S22, the start pixel number is changed to the target pixel number 6, and then the target pixel number Pn is incremented to 7 (step S23). Then, there is no pixel of the target pixel number Pn in the target image data (step S24), the evaluation value is larger than the threshold value 2 (step S25), and the count number is larger than the upper limit number 4 (step S27). ), The increment of the target pixel number Pn (step S23), the increment of the count (step S26), and the update of the reference value (step S28) are repeated.

  In the example at the top of FIG. 10, for convenience of explanation, it is assumed that the target image data includes only 10 pixels (actually, the target image data includes a large number of pixels each composed of a large number of pixels. In the process performed with the pixel number 9 of the last pixel P9 as the target pixel number Pn (step S23), it is confirmed that the pixel of the target pixel number 9 exists in the target image data (step S23). S24) The absolute value 0 of the difference between the value 4 of the pixel P9 of the target pixel number 9 and the current reference value (here, 4) is obtained as the evaluation value (3 from the top in FIG. 10). (Refer to the value immediately below the pixel P9 in the stage). If it is confirmed that the evaluation value 0 is equal to or less than the threshold value 2 (step S25), the count number is incremented to 4 (step S26). When it is confirmed that the count number 4 is equal to or less than the upper limit number 4 (step S27), an average value 4 of pixel values from the start pixel number 6 to the target pixel number 9 is obtained (from the top of FIG. 10 2). In the step, the reference value is updated to the average value (see the value immediately below the pixel P9) (step S28).

  When the reference value is updated, the process returns to step S23, and the target pixel number Pn is incremented to 10. At this time, the data processing unit 22 confirms that the pixel of the target pixel number 10 does not exist in the target image data (step S24), and the pixel number (Pn) from the start pixel number 6 as shown in the bottom row of FIG. -1) The values of the pixels P6 to P9 up to (that is, the pixel number 9) are changed to the current reference value 4 (step S31). Thereby, the values of all the pixels in the processed image data are determined, and the processing in the data processing unit 22 is completed.

  As described above, the data compression unit 23 in FIG. 2 performs reversible compression on the processed image data to obtain the compressed image data, which is used for printing in the printing apparatus 12. Actually, in the print image formed by the printing apparatus 12, the value of the pixel is changed by the data processing unit 22 (it is regarded as a decrease in local gradation information or a decrease in local resolution). The effect is hardly perceived.

  FIG. 11 is a diagram illustrating pixel values included in the target image data and the processed image data. The horizontal axis of FIG. 11 shows one arrangement direction (line direction) in the two-dimensional arrangement of pixels, and the vertical axis shows the pixel values. In FIG. 11, a line indicating the pixel value included in the target image data is denoted by reference symbol L1, and a line indicating the pixel value included in the processed image data is denoted by reference numeral L2.

  Here, FIG. A and FIG. When a set of consecutive pixels that are changed to the same value in the processes of steps S29 and S31 of B is referred to as a specific pixel group, the evaluation value for each pixel (pixel number Pn) included in one specific pixel group Is obtained on the basis of an average value of values from the first pixel of the specific pixel group to a pixel immediately before the pixel (pixel number (Pn-1)), and thus a reference value for each pixel (that is, , The reference value) may fluctuate slightly, but in the repetition of the processing of steps S23 to S28 for the specific pixel group (that is, the process for determining the pixels included in the specific pixel group), the maximum reference If the range of pixel values with the upper limit of the value plus a threshold and the lower limit of the minimum reference value minus the threshold is the determination range, all pixels in the specific pixel group are included in the determination range. It becomes. In FIG. 11, some specific pixel groups are indicated by arrows denoted by B1 and B2, and determination ranges of pixel values of the specific pixel groups B1 and B2 are denoted by arrows denoted by R1 and R2. In practice, the value of the pixel immediately after the specific pixel group B2 in FIG. 11 is also included in the determination range R2 of the pixel value of the specific pixel group B2, but the number of pixels of the specific pixel group B2 is the upper limit number. Therefore, the pixel is not included in the specific pixel group B2.

  By the way, it is conceivable to efficiently compress the target image data with a high-quality irreversible compression (irreversible compression) algorithm such as the JPEG method. In order to ensure a constant transfer speed from the image processing apparatus to the printing apparatus in order to perform at high speed, it is necessary to realize compression processing and expansion processing with expensive hardware, which greatly increases the manufacturing cost of the printing system. Resulting in. On the other hand, a lossless compression algorithm such as the packbits method is generally simple, and the manufacturing cost of the printing system does not increase significantly even when the processing is realized by hardware. However, such reversible compression has a problem that the compression rate for natural images is low.

  On the other hand, in the image processing unit 2, in the two-dimensional array of pixels indicated by the target image data, a set of pixels equal to or less than a predetermined upper limit number continuously arranged in the line direction, the pixels included in the set of pixels The pixel value determination range determined based on the value of the pixel value is specified as a specific pixel group, and all the pixel values of the specific pixel group are included in the determination range. In the above processing example, the processed image data is acquired by changing to the last reference value for the specific pixel group. As a result, the data compression rate in the subsequent data compression unit 23 is improved without increasing the manufacturing cost of the printing system (that is, the value obtained by dividing the size of the compressed image data by the size of the target image data is reduced). Even when compression processing is performed by software, a high transfer speed from the image processing unit 2 to the printing apparatus 12 is ensured, and generation and printing of target image data are performed in parallel and at high speed. Can be done. Further, the number of pixels of the specific pixel group to be changed to the same value is limited to the upper limit number or less (that is, the length of the specific pixel group is limited to a predetermined maximum continuous length or less), and the processed image In the data, it is possible to prevent the change in shading in the line direction from being excessively lost. As a result, it is possible to suppress an excessive decrease in the image quality of the printed image based on the processed image data.

  In the target image data, the number next to the last pixel number of one pixel line is given to the pixel at the end opposite to the pixel of the last pixel number in the pixel line adjacent to the pixel line. Or a pixel near the pixel of the last pixel number (a pixel adjacent in the direction perpendicular to the line direction). A specific pixel group may be set over (that is, the specific pixel group may include pixels of two pixel lines).

  Further, in the technique disclosed in Japanese Patent Laid-Open No. 2003-158635 (Patent Document 1), the pixel value is changed based on a threshold matrix used in a binarization process (halftone process) that is finally performed. As a result, the image data cannot be binarized using another threshold matrix. On the other hand, in the image processing unit 2, since the processed image data acquired by the data processing unit 22 does not depend on the content of the halftone process, the processed image data can be used in another printing apparatus. Become. Therefore, even when the image processing unit 2 is provided in an isolated manner and a printing apparatus that actually performs printing is not specified, compressed image data can be generated, and processing of the entire system related to printing Speed can be improved.

  Further, in the method of Patent Document 1, a protection area is provided before and after the threshold value of the binarization process, and the pixel value changing process is invalidated for the pixel value in this protection area, thereby correcting the state variation of the apparatus in the printing apparatus. However, in this case, depending on the width of the protection area, the compression rate of the image data is greatly reduced. On the other hand, in the processed image data acquired by the data processing unit 22, a local color reduction process (averaging process) is performed on the target image data, and the macroscopic image density change is maintained as it is. As a result, the printing apparatus 12 can correct a change in the printing state (for example, tone correction) while ensuring a high compression ratio in the data compression unit 23.

  Next, another processing example for acquiring processed image data will be described. In this processing example, FIG. The process of step S32 enclosed in the rectangle of a broken line in B is performed. Other processes are the same as those in the above processing example.

  As described above, in the processing for obtaining processed image data, there is no pixel of the target pixel number Pn in the target image data (step S24), the evaluation value is larger than the threshold value (step S25), and the count Until one of the numbers is confirmed to be larger than the upper limit number (step S27), the target pixel number Pn is incremented (step S23), the count number is incremented (step S26), and the reference value is updated (step S28). ) Is repeated. In this case, if it is confirmed in the process of step S27 that the count number is larger than the upper limit number, the data processing unit 22 irregularly sets the upper limit number within a predetermined range according to a random number generated inside. It is changed (randomly) (step S32).

  Subsequently, the value of the pixel from the start pixel number to the pixel number (Pn−1) is changed to the current reference value (step S29), and the reference value and the count number are the value of the pixel of the target pixel number Pn and It is changed to 1 (step S30). After the start pixel number is changed to the target pixel number Pn (step S22), when the processing of steps S23 to S28 is repeated, in the processing of step S27, the count number is greater than the new upper limit number (upper limit number after change). It is confirmed whether it is large or not. When it is confirmed that the count number is larger than the upper limit number, the upper limit number is again irregularly changed (step S32), and pixels from the start pixel number to the pixel number (Pn-1) (that is, the pixel number (Pn-1)). , The specific pixel group) is changed to the current reference value (step S29). The reference value and the count number are respectively changed to the value of the pixel of the target pixel number Pn and 1 (step S30), the start pixel number is changed to the target pixel number Pn (step S22), and then included in the next specific pixel group The processing of steps S23 to S28 for specifying the pixel to be processed is repeated.

  FIG. 12 is a diagram showing one color component of the target image, and FIG. 13 shows one color component of the processed image (that is, the image indicated by the processed image data) when the upper limit number is constant at 32. FIG. 14 is a diagram illustrating one color component of the processed image when the upper limit number is irregularly changed within a range of 12 to 32. Note that the threshold for the evaluation value is set to 32 when the processed images of FIGS. 13 and 14 are acquired.

  The size of the target image data (including all color components) shown in FIG. 12 is 499148 bytes (Bytes), and the processed image data acquired by the data processing unit 22 with the upper limit number being constant. When the packbits compression is applied, the size of the compressed image data is 36683 bytes. In addition, when the processed image data acquired while changing the upper limit number irregularly for each specific pixel group is compressed, the size of the compressed image data becomes 54861 bytes. Therefore, the compression rate for processed image data when the upper limit number is constant at 32 is higher than the compression rate for processed image data when the upper limit number is irregularly changed within the range of 12 to 32. (The size becomes smaller). If the target image data is directly compressed, the size of the compressed image data is 50000175 bytes.

  On the other hand, in a processed image acquired with a constant upper limit number, the position where the pixel value changes in the line direction tends to be constant, and as shown in FIG. 13) becomes apparent in a state that is long in the vertical direction in FIG. 13, and a portion corresponding to the boundary is also visually recognized as a stripe unevenness (or as a tone jump) in the print image in the printing apparatus 12. May end up. On the other hand, in the processed image of FIG. 14 obtained by changing the upper limit number irregularly, the position where the pixel value changes in the line direction is prevented from becoming constant (actually, the pixel value The boundary at which the change occurs can be blurred), and the occurrence of streak unevenness in the printed image can be suppressed while improving the data compression rate.

  In the process of acquiring processed image data, the range of the upper limit number when the upper limit number is irregularly changed may be changed. Specifically, a plurality of pixel value ranges obtained by dividing the entire pixel value range of the target image data into a plurality of ranges are set, and an upper limit number can be taken for each of the plurality of pixel value ranges (hereinafter referred to as “designated range”). Is predetermined). Here, it is assumed that the entire pixel value range of the target image data is 0 to 255 (256 gradations). For example, the pixel value range of the pixel value 0 to 25 (0 to 10% of the total pixel value range) is 10 to 10. 30 specified ranges are determined, and 4 to 12 specified ranges are determined for pixel value ranges of pixel values 26 to 204 (10 to 80% of all pixel value ranges), and pixel values 205 to 255 (all pixel value ranges). The specified range of 2 to 6 is defined in the pixel value range of 80 to 100%. Thus, in the present processing example, the center value of the range that the upper limit number can take in the pixel value range in which the maximum pixel value becomes the highlight value (that is, the pixel value range in which the maximum pixel value is close to the pixel value 255). Has been made smaller.

  In the actual processing of acquiring processed image data, FIG. When the reference value and the count number are changed to the value of the pixel of the target pixel number Pn and 1 in step S30 of B, a process of changing the upper limit number is performed (that is, in step S32 in FIG. 7.B). The process is included in the process of step S30).

  Specifically, in the repetition of the processes of steps S23 to S28, either the evaluation value is larger than the threshold value (step S25) or the count number is larger than the upper limit number (step S27) is confirmed. When the value of the pixel (that is, the specific pixel group) from the start pixel number to the pixel number (Pn−1) is changed to the current reference value (step S29), in the process of step S30, the reference value and the count number Are changed to the value of the pixel of the target pixel number Pn and 1 respectively, and the pixel value range to which the pixel value of the target pixel number Pn belongs is specified, and the designated range corresponding to the pixel value range is selected. . Then, one value (integer) within the specified range is determined according to the random number and set as the upper limit number (that is, the upper limit number is changed). In the data processing unit 22, after the start pixel number is changed to the target pixel number Pn (step S22), the processes of steps S23 to S28 for specifying the pixels included in the next specific pixel group are repeated. At this time, in the process of step S27, a new upper limit number (upper limit number after change) is used.

  As described above, in this processing example, different designated ranges are determined in advance for a plurality of pixel value ranges obtained by dividing the entire pixel value range of the target image data into a plurality, and an upper limit number corresponding to each specific pixel group is set. This is determined based on the pixel value range to which the value of the first pixel of the specific pixel group belongs (in practice, within a specified range corresponding to the pixel value range).

  Here, normally, in a printed image, a change in density is more visible in a highlighted part (bright part), and a change in density is less visible in a shadow part (dark part). In the pixel value range in which the maximum pixel value becomes the highlight side value in the plurality of pixel value ranges, the center value of the range that the upper limit number can take is reduced. As a result, the number of pixels of the specific pixel group is reduced at the highlight side of the processed image, and a certain reproducibility of the target image at the highlight side of the printed image is ensured. In addition, the number of pixels of the specific pixel group increases in the shadow-side portion of the processed image. As a result, in this processing example, the image quality of the print image is reduced (constant) while improving the data compression rate. This can be further suppressed (as compared to the case where the upper limit number is changed irregularly within the range).

  In the processing for obtaining processed image data, the range of the upper limit number when the upper limit number is irregularly changed may be changed in consideration of the spatial frequency of each part of the target image. Also in this case, a plurality of spatial frequency ranges obtained by dividing a spatial frequency range that can be an intensity equal to or higher than a predetermined value in the target image data are set, and a range in which an upper limit number can be taken for each of the plurality of spatial frequency ranges ( That is, the designated range) is predetermined. Actually, the higher the maximum spatial frequency in a plurality of spatial frequency ranges, the smaller the central value of the range that the upper limit number can take.

  In the actual processed image data acquisition process, the evaluation value is larger than the threshold value in the repetition of the processes in steps S23 to S28 (step S25), as in the above-described process example in which a plurality of pixel value ranges are set. Whether the count number is larger than the upper limit number (step S27) is confirmed, and the values of the pixels (that is, the specific pixel group) from the start pixel number to the pixel number (Pn-1) are the current reference When the value is changed (step S29), in the process of step S30, the reference value and the count number are changed to the value of the pixel of the target pixel number Pn and 1, respectively. A spatial frequency spectrum is obtained by FFT (Fast Fourier Transform) or the like from the values of a predetermined number (for example, 16) of pixels including the pixel of the target pixel number Pn and continuous in the line direction, and a representative space of the spatial frequency spectrum is obtained. A spatial frequency range to which the frequency belongs is specified, and a designated range corresponding to the spatial frequency range is selected. Then, one value (integer) within the specified range is determined according to the random number and set as the upper limit number (that is, the upper limit number is changed).

  Here, the representative frequency of the spatial frequency spectrum is specified as, for example, a central value between the minimum value and the maximum value of the spatial frequency that is equal to or higher than a predetermined intensity in the spatial frequency spectrum. In addition, the range (a plurality of pixels) for which the spatial frequency spectrum is obtained includes the pixel of the target pixel number Pn that is the first pixel of the next specific pixel group, and therefore includes at least a part of the specific pixel group. It has become.

  In the data processing unit 22, after the start pixel number is changed to the target pixel number Pn (step S22), the processes of steps S23 to S28 for specifying the pixels included in the next specific pixel group are repeated. At this time, in the process of step S27, a new upper limit number (upper limit number after change) is used.

  As described above, in this processing example, different designated ranges are predetermined for a plurality of spatial frequency ranges of the target image data, and include a plurality of pixels including at least a part of each specific pixel group and continuing in the line direction. The spatial frequency spectrum is obtained from the value of the spatial frequency spectrum, and the upper limit number corresponding to the specific pixel group is determined based on the spatial frequency range to which the representative spatial frequency of the spatial frequency spectrum belongs (in practice, within the specified range corresponding to the spatial frequency range). Determined).

  Here, in the target image containing a lot of texture (roughness), etc., the representative spatial frequency of the spatial frequency spectrum acquired in step S30 becomes relatively high, so that the upper limit number becomes a small value and specified. The number of pixels in the pixel group is reduced. This ensures a certain reproducibility of the target image at the corresponding part of the print image. Further, in a rough part (or a part that is not sharp) in the target image, the representative spatial frequency of the spatial frequency spectrum is relatively low, so that the upper limit number is large and the number of pixels of the specific pixel group is increased. As a result, in this processing example, the reduction in the image quality of the printed image is further suppressed (compared to the case where the upper limit number is changed irregularly within a certain range) while improving the data compression rate. Can do.

  In the process of acquiring processed image data, only one upper limit number may be determined for each pixel value range and each spatial frequency range. In this case, one upper limit number (an upper limit number different from each other in a plurality of pixel value ranges) is predetermined for each of a plurality of pixel value ranges obtained by dividing the entire pixel value range of the target image data into a plurality of ranges, When the upper limit number corresponding to each specific pixel group is determined based on the pixel value range to which the value of the first pixel of the specific pixel group belongs, the maximum pixel value is the highlight side value in the plurality of pixel value ranges. By reducing the upper limit number in the pixel value range, it is possible to further suppress the deterioration of the image quality of the print image compared to the case where the upper limit number is constant.

  Further, one upper limit number (an upper limit number different from each other in the plurality of spatial frequency ranges) is predetermined for each of the plurality of spatial frequency ranges of the target image data, and includes at least a part of each specific pixel group. When obtaining a spatial frequency spectrum from the values of a plurality of pixels continuous in the line direction and determining the upper limit number corresponding to the specific pixel group based on the spatial frequency range to which the representative spatial frequency of the spatial frequency spectrum belongs, By reducing the upper limit number as the maximum spatial frequency is higher in the spatial frequency range, it is possible to further suppress the deterioration of the image quality of the printed image compared to the case where the upper limit number is made constant.

  Next, still another processing example for acquiring processed image data will be described. In the present processing example, a pattern for managing print quality in the printing apparatus 12 and including a plurality of rectangular areas each of which is a set of pixels having the same value (also referred to as a step chart) is included in the target image. It shall be included in The plurality of rectangular regions have the same shape, and the number of pixels in the line direction of the rectangular regions (or a number slightly smaller than the number of pixels) is set in advance as an auxiliary upper limit number described later.

  In this processing example, FIG. A and FIG. In the process of obtaining processed image data in B (when the upper limit number is constant and when the upper limit number is changed (based on the pixel value range or the spatial frequency range)), pixels having the same value are A count number indicating a continuous number (hereinafter referred to as “auxiliary count number”) is also acquired.

  Specifically, when the value of the pixel of the target pixel number Pn is equal to the value of the pixel of the previous pixel number (Pn−1) in the process of step S26 for incrementing the count number, the auxiliary count number is also incremented. If they are not equal, the auxiliary count is changed to 1. Subsequently, in the process of step S27, when it is confirmed that the count number is larger than the upper limit number, or when it is confirmed that the auxiliary count number is equal to or greater than the auxiliary upper limit number, the process proceeds to step S29. When the count number is larger than the upper limit number, the pixel values from the start pixel number to the pixel number (Pn-1) are changed to the current reference value, and the auxiliary count number is equal to or greater than the auxiliary upper limit number. The value of the pixels from the pixel number (Pn−K + 1) to the target pixel number Pn (hereinafter referred to as “a plurality of pixels related to forcible change”) is forcibly set to the target pixel number Pn, where K is the current auxiliary count number. Is changed to the value of

  At this time, even if some of the plurality of pixels related to the forced change are included in the specific pixel group determined immediately before, the value of the pixel is forcibly changed. In addition, a plurality of pixels related to forced change cannot be changed in subsequent processing. When some pixels of the specific pixel group determined immediately before are included in the plurality of pixels related to the forced change, the values of the remaining pixels of the specific pixel group are the original values of these pixels. The average value may be changed again.

  In the data processing unit 22, the reference value and the count number are changed to the value of the pixel of the target pixel number Pn and 1 respectively (step S30), and the start pixel number is changed to the target pixel number Pn (step S22). The processes of S23 to S28 are repeated. As described above, since a plurality of pixels related to forced change cannot be changed in subsequent processing, the processing of steps S23 to S28 repeats the processing of the target pixel number Pn in the target image data. If it is confirmed that there is no pixel (step S24), the evaluation value is larger than the threshold value (step S25), and the count number is larger than the upper limit number (step S27), step S29 is performed. Alternatively, in the process of step S31, the pixel values from the pixel number next to the start pixel number to the pixel number (Pn-1) are changed to the current reference value. In addition, when the value of the pixel immediately after the plurality of pixels related to the forced change is equal to the value of these pixels (the plurality of pixels related to the forced change), the auxiliary count is incremented in the process of step S26. Thereafter, it is confirmed in step S27 that the auxiliary count number is equal to or greater than the auxiliary upper limit number (as described above, the auxiliary count number is determined by the pixel number (Pn− Only when the pixel value is not equal to the pixel value of 1), it is changed to 1.) In the process of step S29, the pixel (that is, the pixel of the target pixel number Pn) is included in the plurality of pixels related to the forced change. It will be.

  Here, if the target image data (part) shown in FIG. 15 represents a part of a pattern for managing print quality, it is processed from the target image data without setting the auxiliary upper limit number. When acquired image data is acquired, the values of all the pixels in FIG. 15 may be changed to one value as shown in FIG. 16 (however, the upper limit number is assumed to be 10 or more). Also, even with the technique disclosed in Japanese Patent Application Laid-Open No. 2003-158635 (Patent Document 1), since the entire image is processed uniformly, the pixel value of a pattern for managing print quality such as a step chart is changed. .

  In contrast, in the present processing example, when pixels equal to or greater than the auxiliary upper limit number having the same value are continuously arranged in the line direction, these pixels are excluded from the specific pixel group, as shown in FIG. In the processed image data, the pixel value indicating the pattern for managing the print quality can be prevented from being changed (that is, the step chart is stored), and the print quality in the printing apparatus 12 can be appropriately set. It becomes possible to manage.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made.

  In the above embodiment, when determining whether or not to include the pixel of the pixel number of interest Pn in the specific pixel group, the average value of the pixels from the start pixel number to the pixel number (Pn−1) is used as the reference value. Although the evaluation value for the pixel of the target pixel number Pn is calculated, the evaluation value for the pixel of the target pixel number Pn may be calculated using the value of the pixel of the start pixel number as a reference value. It can be considered that a pixel value determination range is set in which the upper limit is a value obtained by adding a threshold value to the pixel value, and the lower limit is a value obtained by subtracting the threshold value from the pixel value. Also, refer to a representative value such as a median value or an average value in a predetermined area (a plurality of pixels lined up in the line direction or a plurality of pixels arrayed two-dimensionally) centered on the pixel of the target pixel number Pn. The absolute value of the difference between the pixel value of the target pixel number Pn and the reference value may be used as the evaluation value. In this case, a process for determining the pixels included in the specific pixel group (that is, In the repetition of the processing of steps S23 to S28, the range of pixel values having the upper limit value obtained by adding a threshold value to the maximum reference value and the lower limit value obtained by subtracting the threshold value from the minimum reference value is determined as a specific pixel group. Perceived as a range. As described above, the determination range of the pixel value of the specific pixel group is based on the value of at least one pixel of the specific pixel group.

  In the above embodiment, the values of all the pixels included in the specific pixel group are changed to the average value of all the pixels in the specific pixel group. As described above, the pixel having the target pixel number Pn When the evaluation value for is calculated based on the pixel value of the start pixel number, the values of all the pixels included in the specific pixel group are changed to the values of the pixel of the start pixel number (the first pixel of the specific pixel group) In addition, when the representative value in a predetermined area centered on the pixel of the target pixel number Pn is used as the reference value, the values of all the pixels included in the specific pixel group are all the pixels of the specific pixel group. May be changed to an average value of a plurality of reference values corresponding to. As described above, the values of all the pixels in the specific pixel group may be changed to any one value included in the pixel value determination range.

  By the way, when the median value or the like in a predetermined region centered on the pixel of the target pixel number Pn is used as a reference value as described above, if the upper limit number is not set, the value of the pixel is gradually reduced in the target image data. In the linearly changing portion, the evaluation value becomes a substantially constant value, so that the values of the first pixel and the last pixel in the same specific pixel group in the processed image data are greatly different, and the line Shading changes in direction are lost excessively. However, in practice, by setting the upper limit number, even in such a portion of the target image data, a specific pixel group is set for each pixel equal to or less than the upper limit number. An excessive loss of shading in direction is prevented.

  When a plurality of pixel value ranges obtained by dividing the entire pixel value range of the target image data into a plurality of values are set, in the above embodiment, the value of the first pixel of each specific pixel group is a value that represents the specific pixel group The upper limit number of the specific pixel group is determined based on the pixel value range to which the value of the first pixel belongs, but for example, the upper limit number is made larger than a certain number in all pixel value ranges. In this case, when the count in the process of step S26 for the pixel of the target pixel number Pn becomes a fixed number, the average value of the pixels from the start pixel number to the target pixel number Pn (that is, the specific pixel group) An average value of the values of some of the pixels) and an upper limit number may be determined from a specified range corresponding to the pixel value range to which the average value belongs (only one upper limit number for the pixel value range) Same for matching)As described above, the data processing unit 22 can specify the pixel value range using various values representing the determination range of each specific pixel group, and determine the upper limit number corresponding to the specific pixel group. .

  As described above, the processing in the data processing unit 22 may be realized by a dedicated electrical circuit. Since the data processing unit 22 performs processing by changing the pixel of interest in order of the pixel number, such an electric circuit can be designed relatively easily.

  In the data compression unit 23, if the compression rate for the processed image data is higher than the compression rate for the target image data, in addition to the run length method, other methods such as a lexicographic compression method and a predictive encoding method are used. Lossless compression may be used.

DESCRIPTION OF SYMBOLS 1 Printing system 2 Image processing part 11 Computer 12 Printing apparatus 22 Data processing part 23 Data compression part 24 Data transmission part 92 Program 121 Data reception part 122 Data expansion part B1, B2 Specific pixel group P0-P3, P5, P6, P9 pixel R1, R2 judgment range S12, S13 step

Claims (14)

An image processing apparatus that compresses image data used for variable printing,
In a two-dimensional array of pixels indicated by image data, a set of pixels equal to or less than a predetermined upper limit number continuously arranged in one array direction, and is determined based on the value of at least one pixel of the set of pixels A data processing unit that acquires processed image data by changing the value of all the pixels of a pixel group in which the values of all pixels are included in a determination range of pixel values to one value included in the determination range; ,
A data compression unit that performs reversible compression on the processed image data to obtain compressed image data;
An image processing apparatus comprising: The image processing apparatus according to claim 1,
The image processing apparatus, wherein the data processing unit changes the upper limit number irregularly. The image processing apparatus according to claim 2,
A range that the upper limit number can take for each of a plurality of pixel value ranges obtained by dividing the entire pixel value range of the image data into a plurality is predetermined,
The image processing apparatus, wherein the upper limit number corresponding to each pixel group is determined based on a pixel value range to which a value representative of the determination range of each pixel group belongs. The image processing apparatus according to claim 3,
The image processing apparatus according to claim 1, wherein a central value of a range that the upper limit number can take is smaller in a pixel value range in which a maximum pixel value is a highlight side value in the plurality of pixel value ranges. The image processing apparatus according to claim 2,
A range in which the upper limit number can be set for each of the plurality of spatial frequency ranges of the image data is determined in advance, and a central value of the range that the upper limit number can take as the maximum spatial frequency is higher in the plurality of spatial frequency ranges. Is smaller,
The upper limit number corresponding to each pixel group includes at least a part of each pixel group and a spatial frequency range to which a representative spatial frequency of a spatial frequency spectrum obtained from values of a plurality of pixels continuous in the one arrangement direction belongs An image processing apparatus characterized by being determined based on the above. The image processing apparatus according to claim 1,
The upper limit number is predetermined for each of a plurality of pixel value ranges obtained by dividing the entire pixel value range of the image data into a plurality,
The image processing apparatus, wherein the upper limit number corresponding to each pixel group is determined based on a pixel value range to which a value representative of the determination range of each pixel group belongs. The image processing apparatus according to claim 6,
The image processing apparatus, wherein the upper limit number is smaller in a pixel value range in which a maximum pixel value is a highlight side value in the plurality of pixel value ranges. The image processing apparatus according to claim 1,
In the plurality of spatial frequency ranges of the image data, the upper limit number that is smaller as the maximum spatial frequency is higher is predetermined,
The upper limit number corresponding to each pixel group includes at least a part of each pixel group and a spatial frequency range to which a representative spatial frequency of a spatial frequency spectrum obtained from values of a plurality of pixels continuous in the one arrangement direction belongs An image processing apparatus characterized by being determined based on the above. The image processing apparatus according to claim 1,
An image processing apparatus, wherein when a predetermined number or more pixels having the same value are continuously arranged in the one arrangement direction, the data processing unit excludes the predetermined number or more pixels from the pixel group. . A printing system that performs variable printing,
An image processing apparatus according to any one of claims 1 to 9,
A printing device for printing an image on a predetermined print medium;
With
The image processing apparatus further includes a data transmission unit that transmits the compressed image data to the printing apparatus,
The printing device is
A data receiver for receiving the compressed image data from the data transmitter;
A data decompression unit for decompressing the compressed image data;
A printing system comprising: An image processing method for compressing image data used for variable printing,
a) In a two-dimensional array of pixels indicated by image data, a set of pixels equal to or less than a predetermined upper limit number continuously arranged in one array direction, and determined based on the value of at least one pixel in the set of pixels A step of acquiring processed image data by changing the value of all the pixels of the pixel group in which the values of all pixels are included in the determination range of the pixel value to be one value included in the determination range; ,
b) applying reversible compression to the processed image data to obtain compressed image data;
An image processing method comprising: The image processing method according to claim 11, comprising:
In the step a), the upper limit number is irregularly changed. A program used for compression of image data used for variable printing, and execution of the program by a computer is performed on the computer,
a) In a two-dimensional array of pixels indicated by image data, a set of pixels equal to or less than a predetermined upper limit number continuously arranged in one array direction, and determined based on the value of at least one pixel in the set of pixels A step of acquiring processed image data by changing the value of all the pixels of the pixel group in which the values of all pixels are included in the determination range of the pixel value to be one value included in the determination range; ,
b) applying reversible compression to the processed image data to obtain compressed image data;
A program characterized by having executed. The program according to claim 13,
In the step a), the upper limit number is irregularly changed.