JP2015167307A - Printer, printing data generation device and method therefor, and printing data generation program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve printing of high quality by suppressing a singular dot pattern from being generated in image processing employing both a dither method and an error diffusion method in combination.SOLUTION: A data gradation value as a gradation value of input image is compared with one of a plurality of thresholds and when the data gradation value is equal to or larger than the threshold, a threshold for error diffusion method as a threshold used to determine whether dots are formed by an error diffusion method is set to a low threshold smaller than a high threshold set when the data gradation value is less than the threshold. The threshold for error diffusion method which is thus set is used to generate dot data representing whether dots are formed by the error diffusion method. At this time, a low threshold is determined according to the data gradation value, and has a downward convex singular part within a predetermined gradation range. Consequently, a singular dot pattern is prevented from being generated within the specific gradation range.

Description

  The present invention relates to a technique for printing using image data and a technique for generating printing data.

  For halftone processing that prints multi-tone image data using dot data with a small number of gradations, an error diffusion method that distributes density errors to the surrounding pixels when the number of gradations is converted, and good dispersibility A systematic dither method for generating dot data using a dither mask with a threshold arrangement is known. When using a dither mask with blue noise characteristics or a Bayer-type dither mask, the dispersibility of the dot arrangement is good, and data with relatively small gradation changes has a two-dimensional spread. Excellent image reproducibility. In addition, such a systematic dither method has an advantage that dot formation can be controlled by giving a specific characteristic to the arrangement of threshold values in the dither mask.

  For example, the technique disclosed in Patent Document 1 below independently creates a dither mask used for determination of dot formation at the time of forward movement when performing bidirectional printing and a dither mask used for determination of dot formation at the time of backward movement. . By imparting a blue noise characteristic to each dither mask, it is possible to perform printing with little image quality deterioration with respect to the deviation of the dot formation position between the two printing positions.

  The applicant has proposed a technique that takes advantage of both the dither method and the error diffusion method (see Patent Document 2 below). In this technique, the tone value of the pixel of interest is first compared with the threshold value constituting the dither mask, the threshold value used in the error diffusion method is made different depending on the comparison result, and the threshold value is set to the tone value of the pixel of interest. To change.

JP 2007-15359 A JP 2011-66594 A Japanese Patent No. 3360391

  This method is excellent in that it can control the degree to which the characteristics of the error diffusion method and the dither method are reflected in the dot data to be generated, and its application range is wide. The applicant has further improved this method, and it has been possible to further improve the quality of the printed image without generating a unique texture regardless of the gradation value. By using the method of the present invention, it is possible to solve the problem of generation of a specific texture due to the influence of the error diffusion method at a specific gradation value of image data. Further, in conventional image processing and printing, it has been desired to reduce the size of the apparatus, to reduce the cost, to save resources, to facilitate manufacturing, and to improve usability.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms.

(1) As one embodiment of the present invention, a printing apparatus that prints image data representing a predetermined image is provided. The printing apparatus uses an input unit that inputs the image data, a halftone processing unit that generates dot data indicating the presence or absence of dot formation based on the image data, and the generated dot data. A printing unit that prints the image. The halftone processing unit compares one of a plurality of threshold values with a data gradation value that is a gradation value of the input image data, and the data gradation value is converted into a dot in the error diffusion method. An error diffusion unit that generates the dot data by comparing with an error diffusion threshold value that is a threshold value used for determining the presence or absence of formation may be provided. Here, the threshold value for error diffusion method is a high threshold value set when the comparison result of the comparison unit is less than the threshold value when the data tone value is greater than or equal to the threshold value. Set to a lower low threshold. The lower threshold value may be determined according to the data gradation value, and may have a singular part protruding downward in a predetermined gradation range.

  In this printing apparatus, the low threshold value is determined according to the data gradation value, and has a singular part that protrudes downward in a predetermined gradation range, so that it is singular at a specific gradation by the error diffusion method. Generation of textures can be suppressed.

(2) In such a printing apparatus, the high threshold value may be determined according to the data gradation value, and may have a singular part that protrudes upward in the predetermined gradation range. In this way, the generation of unique texture can be further suppressed.

(3) The plurality of threshold values may be prepared as a dither mask used for a systematic dither method. In this way, when the influence of the dither method is strengthened, it is possible to obtain dot data having characteristics close to the generation of dot data by the systematic dither method.

(4) The dither mask may have a blue noise characteristic. In this way, it is possible to give a characteristic close to the blue noise characteristic to the dot data generated under the influence of the dither method.

(5) The lower threshold value may be set to a value less than the minimum value of the data gradation value when the data gradation value is equal to or less than a predetermined value. In this way, when the presence or absence of dot formation by the error diffusion method is determined using the lower threshold value, dots are always formed, and the characteristics of the dot data can be matched with the characteristics by the dither method.

(6) When the data tone value is equal to or greater than the threshold value in the predetermined range including the predetermined value, the halftone processing unit replaces the processing by the error diffusion unit. The dot data may be generated based on the comparison result of the comparison unit. In this way, part of the error diffusion process can be omitted, and the processing burden can be reduced.

(7) The singular part may include at least one gradation value having an integer ratio with respect to the maximum number of gradations of the data gradation value. This is because it is known that a peculiar texture by the error diffusion method is likely to occur in the vicinity of a gradation value having a relation of an integer ratio with respect to the maximum gradation number of the data gradation value.

(8) The present invention can be implemented as a print data generation device that generates print data for image data representing a predetermined image. Such a printing data generation apparatus may include an input unit that inputs the image data, and a halftone processing unit that generates dot data representing the presence or absence of dot formation based on the image data. The halftone processing unit compares one of a plurality of threshold values with a data gradation value that is a gradation value of the input image data, and the data gradation value is converted into a dot in the error diffusion method. An error diffusion unit that generates the dot data by comparing with an error diffusion threshold value that is a threshold value used for determining the presence or absence of formation may be provided. The threshold value for error diffusion method is smaller than a high threshold value set when the data gradation value is less than the threshold value when the comparison result of the comparison unit is greater than or equal to the threshold value. It may be set to a low threshold. The low threshold value may be determined according to the data gradation value, and may have a singular part protruding downward in a predetermined gradation range.

  In such a printing data generation apparatus, a low threshold is determined according to the data gradation value, and has a singular portion that protrudes downward in a predetermined gradation range. It is possible to generate data that suppresses the occurrence of a unique texture in the tone.

(9) The present invention can also be implemented as a method or a program. For example, it can be implemented as a method for generating print data of image data representing a predetermined image. In this method, the image data is input, one of a plurality of threshold values is compared with a data gradation value that is a gradation value of the input image data, and the data gradation value is determined by the comparison. If the threshold value is less than the threshold value, an error diffusion method threshold value, which is a threshold value used to determine whether or not to form dots in the error diffusion method, A threshold value may be set, and dot data representing the presence or absence of the dot formation may be generated by the error diffusion method using the set threshold value for the error diffusion method. Here, the lower threshold value may be determined according to the data gradation value, and may have a singular part that protrudes downward in a predetermined gradation range. Also by this printing data generation method, the same operational effects as those of the printing apparatus described above can be obtained.

(10) The present invention can also be implemented as a print data generation program for generating print data of image data representing a predetermined image. The program includes a function for inputting the image data, a function for comparing one of a plurality of threshold values with a data gradation value that is a gradation value of the input image data, and the data scale by the comparison. When the tone value is equal to or greater than the threshold value, an error diffusion method threshold value, which is a threshold value used to determine whether or not dots are formed in the error diffusion method, is set to a high level set when the data tone value is less than the threshold value. A function for setting a lower threshold value smaller than a threshold value and a function for generating dot data indicating the presence or absence of dot formation by the error diffusion method using the set error diffusion method threshold value are realized in a computer. You can let it. Here, the lower threshold value may be determined according to the data gradation value, and may have a singular part that protrudes downward in a predetermined gradation range. When such a program is executed by a computer, the same effects as the above method can be obtained.

  The present invention can also be realized in various forms other than the printing apparatus and the image data generation apparatus. For example, the present invention can be realized in the form of a printing apparatus manufacturing method, a printing apparatus control method, a computer program for realizing the control method, a non-temporary recording medium on which the computer program is recorded, and the like.

1 is a schematic configuration diagram illustrating an image processing apparatus according to an embodiment. 4 is a flowchart showing a flow of printing processing in the printer 20. 6 is a flowchart illustrating halftone processing according to the first embodiment. Explanatory drawing which illustrates the dither mask 61 used in the embodiment. Explanatory drawing which illustrates the noise characteristic with which the dither mask 61 used in the embodiment is provided. The graph which shows the relationship between high frequency side threshold value THe_H and low frequency side threshold value THe_L in 1st Embodiment, and attention pixel data Dn. Explanatory drawing which illustrates error diffusion range and weighting. The graph which shows the granularity of the dot data produced | generated by 1st Embodiment. The graph which shows an example of the high threshold value THe_ and the low threshold value THe_L in a comparative example. The graph which shows the granularity of the dot data in a comparative example. 9 is a flowchart illustrating halftone processing according to the second embodiment.

A. First embodiment:
A first embodiment of the present invention will be described.
A-1. Device configuration:
FIG. 1 is a schematic configuration diagram of a printer 20 as a first embodiment of a printing apparatus according to the present invention. The printer 20 is a so-called line printer, and is an ink jet printer that uses four color inks described later. As shown in the figure, the printer 20 drives a paper feed roller 75 by a paper feed motor 74 and conveys the print medium P, and a print head 90 provided at a position facing the print medium P to drive ink. And a mechanism for performing dot formation and dot formation. Further, the printer 20 includes a control unit 30 that controls exchange of signals with the paper feed motor 74, the print head 90, and the operation panel 99. In this embodiment, the paper feed roller 75 also serves as a platen, but the platen may be separated from the paper feed roller. In this case, a flat platen with a flat surface may be used. Further, the paper feed roller 75 may be provided upstream and downstream of the print head 90, respectively.

  The print head 90 also receives a number of nozzles Nz capable of ejecting cyan ink C, magenta ink M, yellow ink Y, and black ink K as color inks across the width of the print medium P. Each of the plurality of nozzles is provided with a piezoelectric element (not shown) as an actuator. The piezo element is driven by a data signal DD corresponding to dot data and a drive signal DS. The actuator that ejects ink from the nozzle Nz is not limited to a piezo element, and various configurations such as a heater type that ejects ink by using the bumping of ink, and those that use a laser are adopted. Can do. Of course, the formation of ink dots is not limited to ink-jet, but thermal transfer or thermal sublimation using an ink ribbon, a method of forming a latent image on a photosensitive drum, or a print head while reciprocating in the width direction of a print medium. Various systems such as a serial printer that ejects ink from nozzles can be employed.

  Each color ink is supplied to the print head 90 from the ink cartridges 82 to 85 for the color inks containing the respective color inks via ink supply pipes 92 to 95. As the ink color, in addition to the above-described CMYK, light cyan ink Lc, light magenta ink Lm, or the like may be used. Of course, special color inks such as red, blue, and green, or so-called metallic inks such as gold and pearl white may be used. Further, it may be provided with an ink system for monochrome printing.

  The control unit 30 includes a CPU 40, a ROM 51, a RAM 52, and an EEPROM 60, and these are connected to each other via a bus. The control unit 30 develops a program stored in the ROM 51 or the EEPROM 60 in the RAM 52 and executes it to control the overall operation of the printer 20, and also functions as the input unit 41, halftone processing unit 42, and printing unit 46. To do. The functions of the halftone processing unit 42 include functions as a comparison unit 43 and an error diffusion unit 44.

  The printing unit 46 is a circuit for driving the print head 90, and outputs a signal DD corresponding to dot data and a drive signal DS for driving a plurality of piezo elements at a time to the print head 90. Piezo elements are grouped for each color of CMYK, and are driven for each group by a signal DD corresponding to dot data held in a latch (not shown) and a drive signal DS output at a predetermined timing. . When the signal DD is on (dot data is 1) and the drive signal DS is given, the piezo element expands, pressurizes ink in an ink chamber (not shown), and ejects ink droplets from the nozzle Nz. . Since the printer 20 of the present embodiment is a line printer, the color nozzles are arranged with a predetermined pitch shifted in the feeding direction of the print medium P. In addition, the nozzles for the same color ink are also arranged in a so-called staggered arrangement in which every other nozzle is displaced in the print medium feeding direction in order to increase the resolution in the print medium width direction. Accordingly, the nozzle drive timings when dots are formed at the same position in the feeding direction of the print medium P are different. For this reason, as will be described later, rearrangement processing is performed in which the dot data obtained by processing the gradation data of the image to be formed is adapted to the nozzle arrangement. Details of processing of each functional unit including processing of the printing unit will be described later with reference to the flowcharts of FIGS.

  In the EEPROM 60, a dither mask 61 and an error diffusion threshold table 62 are stored. The dither mask 61 is used in halftone processing to be described later, and has a size of horizontal (x: print medium width direction) 256 × vertical (y: print medium feed direction) 64, as partly illustrated in FIG. Have The dither mask 61 has a plurality of threshold values THn_d arranged. This threshold value THn_d takes a value from 1 to 255 in this embodiment. Each threshold value THn_d is arranged so that the spatial frequency of dots formed by comparison with this threshold value has a so-called blue noise characteristic.

  FIG. 5 is an explanatory diagram illustrating the noise characteristics provided in such a dither mask 61. The figure conceptually illustrates the spatial frequency characteristics of the threshold values set for each pixel of the dither mask having blue noise characteristics and green noise characteristics. The blue noise characteristic in the dither mask has the largest frequency component in the high frequency region where the length of one cycle is near two pixels. This means that the storage position of the threshold is adjusted so that the largest frequency component is generated in the high frequency region in consideration of the human visual characteristic that the sensitivity is low in the high frequency region. When dots are generated using a dither mask having such blue noise characteristics, an image having excellent dot dispersibility can be obtained.

  FIG. 5 further illustrates the green noise characteristic as a dashed curve. As shown in the figure, the green noise characteristic has the largest frequency component slightly on the low frequency side than the blue noise characteristic, and if the pixel size is sufficiently small, a good image that does not feel graininess even with the green noise characteristic is shown. Is obtained. The dither mask 61 has predetermined spatial frequency characteristics such as blue noise characteristics and green noise characteristics.

  Furthermore, in this embodiment, the dither mask 61 has the following dot formation characteristics. That is, both the dot pattern of the dot group formed by the forward movement of the carriage 80 in the bidirectional printing, the dot pattern of the dot group formed by the backward movement, and the dot pattern of the entire dot group that combines them, It has characteristics close to blue noise characteristics. Such a technique is described in, for example, Patent Document 1 and Japanese Patent Application Laid-Open No. 2007-15359. Note that the size and characteristics of the dither mask 61 are arbitrary, and those having a size and characteristics other than the present embodiment can be adopted. For example, it may have a size of 64 × 32 or more in order to realize a systematic dither method, or may be a dot concentration type dither mask that realizes characteristics close to halftone dots.

  The error diffusion threshold value table 62 stored in the EEPROM 60 is a table in which a high threshold value and a low threshold value used for determination of dot ON / OFF in the error diffusion method are stored. The role of these threshold values will be described in detail later.

  A memory card slot 98 is connected to the control unit 30, and image data ORG can be read and input from the memory card MC inserted into the memory card slot 98. In this embodiment, the image data ORG input from the memory card MC is data composed of three color components of red (R), green (G), and blue (B).

  The printer 20 having the hardware configuration as described above drives the print head 90 by moving the print medium P in the feed direction by driving the paper feed motor 74, so that each color ink dot is printed on the print medium. Form on P. The control unit 30 forms ink dots of appropriate colors at appropriate positions on the print medium P by driving the nozzles at appropriate timing based on the print data in accordance with the paper feed of the print medium P. By doing so, the printer 20 can print the color image input from the memory card MC on the print medium P.

A-2. Printing process:
A printing process in the printer 20 will be described. FIG. 2 is a flowchart showing the flow of printing processing in the printer 20. The printing process here is started when the user performs an instruction to print a predetermined image stored in the memory card MC using the operation panel 99 or the like. When printing processing is started, the CPU 40 first reads and inputs RGB format image data ORG to be printed from the memory card MC via the memory card slot 98 as processing of the input unit 41 (step S110).

  When the image data ORG is input, the CPU 40 refers to a look-up table (not shown) stored in the EEPROM 60 and performs color conversion from RGB format to CMYKLcLm format for the image data ORG (step S120).

  When the color conversion process is performed, the CPU 40 performs a halftone process in which the image data is converted into dot data in which ON / OFF of dots of each color is determined for each pixel as a process of the halftone processing unit 42 (step S130). Details of the halftone process will be described later. In this specification, “halftone processing” is not limited to dot ON / OFF binarization processing, but is a gradation including multi-value processing such as ON / OFF of large and small dots, large, medium, and small dots. This means general number conversion (reduction) processing. Further, the image data provided to step S130 may be subjected to image processing such as resolution conversion processing or smoothing processing.

  When the halftone process is performed, the CPU 40 performs a rearrangement process for rearranging the dot pattern data for simultaneously driving the nozzles of the print head 90 in accordance with the nozzle arrangement of the printer 20 and the paper feed amount (step S150). As described above, the rearrangement process is a process of rearranging the dot data obtained by the halftone process (step S130) according to the arrangement of the nozzles Nz in the print head 90. After performing the rearrangement process (step S150), the CPU 40 drives the print head 90, the motor 74, etc. as a process of the printing unit 46, and executes printing (step S160).

A-3. Details of halftoning:
Details of the above-described halftone process (step S130) will be described with reference to FIG. As shown in the figure, when this process is started, the CPU 40 first sets the coordinate data n (x, y) of the target pixel position and the target pixel data Dn for the image data subjected to the color conversion process in step S120. Are acquired (step S131). The target pixel position starts from the origin (upper left) of the image, and moves one pixel at a time in the main scanning direction (x direction) each time the following processing is repeated. When the target pixel position reaches the right end in the main scanning direction of the image, it moves by one in the sub-scanning direction (y direction), and again moves from the left end in the x direction in the main scanning direction. In the following description, the coordinate data of the target pixel position is expressed as n (x, y), but when used as a subscript indicating the target pixel position, it is expressed directly as (x, y).

  When the coordinate data n (x, y) of the target pixel position and the target pixel data Dn are acquired, the CPU 40 performs a temporary dither process as the process of the comparison unit 43 (step S132). The temporary dither processing here is a threshold value corresponding to the coordinate (x, y) of the target pixel among the plurality of threshold values constituting the dither mask 61 stored in the EEPROM 60 and the gradation value of the target pixel data Dn. This is a process of comparing the magnitude relationship with the value of THn_d. This processing is formally the same as the processing for determining dot ON / OFF by the dither method that is normally performed. In the normal dither method, when the gradation value of the target pixel data Dn is equal to or greater than the threshold value THn_d, it is determined that the dot is turned on, and the gradation value of the target pixel data Dn is less than the threshold value THn_d. In this case, it is determined that the dot is turned off. On the other hand, the temporary dither processing of the present embodiment is a pre-processing for determining dot ON / OFF by an error diffusion method described later, specifically, a process for determining a threshold value of the error diffusion method. There are some differences.

  If the gradation value of the target pixel data Dn is greater than or equal to the threshold value THn_d as a result of the temporary dither processing (step S132: YES), the threshold value THe used for the error diffusion method is set to the lower threshold value THe_L (step S133). On the other hand, if the gradation value of the target pixel data Dn is less than the threshold value THn_d (step S132: NO), the threshold value THe used for the error diffusion method is set to the high threshold value THe_H (step S134). As described above, in this embodiment, the threshold value THe used in the error diffusion method (hereinafter also simply referred to as the threshold value THe) is changed based on the result of the temporary dither process. The threshold THe is set with reference to the error diffusion threshold table 62 stored in the EEPROM 60.

  An example of the error diffusion threshold table 62 is shown as a graph in FIG. As shown in the figure, in the error diffusion threshold table 62, the pixel-of-interest data Dn (here, 0 to 255) is associated with the lower threshold THE_L and the higher threshold THe_H. In the example shown in FIG. 6, the low threshold value THe_L is set to a value smaller than the high threshold value THe_H, and the gradation values 51 and 170 are minimal values, and have a convex relationship downward in a predetermined gradation range before and after that. . The downwardly convex part corresponds to a singular part. On the other hand, the high threshold THe_H has a characteristic of increasing as a whole in accordance with the gradation value of the target pixel data Dn. However, the gradation values 51 and 170 are set to the maximum value and protrude upward in a predetermined gradation range before and after that. With the relationship. The significance of the low threshold value THe_L and the high threshold value THe_H having the characteristics shown in FIG. 6 will be described later through a comparison with a comparative example.

  The CPU 40 refers to the error diffusion threshold table 62, acquires the high threshold THe_H or the low threshold THe_L, and uses it for the setting in step S133 or S134. In this embodiment, the high threshold THe_H and the low threshold THe_L corresponding to the gradation value are set by referring to the error diffusion threshold table 62, but it may be obtained by a function.

  In step S133 or S134 of FIG. 3 described above, when the threshold value THe is set with reference to the error diffusion threshold value table 62, the CPU 40 sets the diffusion error stored in the separately prepared error buffer to the gradation value of the target pixel data Dn. Edn is added (step S135). The diffusion error Edn is calculated in step S139 described later, and the content thereof will be described later.

  When the diffusion error Edn is added to the gradation value of the pixel-of-interest data Dn, the CPU 40 sets the gradation value of the pixel-of-interest data Dn added with the diffusion error Edn (hereinafter also referred to as correction data) in step S133 or step S134. The threshold value THe is compared (step S136). As a result, if the correction data (Dn + Edn) is greater than or equal to the threshold value THe (step S136: YES), the dot data of the pixel of interest is determined to be ON (form a dot) (step S137). On the other hand, if the gradation value of the target pixel data Dn to which the diffusion error Edn is added is less than the threshold value THe (step S136: NO), the dot data of the target pixel is determined to be OFF (no dot is formed) (step S138). .

When the dot ON / OFF is determined, the CPU 40 calculates a binarization error En and a diffusion error Edn (step S139). The binarization error En is a difference between the correction data and the gradation value RSLT (here, value 255 or 0) realized as a result of dot ON / OFF. If expressed by an equation, it is expressed as the following equation (1).
En = {Dn (x, y) + Edn (x, y)} − RSLT (255 or 0) (1)
Generally, if no dot is formed, the binarization error En is a positive value, and if a dot is formed, the binarization error is a negative value.

As a result, if dots are formed by the binarization process by the error diffusion process described below, dots are less likely to be formed in the surrounding pixels, and if dots are not formed, dots are not formed in the surrounding pixels. It becomes easier to form. Error diffusion is a process of obtaining a diffusion error Edn by the following equation (2) and allocating errors generated in the pixel of interest to surrounding pixels. The distributed errors are accumulated and added to the gradation value of the target pixel data Dn in step S135. In the present embodiment, as shown as (A) in FIG. 7, the binarization error En is distributed to four pixels that are peripheral pixels whose dot ON / OFF is undetermined. That is, 7/16 for the pixel right next to the target pixel, 3/16 for the lower left pixel, 5/16 for the lower pixel, and 1/16 for the lower right pixel, The diffusion error Edn was distributed. The diffusion error Edn calculated in this way is stored in an error buffer prepared in the RAM 52.
Edn (x + 1, Y) = Edn (x + 1, y) + En × (7/16)
Edn (x-1, Y + 1) = Edn (x-1, y + 1) + En * (3/16)
Edn (x, Y + 1) = Edn (x, y + 1) + En × (5/16)
Edn (x + 1, Y + 1) = Edn (x + 1, y + 1) + En × (1/16)
(2)

  The processing in steps S135 to S139 is halftone processing by an error diffusion method, and is executed as processing by the error diffusion unit 44. Since the error diffusion method is a well-known technique, a detailed description thereof is omitted, but each image data and a predetermined threshold are added while adding the quantization error of each image data to the surrounding image data at a predetermined distribution ratio. And quantizing each image data. In the above example, steps S135 to S139 are binarization processing that determines only ON / OFF of dots, but even if multilevel processing such as determination of ON / OFF of large dots and small dots is performed. Good.

  After calculating the binarization error En and the diffusion error Edn, the CPU 40 determines whether or not the above processing has been completed for all the pixels (step S140), and the pixel position of interest (x , Y) is incremented, and the processes in steps S131 to S139 are repeated. Thus, the halftone process in step S130 is completed.

A-4. Effects of the first embodiment:
The effect of performing such halftone processing will be described below. As described above, in the processing of steps S132 to S134, if the gradation value of the target pixel data Dn is equal to or greater than the threshold value THn_d, the threshold value THe used for the error diffusion method is set to the lower threshold value THe_L. On the other hand, if the tone value of the target pixel data Dn is less than the threshold value THn_d, the threshold value THe is set to the high threshold value THe_H.

  As a result, it is understood that the determination result using the dither mask 61 strongly reflects the dot data generation result by the error diffusion method. In particular, since the high threshold THe_H and the low threshold THe_L are determined as shown in FIG. 6, dot formation is performed as follows.

(1) When the high threshold value THe_H or the low threshold value THe_L is set as the error diffusion threshold value THe by the temporary dither determination, the determination in step S136 in FIG. Since this is the same as step S132), dots are formed with a strong correlation with the temporary dither determination result. That is, the dot data has dither mask characteristics.
(2) Since the dot formation result (diffusion error Edn) in the surrounding pixels is added to the pixel data of the target pixel by the error diffusion method, the provisional dither determination result in step S132 and the error diffusion method in step S136 The result does not necessarily agree with the result of determination, and some dots are not formed or formed by the error diffusion method so that the density of the formed image matches the density of the original image. Corrections are made. For this reason, the dot formation correction by the error diffusion method is appropriately performed based on the dot formation (1) reflecting the characteristics of the dither mask.

(3) Furthermore, in the first embodiment, as shown in FIG. 6, the low threshold value THe_L is given a characteristic that has a downward convex shape with the gradation values 51 and 170 being the minimum values. Yes. That is, in the vicinity of the gradation values 51 and 170, the low threshold value THe_L is set to a low value with respect to the overall characteristics, and in the vicinity of the gradation values in view of the ratio of dot formation according to the above (1) and (2). Then, when the determination result of the temporary dither is “YES”, it is easier to form dots even if the influence of error diffusion is taken into consideration. In addition, since the reverse characteristic is given to the high threshold value THe_H in accordance with such a characteristic of the low threshold value THe_L, when the determination result of the temporary dither becomes “NO”, the influence of error diffusion is taken into consideration. Further, dots are more difficult to be formed.

  In the first embodiment, by using the low threshold THe_L and the high threshold THe_H having the characteristics shown in FIG. 6, as shown in FIG. 8, the graininess is extended over a wide gradation range of an image to be used for printing. Printing with excellent image quality. FIG. 8 is a graph in which the graininess is plotted when there is a deviation in the nozzle pitch of the print head 90 and when there is no deviation (when precisely aligned). As shown in the figure, in the first embodiment, it can be seen that the graininess is sufficiently suppressed over a wide gradation range whether or not there is a shift.

  On the other hand, FIG. 9 shows an example in which the lower threshold THe_L and the higher threshold THe_H are set so as not to have extreme values near the gradation values 51 and 170. When the halftone processing shown in FIG. 3 is performed using the low threshold THe_L and the high threshold THe_H, the granularity of the image obtained in that case is as shown in FIG. In the example shown in FIG. 10, when there is a deviation in the nozzle arrangement of the print head 90, the graininess is greatly deteriorated in the vicinity of the gradation values 51 and 170 of the image. Therefore, when this point was examined in more detail, it was found that a unique dot pattern was generated in the vicinity of the gradation values 51 and 170, and the graininess deteriorated. The peculiar dot pattern is a pattern that is generated because a certain degree of regularity is generated in the generation of dots due to density error (Edn) when a uniform image having the gradation value 51 or the gradation value 170 is printed.

Such a peculiar dot pattern is, as in the example shown in FIG. 10, an integral number of the maximum value of gradation values (the gradation value is 255 here because the image data is expressed by 8 bits), or It is empirically known that it is likely to occur in the vicinity of a simple integer ratio gradation. In the example shown in FIG.
51 = 255/5
170 = 255 × (2/3)
It is. Of course, the gradation values at which such a unique pattern is generated are not limited to this example, but the gradation range that the image data can take, the error diffusion range and its ratio (see FIG. 7), the low threshold THe_L, and the high threshold THEe_H. It depends on characteristics. Therefore, when the printer 20 first performs a predetermined setting, prints a peta pattern of each gradation value, measures its graininess, and finds a singular point where the graininess is collapsed due to the occurrence of a unique pattern The low threshold value THe_L and the high threshold value THe_H may be corrected so that the gradation value or a gradation value in the vicinity thereof is used as a boundary.

  In this way, when the low threshold THe_L and the high threshold THe_H as shown in FIG. 6 are determined and the image is printed, the generation of a unique dot pattern can be suppressed. At this time, the halftone process shown in FIG. 3 does not make any judgment as to whether the object being processed is a diagram, a character, or a natural image such as a photographic image. The same halftone process (FIG. 3) is performed even if the area of the diagram is shifted to the area of the natural image as it is, or a line drawing or character exists in the natural image. As a result, it is possible to form a high-quality image without occurrence of a peculiar dot pattern in either a line drawing or character area or a natural image area.

  In the present embodiment, as a threshold value (high threshold value THe_H) used in the error diffusion method, as shown in FIG. 6, a value that increases according to the gradation value of the pixel data of interest is used. As described in 3, the phenomenon such as tailing does not occur. Furthermore, if the error diffusion range shown in FIG. 7 is switched according to the gradation value of the pixel data of interest, further improvement in image quality can be expected. Since the technique of performing error diffusion while switching the diffusion range is a known technique, a detailed description is omitted. However, when the error diffusion range is switched according to the combination of the input gradation value and the binarization result, various effects are obtained. Play. For example, if error diffusion is performed over a wide range only when a dot is turned on at a low gradation value, the granularity of the low gradation region can be improved and the occurrence of undesired continuation of dots, so-called worms can be suppressed. .

A-5. Variation:
In the first embodiment, the high threshold THe_H and the low threshold THe_L are set as shown in FIG. 6, but it goes without saying that the setting values of both are not limited thereto. The lower threshold value THe_L may be set to a smaller value, or may be set to a value smaller than the minimum value of the gradation range that can be taken by the pixel data depending on the case. Further, there may be one region having three or more downward convex characteristics, and there may be three or more regions. Similarly, there may be one region or three or more regions having upwardly convex characteristics in the high threshold THe_H. Note that tone value ranges having convex characteristics below the lower threshold value THe_L and the higher threshold value THe_H usually correspond, but it is not always necessary to have the same tone range, and even if they are completely deviated. There is no problem.

  In the first embodiment, the error diffusion range by error diffusion is four pixels around the pixel of interest as shown in FIG. 7, but it may be wider or narrower than this. The determination of diffusion may be changed according to the tone value of the target pixel. For example, it is also preferable to improve the granularity in the low gradation region by decreasing the gradation value so that the diffusion is performed over a wide range. Further, as shown in FIG. 7B, it is also desirable to set the sum of the weights of differences for error diffusion to a value of 1 or less. In the example shown in FIG. 7B, the density error generated in the pixel of interest is diffused to the surrounding four pixels at a ratio of 6/16, 2/16, 4/16, 1/16, and the sum is The generated error is 13/16. As a result, the influence of density errors caused by error diffusion on surrounding pixels is alleviated.

B. Second embodiment:
Next, a second embodiment of the present invention will be described. The printer 20 of the second embodiment has the same hardware configuration as that of the first embodiment, and executes the same printing process (FIG. 2). The printer 20 of the second embodiment is different from the first embodiment in the halftone process. FIG. 11 shows halftone processing executed by the printer 20 of the second embodiment. The halftone process executed by the printer 20 of the second embodiment is different from the first embodiment only in steps S431 and S432 described below, and the other steps S433 to S440 are the same as steps S133 to S133 of the first embodiment. This is the same as S140.

  In the second embodiment, when halftone processing is started, first, the CPU 40 acquires the coordinate data n (x, y) of the target pixel position and the target pixel data Dn, and sets the gradation value of the target pixel data Dn as a predetermined value. The value Dn ′ obtained by multiplying the coefficient αdr (0 <αdr ≦ 1) is obtained (step S431). Since the data calculated in this way is a gradation value related to the gradation value of the target pixel data Dn, it is also referred to as related data Dn ′ (Dn ′ = Dn × αdr). In this embodiment, the coefficient α = 0.9.

  When the related data Dn ′ is calculated, the CPU 40 performs a temporary dither process as the process of the comparison unit 43 (step S432). The difference from the temporary dither processing in step S132 shown in FIG. 3 is that instead of comparing the gradation value of the target pixel data Dn and the threshold value THn_d of the dither mask 61, the related data Dn ′ and the threshold value THn_d are compared. Is a point.

  As a result, if the related data Dn ′ is equal to or greater than the threshold value THn_d (step S432: YES), the threshold value THe used for the error diffusion method is set to the lower threshold value THe_L (step S433). On the other hand, if the related data Dn ′ is less than the threshold THn_d (step S432: NO), the threshold THe used for the error diffusion method is set to the high threshold THe_H (step S434). Set at this time. As the high threshold THe_H and the low threshold THe_L, the same thresholds as illustrated in FIG. 6 of the first embodiment are used. Further, the subsequent error diffusion processing (steps S435 to S439) is the same as that of the first embodiment, and thus the description thereof is omitted. Note that the error diffusion method dot ON / OFF determination is performed using the gradation value of the target pixel data Dn, not the related data Dn ′.

  In the present embodiment, since the coefficient αdr is 0.9, the ratio at which it is determined that a dot is generated by the temporary dither determination is suppressed to 90% of the normal determination described in the first embodiment and the like. As a result, the deficient 10% is generated by the determination by the error diffusion method (step S436). In this way, it is possible to arbitrarily set the ratio between the dots generated by the dither determination and the dots generated by the error diffusion method complementing the dot. In addition, as shown in FIG. 6, the low threshold THe_L and the high threshold THe_H have a downwardly convex characteristic in a predetermined gradation range, so that a unique dot pattern is generated in a specific gradation range. It does not occur. For this reason, as shown in the first embodiment, line drawings, characters, or natural images can be printed with high quality.

  In the second embodiment, since the coefficient αdr can be arbitrarily set between 0 and 1.0, the sum of the dots generated by the dither determination and the dots generated by the error diffusion method should be set appropriately. Is possible. For example, if the coefficient αdr = 1, it is the same as in the first embodiment, but dots by the error diffusion method are generated in addition to the dots generated by the dither determination. Therefore, in this case, there are cases where dots are formed slightly more than the original image density. On the other hand, if the coefficient αdr is set to a value slightly smaller than 1.0, such as 0.9 as in the second embodiment, the number of dots generated by the provisional dither determination is slightly less. The necessary amount can be generated by judgment based on the error diffusion method. It is possible to make the gradation expressed by the dots generated in total more closely match the gradation of the original image.

  Furthermore, if the coefficient αdr is a function of the target pixel data Dn, it is possible to set the ratio of dots generated by dithering to an arbitrary ratio according to the gradation value of the image. If the default value of the coefficient αdr is set to 1 and there is a gradation value for which it is determined that the occurrence of dots is high, if the coefficient αdr is slightly reduced in the vicinity of the gradation of the pixel data of interest, dot generation will occur. The ratio can be made more uniform. In the case where the coefficient αdr is changed according to the gradation value, it is more preferable to gradually increase or decrease the coefficient αdr so that the dot generation rate changes smoothly.

C. Variation:
C-1. Modification 1:
In the above embodiment, a blue noise mask similar in characteristics to error diffusion is used as the dither mask. However, a dot dispersion type systematic dither having a regular pattern such as a Bayer type dither may be used. In this case, the biggest problem with Bayer-type dithering that “lines may disappear” can be solved. In addition, even when dot-concentrated dither such as halftone dot dither or green noise mask is used, problems such as broken lines at halftone dot pitches can be solved and useful. The normal part with a two-dimensional spread is a dither pattern such as a Bayer, halftone dot, green noise mask, etc., but the error diffusion method works in the fine line part, and it can be reproduced without the fine line disappearing or being divided. This is because a tone can be realized.

C-2. Modification 2:
In the second embodiment, when the coefficient αdr multiplied by the pixel-of-interest data Dn is used as a function of the pixel-of-interest data Dn, the coefficient αdr may be obtained by referring to a lookup table. In this way, the coefficient αdr can be set freely. In addition to changing according to the gradation value of the image, the type of image (line image or natural image, etc.) may be determined and changed. The coefficient αdr may be set for each ink color. In this way, in color printing, the ratio of dots generated by dithering and dots generated by error diffusion can be changed for each ink. In addition, in printers that can generate multiple types of ink droplets, such as large, medium, and small, if a configuration that uses halftone processing after determining the occurrence rate of each type of dot from the gradation value using a lookup table, etc. is used for each type of dot The coefficient αdr may be set.

C-3. Modification 3:
When applying the error diffusion method, the error diffusion range may be changed depending on the target pixel data Dn as appropriate, for example, depending on the judgment result of the temporary dither. Alternatively, the result value RSLT when the dot is formed may be changed according to the determination result of the temporary dither. If the result value RSLT when the dot is formed is set to a value larger than 255 when it is determined that the dot is ON based on the determination result of the temporary dither, the distributed diffusion error increases, and the generation of dots in the vicinity Is suppressed, and the total number of dots can be prevented from becoming excessive.

C-4. Modification 4:
In the above-described embodiment, at least the lower threshold value THe_L has a downward convex characteristic with respect to a specific gradation range. In addition, in this specific gradation range, the error diffusion gradation value THe or the pixel Noise may be added to the data Dn. This is because it is possible to make it difficult to generate a specific dot pattern by adding noise. The magnitude of this noise is set to a value depending on the gradation value in accordance with the convex characteristic below the low threshold THe_L, and the low threshold THe_L has a convex characteristic below as illustrated in FIG. It may be a characteristic that does not. This is because adding noise having such characteristics is substantially equivalent to imparting a downward convex characteristic to the lower threshold THe_L in a predetermined gradation range.

C-5. Modification 5:
In the above-described embodiment, the line printer type inkjet printer 20 is used as the printing apparatus. However, other types of printers, such as serial printers and laser printers that form dots while the print head moves in the width direction of the print medium, are used. It may be realized as a page printer. Further, the printer is not limited to a color printer, and may be realized as a printer for monochrome printing. Furthermore, the present invention is not limited to the ink jet type, and can be applied to various types of printers such as a thermal sublimation type printer and a dot impact type. In an ink jet printer, ink ejection by an electrostrictive element such as a piezo, ink ejection by using a heater (so-called bubble jet (registered trademark) system), or the like can be employed.

  Further, the present invention may be applied to an image processing apparatus that performs only image processing. The halftone processing illustrated in FIGS. 3 and 11 may be realized as a dedicated application program executed by a computer, or may be performed in a dedicated device such as RiP. Alternatively, a computer and a printer may be connected and used, and part or all of the image processing may be executed within the printer driver. Furthermore, a dedicated server for performing such image processing may be placed on the network, and the image data may be processed in response to a request from another computer or printer.

  The present invention is not limited to the above-described embodiments, examples, and modifications, and can be realized with various configurations without departing from the spirit thereof. For example, the technical features in the embodiments, examples, and modifications corresponding to the technical features in each embodiment described in the summary section of the invention are to solve some or all of the above-described problems, or In order to achieve part or all of the above-described effects, replacement or combination can be performed as appropriate. Further, if the technical feature is not described as essential in the present specification, it can be deleted as appropriate.

20 ... Printer 30 ... Control unit 40 ... CPU
DESCRIPTION OF SYMBOLS 41 ... Input part 42 ... Halftone processing part 43 ... Comparison part 44 ... Error diffusion part 46 ... Printing part 51 ... ROM
52 ... RAM
60 ... EEPROM
61 ... Dither mask 62 ... Error diffusion threshold table 74 ... Paper feed motor 75 ... Platen 82-85 ... Ink cartridge 90 ... Print head 92-95 ... Supply pipe 98 ... Memory card slot 99 ... Operation panel P ... Printing medium MC ... Memory card

Claims (10)

A printing apparatus for printing image data representing a predetermined image,
An input unit for inputting the image data;
A halftone processing unit for generating dot data representing the presence or absence of dot formation based on the image data;
A printing unit that prints the image using the generated dot data; and
The halftone processing unit
A comparison unit that compares one of a plurality of threshold values with a data gradation value that is a gradation value of the input image data;
An error diffusion unit that generates the dot data by comparing the data gradation value with a threshold value for error diffusion method, which is a threshold value used to determine whether or not dots are formed in the error diffusion method,
The threshold value for error diffusion method is smaller than a high threshold value set when the data gradation value is less than the threshold value when the comparison result of the comparison unit is greater than or equal to the threshold value. Set to the low threshold,
The lower threshold is determined according to the data gradation value, and has a singular portion that protrudes downward in a predetermined gradation range.   The printing apparatus according to claim 1, wherein the high threshold value is determined according to the data gradation value, and has a singular portion that is convex upward in the predetermined gradation range.   The printing apparatus according to claim 1, wherein the plurality of threshold values are prepared as a dither mask used for a systematic dither method.   The printing apparatus according to claim 1, wherein the dither mask has a blue noise characteristic.   The printing apparatus according to any one of claims 1 to 4, wherein the singular part is present in a plurality of locations in a gradation range that the data gradation value can take.   The printing according to any one of claims 1 to 5, wherein the low threshold is set to a value less than a minimum value of the data gradation value when the data gradation value is equal to or less than a predetermined value. apparatus.   The printing apparatus according to claim 1, wherein the singular part includes at least one gradation value having an integer ratio relationship with respect to the maximum number of gradations of the data gradation value. A print data generation device that generates print data of image data representing a predetermined image,
An input unit for inputting the image data;
A halftone processing unit for generating dot data representing the presence or absence of dot formation based on the image data;
With
The halftone processing unit
A comparison unit that compares one of a plurality of threshold values with a data gradation value that is a gradation value of the input image data;
An error diffusion unit that generates the dot data by comparing the data gradation value with a threshold value for error diffusion method, which is a threshold value used to determine whether or not dots are formed in the error diffusion method,
The threshold value for error diffusion method is smaller than a high threshold value set when the data gradation value is less than the threshold value when the comparison result of the comparison unit is greater than or equal to the threshold value. Set to the low threshold,
The low threshold value is determined according to the data gradation value, and has a singular part that protrudes downward in a predetermined gradation range. A method for generating print data of image data representing a predetermined image,
Input the image data,
Comparing one of a plurality of threshold values with a data gradation value which is a gradation value of the input image data;
As a result of the comparison, when the data tone value is equal to or greater than the threshold value, an error diffusion method threshold value, which is a threshold value used to determine whether or not to form dots in the error diffusion method, is set, and the data tone value is less than the threshold value. Set to a lower threshold that is lower than the higher threshold set in some cases,
Using the set error diffusion method threshold, the error diffusion method generates dot data indicating the presence or absence of the dot formation,
The low threshold value is determined in accordance with the data gradation value, and the printing data generation method has a singular part protruding downward in a predetermined gradation range. A print data generation program for generating print data of image data representing a predetermined image,
A function of inputting the image data;
A function of comparing one of a plurality of threshold values with a data gradation value that is a gradation value of the input image data;
As a result of the comparison, when the data tone value is equal to or greater than the threshold value, an error diffusion method threshold value, which is a threshold value used to determine whether or not to form dots in the error diffusion method, A function for setting a lower threshold value that is smaller than the higher threshold value set in some cases;
A function of generating dot data indicating the presence or absence of the dot formation by the error diffusion method using the set threshold value for the error diffusion method;
To the computer,
The low-order threshold value is determined according to the data gradation value, and has a singular part that protrudes downward in a predetermined gradation range.

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