JP5979261B2 - Printing apparatus and printing method - Google Patents

Description

  The present invention relates to a printing technique for printing image data representing a predetermined image, and more particularly to a halftone processing technique.

  In a printing apparatus, dithering and error diffusion are widely known as halftone techniques for expressing gradation. Since the dither method and the error diffusion method have advantages and disadvantages, respectively, there has been a desire to perform halftone processing by combining a dither method element and an error diffusion method element. For example, in Patent Document 1 and Patent Document 2 below, the threshold value of the error diffusion method is periodically varied by using a dither mask of the systematic dither method, so that the dither method element and the error diffusion method are changed. A technique for performing halftone processing in combination with a target element is disclosed.

  However, with the technologies of Patent Document 1 and Patent Document 2, it is difficult to control the contributions of the dither element and the error diffusion element according to the characteristics of the print image data to be subjected to halftone processing. It was. For example, it is difficult to control the degree of contribution according to the input tone value, and to change the degree of contribution smoothly.

JP 2001-352448 A JP 2001-177722 A

  In light of at least some of the problems described above, the problem to be solved by the present invention is to provide a halftoning technique that incorporates dithering and error diffusion elements in a different manner from the prior art. .

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 or application examples.
As a first embodiment of the present invention, printing apparatus is provided for printing of image data representing a predetermined image. 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 of the printing apparatus compares a data gradation value that is a gradation value of the input image data with a first threshold that is one of a plurality of thresholds. An error diffusion unit that applies the error diffusion method to the data gradation value by using a comparison unit, a second threshold value determined according to the data gradation value, and generates the dot data; and the data gradation value and a determining unit to or greater than a predetermined tone value, by the determination unit, is determined data tone value and Ru der than predetermined gradation value lever, the comparison unit wherein the result of the comparison to determine the presence or absence of the dot formation of the by the determination unit, is determined data tone value and Ru der less than the predetermined gray level value lever, the error diffusion using the second threshold value The presence / absence of dot formation is determined by the unit.

Application Example 1 A printing apparatus that prints 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 a dither mask composed of a plurality of threshold values with a related gradation value that is a gradation value related to the input image data;
An error diffusion unit that generates the dot data by an error diffusion method based on a data gradation value that is a gradation value of the input image data, and
The error diffusion unit is a printing apparatus that controls ease of dot formation by the error diffusion method based on a comparison result of the comparison unit.

  When the dot data is generated by the error diffusion method, the printing apparatus configured as described above uses the comparison result between the dither mask and the related gradation value to control the ease of dot formation by the error diffusion method. That is, the ease of dot formation by the error diffusion method is controlled using the determination result of the presence or absence of dot formation when the dither method is used. Therefore, halftone processing incorporating a dithering factor and an error diffusion factor is possible. The data gradation value includes not only the gradation value of the image data itself but also the gradation value of the recording rate that is the ratio of recording dots on pixels in an arbitrary area. In addition to the data gradation value, the related gradation value includes a gradation value obtained by increasing or decreasing a predetermined value with respect to the data gradation value.

Application Example 2 In the printing apparatus according to Application Example 1, in the error diffusion unit, when the comparison result of the comparison unit indicates that the related gradation value is equal to or greater than a threshold value of the dither mask, the dot The control is performed so that the dots are not easily formed when the comparison result of the comparison unit is less than the dither mask threshold. The printing device to perform.

  If the printing apparatus having such a configuration uses the dither method and determines that dots are to be formed, control is performed so that dots are easily formed by the error diffusion method, and if the dither method is used, dots must be formed. If it is determined, control is performed so that dots are not easily formed by the error diffusion method. In any control, since the presence / absence of dot formation approaches the result by the dither method as compared with the dot data by the simple error diffusion method, the dither legal element is strengthened. Therefore, halftone processing incorporating a dithering factor and an error diffusion factor is possible. In addition, by appropriately setting the degree of control of the ease of dot formation, the degree of contribution between the dithering and error diffusion elements in the halftone process can be set to a desired degree.

Application Example 3 The application example in which the error diffusion unit controls the ease with which the dots are formed by changing a threshold value for error diffusion method, which is a threshold value used for determining whether or not dots are formed in the error diffusion method. The printing apparatus according to 2.
The printing apparatus having such a configuration can control the ease of dot formation by the error diffusion method with a simple configuration in which the error diffusion method threshold value is changed.

[Application Example 4] The printing apparatus according to Application Example 2 or Application Example 3, wherein the error diffusion unit changes a degree of control of the ease of formation of the dots based on the size of the data gradation value.

  Since the printing apparatus having such a configuration changes the degree of control of the ease of dot formation based on the magnitude of the data gradation value, dithering factors and errors are determined according to the gradation value of the image data. The degree of contribution with the diffusion law element can be changed. In addition, the contribution can be changed for each arbitrary region of the image data. As a result, it is possible to generate dot data with a good contribution according to the image data and the characteristics of the printing apparatus, and to improve the print image quality.

[Application Example 5] The error diffusion unit is configured such that the data gradation value is relatively high for high gradation data in the predetermined range in which the data gradation value is relatively large in the image data. The printing apparatus according to application example 4, wherein the degree of control of the ease of dot formation is greater than for low gradation data in a small predetermined range.

  The printing apparatus having such a configuration increases the degree of control of dot formation ease for high gradation data than for low gradation data. Rather than a dither legal element. Therefore, the merit of the error diffusion method element can be obtained on the low gradation side, and the advantage of the dither element can be obtained on the high gradation side. That is, printing that makes use of the advantages of both the error diffusion method and the dither method is possible.

Application Example 6 According to Application Example 4 or Application Example 5, in which the error diffusion unit changes the degree of control of the ease of formation of the dots stepwise based on the size of the data gradation value. Printing device.
In the printing apparatus having such a configuration, the degree of control of the ease of dot formation is changed stepwise based on the data gradation value, so that the contribution of the dithering and error diffusion elements The degree can be changed smoothly according to the data gradation value. As a result, the change in the contribution between the dither legal element and the error diffusion legal element is difficult to see in the print result, so that the deterioration of the print image quality due to the change of the dither legal element and the error diffusion legal element is suppressed. Can do.

Application Example 7 In the error diffusion unit, the degree of control of the ease of dot formation increases as the data gradation value increases in at least a part of the range of the data gradation value. A printing apparatus according to any one of Application Example 4 to Application Example 6 in which the control is performed.
The printing apparatus having such a configuration can increase the dithering factor as the gradation value increases in at least a partial range of the data gradation value. Therefore, in this range, the merit of the error diffusion method element can be obtained on the low gradation side, and the advantage of the dither method element can be obtained on the high gradation side. That is, printing that makes use of the advantages of both the error diffusion method and the dither method is possible. Also, as the data gradation value increases, the degree of control of the ease of dot formation increases, so the contribution of the dithering and error diffusion elements is smoothed according to the data gradation value. Can be changed.

Application Example 8 Any one of Application Examples 4 to 7 in which the error diffusion unit prohibits control of the ease of formation of the dots when the data gradation value is in the first range. Printing device.
When the data gradation value is in the first range, the printing apparatus having such a configuration does not control the ease of dot formation by the error diffusion method. Can be formed. Therefore, when there is a gradation region where the print image quality is improved when dot data is formed using only the error diffusion method element, it is possible to generate dot data with good print image quality even in the region.

Application Example 9 When the data tone value is in the second range, the halftone processing unit prohibits the generation of the dot data by the error diffusion unit and uses the comparison result of the comparison unit. The printing apparatus according to any one of Application Examples 4 to 8, wherein the dot data is generated by a dither method.

  In the printing apparatus having such a configuration, when the data gradation value is in the second range, generation of dot data by the error diffusion unit is prohibited, and dot data is generated by the dither method. Dot data can be generated. Therefore, when there is a gradation area where the print image quality is improved when dot data is formed using only the dither element, dot data with good print image quality can be generated even in the area. Further, in the dither method, unlike the error diffusion method, it is not necessary to calculate a diffusion error, so that halftone processing can be speeded up.

Application Example 10 The printing apparatus according to Application Example 9, wherein the second range is a range having a relatively large value when the entire range of the data gradation value is divided into two ranges.
The printing apparatus having such a configuration generates dot data using only dithering elements in a range where the data gradation value is relatively large. The error diffusion method is superior to the dither method from the viewpoint of the granularity of the print image quality, but since the granularity of the print image quality is less noticeable on the high gradation side, even if dot data is generated using only the dither legal element, The deterioration of graininess can be suppressed. Therefore, it is possible to speed up the halftone process while suppressing a decrease in print image quality.

Application Example 11 The printing apparatus according to any one of Application Example 2 to Application Example 10, wherein the related gradation value is the data gradation value.
In the printing apparatus having such a configuration, since the related gradation value is the gradation value of the input image data, no special calculation for calculating the related gradation value is performed. Therefore, halftone processing can be simplified and the processing speed can be increased.

Application Example 12 Any one of Application Examples 2 to 10 in which the related gradation value includes a gradation value that is increased or decreased by a predetermined positive number with respect to the data gradation value. The printing apparatus as described.
In the printing apparatus having such a configuration, the related gradation value may be a gradation value obtained by increasing or decreasing the gradation value of the input image data by a predetermined value. In this way, since the comparison result of the comparison unit can be controlled, the degree of reflection of the dot generation pattern of the dither mask can be controlled. As a result, it is possible to more flexibly control the contributions of the dither legal element and the error diffusion legal element.

[Application Example 13] The printing apparatus according to Application Example 12, wherein the related gradation value is a gradation value obtained by increasing the data gradation value by the positive predetermined value, The error diffusion unit may prevent the dots from being formed or the dots from being formed when the comparison result of the comparison unit indicates that the related gradation value is less than the threshold value of the dither mask. A printing apparatus that performs the above control.

  Since the printing apparatus having such a configuration uses the gradation value obtained by increasing the related gradation value by a predetermined positive number, the comparison result of the comparison unit is compared with the case where the related gradation value is the data gradation value. However, there are many cases where the related gradation value is equal to or greater than the threshold of the dither mask (also referred to as the first case). In the first case, control is performed so that dots are easily formed by the error diffusion method. On the other hand, in the printing apparatus, when the comparison result of the comparison unit indicates that the related gradation value is less than the dither mask threshold (also referred to as the second case), dots are hardly formed by the error diffusion method. In addition, control is performed so that dots are not formed. Therefore, it is highly likely that a pixel in which a dot is to be formed is determined from among the pixels in the first case, and it is possible to generate dot data that is close to the dot generation pattern of the dither mask. In the second case, if control is performed so that dots are not formed, the pixels in which dots are always formed are determined from the pixels in the first case. Therefore, dot data closer to the dot generation pattern of the dither mask can be generated.

[Application Example 14] The printing apparatus according to any one of Application Examples 1 to 13, wherein the dither mask has a blue noise characteristic.
In the printing apparatus having such a configuration, the dither mask may have a blue noise characteristic. In this way, it is possible to incorporate a dithering element with excellent print image quality into halftone processing. In addition, because of the blue noise characteristics, the dithering element has excellent print quality graininess, so the change of contribution with the error diffusion method element that also has excellent print quality graininess can be made more smoothly. Can do.

  In addition to the configuration as a printing apparatus, the present invention can also be realized as a print data generation apparatus, a printing method for printing with the printing apparatus, a printing program, a storage medium storing the program, and the like.

1 is a schematic configuration diagram of a printer 20 as a first embodiment of the present invention. 3 is a flowchart showing a flow of printing processing in the printer 20. It is a flowchart which shows the flow of the halftone process as 1st Example. It is explanatory drawing which shows the characteristic of the error diffusion threshold value table 62 as 1st Example. It is explanatory drawing which shows the characteristic of the error diffusion threshold value table 62 as 2nd Example. It is explanatory drawing which shows the characteristic of the error diffusion threshold value table 62 as 3rd Example. It is explanatory drawing which shows the characteristic of the error diffusion threshold value table 62 as 4th Example. It is a flowchart of the halftone process as 5th Example. It is a flowchart which shows the modification of the halftone process as 5th Example. It is a flowchart of the halftone process as 6th Example. It is a flowchart which shows the modification of the halftone process as 6th Example.

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 the present invention. The printer 20 is a serial inkjet printer that performs bi-directional printing. As illustrated, the printer 20 includes a mechanism that transports the print medium P by a paper feed motor 74 and a carriage 80 that moves the carriage 80 by the carriage motor 70. A mechanism for reciprocating in the direction, a mechanism for driving the print head 90 mounted on the carriage 80 to discharge ink and forming dots, the paper feed motor 74, the carriage motor 70, the print head 90, and the operation panel 99. The control unit 30 is responsible for exchanging signals with the control unit 30.

  A mechanism for reciprocating the carriage 80 in the axial direction of the platen 75 is installed in parallel with the axis of the platen 75, and is driven endlessly between the slide shaft 73 that holds the carriage 80 slidably and the carriage motor 70. A pulley 72 and the like for stretching the belt 71 are included.

  On the carriage 80, ink cartridges 82 to 87 for color ink respectively containing cyan ink C, magenta ink M, yellow ink Y, black ink K, light cyan ink Lc, and light magenta ink Lm are mounted as color inks. . In the print head 90 below the carriage 80, nozzle rows corresponding to the above-described color inks are formed. When these ink cartridges 82 to 87 are mounted on the carriage 80 from above, ink can be supplied from each cartridge to the print head 90.

  The control unit 30 includes a CPU 40, a ROM 51, a RAM 52, and an EEPROM 60 that 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. Details of these functional units will be described later.

  In the EEPROM 60, a dither mask 61 and an error diffusion threshold table 62 are stored. The dither mask 61 is used for halftone processing by a systematic dither method, and includes a plurality of threshold values. In this embodiment, the dither mask 61 has a so-called blue noise characteristic.

  In this embodiment, the dither mask 61 has predetermined dot formation characteristics. That is, both the dot pattern of the dot group formed by the forward movement of the carriage 80 in bidirectional printing, the dot pattern of the dot group formed by the backward movement, and the dot pattern of the entire dot group combining these are good. It has a characteristic of having excellent dot dispersibility. Such a technique is described in, for example, Japanese Patent Application Laid-Open No. 2007-15359. It should be noted that the dither mask 61 is a main scanning group that indicates in which main scanning of a plurality of main scannings of the carriage 80 dots are formed instead of or in addition to the group for each reciprocation described above. In each case, good dot dispersibility may be obtained.

  Good dot dispersibility can be specified as a dot pattern having blue noise characteristics or green noise characteristics. Alternatively, each of the spatial frequency distributions of the threshold values of the dither mask set for the pixels belonging to each of the plurality of groups and the spatial frequency distribution of the print image have a positive correlation coefficient, preferably 0. It can be specified as having a correlation coefficient of .7 or more. The error diffusion threshold value table 62 is a table that stores threshold values used for determination of dot ON / OFF in the error diffusion method, and details thereof will be described 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 above hardware configuration drives the carriage motor 70 to reciprocate the print head 90 with respect to the print medium P in the main scanning direction, and also drives the paper feed motor 74. Thus, the print medium P is moved in the sub-scanning direction. The control unit 30 performs printing by driving the nozzles at an appropriate timing based on the print data in accordance with the movement of the carriage 80 in the reciprocating motion (main scanning) and the paper feeding movement of the printing medium (sub scanning). Ink dots of appropriate colors are formed at appropriate positions on the 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 for converting the image data into ON / OFF data of dots of each color as a process of the halftone processing unit 42 (step S130). Details of the halftone process will be described later. The halftone process is not limited to the binarization process of dot ON / OFF, but may be a multi-value process such as ON / OFF of large dots and small dots. 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 processing is performed, the CPU 40 performs interlace processing for rearranging the dot pattern data to be printed in one main scanning unit in accordance with the nozzle arrangement of the printer 20 and the paper feed amount (step S150). When the interlace process is performed, the CPU 40 drives the print head 90, the carriage motor 70, the motor 74, and the like as the 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).

  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 refers to the gradation value of the target pixel data Dn and the value of the threshold value THn_d corresponding to the target pixel data Dn among the plurality of threshold values constituting the dither mask 61 stored in the EEPROM 60. This is a process of comparing the magnitude relationship. This processing is formally the same as the processing for determining dot ON / OFF by the dither method that is normally performed. In practice, 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 the threshold value THn_d. If the value is less than the value, it is determined that the dot is turned OFF. However, the provisional dither processing of this embodiment is a pre-processing for determining ON / OFF of the dot by an error diffusion method described later, specifically, The difference is that this is a process for determining the threshold value of the error diffusion method.

  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 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.

  A specific example of the error diffusion threshold table 62 is conceptually shown in FIG. As shown in the drawing, in the error diffusion threshold table 62, the gradation value (0 to 255 in this case) of the target pixel data Dn is associated with the low threshold THe_L and the high threshold THe_H, respectively. The threshold value THe_N shown in the figure is displayed with reference to an example of the threshold value used in the normal error diffusion method. In this example, the threshold value THe_N is a constant value 127.N regardless of the gradation of the target pixel data Dn. 5

  In the example shown in FIG. 4, the high threshold THe_H takes a value of 127.5 when the gradation value of the pixel-of-interest data Dn is 0, and increases as the gradation value increases from 0. When the tone value is 255, the value is 207.5. The lower threshold THe_L takes the same value 127.5 as the higher threshold THe_H when the pixel value Dn of the pixel-of-interest data Dn is 0, and decreases as the gradation value increases from 0, and the gradation value is 255. In this case, the value is 47.5.

  In other words, the difference between the high threshold THe_H and the low threshold THe_L (hereinafter also referred to as threshold difference ΔTHe) is 0 when the gradation value is 0, and increases as the gradation value increases. When the value is 255, the value is 80. That is, in the example of FIG. 4, the error diffusion threshold value table 62 is set so that the threshold value difference ΔTHe increases as the gradation value increases over the entire range of gradation values (in this case, in a directly proportional relationship). Yes. Thus, the magnitude relationship between the high threshold THe_H and the low threshold THe_L is set such that the mutual values are equal or the high threshold THe_H is relatively larger than the low threshold THe_L. However, the case where the high threshold THe_H and the low threshold THe_L are the same in all gradation values is excluded. Note that the values of the high threshold THe_H and the low threshold THe_L for each gradation value can be set in any way as long as such a magnitude relationship is maintained. In this embodiment, the high threshold value THe_H and the low threshold value THe_L corresponding to the gradation value are obtained by referring to the table, but may be obtained by a function.

  Here, the description returns to the halftone process of FIG. When the threshold value THe is set with reference to the error diffusion threshold value table 62, the CPU 40 adds the diffusion error Edn stored in the separately prepared error buffer to the gradation value of the target pixel data Dn (step S135). Here, 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 compares the gradation value of the pixel-of-interest data Dn added with the diffusion error Edn with the threshold value THe set in step S133 or step S134 ( Step S136). As a result, if the gradation value of the target pixel data Dn to which the diffusion error Edn is added is equal to or greater than the threshold value THe (step S136: YES), the dot of the target pixel is determined to be ON (step S137), and the diffusion error Edn is added. If the gradation value of the target pixel data Dn is less than the threshold value THe (step S136: NO), the dot of the target pixel is determined to be OFF (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 tone value of the target pixel data Dn to which the diffusion error Edn is added and the dot ON / OFF result (the tone value 255 or 0 in this case). The diffusion error Edn is an error that is added to the gradation value of the target pixel data Dn in step S135. In this embodiment, the binarization error En is set to 7/16 for the pixel adjacent to the right of the pixel of interest, which is a peripheral pixel for which ON / OFF of the dot has not been determined, and 3/16 for the pixel on the lower left. The diffusion error Edn is distributed at a ratio of 5/16 for the lower pixel and 1/16 for the lower right pixel. The diffusion error Edn calculated in this way is stored in the error buffer.

  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.

  When the binarization error En and the diffusion error Edn are calculated, the CPU 40 repeats the processes of steps S131 to S139 with all pixels as the target pixel (step S140). Thus, the halftone process in step S130 is completed.

  The principle of such halftone processing will be described below. As described above, in the processing of steps S132 to S134, if the tone 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, and the target pixel If the gradation value of the data Dn is less than the threshold value THn_d, the threshold value THe is set to the high threshold value THe_H. The threshold difference ΔTHe (= THe_H−THe_L) is a value of 0 or more.

  Here, a case where the threshold difference ΔTHe is 0 (THe_H = THe_L) is considered. In this case, since the result of the temporary dither processing does not affect the threshold value THe, the processing of steps S132 to S134 is performed to determine the final dot ON / OFF by the error diffusion method (steps S135 to S139). It has no meaning for it. This means that in the halftone process of step S130, the final dot ON / OFF is determined only by the error diffusion method element.

  Next, consider a case where the threshold difference ΔTHe is greater than 0 (THe_H> THe_L). In this case, when the CPU 40 determines that the dot is ON by the provisional dither processing (which means that the gradation value of the target pixel data Dn is equal to or greater than the threshold value THn_d), the CPU 40 sets the threshold value THe to a relatively small lower threshold value THe_L. . On the other hand, when it is determined that the dot is OFF by the temporary dither processing (which means that the gradation value of the target pixel data Dn is less than the threshold value THn_d), the threshold value THe is set to a relatively large high threshold value THe_H. That is, if the CPU 40 determines that the dot is ON by the temporary dither process, the CPU 40 controls the dot to be easily turned ON by the error diffusion method. If the CPU 40 determines that the dot is OFF by the temporary dither process, the dot is likely to be OFF by the error diffusion method. To control. This means that the final dot ON / OFF determination result by the error diffusion method is closer to the dot ON / OFF determination result by the temporary dither process than when the threshold difference ΔTHe is 0. doing. In other words, the final dot ON / OFF is determined by adding the dither legal element in addition to the error diffusion legal element.

  As the threshold difference ΔTHe increases, the dithering factor increases, and when the threshold difference ΔTHe becomes infinite, only the dithering factor only determines whether the dot is ON / OFF. It becomes. When the threshold difference ΔTHe is infinite, if it is determined that the dot is ON by the temporary dither process, it is always determined that the dot is ON by the subsequent error diffusion method, and if the dot is determined to be OFF by the temporary dither process, This is because it is always determined that the dot is OFF by the error diffusion method.

  In short, by changing the threshold value THe according to the result of the temporary dither processing, specifically, by changing the magnitude of the threshold difference ΔTHe, It is possible to control the respective contributions. In the present embodiment, using such a principle, the dithering element and the error diffusion element in the halftone process are dynamically controlled according to the gradation value of the target pixel data Dn. This can also be understood as controlling the degree of control of the ease of dot formation by the error diffusion method based on the magnitude of the threshold difference ΔTHe.

  When the printer 20 having such a configuration generates dot data by the error diffusion method, it controls the ease of dot formation by the error diffusion method using the result of the temporary dither processing. In other words, the ease of dot formation by the error diffusion method is controlled using the dot ON / OFF determination result when the dither method is used. Therefore, halftone processing incorporating a dithering factor and an error diffusion factor is possible.

  In addition, when the result of the temporary dither processing is dot ON, the printer 20 sets the threshold value THe used for the error diffusion method to the lower threshold value THe_L, and performs control so that dots are easily formed by the error diffusion method. Further, when the result of the temporary dither processing is dot OFF, the threshold value THe is set to the high threshold value THe_H, and control is performed so that dots are not easily formed by the error diffusion method. In any control, since the presence / absence of dot formation approaches the result by the dither method as compared with the dot data by the simple error diffusion method, the dither legal element is strengthened. Therefore, by appropriately setting the degree of these controls, that is, the threshold difference ΔTHe, it is possible to set the degree of contribution between the dithering element and the error diffusion element in the halftone process to a desired degree. Further, since the threshold THe is changed based on the result of the temporary dither process, the ease of dot formation by the error diffusion method is controlled, so the configuration is simple and contributes to the speeding up of the process.

  Further, the printer 20 changes the threshold value THe based on the magnitude of the gradation value of the pixel-of-interest data Dn. Specifically, the printer 20 changes the threshold value difference ΔTHe to change the dot by the error diffusion method. Since the ease of formation is controlled, the degree of contribution between the dither element and the error diffusion element can be changed according to the gradation value of the image data. In addition, the contribution can be changed for each arbitrary region of the image data. As a result, it is possible to generate dot data with a good contribution according to the image data and the characteristics of the printing apparatus, and to improve the print image quality.

  In the present embodiment, the error diffusion threshold value table 62 is set so that the threshold difference ΔTHe becomes larger toward the higher gradation side. In other words, the printer 20 increases the degree of control of the ease of dot formation by the error diffusion method for high gradation image data than for low gradation image data. On the side, the dithering factor can be strengthened compared to the low gradation side. Therefore, the merit of the error diffusion method element can be obtained on the low gradation side, and the advantage of the dither element can be obtained on the high gradation side.

  As an advantage of the error diffusion method element on the low gradation side, for example, it is possible to obtain a granularity with good print image quality. If a diffusion range switching error diffusion method or the like is used as the error diffusion method, further improvement in image quality can be expected. Since the diffusion range switching error diffusion method is a known technique, a detailed description thereof is omitted, but the error diffusion range is switched according to the combination of the input gradation value and the binarization result. By diffusing the error over a wide range only when the dot is ON, it is possible to improve the granularity of the low gradation region and suppress the occurrence of undesired continuity of dots, so-called worms.

  As an advantage of the dithering element on the high gradation side, for example, it is possible to suppress image quality deterioration due to deviation of the dot landing position. This merit is due to the above-described predetermined dot formation characteristics of the dither mask 61. In the high gradation region, even if dot data is generated by the dither method, the graininess of the print image quality is not conspicuous due to ink bleeding, and this does not cause a big problem.

  Moreover, the error diffusion threshold value table 62 is set so that the threshold difference ΔTHe changes stepwise based on the magnitude of the gradation value. That is, the printer 20 changes the degree of control of the ease of dot formation by the error diffusion method in a stepwise manner based on the magnitude of the gradation value. Therefore, it is possible to smoothly change the contributions of the dithering element and the error diffusion element according to the data gradation value. As a result, the change in the contribution between the dithering and error diffusion elements is difficult to see in the printed result, so the change in the contribution of the dithering and error diffusion elements in the same print image The accompanying deterioration in print image quality can be suppressed.

  In particular, in this embodiment, since the dither mask 61 has a blue noise characteristic that is excellent in the graininess of the print image quality, the error diffusion method element that is also excellent in the graininess of the print image quality is used. The change in contribution can be seen more smoothly. Even if the dither mask 61 does not have a blue noise characteristic, the degree of control of dot formation ease by the error diffusion method is changed stepwise based on the magnitude of the gradation value. By doing so, it is possible to smoothly change the contribution of the dithering and error diffusion elements according to the data gradation value.

  From the viewpoint of the printed matter of the printer 20, the printer 20 is characterized by dots formed by dithering elements when one image is printed by changing the gradation value from the minimum value to the maximum value, It can be said that the joints with the dots formed by the error diffusion method element are invisible to the naked eye. If the dither method and the error diffusion method are switched at a certain gradation, such a feature does not appear.

B. Second embodiment:
A second embodiment of the present invention will be described. The apparatus configuration of the printer 20 and the flow of printing processing are the same as in the first embodiment, and only the characteristics of the error diffusion threshold table 62 are different from those in the first embodiment. In the following, description of the same points as in the first embodiment will be omitted, and only different points will be described. A specific example of the error diffusion threshold table 62 as the second embodiment is conceptually shown in FIG. This example is different from the first embodiment in that the increase rate of the high threshold THe_H and the decrease rate of the low threshold THe_L accompanying the increase of the gradation value are large and the gradation value is constant above a predetermined value. .

  If this point is seen as the threshold value difference ΔTHe, the threshold value difference ΔTHe is 0 when the gradation value is 0, increases as the gradation value increases, becomes the value 255 with the gradation value 128, and thereafter. Until the gradation value reaches 255, the value transitions constant at 255. That is, in the example of FIG. 5, the error diffusion threshold value table 62 is set so that the threshold value difference ΔTHe increases as the gradation value increases in the range of gradation values from 0 to 128.

  When the error diffusion threshold table 62 has such characteristics, the amount of change in the threshold difference ΔTHe with respect to the change in the gradation value of the target pixel data Dn is larger than that in the first embodiment (FIG. 4). Since the final threshold value difference ΔTHe becomes large, the dithering factor becomes strong at each gradation value. In particular, in the range where the gradation value is 128 or more, the dithering factor is extremely strong. In this embodiment, when the threshold difference ΔTHe is a value of 255, the final dot ON / OFF is determined by almost only the dithering factor. Therefore, in the range where the gradation value is 128 or more, the threshold value The difference ΔTHe is constant. If the threshold difference ΔTHe is further increased, it is possible to determine the final dot ON / OFF with only dithering factors.

  The threshold difference ΔTHe necessary for determining ON / OFF of the dot by almost only the dithering factor varies depending on the characteristics of the dither mask 61. Therefore, the actual output result may be examined and set as appropriate. The high threshold THE_H The difference between the threshold value difference ΔTHe and the threshold value difference ΔTHe may be further increased by setting the maximum value of the threshold value to a value larger than 255 or the minimum value of the lower threshold value THe_L to a value smaller than 0.

  Further, when the characteristics of the error diffusion threshold table 62 are of a dot concentration type such as halftone dither, the dots are continuously turned on or off in adjacent pixels, so that the surrounding pixels Since the distributed diffusion error Edn tends to increase cumulatively, it is desirable to further increase the threshold difference ΔTHe in order to determine dot ON / OFF with only dithering factors.

  The printer 20 that stores the error diffusion threshold value table 62 having the above-described characteristics has some gradation values (0 to 128 here) among all the gradation values (0 to 255 here) of the target pixel data Dn. In this range, the threshold difference ΔTHe increases as the gradation value increases. In this range, the contribution of the dithering and error diffusion elements in the dot ON / OFF determination is smoothly changed. be able to. Further, for a high gradation printing region (in this case, gradation values of 128 to 255), the final dot ON / OFF can be determined by only a dithering factor. Accordingly, it is possible to obtain a good print image quality by making the best use of the features of the dither mask 61 of the present embodiment, which has a great effect of suppressing image quality deterioration due to the deviation of the dot landing position in the high gradation region.

C. Third embodiment:
A third embodiment of the present invention will be described. The apparatus configuration of the printer 20 and the flow of printing processing are the same as in the first embodiment, and only the characteristics of the error diffusion threshold table 62 are different from those in the first embodiment. In the following, description of the same points as in the first embodiment will be omitted, and only different points will be described. A specific example of the error diffusion threshold table 62 as the third embodiment is conceptually shown in FIG. FIG. 6 is an example of the high threshold THe_H and the low threshold THe_L when the threshold optimization error diffusion method is adopted as the error diffusion method used in step S130. Since the threshold optimization error diffusion method is a known technique, a detailed description thereof is omitted. However, the threshold optimization error diffusion method is a method of changing the threshold value THe according to the input gradation value, and suppresses so-called dot formation delay and tailing. it can.

  As shown in FIG. 6, the threshold value THe_N, which is an example of the threshold value in the threshold optimization error diffusion method, is controlled to increase as the gradation value of the target pixel data Dn increases by the threshold optimization error diffusion method. The Here, in the range where the gradation value of the target pixel data Dn is 0 to 16, the threshold difference ΔTHe is 0, and the values of the high threshold THe_H and the low threshold THe_L coincide with the threshold THe_N. In the range of the gradation value from 16 to 192, the high threshold value THe_H is set larger than the threshold value THe_N, the low threshold value THEe_L is set smaller than the threshold value THe_N, and the high threshold value ΔTHe increases as the gradation value increases. A threshold value THe_H and a lower threshold value THe_L are set. In the range of gradation values from 192 to 255, the high threshold value THe_H and the low threshold value THe_L are set so that the threshold difference ΔTHe is constant at a value of 255.

  If the error diffusion threshold value table 62 has such a characteristic, in the low gradation region (here, gradation values 0 to 16), the threshold value difference ΔTHe is set to 0, and the error diffusion method is performed by the threshold optimization error diffusion method. By performing halftone processing using only the target element, the merit of the threshold optimization error diffusion method, which is superior in graininess in a low gradation region as compared with the dither method, can be sufficiently exhibited.

  In a high gradation region (here, gradation values 192 to 255), the threshold difference ΔTHe is set to a value substantially corresponding to only a dither legal element, thereby suppressing deterioration in image quality due to dot landing position deviation. The advantages of the dither legal element of the present embodiment can be fully exhibited. In terms of graininess, the error diffusion legal element is superior to the dither legal element, but in high gradation areas, even if the dither legal element is strengthened in this way, the print image quality is reduced due to ink bleeding. Granularity is not a big problem.

  Also, in the middle gradation region (here, gradation values 16 to 192), the error diffusion method element and the dither method element are combined, and the dither method element becomes stronger as the gradation value increases. By performing halftone processing in this way, the contributions of the dithering and error diffusion elements can be changed smoothly. As described above, the characteristics of the error diffusion threshold value table 62 according to the present embodiment can perform the optimum halftone process according to the gradation value.

D. Fourth embodiment:
A fourth embodiment of the present invention will be described. The apparatus configuration of the printer 20 and the flow of printing processing are the same as in the first embodiment, and only the characteristics of the error diffusion threshold table 62 are different from those in the first embodiment. In the following, description of the same points as in the first embodiment will be omitted, and only different points will be described. A specific example of the error diffusion threshold table 62 as the fourth embodiment is conceptually shown in FIG. FIG. 7 is an example of the high threshold THe_H and the low threshold THe_L when a technique that does not take worm countermeasures such as the above-described diffusion range switching error diffusion method is adopted as the error diffusion method used in step S130.

  In this example, as shown in FIG. 7, the threshold difference ΔTHe takes a value of 255 when the tone value of the pixel-of-interest data Dn is 0, decreases as the tone value increases, and the tone value becomes 96. When the value reaches 0, the value becomes 0. Thereafter, the value becomes 0 until the gradation value becomes 255. Depending on the characteristics of the error diffusion and dithering elements used for halftone processing, the dithering element is stronger in the low gradation area, and the error diffusion element becomes stronger as the gradation becomes higher. Even when the above control is performed, the optimum halftone process according to the gradation value can be performed. In the error diffusion method for which worm countermeasures have not been taken, image quality degradation due to worms in the low gradation region is a major problem, but image quality degradation due to worms is not a problem when the gradation value is larger than a predetermined value. .

E. Example 5:
A fifth embodiment of the present invention will be described. The apparatus configuration of the printer 20 is the same as that of the first embodiment, and only the flow of printing processing, specifically, the flow of halftone processing is different from that of the first embodiment. Further, the error diffusion threshold table 62 employs the characteristic having the characteristics shown as the second embodiment (see FIG. 5). In the following, description of the same points as in the above-described embodiment will be omitted, and only different points will be described. FIG. 8 shows the flow of halftone processing as the fifth embodiment. In FIG. 8, processes having the same contents as the halftone process shown in FIG. 3 are denoted by the same reference numerals as those in FIG. 3 to simplify the description.

  As shown in FIG. 8, when the halftone process according to the fifth embodiment is started, the CPU 40 acquires the coordinate data n (x, y) of the target pixel position and the target pixel data Dn (step S131). ). Then, the CPU 40 determines whether or not the gradation value of the target pixel data Dn is in a predetermined range (step S231). In this embodiment, it is determined whether or not the gradation value of the target pixel data Dn is 128 or more.

  As a result, if the gradation value is less than 128, the CPU 40 executes the processing of steps S132 to S139 (step SST100) shown in the first embodiment (see FIG. 3). That is, the dot ON / OFF determination is performed while changing the strength of the dithering and error diffusion elements according to the gradation value.

  On the other hand, if the gradation value is 128 or more, the CPU 40 determines ON / OFF of the dot by the dither method using the dither mask 61. Specifically, the gradation value of the target pixel data Dn is compared with the threshold value THn_d (step S232). If the gradation value is equal to or greater than the threshold value THn_d (step S232: YES), the dot of the target pixel is turned on. If the gradation value is less than the threshold value THn_d (step S232: NO), the dot of the target pixel is determined to be OFF (step S234). That is, for the gradation value of high gradation (here, 128 or more), determination of dot ON / OFF by the error diffusion method shown in the above-described embodiment is prohibited, and dot ON / OFF by only the dither method. To decide.

  The CPU 40 repeatedly executes the dot ON / OFF determination process until all the pixels are the target pixel (step S140). Note that the diffusion error Edn calculated in step S139 (see FIG. 3) is not reflected in the processing in steps S232 to S234.

  The printer 20 having such a configuration prohibits the determination of dot ON / OFF by the error diffusion method (step SST100) when the gradation value of the target pixel data Dn is in a predetermined range, and turns the dot ON / OFF by the dither method. Determine OFF. Therefore, when there is a gradation area where the print image quality is improved when dot data is formed using only the dither element, dot data with good print image quality can be generated even in the area. Further, since it is not necessary to perform error diffusion calculation for a predetermined range of gradation values, only simple calculation for comparing the gradation value of the target pixel data Dn with the threshold value THn_d constituting the dither mask 61 is performed. Can determine ON / OFF of dots, and can speed up halftone processing.

  In addition, in the printer 20, a predetermined range in which determination of dot ON / OFF by the error diffusion method is prohibited is a range where the gradation value is 128 or more. Here, in the error diffusion threshold value table 62 of the present embodiment, as shown in FIG. 5, after the gradation value 128, the threshold difference ΔTHe is the final dot ON / OFF only with the dithering factor only. The value 255 can be determined. That is, in the error diffusion threshold value table 62, the threshold value difference ΔTHe is a range of gradation values that are equal to or larger than the gradation value that is large enough to determine the final dot ON / OFF with only the dithering factor. In this, the ON / OFF determination of the dot by the error diffusion method is prohibited (the ON / OFF of the dot is determined only by the dither method), so the switching between the error diffusion method and the dither method is smooth from the viewpoint of dot formation characteristics. Can be done. As a result, such switching is not visually recognized on the print image, and deterioration of the print image quality can be suppressed.

  The predetermined range of gradation values that prohibits the ON / OFF determination of dots by the error diffusion method described above may be set as appropriate according to the desired printing characteristics. For example, the range of gradation values is 96 or more. Alternatively, the range may be 160 or more. In addition, as shown in FIG. 7, when it is desired to strengthen the dithering factor in a range where the gradation value is relatively small, the dot is turned on by the error diffusion method in a range where the gradation value is smaller than a predetermined value. The determination of / OFF may be prohibited. It should be noted that the high threshold THe_H and the low threshold THe_L need not be set in the error diffusion threshold table 62 in the range of gradation values that prohibits ON / OFF determination of dots by the error diffusion method. .

  Another configuration of halftone processing that prohibits the determination of dot ON / OFF by the error diffusion method within a predetermined range of gradation values will be described with reference to FIG. In FIG. 9, processes having the same contents as the halftone processes shown in FIGS. 3 and 8 are denoted by the same reference numerals as those in FIG. 3 or FIG. Also in this example, the error diffusion threshold value table 62 has the characteristics shown as the second embodiment.

  As shown in FIG. 9, when the halftone process is started, the CPU 40 acquires the coordinate data n (x, y) of the target pixel position and the target pixel data Dn (step S131), and the target pixel data Dn. It is determined whether or not the gradation value is within a predetermined range (step S231). In this embodiment, it is determined whether the gradation value of the target pixel data Dn corresponds to one of the three ranges of less than 128, less than 128 and less than 144, and 144 or more.

  As a result, if the gradation value is less than 128, the CPU 40 executes the processing of steps S132 to S139 (step SST100) shown in the first embodiment (FIG. 3). On the other hand, if the gradation value is 144 or more, the CPU 40 executes the processing of steps S232 to S234 (step SST200) shown in the fifth embodiment (FIG. 8).

  On the other hand, if the gradation value is in the range of 128 or more and less than 144, the CPU 40 performs the processing of steps S232 to S234 (step SST200) and then calculates the binarization error En and the diffusion error Edn (step). S331). The diffusion error Edn calculated here is reflected in step S135 of step SST100. The CPU 40 repeatedly executes the dot ON / OFF determination process until all the pixels are the target pixel (step S140).

  As described above, in the halftone process of FIG. 9, in the first gradation value range (less than 128 in this case), the error diffusion process in step SST100 is performed, and the first gradation value range is continuous. In a range of gradation values of 2 (here, 128 or more and less than 144), processing for diffusing binarization error En to surrounding pixels while determining dot ON / OFF by the dither method (hereinafter, processing of diffusion dither method) In a third tone value range (here, 144 or more) that is continuous with the second tone value range, a normal dithering process (step SST200) is performed to turn dots on. / OFF is determined. Therefore, it is possible to switch the halftone method more smoothly and suppress deterioration of the printed image.

  For example, in the halftone process shown in FIG. 8, when the input image data includes an area whose gradation value fluctuates around 128, in the halftone process of the area, the process of step SST100 and step SST200 are performed. This process may be frequently replaced for each successive pixel. Then, in these pixels, although the gradations are almost the same level, pixels in which the diffusion error is reflected and pixels in which the reflection error is not sufficiently reflected are mixed in close proximity, which may cause deterioration in image quality. .

  On the other hand, as shown in FIG. 9, in the middle of switching the dot ON / OFF determination process from the error diffusion process in step SST100 to the normal dither process (step SST200) according to the gradation value. If the process of the diffusion dither method is interposed, it is possible to suppress a pixel in which a diffusion error is reflected and a pixel in which a diffusion error is not sufficiently reflected from being mixed in close proximity even though the gradation is substantially the same level. be able to. As a result, deterioration of image quality can be suppressed.

F. Example 6:
A sixth embodiment of the present invention will be described. The configuration of the printer 20 is the same as that of the first embodiment, and only the flow of printing processing, specifically, the flow of halftone processing is different from that of the first embodiment. In the following, description of the same points as in the first embodiment will be omitted, and only different points will be described. FIG. 10 shows the flow of halftone processing as the sixth embodiment. In FIG. 10, processes having the same contents as the halftone process shown in FIG. 3 are denoted by the same reference numerals as those in FIG. 3 to simplify the description.

  As shown in FIG. 10, when the halftone process according to the sixth embodiment is started, the CPU 40 acquires the coordinate data n (x, y) of the target pixel position and the target pixel data Dn, and the target pixel. A predetermined value α (α> 0) is added to the gradation value of the data Dn (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 + α). In the present embodiment, the predetermined value α = 4.

  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 S133). 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 S134). Since the subsequent error diffusion processing (steps S135 to S139) is the same as that of the first embodiment, description thereof is omitted. Note that the ON / OFF determination of the dot in the error diffusion method is performed using the gradation value of the target pixel data Dn, not the related data Dn ′.

  In the halftone process having such a configuration, the predetermined value α is added to the gradation value, and the temporary dither process is performed, so that the number of pixels determined to be dot ON (Dn ′ ≧ THn_d) by the temporary dither process is increased by the error diffusion. More than the number of pixels determined to be dot ON by the law. That is, it is less likely that pixels other than the pixel determined to be dot ON by the temporary dither processing are finally determined to be dot ON by the error diffusion method at the subsequent stage. As a result, since it is possible to reduce the formation of dots at pixel positions unrelated to the dot generation pattern of the dither mask 61, the dither legal element can be further strengthened without increasing the threshold difference ΔTHe so much. . That is, the degree of reflection of the dot generation pattern of the dither mask can be controlled to more flexibly control the contribution of the dither legal element and the error diffusion legal element. In other words, the threshold difference ΔTHe can be reduced to obtain the same degree of dithering factor. If the threshold difference ΔTHe is reduced, the magnitude of the binarization error En in the error diffusion method is also reduced, leading to an improvement in print image quality.

  The predetermined value α described above is not necessarily constant, and may be a value that changes according to the gradation value. Further, the predetermined value α may be set to 0 depending on the gradation value. In this way, the dither legal element can be controlled more flexibly. Of course, the predetermined value α is not necessarily a positive number, and may be a negative number. For example, when the gradation value is in the low gradation side (here 127 or less), the predetermined value α is a positive number, and when the gradation value is in the high gradation side (here 128 or more), the predetermined value α is It may be a negative number. Since the relationship between dot ON and dot OFF is reversed, from this point of view, for example, the predetermined value α is controlled symmetrically on the low gradation side and the high gradation side, and the predetermined value is set on the low gradation side. α may be a positive number, and the predetermined value α may be a negative number on the high gradation side.

  Further, if the predetermined threshold value α is set to a positive number and the high threshold value THe_H is considerably increased, when the threshold value THe is set to the high threshold value THe_H (step S134), the dots are turned ON by the error diffusion method. Therefore, it is possible to further reduce the case where pixels other than those determined to be dot ON by the temporary dither processing are finally determined to be dot ON by the error diffusion method at the subsequent stage. That is, the final halftone result can be brought close to the dot generation pattern of the dither mask 61.

  FIG. 11 shows another configuration of halftone processing for controlling the correlation with the dot generation pattern conforming to the dither mask 61. In FIG. 11, the same reference numerals are assigned to the same processes as the processes of the halftone processes described above. The difference between the process shown in FIG. 11 and the process shown in FIG. 10 is that the threshold value THe used in the error diffusion method is not affected by the result of the temporary dither process (step S432). If Dn ′ is less than the threshold value THn_d (step S432: NO), the final dot ON / OFF is determined to be dot ON (step S138). The predetermined value α is set as a positive number.

  In this way, in the error diffusion method in steps S135 to S139, only the pixels that are dot-ON finally selected from the pixels that are dot-ON in the temporary dither process. Therefore, a halftone processing result closer to the dot generation pattern of the dither mask 61 can be obtained. Further, in the temporary dither process, if the related data Dn ′ is less than the threshold value THn_d, the processes in steps S135 and S136 can be omitted, so that the process can be speeded up.

G. Variations:
A modification of the above embodiment will be described.
G-1. Modification 1:
In the halftone process of the above-described embodiment, the provisional dither process is performed for each pixel when determining the ON / OFF state of the dot for each pixel. However, the temporary dither process is performed for a predetermined range of pixels collectively. May be. That is, the result of provisional dither processing for one pixel may be applied to surrounding pixels. For example, in a 3 pixel × 3 pixel region, provisional dither processing may be performed only for the central pixel, and the result may be applied to all the pixels in the region. In this way, halftone processing can be speeded up. Even in this case, the print image quality is not greatly reduced if the photographic image has a smooth gradation change for each local region.

G-2. Modification 2:
In the halftone processing of the above-described embodiment, the gradation value of the pixel-of-interest data Dn is compared with various threshold values, and the ON / OFF determination of the dot by the temporary dither processing or the error diffusion method is performed. The gradation value of the data Dn may be converted into a recording rate based on a predetermined conversion rule, and the gradation value of the recording rate may be compared with various threshold values. The recording rate refers to the rate at which dots are recorded on pixels in an arbitrary area. For example, when the printer 20 forms an image with dots of a plurality of sizes, such as large dots and small dots, the recording rate scale calculated for each dot size based on the gradation value of the target pixel data Dn. The adjustment value may be compared with various threshold values.

G-3. Modification 3:
In the embodiment described above, the error diffusion threshold value table 62 is configured such that the threshold value difference ΔTHe increases as the gradation value increases or decreases as the gradation value decreases with respect to a predetermined range of gradation values. However, in the error diffusion method, it may be set so that the dithering factor is particularly strong, that is, the threshold difference ΔTHe is large in the vicinity of a gradation value at which a texture pattern is likely to occur. The gradation value at which a texture pattern is likely to occur varies depending on the characteristics of the printer 20, but in general, the gradation value at which the ink duty is 1 / N (N is an integer equal to or greater than 2)% or in the vicinity thereof. is there. By so doing, it is possible to suppress the generation of texture patterns by the error diffusion method. Even in such a case, if the threshold difference ΔTHe is smoothly changed stepwise, the contribution degree between the dithering element and the error diffusion element can be changed smoothly.

G-4. Modification 4:
The halftone process of the present invention may adopt a different form for each ink color. For example, the printer 20 may change the change characteristic of the threshold difference ΔTHe according to the gradation value according to the ink color. Such a configuration can be realized, for example, by the printer 20 storing a plurality of error diffusion threshold tables 62 having different characteristics and switching the error diffusion threshold tables 62 used in the halftone process according to the ink color.

  Alternatively, the halftone processing method may be switched for each ink color. For example, the printer 20 performs the halftone process shown in FIG. 8 and FIG. 9 for ink that is relatively difficult to be seen by human eyes, such as yellow ink, and FIG. 3 shows other inks. Alternatively, a configuration that performs halftone processing may be used. This is because ink that is difficult to visually recognize hardly affects the graininess of the print image quality, and therefore, even if a dither method that is inferior to the error diffusion method from the viewpoint of graininess is used, the print image quality is hardly affected. In this way, the halftone process can be speeded up.

  Alternatively, dark ink with relatively dark color material (in this embodiment, cyan ink and magenta ink) and light ink with relatively low color material density (in this embodiment, light cyan ink and light magenta ink) 8 and FIG. 9, the halftone process shown in FIG. 8 and FIG. 9 is performed, and the ON / OFF determination of the dots by the dither method alone is performed (steps S232 to S234) from the gradation value of the dark ink lower than that of the light ink. It is good also as a structure to switch. In this way, halftone processing can be speeded up with a small influence on print quality. Of the dark and light inks, the dark ink is rarely used alone, so it is better to improve the image quality by relatively improving the graininess of the light ink dots that are less noticeable by the error diffusion method. This is because the effect is great.

G-5. Modification 5:
In the halftone processing of the above-described embodiment, the configuration in which the threshold difference ΔTHe is changed according to the gradation value has been described. However, the threshold difference ΔTHe may be a constant value that does not depend on the gradation value. Even in this case, it is possible to perform halftone processing that incorporates both error diffusion and dithering elements.

G-6. Modification 6:
In the above-described embodiment, the printer 20 controls the ease with which dots are formed in the error diffusion method by changing the threshold value THe used for dot ON / OFF determination in the error diffusion method, and error in the halftone process. Although the configuration in which the contribution degree of the diffusion law element and the dither law element is changed is shown, the control of the ease of dot formation by the error diffusion method is not limited to this mode. For example, on the basis of the result of the temporary dither processing, the dot value is turned on by adding a predetermined value β (β> 0) or subtracting the predetermined value β to the gradation value of the target pixel data Dn to which the error diffusion method is applied. It may be configured to determine / OFF. At this time, the binarization error En may be calculated by excluding the predetermined value β in order to accurately reflect the gradation of the entire image data. Of course, the predetermined value β may be given as a value that changes according to the gradation value of the input target pixel data Dn. Even with such a configuration, it is possible to suitably control the ease of dot formation in the error diffusion method, as in the case of changing the threshold value THe.

G-7. Modification 7:
In the above-described embodiment, the printer 20 is configured to execute all of the print processing illustrated in FIG. 2, but the print processing is performed in a printing system (broadly defined printing apparatus) in which the printer and the computer are connected. For example, all or part of the printing process and the halftone process may be performed by either a computer or a printer.

  The embodiment of the present invention has been described above, but among the components of the present invention in the above-described embodiment, elements other than the elements described in the independent claims are additional elements and can be omitted as appropriate. In addition, the present invention is not limited to such an embodiment, and it is needless to say that the present invention can be implemented in various modes without departing from the gist of the present invention. For example, the present invention is not limited to the serial ink jet printer shown in the above-described embodiment, but can be applied to various types of printing apparatuses such as an ink jet line printer and a laser printer. In addition to the configuration as a printing apparatus, the present invention can be realized as a printing method, a program, a storage medium, and the like.

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 70 ... Carriage motor 71 ... Drive belt 72 ... Pulley 73 ... Sliding shaft 74 ... Paper feed motor 75 ... Platen 80 ... Carriage 82-87 ... Ink cartridge 90 ... Print head 98 ... Memory Card slot 99 ... Operation panel P ... Print medium MC ... Memory card

Claims (7)

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 a data gradation value that is a gradation value of the input image data with a first threshold that is one of a plurality of thresholds;
An error diffusion unit that generates the dot data by applying an error diffusion method to the data gradation value using a second threshold value determined according to the data gradation value ;
A determination unit that determines whether or not the data gradation value is equal to or greater than a predetermined gradation value ;
Wherein the determination unit, is determined data tone value and Ru der than predetermined gradation value lever, to determine the presence or absence of the dot formed by the result of the comparison of the comparison unit,
Wherein the determination unit, is determined data tone value and Ru der less than the predetermined gray level value lever, a printing apparatus for determining the presence or absence of the dot formed by the error diffusion unit using the second threshold value.   The printing apparatus according to claim 1, wherein the error diffusion unit controls the ease with which the dots are formed by changing the second threshold value in the error diffusion method. The error diffusion unit, on the basis of the magnitude of the gray scale values, the printing apparatus of the dot formation of the easiness of controlling the degree of the change is allowed Ru claim 2 in.   The error diffusing unit has a data gradation value in a predetermined range that is relatively small for high gradation data in the image data that is in a predetermined range in which the data gradation value is relatively large. The printing apparatus according to claim 3, wherein the degree of control of the ease with which the dots are formed is greater than for certain low gradation data.   5. The printing apparatus according to claim 3, wherein the error diffusion unit changes the degree of control of the ease of forming the dots in a stepwise manner based on the magnitude of the data gradation value.   The error diffusing unit performs the control so that the degree of control of the ease of dot formation increases as the data gradation value increases in at least a part of the range of the data gradation value. The printing apparatus according to claim 3, wherein the printing apparatus is performed. A printing method for performing printing based on image data representing a predetermined image,
Input the image data,
A process of comparing the magnitude of the data gradation value that is the gradation value of the input image data and the first threshold value that is one threshold value among a plurality of threshold values, and depending on the data gradation value Performing a process of applying an error diffusion method to the data gradation value using a second threshold value for which a value is determined;
If the data grayscale value is equal to or higher than a predetermined gradation value, as a result of the previous SL comparison generates dot data representing the presence or absence of the dot formation, wherein the data grayscale value is less than the predetermined gray level value, the Generating dot data indicating the presence or absence of the dot formation by a process of applying the error diffusion method using a second threshold;
A printing method for printing the image using the generated dot data.

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