JP2017114033A - Printing device, printing method, image processing device and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a printing device which can improve the quality of characters and lines in a pale background part while taking advantage of a dither method and error diffusion method.SOLUTION: A printing device which prints an image on the basis of image data includes: an input unit 41 which inputs the image data; a conversion unit 51 which converts an input gradation value being a gradation value of the input image data to generate a converted gradation value; a comparison unit 53 which compares the converted gradation value with the threshold of a dither mask to output a comparison result; an error diffusion processing unit 55 which generates dot data by an error diffusion method on the basis of the comparison result and input gradation value; and a printing unit 57 which prints an image by using the generated dot data. The conversion unit 51 generates the converted gradation value by reducing a prescribed gradation value from the input gradation value when the input gradation value is within a prescribed range.SELECTED DRAWING: Figure 1

Description

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

  Conventionally, as shown in Patent Document 1, when printing an object including a photograph, a figure, a graph, a character, and a line in a printing apparatus, as a halftone technique that achieves both print qualities, the photographic image quality is improved. A hybrid halftone technique that combines an effective dithering method and an error diffusion method effective for printing quality of characters and lines has been proposed.

JP 2014-78980 A

  However, in the printing apparatus described in Patent Document 1, when the hybrid halftone technique is applied to characters / lines arranged on a light background, dots by the dither method are generated in the light background region. When the error diffusion method is applied to such a dither result, a negative error value is accumulated, and there is a problem that it is difficult to generate dots in the character / line portion. As a result, there has been a problem that the line in the light background portion is interrupted or a part of the outline portion of the character is missing.

  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.

  Application Example 1 A printing apparatus according to this application example is a printing apparatus that prints an image based on image data, and includes an input unit that inputs the image data and a gradation value of the input image data. A conversion unit that converts the input gradation value to generate a converted gradation value; a comparison unit that outputs a comparison result by comparing with a threshold value of a dither mask based on the converted gradation value; and the comparison result and the input An error diffusion processing unit that generates dot data by an error diffusion method based on a gradation value, and a printing unit that prints the image using the generated dot data, and the conversion unit includes the input When the gradation value is within a predetermined range, the conversion gradation value is generated by subtracting the predetermined gradation value from the input gradation value.

  According to this application example, the conversion unit subtracts a predetermined gradation value from the input gradation value to generate a converted gradation value. Since the subtracted converted gradation value is less likely to exceed the dither mask threshold than the input gradation value, the occurrence of dots in the comparison result is reduced. As a result, it is difficult for the error diffusion processing unit to accumulate negative error values, and it is possible to easily generate dots existing in the input image data. Further, the conversion unit subtracts the input gradation value in a predetermined range. For example, by specifying a range other than the gradation value that the dot of the character / line part has as a predetermined range, the occurrence of dots in the comparison result of the area other than the character / line part is reduced, and it exists from the input image data. It becomes difficult to lack the dots in the letters and lines. Therefore, it is less likely that the line in the light background portion is interrupted or a part of the outline portion of the character is lost.

  Application Example 2 In the printing apparatus according to the application example, in the first range in which the input gradation value is relatively minimum in the range obtained by dividing the predetermined range, the conversion unit may perform the predetermined gradation. The converted gradation value is generated using a value as the input gradation value.

  According to this application example, in the first range, the value of the converted gradation value generated using the predetermined gradation value as the input gradation value is zero, and therefore does not exceed the threshold value of the dither mask. In the first range, dot generation due to dither is eliminated. Further, since the first range is a relatively minimum range, for example, it can be a light background portion. This eliminates the occurrence of dots in the light background portion, so that the ease of dot generation can be maximized in the character / line portion having the light background portion.

  Application Example 3 In the printing apparatus according to the application example, in the second range other than the range in which the input tone value is relatively minimum in the range obtained by dividing the predetermined range, the conversion unit The converted gradation value is generated by setting a predetermined gradation value from the input gradation value to a gradation value equal to or lower than the input gradation value.

  According to this application example, the converted gradation value in the second range is calculated by subtracting the gradation value less than or equal to the input gradation value, so that the maximum value in the first range and the minimum value other than the predetermined range are A value between can be taken. The calculated converted gradation value can smoothly increase between the maximum value in the first range and the minimum value outside the predetermined range. As described above, for example, a light portion of a gradation image can be smoothly expressed by the converted gradation value in the second range.

  Application Example 4 In the printing apparatus according to the application example described above, the predetermined range includes a maximum gradation value that is a maximum value indicating the maximum density of gradation values and a minimum value that indicates the lightness of the gradation value. It is not more than an intermediate gradation value indicating the middle of a certain minimum gradation value, and is not less than the minimum gradation value.

  According to this application example, the predetermined range can take a light area equal to or lower than the intermediate gradation value. For example, when the maximum gradation value is 255 and the minimum gradation value is 0, the gradation value in the predetermined range is effective for a light background portion having a light gradation value of 0 to 127.

  Application Example 5 In the printing apparatus according to the application example described above, the predetermined range is a maximum gradation value that is a maximum value that indicates the maximum density of gradation values and a minimum value that indicates the lightness of the gradation value. It is characterized in that it is the range closest to the minimum gradation value among the gradation value ranges indicating the quarter of the minimum gradation value.

  According to this application example, the predetermined range can take a light area closest to the minimum gradation value. For example, when the maximum gradation value is 255 and the minimum gradation value is 0, the gradation value in the predetermined range is effective for a light background portion having a light gradation value of 0 to 63.

  Application Example 6 In the printing apparatus according to the application example, in the error diffusion processing unit, an error value between the comparison result and the input gradation value is calculated for pixels around the pixel in the pixels constituting the image data. Then, the corrected gradation value generated by adding the error value is compared with a threshold value for error diffusion to generate dot data.

  According to this application example, by using the corrected gradation value, the error value generated in the binarization process in the error diffusion is not discarded, so that dot data in which the error value is reflected on the surrounding pixels is generated. Can do.

  Application Example 7 The printing method described in this application example is a printing method for printing an image based on image data, and includes an input step for inputting the image data, and a gradation value of the input image data. A conversion step of converting a certain input gradation value to generate a conversion gradation value; a comparison step of comparing a dither mask threshold value based on the conversion gradation value and outputting a comparison result; and the comparison result and the input An error diffusion processing step of generating dot data by an error diffusion method based on a gradation value; and a printing step of printing the image using the generated dot data, wherein the conversion step includes the input When the gradation value is within a predetermined range, the conversion gradation value is generated by subtracting the predetermined gradation value from the input gradation value.

  According to this application example, since the converted gradation value obtained by subtracting the predetermined gradation value from the input gradation value is less likely to exceed the threshold value of the dither mask, the occurrence of dots in the comparison result is reduced. This makes it difficult for negative error values to accumulate in the error diffusion processing step, making it easier to generate dots that existed from the original image data. In the conversion process, for example, by specifying a range other than the gradation value that the dot of the character / line part has as the predetermined range, the occurrence of dots in the comparison result of the area other than the character / line part is reduced, and the original exists. It becomes difficult to lack the dots in the characters and lines. Therefore, it is less likely that the line in the light background portion is interrupted or a part of the outline portion of the character is lost.

  Application Example 8 A program described in this application example includes an input step for inputting image data, and a conversion for generating a converted gradation value by converting an input gradation value that is a gradation value of the input image data. Dot data is generated by an error diffusion method based on the step, a comparison step of comparing with the threshold value of the dither mask based on the converted gradation value and outputting a comparison result, and the comparison result and the input gradation value An error diffusion processing step, and a printing step for printing an image using the generated dot data, and the conversion step starts from the input tone value when the input tone value is within a predetermined range. A computer is caused to generate the converted gradation value by subtracting a predetermined gradation value.

  According to this application example, since the converted gradation value obtained by subtracting the predetermined gradation value from the input gradation value is less likely to exceed the threshold value of the dither mask, the occurrence of dots in the comparison result is reduced. This makes it difficult for negative error values to accumulate in the error diffusion processing step, making it easier to generate dots that existed from the original image data. In the conversion process, for example, by specifying a range other than the gradation value that the dot of the character / line part has as the predetermined range, the occurrence of dots in the comparison result of the area other than the character / line part is reduced, and the original exists. It becomes difficult to lack the dots in the characters and lines. Therefore, it is less likely that the line in the light background portion is interrupted or a part of the outline portion of the character is lost.

  Application Example 9 The image processing apparatus described in this application example is an image processing apparatus that generates dot data based on image data, and includes an input unit that inputs the image data, and a level of the input image data. A conversion unit that converts an input gradation value that is a tone value to generate a converted gradation value; a comparison unit that compares a dither mask threshold value based on the converted gradation value and outputs a comparison result; and the comparison result And an error diffusion processing unit that generates the dot data by an error diffusion method based on the input gradation value, and the conversion unit, when the input gradation value is within a predetermined range, The converted gradation value is generated by subtracting a predetermined gradation value from the gradation value.

  According to this application example, since the converted gradation value obtained by subtracting the predetermined gradation value from the input gradation value does not easily exceed the threshold value of the dither mask, the occurrence of dots due to dither is reduced. As a result, it is difficult for the error diffusion processing unit to accumulate negative error values, and it is possible to easily generate dots existing from the original image data. In the conversion unit, for example, by specifying a range other than the gradation value of the dot of the character / line part as the predetermined range, the occurrence of dots in the comparison result of the area other than the character / line part is reduced, and the original exists. It becomes difficult to lack the dots in the characters and lines. Therefore, it is less likely that the line in the light background portion is interrupted or a part of the outline portion of the character is lost.

FIG. 2 is a block diagram illustrating a schematic configuration of a printer. The flowchart figure which shows the flow of the printing process in a printer. The flowchart figure which shows the flow of a halftone process. The flowchart figure which shows the flow of a dither halftone process. The graph which shows the relationship between an input gradation value and a conversion gradation value. The dot pattern figure (original image data) explaining an effect. The dot pattern figure explaining an effect (halftone result by the conventional method). The dot pattern figure explaining an effect (halftone result by an embodiment).

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, the scale of each part is made different from the actual scale in order to make each part recognizable.

(Embodiment 1)
(Configuration of printing device)
FIG. 1 is a block diagram illustrating a schematic configuration of a printer. The printer 20 as a printing apparatus is a serial ink jet printer that performs bidirectional printing and includes an operation panel 99, a memory card slot 98, a control unit 30, a printing mechanism unit 70, and the like.

  The control unit 30 is a control circuit or an ASIC (Application Specific Integrated Circuit) configured by the control unit 40, the storage unit 60, and the like.

The control unit 40 is a CPU (Central Processing Unit), and comprehensively controls each unit such as the storage unit 60, the printing mechanism unit 70, the memory card slot 98, and the operation panel 99.
The control unit 40 includes an input unit 41, a halftone processing unit 50, a printing unit 57, and the like as functional units. However, these functional units are merely described as one embodiment, and all of these functional units do not necessarily have to be essential components. Moreover, it is good also considering a function part other than these as an essential component.
Further, not only the example of FIG. 1 but also the whole or a part of the operation panel 99, the memory card slot 98, and the control unit 30 may be configured in the host device. The host device includes a computer such as a personal computer, a digital camera, a digital video camera, a mobile phone such as a smartphone, and the like.

  The input unit 41 reads RGB (red, green, blue) format image data to be printed from the memory card MC in the memory card slot 98. The input unit 41 converts image data in the RGB color space into a color space of a recording material such as ink. The color space of the recording material is, for example, a color space of CMYK (cyan, magenta, yellow, black). In the color space conversion process, the gradation value of each color of RGB is converted into the gradation value of each color of CMYK using a color conversion table (not shown). The converted image data has a gradation value of each color of CMYK for each pixel. The color space of the recording material is not limited to four colors of CMYK, and may be six colors including Lc (light cyan), Lm (light magenta), etc., and other colors including a single color (one color). It may be the number of colors.

  The halftone processing unit 50 includes a conversion unit 51, a comparison unit 53, an error diffusion processing unit 55, and the like, and performs halftone processing on input gradation values that are gradation values of input image data (for details, see FIG. Then, dot data is generated. The dot data is data representing the dot formation state (dot pattern) in the pixels constituting the image data. Specifically, dots are formed for each gradation value of the color space of the recording material of the pixels. This is data indicating whether or not (or which size dot is formed). Here, “dot” refers to one area formed by landing ink droplets or ink droplets ejected from ink nozzles provided in the ink head of the printer 20 on a printing medium such as printing paper. The dot data is generated for each color, but in the halftone process, the same process is applied to each color. In the following description, it will be described as a monochromatic process.

The conversion unit 51 converts an input gradation value into a converted gradation value using an input gradation value conversion table 61 (details will be described later). Specifically, an input gradation value corresponding to a predetermined range is subtracted and converted to a converted gradation value. For example, when the gradation value is 256, the lightest gradation value is 0, and the darkest gradation value is 255, the range where the gradation value representing the light density is 0 to 64 is set as the predetermined range. In this way, the gradation value representing the light density is subtracted, and the subtraction is not performed for the other ranges. In the following description, the gradation value is described in 256 stages unless otherwise specified.
The method of converting the input gradation value corresponding to the predetermined range in the subtraction direction refers to the input gradation value conversion table 61, converts the gradation value of each pixel of the image data, and converts the conversion gradation for each pixel. Calculate the value. The input tone value conversion table 61 stores converted tone values corresponding to the input tone values. Details will be described later.

  The comparison unit 53 outputs a comparison result between the converted gradation value and the threshold value of the dither mask 63. Specifically, the converted gradation value of each pixel is compared with the threshold value of the dither mask 63. The dither mask 63 is compared with each pixel having a converted gradation value. When the converted gradation value exceeds the threshold value, the dither result is ON (dot generation). Otherwise, the dither result is OFF (no dot). To do.

  The error diffusion processing unit 55 performs error diffusion processing using the comparison result subjected to the dither mask processing by the comparison unit 53 and the error diffusion threshold value table 65 to generate dot data. The error diffusion threshold value table 65 has a plurality of types of threshold value tables, and may be selected from them. For example, when the comparison result is the dither result ON, the dither result ON error diffusion threshold table is selected, and when the comparison result is the dither result OFF, the dither result OFF error diffusion threshold table is selected.

  The printing unit 57 controls the printing mechanism unit 70 to output dot data to the printing mechanism unit 70. Specifically, the dot data generated for each color of the recording material by the error diffusion processing unit 55 is output to a printing mechanism unit 70 having a print head (not shown), and printed on a print medium by the print head.

  The storage unit 60 includes an arithmetic processing unit such as a ROM (Read Only Memory), a RAM (Random Access Memory), an NVRAM (Non Volatile Random Access Memory), and an EEPROM (Electrically Erasable Programmable Read Only Memory), a volatile memory, and a nonvolatile memory. An input tone value conversion table 61, a dither mask 63, an error diffusion threshold table 65, a program 67, and the like are stored.

  The input tone value conversion table 61 converts the input tone value to be compared with the threshold value of the dither mask 63, and the maximum tone value indicating the highest density of the input tone value and the minimum tone value indicating the lightness. Consists of a range of values. In order to extend the accuracy, the resolution may be higher than the input gradation value. Details of the input tone value conversion table 61 will be described later with reference to FIG.

  The dither mask 63 is a table used for halftone processing by a known systematic dither method, and includes a plurality of threshold values arranged in a matrix.

  The error diffusion threshold table 65 is used for halftone processing by the error diffusion method, and stores thresholds corresponding to input tone values.

  The program 67 is a program in which processing from halftone processing to output of dot data is described from input image data, and is read and executed by the control unit 40.

  The printing mechanism unit 70 includes a transport mechanism that transports the print medium, a reciprocating mechanism that reciprocates the carriage, and a dot formation mechanism that discharges ink droplets onto the print medium to form dots (all of which are not shown). ing. In the printing mechanism unit 70, when ink droplets are ejected onto the print medium based on the dot data output from the printing unit 57, images and characters are printed on the print medium. These show the configuration examples of the serial type ink jet printer, but the ink jet printer to which the present technology can be applied may be a line type ink jet printer or the like. A printing apparatus to which the present technology can be applied may be an electrophotographic printer such as a laser printer, a copying machine, a facsimile, a multifunction machine having these functions, or the like.

(Printing process)
FIG. 2 is a flowchart showing the flow of printing processing in the printer. For example, the print process is triggered by a user performing a print instruction operation on a predetermined image stored in the memory card MC using the operation panel 99 or the like. The following flow corresponds to a printing method, and is executed by the control unit 40 controlling each unit including the storage unit 60 and the printing mechanism unit 70 based on the program 67 shown in FIG.

  In step S100, image data is input. Specifically, when it is detected that the memory card MC is inserted into the memory card slot 98, the RGB format image data ORG to be printed is read. This step corresponds to an input process.

  In step S110, color conversion processing is performed. Specifically, referring to a color conversion table (not shown) stored in the storage unit 60, the RGB image data is color-converted to the CMYKLcLm format. In this step, image processing such as resolution conversion and smoothing processing may be performed before and after the color conversion processing.

  In step S120, halftone processing is executed. Halftone processing is to determine whether or not to eject small / medium / large ink droplets ejected from each color ink nozzle mounted on the carriage using gradation values (input gradation values) given to each pixel of image data. This is a process of converting into predetermined data (the dot data described above). In the halftone process of this embodiment, the dot data is converted into dot data that determines whether small, medium, and large ink droplets are ejected or not. However, in the halftone process, tone number conversion (including multi-value processing) The reduction is not limited to this example. For example, the dot data may be converted into dot data that defines ON / OFF of light and dark dots, or may be converted into dot data that defines ON / OFF of small, medium, large, and non-light dots.

  In step S130, an interlace process is executed. More specifically, the halftone processed dot arrangement is rearranged into dot pattern data formed in one main scanning unit in accordance with the nozzle arrangement of the printer 20 and the paper feed amount.

In step S140, printing is executed. Specifically, the printing mechanism unit 70 is controlled to drive a print head, a carriage motor, a paper feed motor, and the like (all not shown) to execute printing.
Steps S130 and S140 correspond to a printing process.

(Halftone processing)
FIG. 3 is a flowchart showing the flow of halftone processing. In the halftone processing, error diffusion processing is performed based on the dither halftone result, and dot ON / OFF data is generated. This flow is a flow for realizing the halftone processing in step S120 (FIG. 2). When this flow ends, the process proceeds to step S130 (FIG. 2).

  In step S200, dither halftone processing is executed. The dither halftone is a process of comparing the threshold value of the dither mask 63 stored in the storage unit 60 with the input tone value. As a result of comparison for each pixel, when the input gradation value is larger than the threshold value of the dither mask 63, the pixel is output as “dot ON”, and otherwise, it is output as “dot OFF”.

  In step S210, it is determined whether the dither comparison result in step S200 is ON or OFF. In this step, referring to the dot ON / OFF of the pixel output in step S200, if the dot is ON (ON), the process proceeds to step S220. If the dot is OFF (OFF), the process proceeds to step S230.

  In step S220, the dither result ON-use error diffusion threshold table is selected with reference to the output indicating that the dither comparison result in step S210 is ON. The error diffusion ON threshold table is one of the error diffusion threshold tables 65 stored in the storage unit 60, and the threshold is set low (−255) so as to coincide with the result determined to be ON by the dither comparison. Error diffusion threshold.

  In step S230, an error diffusion threshold table for dither result OFF is selected with reference to the output that the dither comparison result in step S210 is OFF. The error diffusion OFF threshold table is one of the error diffusion threshold tables 65 stored in the storage unit 60, and is a threshold table equivalent to a normal error diffusion threshold table. Accordingly, when it is determined that the dither comparison is OFF, the binarization error is diffused to the surrounding pixels and the error is not discarded, so that the error diffusion characteristic that the line connection is improved is exhibited.

  In step S240, the input tone value and the error diffusion threshold are compared to determine whether or not the input tone value is greater than the error diffusion threshold. As the error diffusion threshold at this time, the value of the error diffusion threshold table selected in step S220 and step S230 is used. When the input tone value of the target pixel is larger than the error diffusion threshold (Yes), the process proceeds to step S250, and when the input tone value is equal to or less than the error diffusion threshold (No), the process proceeds to step S260.

  In step S250, the dot of the target pixel is turned ON. If the input tone value exceeds the error diffusion threshold as a result of comparing the input tone value with the error diffusion threshold in step S240, the dot data to be generated is turned ON.

  In step S260, the dot of the target pixel is turned off. If the input tone value is equal to or smaller than the error diffusion threshold as a result of comparing the input tone value with the error diffusion threshold in step S240, the dot data to be generated is turned OFF.

In step S270, the error value of the binarization result generated in step S250 or step S260 is diffused and applied to the gradation values of surrounding pixels.
The processing from step S200 to S270 is repeatedly performed for each pixel of the image data, and when the processing of all the pixels of the image data is completed, the processing proceeds to step S130. Note that the gradation value of the pixel to which the error value is applied in step S270 is reflected in the input gradation value in step S240, and the process is repeated.
Note that the processing from steps S220 to S270 corresponds to an error diffusion processing step.

(Dither halftone)
FIG. 4 is a flowchart showing the flow of dither halftone. In dither halftone, an input tone value is converted into a converted tone value using an input tone value conversion table 61 described in FIG. The converted converted gradation value is compared with the threshold value of the dither mask 63, and if the converted gradation value exceeds the threshold value of the dither mask 63, it is determined that the dither result is ON, and otherwise, the dither result is OFF.

  In step S300, input tone value conversion is executed. In the conversion of the input gradation value, the input gradation value conversion table 61 is used to convert the input gradation value into the converted gradation value. This step corresponds to a conversion process.

  In step S310, the converted gradation value converted in step S300 is compared with the threshold value of the dither mask 63. If the converted gradation value exceeds the threshold value of the dither mask 63 (Yes) as a result of comparing the two, the process proceeds to step S320, and if the converted gradation value does not exceed the dither mask 63 (No), step S320 is performed. Proceed to S330.

  In step S320, the dither result is turned ON. Specifically, if the converted gradation value exceeds the threshold value of the dither mask 63 as a result of the comparison in step S310, the dither result is set to “ON”. The pixel set to “ON” in this step is not the dot data to be finally output, but when the error diffusion threshold table for dither result ON is selected in the error diffusion processing to be processed later (steps S210 and S220). ).

In step S330, the dither result is turned off. Specifically, if the converted gradation value is equal to or less than the threshold value of the dither mask 63 as a result of the comparison in step S310, the dither result is set to “OFF”. The pixel set to “OFF” in this step is not the dot data to be finally output, but when the error diffusion threshold table for dither result OFF is selected in the error diffusion processing to be processed later (steps S210 and S230). ).
Steps S310 to S330 correspond to a comparison process.

  In this way, the input gradation value corresponding to the light gradation value is subtracted, and the dither mask process is applied to the subtracted converted gradation value, thereby making it difficult to turn on the dither result of the light gradation value. Can do.

(Conversion from input gradation value to converted gradation value)
Next, the relationship between the input gradation value converted by the conversion unit 51 and the converted gradation value will be described with reference to FIG.
FIG. 5 is a graph showing the relationship between the input gradation value and the converted gradation value.
The graph 100 is a graph showing the relationship between the input gradation value converted by the input gradation value conversion table 61 stored in the storage unit 60 and the converted gradation value. The horizontal axis is the input gradation value, the vertical axis is the converted gradation value, and both axes indicate integer values from “0” (light) to “255” (dark). A solid line 101 indicates the relationship between the input gradation value and the converted gradation value, and a point where the input gradation value intersects the solid line 101 is converted as a converted gradation value. A dotted line 103 is an auxiliary line provided for comparison with the solid line 101, and shows a relationship when the input gradation value and the converted gradation value are equal.
A range A indicates a range where the input gradation value is “0” or more and “63” or less. A range D indicates a range where the input gradation value is “64” or more and “255” or less. A range B indicates a range where the input gradation value is “0” or more and “15” or less. A range C indicates a range where the input gradation value is “16” or more and “63” or less.

In the range A, the solid line 101 is biased toward the converted gradation value “0” side than the dotted line 103. Further, as the bias tendency approaches the end of the range A, the dotted line 103 approaches the solid line 101, and when the input tone value at the tip of the range D is “64”, the converted tone value is “64”. Shows a match.
A range B is included in the range A and indicates a range where the input gradation value is “0” or more and “15” or less. In this range, in the solid line 101, the converted gradation value indicates “0”. That is, the converted gradation value in the range B (input gradation value “0” or more and “15” or less) is “0” (the lightest density).
In the range C, the solid line 101 gradually increases from the converted gradation value “0” at the end of the range B to the value of the converted gradation value equal to the dotted line 103 toward the end of the range A.
In the range D, since the solid line 101 overlaps the dotted line 103, the input gradation value is output as it is as the converted gradation value.

  By such conversion, in the range B, the input gradation value is converted to zero so that the converted gradation value does not exceed the threshold value of the dither mask 63 in step S310. Thereby, in the input gradation value in the range B, for example, in the light background portion where the input gradation is small, the generation of dots due to dither can be eliminated.

  In the range C, the discontinuous region from the end of the range B to the tip of the range D is interpolated so as to be continuous. Thereby, a gradation image etc. can be expressed smoothly.

  The input tone value conversion table 61 includes an input item corresponding to the input tone value and an output item corresponding to the converted tone value. The input items include numerical values from “0” to “255” and output. In the item, the numerical value of the converted gradation value of the solid line 101 corresponding to the numerical value (input gradation value) of the input item is stored. The conversion unit 51 can acquire the numerical value of the converted gradation value by inputting the input gradation value into the input item of the input gradation value conversion table 61.

The range A corresponds to a predetermined range, and the relatively minimum range B corresponds to the first range. A range C other than the range B corresponds to the second range. The difference between the dotted line 103 and the solid line 101 corresponds to a predetermined gradation value obtained by subtracting the converted gradation value from the input gradation value.
The maximum gradation value is “255”, and the minimum gradation value is “0”. The gradation value ranges indicating quarters (quarters) in the gradation values from “0” to “255” are “0” to “63”, “64” to “127”, and “128” to “128”. "191", "192" to "255", and the range closest to the minimum gradation value "0" is the range "0" to "63", and this range corresponds to a predetermined range.

(Explanation of effect)
Next, the effects when image data is printed by the printer 20 will be described by way of example with reference to FIGS.
6A to 6C are dot pattern diagrams for explaining the effect. FIG. 6A shows original image data, FIG. 6B shows a halftone result by a conventional method, and FIG. 6C shows an example of a halftone result by this embodiment.

  FIG. 6A shows the input original image data, and each grid represents each pixel of the original image. Each pixel has PA (1, 1) from the upper left and PA (6, 6) from the lower right. Pixels (PA (1, 1), PA (2, 1), etc.) indicated by stippling are light background portions with small input gradation values (“16” or less). Pixels (PA (6, 1), PA (5, 2), etc.) indicated by diagonal grids have an input tone value larger than the background portion (input tone value “over 64” and “255” or less). Represents. For example, when the original image data is displayed on a display screen or the like, the original image data is displayed such that a diagonal line having a high density is connected to a background part having a low density. The result of binarizing each pixel of the original image and determining dot ON and dot OFF will be described with reference to FIGS. 6B and 6C.

  6B and 6C are the results of halftoning the original image data of FIG. 6A, FIG. 6B is a halftone process performed by the conventional method, and FIG. 6C is a dot pattern resulting from the halftone process performed in the present embodiment. FIG. Each pixel in FIG. 6B and FIG. 6C represents the same area as the dot pattern in FIG. 6A, and the position of each pixel is shown as PB (1,1) to PB (6,6) in FIG. 6B. Shown as PC (1,1) to PC (6,6). Pixels indicated by diagonal lines are pixels whose dots are ON, and pixels indicated by white are pixels whose dots are OFF.

  In FIG. 6B, since the input gradation value of the background portion is compared with the threshold value of the dither mask 63 without being converted, if the input gradation value of the background portion is larger than the threshold value of the dither mask 63, the dot is turned ON. For example, PB (2, 3) indicated by hatching is a pixel with the dot ON. As a result, a negative error value is accumulated in the error diffusion processing unit 55, and it becomes difficult for the dot in the line portion immediately after PB (2, 3) to turn on. That is, there is a possibility that the dots of dark density PA (4,3) and PA (3,4) in the original image data will be turned off at PB (4,3) and PB (3,4). .

On the other hand, in FIG. 6C, since the input gradation value of the background portion is converted into the converted gradation value by the input gradation value conversion table 61, and the converted gradation value does not exceed the threshold value of the dither mask 63, no dots are generated. . For example, the PC (2, 3) shown in white is the dot OFF pixel. As a result, positive error values are accumulated in the error diffusion processing unit 55, and dots (PB (4,3), PB (3,4)) that did not occur in FIG. 6B are generated in FIG. 6C (FIG. 6C). PC (4,3), (3,4)).
6C, dots are generated in the diagonally shaded pixels of PC (1, 1). However, in the background portion with a light density, some dots ON pixels are necessary, which is the occurrence of dots. Note that this dot ON does not affect a line portion having a high density.

  According to such a halftone process of the present embodiment, since the pixel having a small input gradation value does not exceed the threshold value of the dither mask 63 in the conversion unit 51, the dither result is not easily turned on. As a result, it is difficult for the error diffusion processing unit 55 to accumulate negative error values, and it is possible to easily generate dots existing from the original image data.

As described above, the printer 20 in this embodiment can obtain the following effects.
Based on the input tone value conversion table 61, the conversion unit 51 converts the tone value of a pixel having a tone value with a light density (“0” to “64”) among the input tone values. The converted gradation value is output by subtraction. Since the converted gradation value obtained by subtracting the input gradation value is less likely to exceed the threshold value of the dither mask 63, the occurrence of dots due to dither is reduced. As a result, it is difficult for the error diffusion processing unit 55 to accumulate negative error values, and it is possible to easily generate dots existing from the original image data. As shown in FIG. 6, the relatively dark line portion having a light background portion has been interrupted in the conventional method, but in the present embodiment, the line can be connected without being interrupted.
In this way, by subtracting the input tone value in the background area with the light density, the dark density line breaks that occurred near the boundary with the background area with the light density, or the outline of characters, etc. The lack of dots will be less.

  Note that the present invention is not limited to the above-described embodiment, and various modifications and improvements can be added to the above-described embodiment. A modification will be described below.

(Modification 1)
In the above-described embodiment, the printer 20 has been described as an example of a printing apparatus, but is not limited to such a configuration. Not only an image printing apparatus but also an image processing apparatus with a different display format may be used. The configuration of the image processing apparatus will be described with reference to FIG. The image processing apparatus may include at least the input unit 41, the conversion unit 51, the comparison unit 53, the error diffusion processing unit 55, and the storage unit 60. In this case, the control unit 40 (CPU) inputs image data from the input unit 41, and the dot data is generated by realizing the functions of the conversion unit 51, the comparison unit 53, and the error diffusion processing unit 55. . The dot data can be output as image data generated for each color of the recording material. The same effect as the above-described embodiment can be obtained. Further, the image processing apparatus may be provided with a display device (not shown) such as an electronic paper or a liquid crystal display, for example, and information such as figures, graphs, characters, and lines is displayed on the display device. A device that displays only binary values may be used. Since the print medium of the above-described embodiment is merely replaced with a display device, the same effect as that of the above-described embodiment can be obtained.

(Modification 2)
In the flow of halftone processing (FIGS. 3 and 4) in the embodiment and the modification described above, the procedure of halftone processing is merely an example, some steps are omitted, and other steps are added and executed. Various modifications such as changing the order of the steps to be performed may be made. For example, in the case where the dither result is ON in the dither result determination step in step S210, since “dot ON” is always set in subsequent step S240, the dot data ON in step S250 may be performed first. Similar results can be output.

(Modification 3)
The input tone value conversion table 61 stored in the storage unit 60 (FIG. 1) in the above-described embodiment and modification is assumed to convert the input tone value to the converted tone value in the table. It is not limited to an arithmetic expression. As the arithmetic expression, for example, when the input gradation value of the processing target pixel (x, y) of 256 gradations is 16 or less, (Expression 1) is used.
Conversion gradation value (x, y) = input gradation value (x, y) −input gradation value (x, y) (Expression 1)
When the input gradation value is 17 or more and 64 or less, (Expression 2) is used.
Conversion gradation value (x, y) = input gradation value (x, y) −X (Expression 2)
However, X is a value equal to or smaller than the input gradation value (x, y).
Even in such (Equation 1) and (Equation 2), since the input gradation value can be subtracted, the same effect as the input gradation value conversion table 61 can be obtained.

(Modification 4)
In the above-described embodiment and modification, the predetermined range of the input gradation value is 0 to 64. However, the predetermined range may be another range as long as it corresponds to the gradation value of the light background portion. The most effective range is from 0 to 16 in which positive errors are unlikely to accumulate and dots are unlikely to turn on in characters / line portions. In a range larger than 16, positive errors are likely to accumulate. The value may be widened or narrowed from 64.

(Modification 5)
In the above-described embodiment and modification, in order to reduce the occurrence of dots due to dither in a light background portion, only one predetermined range of input gradation values is illustrated as 0 to 64, but there may be a plurality of predetermined ranges. . For example, the second predetermined range 80 to 128 may be added when it is desired to improve the quality of characters and lines by increasing the generation of dots due to error diffusion at a higher density (gradation values 80 to 128).

(Modification 6)
The predetermined range of the above-described embodiment may be a different value for each recording material such as CMYK. For example, even if the predetermined range of C is 0 to 48 and the predetermined range of M is 0 to 80, as long as the range of 0 to 16 where the maximum effect is obtained is included, the same effect as the above-described embodiment is obtained. can get.

(Modification 7)
In the first range of the above-described embodiment, the input gradation value is subtracted from the input gradation value, and the converted gradation value after subtraction is set to 0, but it may be a negative value. Since the minimum value of the threshold value of the dither mask 63 is a positive value, if the converted gradation value is 0 or less, the threshold value of the dither mask 63 is not exceeded, and the same effect as in the above embodiment can be obtained.

  DESCRIPTION OF SYMBOLS 20 ... Printer, 30 ... Control unit, 40 ... Control part, 41 ... Input part, 50 ... Halftone processing part, 51 ... Conversion part, 53 ... Comparison part, 55 ... Error diffusion processing part, 57 ... Printing part, 60 ... Storage unit 61 ... Input gradation value conversion table 63 ... Dither mask 65 ... Error diffusion threshold table 67 67 Program 70 Printing mechanism unit 98 Memory card slot 99 Operation panel MC Memory card

Claims (9)

A printing device that prints an image based on image data,
An input unit for inputting the image data;
A conversion unit that converts an input gradation value that is a gradation value of the input image data to generate a converted gradation value;
A comparison unit that outputs a comparison result by comparing with a threshold value of a dither mask based on the converted gradation value;
An error diffusion processing unit that generates dot data by an error diffusion method based on the comparison result and the input gradation value;
A printing unit that prints the image using the generated dot data;
With
When the input gradation value is within a predetermined range, the conversion unit generates the converted gradation value by subtracting a predetermined gradation value from the input gradation value.   In the first range in which the input gradation value is relatively minimum in a range obtained by dividing the predetermined range, the conversion unit sets the conversion gradation value as the input gradation value and uses the predetermined gradation value as the input gradation value. The printing apparatus according to claim 1, wherein the printing apparatus generates the printing apparatus.   In the second range other than the range where the input gradation value is relatively minimum in the range obtained by dividing the predetermined range, the conversion unit converts the predetermined gradation value to a gradation value equal to or less than the input gradation value. The printing apparatus according to claim 1, wherein the conversion gradation value is generated.   The predetermined range is equal to or less than the intermediate gradation value indicating the middle between the maximum gradation value which is the maximum value indicating the maximum density of the gradation values and the minimum gradation value which is the minimum value indicating the lightest degree of the gradation value. 4. The printing apparatus according to claim 1, wherein the printing apparatus is equal to or greater than the minimum gradation value. 5.   The predetermined range is a range of gradation values indicating a quarter of a maximum gradation value that is a maximum value indicating the maximum density of gradation values and a minimum value that is a minimum value indicating the lightest degree of gradation values, The printing apparatus according to claim 1, wherein the printing apparatus is in a range closest to the minimum gradation value.   The error diffusion processing unit diffuses an error value between the comparison result and the input gradation value in pixels constituting the image data to pixels around the pixel, and a correction generated by adding the error value The printing apparatus according to claim 1, wherein dot data is generated by comparing a gradation value with an error diffusion threshold value. A printing method for printing an image based on image data,
An input step of inputting the image data;
A conversion step of converting the input gradation value, which is the gradation value of the input image data, to generate a converted gradation value;
A comparison step of comparing a dither mask threshold value based on the converted gradation value and outputting a comparison result;
Based on the comparison result and the input gradation value, an error diffusion processing step of generating dot data by an error diffusion method;
A printing step of printing the image using the generated dot data;
With
In the conversion step, when the input gradation value is within a predetermined range, the conversion gradation value is generated by subtracting the predetermined gradation value from the input gradation value. An input process for inputting image data;
A conversion step of converting the input gradation value, which is the gradation value of the input image data, to generate a converted gradation value;
A comparison step of comparing a dither mask threshold value based on the converted gradation value and outputting a comparison result;
Based on the comparison result and the input gradation value, an error diffusion processing step of generating dot data by an error diffusion method;
A printing step of printing an image using the generated dot data;
With
The converting step generates the converted gradation value by subtracting a predetermined gradation value from the input gradation value when the input gradation value is within a predetermined range;
A program characterized by causing a computer to execute. An image processing device that generates dot data based on image data,
An input unit for inputting the image data;
A conversion unit that converts an input gradation value that is a gradation value of the input image data to generate a converted gradation value;
A comparison unit that compares the threshold value of the dither mask based on the converted gradation value and outputs a comparison result;
An error diffusion processing unit that generates the dot data by an error diffusion method based on the comparison result and the input gradation value;
With
The image processing apparatus according to claim 1, wherein, when the input gradation value is within a predetermined range, the conversion unit generates the converted gradation value by subtracting the predetermined gradation value from the input gradation value.

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