JP2008100529A - Printing for changing recording rate of dots in accordance with size error of ink droplet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress a generation of granularity or banding caused by errors of ink discharge amounts. <P>SOLUTION: In the printing of this invention, a plurality of types of ink droplets having different ink amounts are selectively discharged, thereby forming dots having different sizes in the region of one pixel. A printing device of this invention receives error information which represents the errors of the ink amounts of particular ink droplets of at least one type among these and adjusts the recording rate of the dots formed by the particular ink droplets according to the error information. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for printing an image on a print medium by discharging ink.

  2. Description of the Related Art In recent years, color printers that eject several colors of ink from a print head have become widespread as computer output devices, and are widely used for printing images processed by a computer in multiple colors and multiple gradations. Such gradation expression is also performed by a method of selectively forming any one of a plurality of types of dots having different sizes in one pixel area on the print medium.

  In such a gradation expression method, each gradation value is expressed by recording dots of each size at an appropriate recording rate. This dot recording rate is set in advance to an optimum value that reduces graininess (roughness of the image) and banding (striped image quality degradation) in consideration of the size of each dot.

  However, if there is an error in the ink ejection amount, an error also occurs in the size of each dot. When an error occurs in the size of each dot, there is a problem that the dot recording rate set in advance on the assumption of the size of each dot is not an optimum value.

  The present invention has been made to solve the above-described problems in the prior art, and an object of the present invention is to provide a technique for suppressing the occurrence of graininess and banding due to an error in ink discharge amount.

  In order to solve at least a part of the above-described problems, the present invention provides an area of one pixel by selectively ejecting N types (N is an integer of 2 or more) of ink droplets having different ink amounts onto a print medium. A printing control device that generates print data to be supplied to the printing unit in order to perform printing using a printing unit capable of forming the N types of dots having different sizes, and includes the N types of ink droplets. An error information receiving unit that receives error information indicating an error in the ink amount of at least one type of specific ink droplets, and processing the given original image data, thereby representing the dot formation state of each pixel in the printed image A dot data generation unit that generates dot data, wherein the dot data generation unit is a specific dot recording that is a recording rate of dots formed by the specific ink droplets according to the error information There, characterized in that it is configured to generate dot data adjusted.

  According to the printing control apparatus of the present invention, since the recording rate of at least one of the plurality of types of dots is adjusted according to the error information, graininess and banding caused by an error in the ink ejection amount are adjusted. Occurrence can be suppressed.

  In the printing control apparatus, the specific ink droplet is an ink droplet for forming a relatively small specific dot among the N types of dots, and the dot data generation unit is configured to set an upper limit value of the specific dot recording rate. Is preferably configured to generate dot data that has been changed.

  Since the maximum value of the recording rate of a relatively small dot among a plurality of types of dots has a relatively large effect on the occurrence of banding, changing the upper limit of the recording rate of such a dot effectively improves the graininess and Banding can be suppressed.

  In the printing control apparatus, the dot data generation unit increases the upper limit value of the specific dot recording rate when the error information indicates that the specific ink droplet is large due to an error. Is preferred.

  When ink droplets of relatively small dots increase, banding tends not to occur even if the dot recording rate is increased. Therefore, recording of relatively large dots that are easy to see by increasing the upper limit of the dot recording rate The rate can be lowered. As a result, it is possible to improve the print quality by reducing the graininess while suppressing the occurrence of banding.

  In the print control apparatus, the dot data generation unit reduces the upper limit value of the specific dot recording rate when the error information indicates that the specific ink droplet is small due to an error. Is preferred.

  When the ink droplet of a relatively small dot becomes small, banding tends to easily occur due to the recording of the dot. However, even in such a case, with the above configuration, the upper limit of the dot recording rate can be lowered to suppress the occurrence of banding.

  In the printing control apparatus, the specific ink droplet may be an ink droplet having the smallest ink amount among the N types of ink droplets.

  In the printing control apparatus, the dot data generation unit includes a table storage unit that stores a plurality of dot recording rate tables that can be used for determining the dot recording rate of the N types of dots, and the error data according to the error information. And a table selection unit that selects one of the plurality of dot recording rate tables, and the plurality of dot recording rate tables preferably include a specific table selected according to the error information. .

  In this way, the dot recording rate can be adjusted only by selecting the dot recording rate table according to the error information, so that the processing burden associated with the application of the present invention is small. Furthermore, since the present invention can be applied without changing the halftone processing and other color reduction processing methods, the present invention can be easily applied.

  In the printing control apparatus, the dot data generation unit may be configured to generate dot data in which an error in the ink amount of the specific ink droplet is further compensated according to the error information. preferable.

  Alternatively, in the print control apparatus, it is preferable that the specific table is further adjusted to compensate for an error in the ink amount of the specific ink droplet according to the error information.

  In this way, it is possible to reproduce an accurate color by compensating for an ink amount error as well as improving the graininess and the like.

  In the print control apparatus, it is preferable that the dot generation unit is configured to generate dot data in which the specific dot recording rate is adjusted according to the error information and a type of a print medium. .

  In this way, even when the preferred dot recording rate differs for each print medium, it is possible to approach the preferred dot recording rate for each print medium.

  The present invention can be realized in various modes. For example, a printing method and a printing apparatus, a computer program for realizing the function of the method or apparatus, a recording medium on which the computer program is recorded, It can be realized in a form such as a data signal including the computer program and embodied in a carrier wave.

Next, embodiments of the present invention will be described in the following order based on examples.
A. Device configuration:
B. Dot formation control process:
C. Setting the dot recording rate table:
D. Adjustment of dot recording rate in the first embodiment:
E. Adjustment of dot recording rate in the second embodiment:
F. Variation:

A. Device configuration:
FIG. 1 is a block diagram showing the configuration of a printing system as an embodiment of the present invention. This printing system includes a computer 88 as a print control device and a color printer 20 as a printing unit. The combination of the color printer 20 and the computer 88 can be called a “printing apparatus” in a broad sense.

  In the computer 88, an application program 95 operates under a predetermined operating system. A video driver 94 and a printer driver 96 are incorporated in the operating system, and print data PD to be transferred to the color printer 20 is output from the application program 95 via these drivers. The application program 95 performs desired processing on the image to be processed, and displays the image on the CRT 21 via the video driver 94.

  When the application program 95 issues a print command, the printer driver 96 of the computer 88 receives image data from the application program 95 and converts it into print data PD to be supplied to the color printer 20. In the example shown in FIG. 1, the printer driver 96 includes a resolution conversion module 97, a color conversion module 98, a halftone module 99, a print data generation module 100, a color conversion table LUT, and a plurality of dots. A recording rate table DT and an error information receiving unit 102 are provided. These functions will be described later.

  The printer driver 96 corresponds to a program for realizing a function for generating the print data PD. A program for realizing the function of the printer driver 96 is supplied in a form recorded on a computer-readable recording medium. Such recording media include flexible disks, CD-ROMs, magneto-optical disks, IC cards, ROM cartridges, punch cards, printed matter on which codes such as bar codes are printed, computer internal storage devices (such as RAM and ROM). A variety of computer-readable media such as a memory) and an external storage device can be used.

  FIG. 2 is a schematic configuration diagram of the color printer 20. The color printer 20 includes a sub-scan feed mechanism that transports the printing paper P in the sub-scanning direction by the paper feed motor 22, and an axial direction of the paper discharge-side sub-scan feed mechanism 27 that includes the carriage 30 by the carriage motor 24. A main scanning feed mechanism that reciprocates in the (main scanning direction) and a head drive that drives a print head unit 60 (also referred to as a “print head assembly”) mounted on the carriage 30 to control ink ejection and dot formation. A mechanism and a control circuit 40 that controls the exchange of signals with the paper feed motor 22, the carriage motor 24, the print head unit 60, and the operation panel 32 are provided. The control circuit 40 is connected to the computer 88 via the connector 56.

  The sub-scan feed mechanism that transports the printing paper P includes a gear train (not shown) that transmits the rotation of the paper feed motor 22 to the platen 27 and a paper transport roller (not shown). Further, the main scanning feed mechanism for reciprocating the carriage 30 is an endless drive belt 36 between the carriage motor 24 and a slide shaft 34 that is installed in parallel with the axis of the platen 27 and slidably holds the carriage 30. And a position sensor 39 for detecting the origin position of the carriage 30.

  The print head unit 60 includes a print head 28 (to be described later) and a memory (not shown) that stores error information indicating an error in ink discharge amount. The control circuit 40 reads error information from the memory and transmits it to the computer 88 via the connector 56. The transmitted error information is received by the error information receiving unit 102 (FIG. 1) in the computer 88.

  FIG. 3 is an explanatory diagram showing a schematic configuration inside the print head 28. When the ink cartridges 71 and 72 (FIG. 2) are mounted on the carriage 30, the ink in the ink cartridge is sucked out through the introduction pipe 67, and the respective color heads 61 to 66 of the print head 28 provided at the lower portion of the carriage 30. Led to.

  When an ink cartridge is first installed, an operation of sucking ink to the respective color heads 61 to 66 is performed by a dedicated pump. In this embodiment, a pump for suction, a cap that covers the print head 28 at the time of suction is performed. The illustration and description of such a configuration is omitted.

  The heads 61 to 66 of each color are provided with a plurality of nozzles Nz for each color, and a piezo element PE that is one of electrostrictive elements and excellent in responsiveness is arranged for each nozzle. FIG. 4 shows the structure of the piezo element PE and the nozzle Nz in detail. As shown in the upper part of FIG. 4, the piezo element PE is installed at a position in contact with the ink passage 68 that guides ink to the nozzle Nz. The piezo element PE is an element that transforms electro-mechanical energy at an extremely high speed because the crystal structure is distorted by application of a voltage.

  In the present embodiment, by applying a voltage having a predetermined time width between the electrodes provided at both ends of the piezo element PE, the piezo element PE extends for the voltage application time as shown in the lower part of FIG. One side wall of 68 is deformed. As a result, the volume of the ink passage 68 contracts according to the expansion of the piezo element PE, and the ink corresponding to the contraction becomes an ink droplet Ip and is ejected from the tip of the nozzle Nz at high speed. Printing is performed by the ink droplet Ip soaking into the paper P mounted on the platen 27. The size of the ink droplet Ip can be changed by a method of applying a voltage to the piezo element PE. As a result, it is possible to form, for example, three types of large, medium, and small dots.

  Further, the size of the ink droplet Ip also varies due to manufacturing errors of the ink passage 68 and individual differences of the piezo elements PE. This fluctuation amount is stored as error information in a memory included in the print head unit 60. The error information is information regarding the ink droplet Ip for forming a small dot.

  The printer 20 having the hardware configuration described above transports the paper P by the paper feed motor 23 (hereinafter referred to as sub-scanning), reciprocates the carriage 30 by the carriage motor 24 (hereinafter referred to as main scanning), and at the same time. The piezo elements PE of the respective color heads 61 to 64 of the print head 28 are driven to discharge the respective color inks to form dots and form a multicolor image on the paper P.

B. Dot formation control process:
FIG. 5 is a flowchart showing the flow of the dot formation control routine in this embodiment. This process is executed in the computer 88. In step S100, the printer driver 96 receives RGB image data. This image data is data delivered from the application program 95 shown in FIG. 1, and 256-level gradation values from 0 to 255 for each color of R, G, and B for each pixel constituting the image. It is data which has.

  In step S105, the resolution conversion module 97 converts the resolution of the input image data into the resolution in the printer 20. When the image data is lower than the printing resolution, resolution conversion is performed by generating new data between adjacent original image data by linear interpolation. Conversely, when the image data is higher than the print resolution, resolution conversion is performed by thinning out the data at a constant rate.

  In step S110, the color conversion module 98 performs color conversion processing. The color conversion process is a process of converting image data composed of R, G, B gradation values into multi-gradation data representing gradation values of C, M, Y, K colors used in the printer 20. This processing is performed by using a color conversion table LUT (FIG. 1) that stores combinations of C, M, Y, and K for expressing the color composed of each combination of R, G, and B by the printer 20.

  In step S200, the halftone module 99 performs halftone processing on the image data thus color-converted. The halftone process is a process of reducing the gradation value of original image data (256 gradations in this embodiment) to a gradation value that can be expressed by the printer 20 for each pixel. Here, “color reduction” refers to reducing the number of gradations expressing a color. In this embodiment, color reduction is performed to four gradations: “no dot formation”, “small dot formation”, “medium dot formation”, and “large dot formation”.

  FIG. 6 is a flowchart showing the flow of halftone processing in this embodiment. In step S <b> 210, the halftone module 99 receives multi-gradation data from the color conversion module 98. The multi-gradation data input here is data that has undergone color conversion processing (step S110 in FIG. 5) and has 256 gradations for each color of C, M, Y, and K. In step S220, the large dot level data LVL is determined as follows according to the gradation of the image data.

  FIG. 7 is an explanatory diagram showing a plurality of dot recording rate tables used for determining the level data of large, medium, and small dots. The horizontal axis in FIG. 7 is the gradation value (0 to 255), the left vertical axis is the dot recording rate (%), and the right vertical axis is the level data (0 to 255). Here, the “dot recording rate” means a ratio of pixels in which dots are formed among pixels in the region when a uniform region is reproduced according to a certain gradation value. The profile SD in FIG. 7 indicates the recording rate of small dots, the profile MD indicates the recording rate of medium dots, and the profile LD indicates the recording rate of large dots. The level data refers to data obtained by converting the dot recording rate into 256 levels from 0 to 255. A method for setting the dot recording rate table will be described later.

  In step S220, level data LVL corresponding to the gradation value is read from the large dot profile LD. For example, as shown in FIG. 7, if the gradation value of multi-gradation data is gr, the level data LVL is obtained as ld using the profile LD. Actually, the profile LD is stored in a memory (not shown) as a one-dimensional table, and level data is obtained by referring to this table. This table is the dot recording rate table DT (FIG. 1).

  In step S230, it is determined whether or not the level data LVL set in this way is larger than the threshold value THL. Here, for example, dot on / off determination is performed by the dither method. The threshold THL is set to a different value for each pixel by a so-called dither matrix. In this embodiment, a matrix in which values 0 to 254 appear in a 16 × 16 square pixel block is used.

  FIG. 8 is an explanatory diagram showing the concept of dot on / off determination by the dither method. For the sake of illustration, only some pixels are shown. As shown in FIG. 8, each pixel of the level data LVL is compared with the size of the corresponding portion of the dither table. When the level data LVL is larger than the threshold value THL shown in the dither table, the dot is turned on, and when the level data LVL is smaller, the dot is turned off. In FIG. 8, a hatched pixel means a pixel that turns on a dot.

  When the level data LVL is larger than the threshold value THL, the halftone module 99 determines that the large dot should be turned on, and substitutes the value 11 in binary for the variable RE indicating the result value (step S300). . The result value RE is a variable representing the size of the dot formed on the pixel. A large dot is formed in a pixel having this variable of 11.

  On the other hand, in step S230, if the level data LVL is smaller than the threshold value THL, the halftone module 99 determines that a large dot should not be formed, and proceeds to step S240. In step S240, medium dot level data LVM is set. The medium dot level data LVM is set by the above-described recording rate table DT based on the gradation value. The setting method is the same as that for setting the large dot level data LVL.

  In step S250, the medium dot level data LVM and the threshold value THM are compared to determine whether the medium dot is on or off. The on / off determination method is the same as that for large dots, but the threshold value LVM used for determination is different from the threshold value LVL for large dots as shown below.

  If ON / OFF determination is performed using the same dither matrix for large dots and medium dots, the pixels where the dots are likely to turn on match in both. That is, when the large dot is turned off, the medium dot is likely to be turned off. As a result, the medium dot recording rate may be lower than the desired recording rate. In this embodiment, in order to avoid such a phenomenon, the dither matrix is changed for both. That is, by changing the position of the pixel that is likely to be turned on between the large dot and the medium dot, it is ensured that each pixel is appropriately formed.

  FIG. 9 is an explanatory diagram showing a relationship between a dither matrix used for large dot determination and a dither matrix used for medium dot determination. In this embodiment, as shown in the figure, the first dither matrix TM is used for large dots, and the second dither matrix UM in which the respective threshold values are moved symmetrically in the sub-scanning direction is used for medium dots. . In this embodiment, a 64 × 64 matrix is used as described above, but FIG. 9 shows a 4 × 4 matrix for convenience of illustration. Note that dither matrices that are completely different for large dots and medium dots may be used.

  If the medium dot level data LVM is larger than the threshold value THM in step S250, it is determined that the medium dot should be turned on, and the value 10 is substituted in binary for the result value RE (step S290). On the other hand, if the medium dot level data LVM is smaller than the threshold value THM in step S250, it is determined that a medium dot should not be formed, and the process proceeds to step S255.

  In step S255, small dot level data LVS is set in the same manner as the setting of the level data for large dots and medium dots. In step S260, if the level data LVS is larger than the threshold value THS, the halftone module 99 determines that the small dot should be turned on, and substitutes the value 01 in binary for the variable RE indicating the result value. (Step S280). On the other hand, if the small dot level data LVS is smaller than the threshold value THS in step S260, it is determined that a dot should not be formed, and the value 00 is substituted in binary for the variable RE indicating the result value (step S260). S270). The dither matrix for small dots is preferably different from that for medium dots and large dots in order to suppress a decrease in the recording rate of small dots as described above.

  With the above processing, it is determined which dot should be formed for one pixel. The halftone module 99 repeats the processes from step S220 to S300 until the process is completed for all pixels (step S310). When the processing is completed for all the pixels, the halftone processing routine is temporarily terminated and the process returns to the dot formation control processing routine. The halftone data corresponds to “dot data” in the claims.

  In step S400, the print data generation module 100 generates print data PD from the halftone data generated in this way. The print data PD is data including raster data indicating the dot recording state during each main scan and data indicating the sub-scan feed amount, and is output to the printer 20 (S410). The printer 20 receives this data, forms large, medium and small dots on each pixel and prints an image.

C. Setting the dot recording rate table:
FIG. 10 is an explanatory diagram showing a relationship between the dot recording rate table and the ink discharge amount. FIG. 10A is a diagram showing the relationship between the gradation value of multi-gradation data and the dot recording rate of dots of each size, and is the same as FIG. FIG. 10B is an explanatory diagram showing the relationship between the gradation value and the weight of ink ejected to a predetermined area. The predetermined area is assumed to be an area composed of 255 pixels. The ink weight is assumed to be 10 ng for small dots, 20 ng for medium dots, and 30 ng for large dots.

FIG. 10B is a plot of the following product for each gradation value.
(1) The recording rate of dots of each size (for example, in the gradation value G2, small dots are 25% and medium dots are 50%)
(2) Ink weight (10 ng for small dots, 20 ng for medium dots, 30 ng for large dots)
(3) Number of pixels in a predetermined area (255 pixels)
For example, when the gradation value is 255 (maximum gradation value), the above product is 7650 ng (= 100% × 30 ng × 255 pixels).

  As can be seen from FIG. 10B, when the gradation value increases from 0 to 255, the ink discharge amount increases from 0 ng to 7650 ng along the straight line Wi. As described above, in this embodiment, in order to make the explanation easy to understand, it is assumed that the ink weight ejected to a predetermined region and the gradation value have a linear relationship.

As can be seen from FIGS. 10A and 10B, the weight of the ink ejected to the predetermined area increases as follows according to the increase in the gradation value.
(1) In the region from the gradation value 0 to the gradation value G1, the ink weight increases linearly as the dot recording rate of small dots increases.
(2) In the area from the gradation value G1 to the gradation value G2, the dot recording rate of small dots is constant, and the ink weight increases linearly as the dot recording rate of medium dots increases.
(3) In the region from the gradation value G2 to the gradation value G3, the dot recording rate of small dots and medium dots is constant, and the ink weight increases linearly as the dot recording rate of large dots increases.
(4) In the area from the gradation value G3 to the maximum gradation value, the dot recording rate of small dots and medium dots starts to decrease, and the ink weight is linearly replaced by replacing the small dots and medium dots with large dots. Will increase.

In the present embodiment, such a dot recording rate profile is generated as a result of the following trade-off.
(1) In order to suppress graininess (roughness of the image), it is preferable to increase the dot recording rate of relatively small dots by decreasing the recording rate of relatively large dots that are easily visible. Such characteristics are particularly remarkable in a low gradation region.
(2) In order to reduce banding (striped image quality degradation), it is preferable to reduce the dot recording rate of relatively small dots by replacing relatively small dots with relatively large dots. Such characteristics are particularly remarkable in a high gradation region.
As a result of such a trade-off, in this embodiment, the profile is set with the upper limit value of the dot recording rate of small dots being 25% and the upper limit value of the dot recording rate of medium dots being 50%.

  FIG. 11 is an explanatory diagram showing the relationship between the dot recording rate of small dots and medium dots and the occurrence of banding. The numbers in the circles indicate the numbers of the nozzles in charge of forming dots at the pixel positions. In this example, banding occurs because the dots formed by the No. 5 nozzle are shifted upward due to a manufacturing error or the like. As can be seen from FIG. 11A, it can be clearly seen that banding occurs when the recording rate of small dots is 100%. It can be seen that such banding occurs even when the dot recording rate is reduced to some extent as shown in FIG.

  FIG. 11C shows a state where the same ink amount is ejected to the same area as FIG. 11B. For this reason, the dot pattern of FIG. 11C and the dot pattern of FIG. 11B express the same gradation. However, in FIG. 11C, the six small dots in FIG. 11B are replaced with three medium dots.

  In the dot pattern of FIG. 11 (c), it can be seen that banding is suppressed as compared with that of FIG. 11 (b). This is because the white streak generated in FIG. 11B is divided by two medium dots formed by the fourth nozzle and the fifth nozzle. It has been found that white stripes are less noticeable as image quality degradation when divided as shown below.

  FIG. 12 is a graph showing the relationship between the spatial frequency and the number of identifiable gradations in human visual characteristics. This graph shows that the number of gradations that can be identified by humans decreases as the spatial frequency increases.

  For example, assuming that the print resolution is 720 dpi in the main scanning direction, the spatial frequency of each pixel is 28 cycles / mm (= 720 dpi ÷ 25.4 mm). Here, when a white streak having a length of 10 pixels is generated in the main scanning direction, a white streak corresponding to 2.8 cycles / mm is generated in the spatial frequency. This white streak is generated in an area where about 100 gradations can be identified in human visual characteristics. As a result, it can be seen that image quality degradation due to white stripes is easily recognized by humans as banding.

  On the other hand, when white streaks are divided as shown in FIG. This is because, when the length of white stripes becomes, for example, 3 pixels due to the division, the spatial frequency exceeds 9 cycles / mm, and the image quality deteriorates in an area that is almost indistinguishable by human eyes.

  Thus, it can be seen that banding is likely to occur if the small dot recording rate is increased alone, or if the small dot recording rate is increased while the medium dot recording rate is extremely low. However, when medium dots are frequently used, the graininess (roughness of the image) tends to occur. This is because medium dots are easier to visually recognize than small dots.

  For this reason, the dot recording rate for each size is determined to an optimum value as a result of the trade-off between banding and graininess as described above. However, when the dot size fluctuates due to an error, there arises a problem that a value that should have been optimally set is not optimal. In the following embodiment, this problem is solved.

D. Adjustment of dot recording rate in the first embodiment:
FIG. 13 is an explanatory diagram showing a plurality of dot recording rate tables DT (n) used in the first embodiment of the present invention. FIG. 13A shows that the dot recording rate tables DT (n) include three dot recording rate tables DTn, DT1, and DT2.

  FIG. 13B shows a relationship between the error information received by the error information receiving unit 102 and the dot recording rate tables DTn, DT1, and DT2 selected in the color reduction process according to the error information. For example, in the case of 0.1, the error information means that the ink amount of the ink droplet is larger by 10% than the nominal value. That is, when the error information is 0.1, it is predicted that the ink droplets of small dots that are actually ejected are 11 ng.

  In this embodiment, when the error information is −0.1 to 0.1, the dot recording rate table DTn is selected, and when the error information is greater than 0.1, the dot recording rate table DT1 is error information is −0. When smaller than .1, the dot recording rate table DT2 is selected.

  FIG. 14 is an explanatory diagram showing a dot recording rate table setting method in the first embodiment of the present invention. FIG. 14A shows a dot recording rate profile of each size used for setting the dot recording rate table. For example, the dot recording rate profiles for small dots include profiles SDn, SD1, and SD2. The profile SDn is used as a dot recording rate for small dots when the error information is between -0.1 and 0.1. The profile SD1 is used when the error information is greater than 0.1. When the error information is smaller than −0.1, each is used to determine the dot recording rate of small dots.

  The difference between these profiles SDn, SD1, and SD2 is the upper limit value of the dot recording rate. The upper limit of the dot recording rate is set to 25% in the profile SDn, but is set to 30% (L1) which is 5% higher than the profile SDn in the profile SD1, and only 5% higher than the profile SDn in the profile SD2. Low 20% (L2) is set.

  With the adjustment of the small dot recording rate profile, the medium dot recording rate profiles MDn, MD1, and MD2 are also adjusted. For example, when the error information is greater than 0.1, the profile MD1 is selected as the profile of the dot recording rate for medium dots. As a result, when the gradation value reaches GS1 and the small dot recording rate reaches the upper limit value, medium dot recording is started. Further, as can be seen from the profile MD1, the dot recording rate of medium dots is reduced as a whole as the maximum recording rate of small dots increases. As a result, the number of medium dots is reduced and the graininess is improved.

  Note that the printing process in this embodiment is performed as follows. When the application program 95 issues a print command, first, the computer 88 reads error information from the memory of the print head unit 60 via the control circuit 40 of the printer 20. This error information is received by the error information receiving unit 102 in the computer 88 and sent to the halftone module 99.

  The halftone module 99 selects the dot recording rate table DT (n) according to the error information. As a result, an optimal dot recording table is selected according to the ink droplet error, and printing can be performed in a state in which the occurrence of graininess and banding due to the error in the ink discharge amount is suppressed.

  As described above, in the first embodiment, the dot recording rate of the small dots is adjusted by changing the upper limit of the recording rate of the small dots according to the error information. The occurrence of banding can be suppressed. The dot recording rate tables DT1 and DT2 correspond to “specific tables” in the claims.

E. Adjustment of dot recording rate in the second embodiment:
FIG. 15 is an explanatory diagram showing a dot recording rate table setting method in the second embodiment of the present invention. In the second embodiment, not only the graininess and banding due to the ink amount error are suppressed, but also the error in the ink amount discharged onto the printing medium is compensated to reproduce an accurate color.

  In this embodiment, there are profiles SDn, SD3, and SD4 as dot recording rate profiles for small dots (FIG. 15A). The profile SDn is the same as the profile SDn shown in FIG. 14A, the profile SD3 is when the error information is larger than 0.1, and the profile SD4 is when the error information is smaller than −0.1. Each profile is used as a dot recording rate for small dots.

  Profile SD3 is common to profile SD1 in that it is used when error information is greater than 0.1 and the upper limit of the dot recording rate is increased to 30%. However, the profile SD3 is different in that the gradient of the profile is smaller than that of the profile SD1 in the region where the gradation is expressed only with small dots (region where the gradation value is 0 to GS3).

  In this way, the slope is set to be small because the profile is used when small dots are large due to an error. Therefore, the recording rate at each gradation value is reduced and ejected onto the print medium. This is to compensate for an ink amount error. On the other hand, the profile SD4 used when the error information is smaller than −0.1 has a large slope to compensate for the ink amount by increasing the dot recording rate.

  As described above, the second embodiment not only suppresses the graininess and banding caused by the ink amount error according to the error information, but also compensates for the error in the ink amount discharged onto the print medium. There is an advantage that can be.

  In the claims, “compensation of ink amount error of specific ink droplet” is adjusted so that the weight of ink ejected to a predetermined region is less likely to fluctuate due to the error of ink amount of specific ink droplet. It means that.

F. Variation:
The present invention is not limited to the above-described examples and embodiments, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible.

F-1. In each of the above embodiments, one is selected from the three dot recording rate tables according to the error information. However, the number of dot recording rate tables to be selected may be two or four or more.

F-2. In each of the above-described embodiments, the upper limit value of the recording rate of the smallest dot among the three types of dots is varied according to the error information. For example, the second smallest in a printing apparatus capable of forming four types of dots. The upper limit value of the dot recording rate may be changed. In general, the specific ink droplet whose upper limit of the recording rate is adjusted in the present invention may be a relatively small dot among a plurality of types of dots that can be formed.

F-3. In each of the above embodiments, the upper limit of the dot recording rate is changed only for ink droplets for forming small dots according to the error information. However, other types of ink droplets may be changed simultaneously. . For example, the upper limit of the dot recording rate for medium dots may be changed. The present invention is generally applicable when a problem of banding or graininess occurs due to an error in the ink amount of at least one type of specific ink droplets among a plurality of types of ink droplets that can be ejected by the printing apparatus. It is.

F-4. In each of the above embodiments, the dot recording rate is adjusted only according to the error information, but may be adjusted according to the error information and the type of the print medium. In this way, even when the preferred dot recording rate differs for each print medium, it is possible to approach the preferred dot recording rate for each print medium.

F-5. In each of the above embodiments, a dither (diffusion) process is used for the halftone process, but an error diffusion process may be used. In general, any halftone processing method may be used as long as multi-tone data of each ink color can be reduced to the number of tones that can be formed by N types of dots.

F-6. In the above-described embodiment, a printer including a head that ejects ink using a piezo element is used. However, a printer that ejects ink by another method may be used. For example, the present invention may be applied to a printer of a type in which electricity is supplied to a heater arranged in the ink passage and ink is ejected by bubbles generated in the ink passage.

  The processing in the printing apparatus described above can also be realized by a computer program. The recording medium on which such a computer program is recorded includes a flexible disk, a CD-ROM, a magneto-optical disk, an IC card, a ROM cartridge, a punch card, a printed matter on which a code such as a barcode is printed, an internal storage device of a computer ( A variety of computer-readable media such as RAM and ROM) and external storage devices can be used. In addition, an aspect as a program supply apparatus that supplies a computer program for performing the above-described image processing or the like to a computer via a communication path is also possible.

1 is a block diagram showing a configuration of a printing system as an embodiment of the present invention. 1 is a schematic configuration diagram of a color printer 20. FIG. FIG. 3 is an explanatory diagram illustrating a schematic configuration of a dot recording head of a printer according to the invention. FIG. 3 is an explanatory diagram illustrating a dot formation principle in the printer of the present invention. 6 is a flowchart showing the flow of a dot formation control routine. The flowchart which shows the flow of a halftone process. Explanatory drawing which shows a dot recording rate table. Explanatory drawing which shows the view of the dot on / off determination by the dither method. Explanatory drawing which shows the relationship between the dither matrix used for determination of a large dot, and the dither matrix used for determination of a small dot. FIG. 4 is an explanatory diagram showing a relationship between a dot recording rate table and an ink discharge amount. Explanatory drawing which shows the relationship between the dot recording rate of the dot of a comparatively small size, and generation | occurrence | production of banding. The graph which shows the relationship between the spatial frequency and the number of identifiable gradations in human visual characteristics. FIG. 3 is an explanatory diagram showing a plurality of dot recording rate tables used in the first embodiment of the present invention. Explanatory drawing which shows the setting method of the dot recording rate table in 1st Example of this invention. Explanatory drawing which shows the setting method of the dot recording rate table in 2nd Example of this invention.

Explanation of symbols

20 ... Color printer 21 ... CRT
22 ... paper feed motor 23 ... paper feed motor 24 ... carriage motor 27 ... platen 27 ... discharge side sub-scan feed mechanism 28 ... print head 30 ... carriage 32 .. Operation panel 34 ... Sliding shaft 36 ... Drive belt 38 ... Pulley 39 ... Position sensor 40 ... Control circuit 56 ... Connector 60 ... Print head unit 67 ... Introduction Pipe 68 ... Ink passage 71, 72 ... Ink cartridge 88 ... Computer 94 ... Video driver 95 ... Application program 96 ... Printer driver 97 ... Resolution conversion module 98 ... Color Conversion module 99 ... Halftone module 100 ... Print data generation module 102 ... Error information receiver

Claims (13)

Using a printing unit capable of forming N types of dots having different sizes in a region of one pixel by selectively ejecting N types of ink droplets (N is an integer of 2 or more) having different ink amounts on a print medium In order to perform printing, a print control apparatus that generates print data to be supplied to the printing unit,
An error information receiving unit that receives error information indicating an error in the ink amount of at least one type of specific ink droplets of the N types of ink droplets;
A dot data generation unit that generates dot data representing the dot formation state of each pixel in the print image by processing the given original image data;
With
The dot data generation unit is configured to generate dot data in which a specific dot recording rate that is a recording rate of dots formed by the specific ink droplets is adjusted according to the error information. A print control device. The print control apparatus according to claim 1,
The specific ink droplet is an ink droplet for forming a relatively small specific dot among the N types of dots,
The printing control apparatus, wherein the dot data generation unit is configured to generate dot data in which an upper limit value of the specific dot recording rate is changed. The print control apparatus according to claim 2,
The dot data generation unit is configured to increase an upper limit value of the specific dot recording rate when the error information indicates that the specific ink droplet is large due to an error. The print control apparatus according to claim 2, wherein:
The dot data generation unit is configured to reduce an upper limit of the specific dot recording rate when the error information indicates that the specific ink droplet is small due to an error. The print control apparatus according to any one of claims 1 to 4,
The printing control apparatus, wherein the specific ink droplet is an ink droplet having the smallest ink amount among the N types of ink droplets. The print control apparatus according to any one of claims 1 to 5,
The dot data generation unit
A table storage unit for storing a plurality of dot recording rate tables usable for determining the dot recording rate of the N types of dots;
A table selection unit that selects one of the plurality of dot recording rate tables according to the error information;
With
The plurality of dot recording rate tables include a specific table selected according to the error information. The printing control apparatus according to any one of claims 1 to 6,
The dot data generation unit is configured to generate dot data in which an error in the ink amount of the specific ink droplet is further compensated according to the error information. The print control apparatus according to claim 6,
The print control apparatus, wherein the specific table is further adjusted to compensate for an error in the ink amount of the specific ink droplet according to the error information. The print control apparatus according to any one of claims 1 to 8,
The dot generation unit is configured to generate dot data in which the specific dot recording rate is adjusted according to the error information and the type of print medium. A printing apparatus capable of forming N types of dots having different sizes in a region of one pixel by selectively ejecting N types of ink droplets (N is an integer of 2 or more) having different ink amounts on a print medium. And
An error information receiving unit that receives error information indicating an error in the ink amount of at least one type of specific ink droplets of the N types of ink droplets;
A dot data generation unit that generates dot data representing the dot formation state of each pixel in the print image by processing the given original image data;
In accordance with the dot data, a dot forming unit that ejects the N types of ink droplets on the print medium;
With
The dot data generation unit is configured to generate dot data in which a specific dot recording rate that is a recording rate of dots formed by the specific ink droplets is adjusted according to the error information. A printing device. Using a printing unit capable of forming N types of dots having different sizes in a region of one pixel by selectively ejecting N types of ink droplets (N is an integer of 2 or more) having different ink amounts on a print medium In order to perform printing, a print control method for generating print data to be supplied to the printing unit,
(A) receiving error information indicating an error in ink amount of at least one type of specific ink droplet among the N types of ink droplets;
(B) generating dot data representing the dot formation state of each pixel in the printed image by processing the given original image data;
With
The step (b) includes a step of generating dot data in which a specific dot recording rate that is a recording rate of dots formed by the specific ink droplets is adjusted according to the error information. Control method. Using a printing unit capable of forming N types of dots having different sizes in a region of one pixel by selectively ejecting N types of ink droplets (N is an integer of 2 or more) having different ink amounts on a print medium A printing method for performing printing,
A specific dot recording rate, which is a recording rate of dots formed by the specific ink droplets, is adjusted according to error information indicating an ink amount error of at least one specific ink droplet among the N types of ink droplets. A printing method comprising the steps. Using a printing unit capable of forming N types of dots having different sizes in a region of one pixel by selectively ejecting N types of ink droplets (N is an integer of 2 or more) having different ink amounts on a print medium In order to perform printing, a computer program for generating print data to be supplied to the printing unit,
A function of receiving error information indicating an error in the ink amount of at least one specific ink droplet of the N types of ink droplets;
A function of generating dot data representing the dot formation state of each pixel in the print image by processing the given original image data;
Including a program for causing the computer to realize
The function of generating dot data includes a function of generating dot data in which a specific dot recording rate that is a recording rate of dots formed by the specific ink droplets is adjusted according to the error information. , Computer program.

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