JP2005041041A - Edge treatment for inkjet printing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain collapse of an edge part when ink dots having several sizes are used for printing. <P>SOLUTION: The edge part in an image is detected and is subjected to an edge treatment for generating dot data so that the recording rate of small dots increases, while the recording rate of large dots decreases as compared with the non-edge part other than the edge part. For example, either a dot recording rate table for the edge or a dot recording rate table for the non-edge can be selectively used to change the recording rate of each dot. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an edge processing technique for inkjet printing.
[0002]
[Prior art]
A technique using a plurality of dots of different sizes for the same ink (hereinafter referred to as “multi-size dot printing”) is known (for example, Patent Document 1).
[0003]
[Patent Document 1] JP 2000-6445 [Patent Document 2] JP 2000-125121 [Patent Document 3] JP 2000-177178 [Patent Document 4] JP 2000-184215 A. ]
[Problems to be solved by the invention]
Multi-size dot printing often reproduces the edge part (ie, contour) in an image more clearly than when only one size dot is used, but the edge part cannot be reproduced neatly even after multi-size dot printing. There is. FIG. 6 is an explanatory diagram showing an example of edge portions and dot arrangements in an image according to the prior art. FIG. 6A shows an image represented by image data. Here, a square frame with hatching indicates a print pixel painted with a certain color (for example, gray), and a square frame without hatching indicates a white print pixel. FIG. 6B shows an example of dot arrangement when printing this image using large and small dots of three types of ink, C (cyan), M (magenta), and Y (yellow). . Note that the positions and sizes of the dots are not accurate, and are drawn at positions and sizes so that the types of ink dots recorded in each pixel can be understood.
[0005]
In the dot arrangement in FIG. 6B, no dots are formed in the two pixels PP1 and PP2 among the pixels constituting the edge portion of the original image in FIG. When this image is observed with the naked eye, an edge portion (contour) in the image is deformed and viewed. When the total dot generation rate (recording rate) does not reach 100%, such a dot arrangement may occur. In particular, when a low density image is subjected to halftone processing, the total dot generation rate is 100. % Is often not reached. Also, when severe restrictions are imposed on the amount of ink discharged due to problems such as ink bleeding, the amount of small dots used (recording rate) due to color unevenness when small dots are used in large amounts over a wide range. Is restricted, the total generation rate of all dots is less than 100% even at high image density. As a result, as shown in FIG. 6B, when no dot is formed in a part of the pixels constituting the edge portion, there arises a problem that the outline is broken.
[0006]
An object of the present invention is to provide a technique capable of reducing the collapse of an edge portion when printing is performed using a plurality of sizes of ink dots.
[0007]
[Means for solving the problems and their functions and effects]
In order to achieve at least part of the above object, an image processing apparatus of the present invention is an image processing apparatus for inkjet printing,
An edge detection unit for detecting an edge portion present in the target image;
A dot data generation unit that generates dot data indicating a recording state of a plurality of types of ink dots for each print pixel of the target image;
With
The dot data is data indicating whether or not dots of a plurality of sizes including a relatively small first dot and a relatively large second dot are recorded for each print pixel with respect to the same ink,
The dot data generation unit generates the dot data so that the recording rate of the first dot is larger and the recording rate of the second dot is smaller in the edge portion than in the non-edge portion of the same color. Execute the process.
[0008]
According to this image processing apparatus, since the recording rate of the relatively small first dots is increased at the edge portion, the dots are reliably recorded at the edge portion. As a result, it is possible to alleviate the collapse of the edge portion due to no dots recorded at the edge portion.
[0009]
In the edge processing, the dot data generation unit converts the dot data so that the recording rate of dots other than the maximum size among the plurality of size dots is large and the recording rate of the maximum size dots is small. You may make it produce | generate.
[0010]
According to this configuration, since the recording rate of the maximum size dot is reduced, the collapse of the edge portion can be more effectively mitigated.
[0011]
The dot data generation unit may execute the edge process for each of cyan ink, magenta ink, and yellow ink.
[0012]
If edge processing is executed for each of these three basic primary color inks, since relatively small dots of each ink are recorded on the edge portion, it is possible to more reliably alleviate the collapse of the edge portion.
[0013]
The dot data generation unit may perform the edge process when an ink duty limit, which is a total amount of ink that can be ejected in a unit area of a printing medium used for printing, is less than 100%.
[0014]
When the ink duty limit is less than 100%, when a large dot is recorded on a pixel at an edge portion, there is a high possibility that no dot is recorded at the edge portion adjacent thereto. Therefore, if edge processing is executed in this case, the effect of reducing the collapse of the outline is great.
[0015]
It is preferable that the edge portion is composed of pixels having a lower lightness among pixels existing at a boundary between adjacent pixels whose image lightness is a predetermined amount or more apart.
[0016]
The dot data generation unit
As a dot recording rate table that inputs the ink amount of each pixel and outputs the dot recording rate of the plurality of dots, it has an edge dot recording rate table and a non-edge dot recording rate table,
A dot recording rate determining unit that determines the dot recording rate of each ink dot using the dot recording rate table for edges and the dot recording rate table for non-edges;
The dot recording rate determination unit may selectively use one of the edge dot recording rate table and the non-edge dot recording rate table in accordance with a detection result from the edge detection unit.
[0017]
According to this configuration, the dot recording rate can be easily changed in the edge portion and the non-edge portion.
[0018]
The present invention can be realized in various forms, for example, an image processing method and an image processing apparatus, a printing method and a printing apparatus, a computer program for realizing the functions of these methods or apparatuses, The present invention can be realized in the form of a recording medium on which a computer program is recorded.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described in the following order based on examples.
A. Device configuration:
B. Example operation:
C. Modification [0020]
A. Device configuration:
FIG. 1 is an explanatory diagram showing the configuration of an image processing system as an embodiment of the present invention. This system includes a computer 200 as an image processing apparatus and a printer 300 as an image output apparatus.
[0021]
The computer 200 is installed with a printer driver 210 for generating print data based on the image data file MF. The printer driver 210 includes an edge detection unit 212, a color conversion unit 214, a halftone processing unit 216, and an output processing unit 218. The edge detection unit 212 is a module having a function of detecting an edge portion in the target image. The color conversion unit 214 and the halftone processing unit 216 are modules that realize a function as a dot data generation unit that generates dot data representing a recording state (formation state) of dots for each print pixel.
[0022]
The print data PD generated by the printer driver 210 is supplied from the output processing unit 218 to the printer 300. The print data PD includes dot data representing the ink dot recording state for each pixel on the main scan line having the print resolution, and sub-scan feed amount data specifying the sub-scan feed amount.
[0023]
A program for realizing the function of the printer driver 210 may be supplied in a form recorded on a computer-readable recording medium. As such a recording medium, various computer-readable media such as a flexible disk, a CD-ROM, a magneto-optical disk, an IC card, a ROM cartridge, a punch card, and a printed matter on which a code such as a barcode is printed are used. it can.
[0024]
FIG. 2 is a block diagram showing the internal configuration of the color conversion unit 214 and the halftone processing unit 216. The color conversion execution unit 230 uses the color conversion lookup table 232 to convert the RGB multi-tone data of the image data file MF into CMYK multi-tone data. The dot recording rate determination unit 240 of the halftone processing unit 216 selectively uses the non-edge dot recording rate table 242a and the edge dot recording rate table 242b, from the multi-tone data of each color of CMYK. Printing rate data for dots and printing rate data for small dots are generated. At this time, the selection of the tables 242a and 242b is performed according to the detection result by the edge detection unit 212. The large dot recording rate data and the small dot recording rate data for each color of CMYK are each subjected to halftone processing by the halftone processing execution unit 244 and converted to dot data.
[0025]
B. Example operation:
FIG. 3 is an explanatory diagram illustrating processing contents in the embodiment. The image data shown in FIG. 3A is the same as that shown in FIG. 6A used for explaining the conventional example. That is, a square frame with hatching indicates a print pixel painted with a certain color (for example, gray), and a square frame without hatching indicates a white print pixel.
[0026]
FIG. 3B shows an edge portion detected by the edge detection unit 212. Here, “edge” means a boundary between adjacent pixels whose image brightness is a predetermined amount or more apart. The brightness difference for detection as an edge can be arbitrarily set. For example, it is possible to define a boundary between an area where one or more types of ink are solid and a paper white area as an “edge”. Edge detection can be performed using various known edge detection filters. As shown in FIG. 3B, the edge portion in the image is composed of edge pixels. Here, the “edge pixel” means a pixel having a lower brightness among pixels existing on both sides of a pixel boundary constituting an edge (contour) in the image. In the example of FIG. 3B, nine edge pixels are included. A pixel that is not an edge pixel is referred to as a “non-edge pixel”.
[0027]
FIG. 3C shows an example of dot arrangement when printing this image using large and small dots of three types of CMY inks. Here, an example in which black ink is not used is shown. As can be understood by comparing with the conventional example shown in FIG. 6B, in the embodiment shown in FIG. 3C, no large dots are formed in the edge pixels, and only small dots are recorded. . As a result, in the embodiment, it can be understood that the edge portion (that is, the contour of the image) is reproduced exactly, and there is no collapse of the contour as shown in FIG. In FIG. 3C and FIG. 6B, the ink amounts of the respective inks are substantially the same, so that they appear to be almost the same color when observed with the naked eye.
[0028]
FIG. 3D shows the actual size of large dots and small dots. As can be understood from this, the large dot has a size including one pixel, and is set to a size capable of filling the surface of the print medium if the large dot is recorded in all the pixels. Further, the size of the small dot is slightly smaller than the pixel. Therefore, if the edge pixel is recorded with small dots, the outline of the edge portion can be reproduced fairly beautifully.
[0029]
In FIG. 3C, there is a pixel in which no ink dot is recorded inside the edge pixel, but it is unlikely that this is recognized as a white spot (a portion where no ink dot is recorded). The reason is that ink flows from the surrounding pixels into pixels where no ink dots are recorded. In particular, as shown in FIG. 3D, a large dot usually has a size larger than a printing pixel. Therefore, as shown in FIG. 3C, even if there is a pixel in which no ink dot is recorded in the pixel inside the outline, the pixel is covered by the ink from the outside pixel, and white spots occur. The possibility is practically negligible. On the other hand, as shown in FIG. 6B, when ink is not ejected to the edge portion, the amount of ink that flows from the surrounding pixels is small, and thus there is a high possibility of white spots. Therefore, in order to reproduce the edge portion neatly, it is preferable to record small dots by increasing the recording rate of small dots. In this specification, processing for adjusting the dot recording rate at the edge portion in this way is referred to as “edge processing”.
[0030]
4A shows the contents of the non-edge dot recording rate table 242a, and FIG. 4B shows the contents of the edge dot recording rate table 242b. The horizontal axis of these graphs is the ink amount, and the horizontal axis is the dot recording rate. Here, the “ink amount” means an ink discharge amount for one pixel with 100% solid printing. “Dot recording rate” means the probability that a dot is recorded in a pixel. For example, when 10 pixels are recorded at a dot recording rate of 10%, dots are recorded at a rate of 1 pixel per 10 pixels. When the ink amount is 100%, the recording rate of large dots is 100%, and large dots are recorded in all pixels.
[0031]
In the edge dot recording rate table 242b of FIG. 4B, the recording rate of small dots (broken line) is larger than that of the non-edge dot recording rate table 242a of FIG. It can be understood that the solid line is small (except when the ink amount is 0% and 100%). If such an edge dot recording rate table 242b is used, the probability that a large dot is recorded at the edge portion is reduced, and the probability that a small dot is recorded is increased. As a result, as shown in FIG. 3C, small dots are easily formed at the edge portion, and the contour can be reproduced clearly.
[0032]
FIG. 5 is a flowchart showing the flow of halftone processing in the embodiment of the present invention. In this embodiment, the systematic dither method (threshold matrix method) is used as the halftone process. In step S210, the dot recording rate determination unit 240 selects either the edge or non-edge dot recording rate table. In step S220, the large dot level data LVL is set using the selected table. . Level data is data representing the dot recording rate, for example, 8-bit data expressing 0 to 100% in 256 levels from 0 to 255.
[0033]
In step S230, the large dot level data LVL and the large dot threshold value THL are compared. Here, for example, dot on / off determination is performed by a systematic dither method. The dither threshold THL used in the systematic dither method is set to a different value for each pixel by a so-called dither matrix. As the threshold matrix, for example, a matrix in which values 0 to 254 appear in a 16 × 16 square pixel block can be used. The same threshold matrix is used for both edge pixels and non-edge pixels. As the threshold matrix for large dots and the threshold matrix for small dots, different ones can be used, or the same one can be used. Note that there are cases where dot dispersion is better when the same threshold matrix is used for large dots and small dots.
[0034]
In step S230, if the level data LVL is larger than the threshold value THL, it is determined that a large dot should be formed (step S261). On the other hand, if the level data LVL is smaller than the threshold value THL in step S230, it is determined that a large dot should not be formed, and the process proceeds to step S240.
[0035]
In step S240, small dot level data LVS is set. The setting method is the same as the setting of the small dot level data LVL. In step S250, if the small dot level data LVS is larger than the small dot threshold THS, it is determined that a small dot should be formed (step S262). On the other hand, if the level data LVS is smaller than the threshold value THS in step S250, it is determined that no dot should be formed (step S263).
[0036]
By performing the above processing for all the pixels, dot data for all the pixels is generated (step S270).
[0037]
As described above, according to the present embodiment, the edge portion (contour) of the target image is detected, and the pixel of the edge portion increases the recording rate of small dots as compared with the pixel of the non-edge portion of the same color. Since the dot recording rate is reduced, the possibility that no ink dot is recorded at the edge portion can be reduced. As a result, ink dots are recorded at a higher probability in the pixels at the edge portion, so that it is possible to alleviate the collapse of the contour of the edge portion.
[0038]
C. 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.
[0039]
C1. Modification 1:
In the above embodiment, two types of dots, small dots and large dots, are used. However, the present invention can also be applied to the case of using three or more different sizes of dots. In general, when using dots of multiple sizes, in edge processing at the edge portion, the recording rate of dots other than the maximum size is greater than that of the non-edge portion, and the recording rate of dots of the maximum size is greater than that of the non-edge portion. It is preferable to make it smaller. However, for example, when three types of small, medium, and large dots are used, the dot recording rate of the edge portion may be kept the same as that of the non-edge portion with respect to the medium dot. However, if the recording rate of dots other than the maximum size is reduced at the edge portion, it is expected that the effect will be greater in reproducing the edge portion neatly.
[0040]
C2. Modification 2:
In the above embodiment, the dot recording rate of each size is adjusted by selectively using the dot recording rate table. Instead, the dot recording rate is adjusted by changing the threshold value matrix of each size. You may do it. Alternatively, when the error diffusion method is used, the dot recording rate may be adjusted by changing the on / off threshold for each dot. However, if the dot recording rate table is selectively used, the actual dot recording rate can be changed more reliably.
[0041]
C3. Modification 3:
In the above-described embodiments, the case where printing is performed using three types of CMY inks has been described. However, the present invention is applicable to cases where arbitrary types of ink are used. In this case, it is preferable to perform the edge processing described above for each ink used for printing. However, it is also possible to perform the above-described edge processing only on some inks and not perform edge processing on other inks. Even in this case, if edge processing is performed on at least cyan ink, magenta ink, and yellow ink, the probability that a relatively small dot of these inks is recorded at the edge portion increases, so that it is possible to prevent the collapse of the outline. It is.
[0042]
C4. Modification 4:
The present invention is particularly effective when printing is performed using a print medium in which the ink discharge amount is severely limited. Here, the restriction on the ink ejection amount means the total amount of ink that can be ejected in a unit area, and is also referred to as “ink duty restriction”. The unit of ink duty limit is 100% when dots used for solid printing (large dots in the above embodiment) are recorded on all pixels. For example, when the ink duty limit is 60%, the total amount of all inks is limited to 60% or less. As shown in FIG. 3D, the large dot is considerably larger than the size of one pixel, so if a large dot is recorded at a dot recording rate of 60%, for example, a solid image can be printed. It is.
[0043]
When the ink duty limit is less than 100%, there is a very high probability that no dot is recorded in a pixel adjacent to a pixel where a large dot is recorded. Therefore, when the present invention is applied to the case where such a print medium is used, the effect of neatly reproducing the contour is greater.
[0044]
C5. Modification 5:
The halftone processing method is not limited to the dither method, and various methods such as an error diffusion method can be used. Further, in this specification, the term “halftone processing” is used in the same meaning as “subtractive color processing”.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a configuration of an image processing system as an embodiment of the present invention.
FIG. 2 is a block diagram showing an internal configuration of a color conversion unit 214 and a halftone processing unit 216.
FIG. 3 is an explanatory diagram showing processing details in an embodiment.
FIG. 4 is a graph showing the contents of a non-edge dot recording rate table 242a and an edge dot recording rate table 242b.
FIG. 5 is a flowchart showing a flow of halftone processing in the embodiment of the present invention.
FIG. 6 is an explanatory diagram illustrating an example of an edge portion and dot arrangement in an image according to a conventional technique.
[Explanation of symbols]
200: Computer 210 ... Printer driver 212 ... Edge detection unit 214 ... Color conversion unit 216 ... Halftone processing unit 218 ... Output processing unit 230 ... Color conversion execution unit 232 ... Color conversion lookup table 240 ... Dot recording rate determination unit 242a ... Non-edge dot recording rate table 242b... Edge dot recording rate table 244... Halftone processing execution unit 300.

Claims (8)

An image processing apparatus for inkjet printing,
An edge detection unit for detecting an edge portion present in the target image;
A dot data generation unit that generates dot data indicating a recording state of a plurality of types of ink dots for each print pixel of the target image;
With
The dot data is data indicating whether or not dots of a plurality of sizes including a relatively small first dot and a relatively large second dot are recorded for each print pixel with respect to the same ink,
The dot data generation unit generates the dot data so that the recording rate of the first dot is larger and the recording rate of the second dot is smaller in the edge portion than in the non-edge portion of the same color. An image processing apparatus that executes processing. The image processing apparatus according to claim 1,
In the edge processing, the dot data generation unit generates the dot data so that a recording rate of dots other than the maximum size among the plurality of size dots is large and a recording rate of the dots of the maximum size is small. , Image processing device. The image processing apparatus according to claim 1 or 2,
The dot data generation unit is an image processing apparatus that executes the edge processing for each of cyan ink, magenta ink, and yellow ink. The image processing apparatus according to any one of claims 1 to 3,
The dot data generation unit performs the edge processing when an ink duty limit, which is a total amount of ink that can be ejected in a unit area of a printing medium used for printing, is less than 100%. The image processing apparatus according to any one of claims 1 to 4,
The image processing apparatus, wherein the edge portion is composed of pixels having a lower lightness among pixels existing at a boundary between adjacent pixels whose image brightness is a predetermined amount or more apart. An image processing apparatus according to any one of claims 1 to 5,
The dot data generation unit
As a dot recording rate table that inputs the ink amount of each pixel and outputs the dot recording rate of the plurality of dots, it has an edge dot recording rate table and a non-edge dot recording rate table,
A dot recording rate determining unit that determines the dot recording rate of each ink dot using the dot recording rate table for edges and the dot recording rate table for non-edges;
The dot recording rate determining unit selectively uses one of the edge dot recording rate table and the non-edge dot recording rate table according to a detection result from the edge detecting unit. An image processing method for printing ink dots,
(A) detecting an edge portion present in the target image;
(B) generating dot data indicating a recording state of a plurality of types of ink dots for each print pixel of the target image;
With
The dot data is data indicating whether or not a plurality of size dots including a relatively small first dot and a relatively large second dot are formed for each print pixel with respect to the same ink,
In the step (b), an edge for generating the dot data so that the recording rate of the first dot is larger and the recording rate of the second dot is smaller in the edge portion than in the non-edge portion of the same color. An image processing method including a step of executing processing. A computer program for performing image processing for ink dot printing,
An edge detection function for detecting edge portions existing in the target image;
A dot data generation function for generating dot data indicating a recording state of a plurality of types of ink dots for each print pixel of the target image;
A computer program for causing a computer to
The dot data is data indicating whether or not a plurality of size dots including a relatively small first dot and a relatively large second dot are formed for each print pixel with respect to the same ink,
The dot data generation function generates an edge for generating the dot data so that the recording rate of the first dot is larger and the recording rate of the second dot is smaller in the edge portion than in the non-edge portion of the same color. A computer program including a function for executing processing.

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