JP2005072748A - Edge processing for printing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To ease the collapse of an edge in printing using a dot. <P>SOLUTION: An edge existing in a target image is detected, a first halftone processing by a dither method is executed at the edge, and a second halftone processing by an error diffusion method is executed at a non-edge. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an edge processing technique for printing.

  In inkjet printing, so-called halftone processing is performed in order to reproduce various colors using a plurality of types of ink (for example, Patent Document 1). As the halftone process, an error diffusion method and a dither method (organized dither method) are often used. The error diffusion method has a good image quality but a long processing time, while the dither method has a feature that the image quality is inferior but the processing time is short.

Japanese Patent Laid-Open No. 2000-6445 JP 2000-125121 A JP 2000-177178 A JP 2000-184215 A

  However, when the error diffusion method is adopted to improve the image quality, the shape of the edge portion (contour) of the image may not be accurately reproduced. FIG. 8 is an explanatory diagram showing an example of edge portions and dot arrangements in an image according to the prior art. FIG. 8A 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. The contour lines BR1 and BR2 indicate the boundaries between the pixels of the gray area and the blank area. Note that the outlines BR1 and BR2 are virtual lines.

  FIG. 8B shows an example of dot arrangement determined using the error diffusion method. Here, the error diffusion method is sequentially applied to all pixels along the main scanning direction MS. As is well known, in the error diffusion method, dots may not be generated until errors are accumulated. Therefore, in the example of FIG. 8B, no dot is generated in the pixel in contact with the left boundary line BR1, and the contour along the boundary line BR1 is not correctly reproduced. That is, when the error diffusion method is used, there is a problem that the shape of the edge portion is broken. The problem that the thin line disappearance and the generation of dots in the error diffusion method become very small can also be regarded as a special case of the problem of dot generation delay at the edge portion.

  An object of the present invention is to provide a technique capable of alleviating collapse of an edge portion when printing is performed using dots.

In order to achieve at least a part of the above object, an image processing apparatus of the present invention is an image processing apparatus for 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 dot recording state for each print pixel of the target image;
With
The dot data generation unit includes a halftone processing unit that performs halftone processing,
The halftone processing unit
A first halftone process by the dither method is executed at the edge portion, and a second halftone process by the error diffusion method is executed at the non-edge portion.

  According to this configuration, since the dot recording state is determined using the dither method at the edge portion, it is possible to avoid the problem that the dot does not occur until the error is accumulated in the error diffusion method. As a result, it is possible to reduce the collapse of the edge portion. Since the thin line can be handled as if only the edge part is long and spread, the problem that the generation of dots in the fine line part is eliminated or extremely reduced by using error diffusion is also to use the dither method for the edge part. Can prevent.

The halftone processing unit
An error buffer commonly used for the first and second halftone processes;
In the first halftone process using the dither method, the recording state of the dots in the processing target pixel is determined by comparing the threshold value read from the predetermined threshold value matrix with the pixel value of the processing target pixel. Stored in the error buffer so as to diffuse the error generated according to the recorded state of the dots to a predetermined adjacent pixel,
In the second halftone process using the error diffusion method, a sum of a pixel value of the processing target pixel and an error for the target pixel stored in the error buffer is calculated, and the sum value and the error diffusion are calculated. The dot recording state in the processing target pixel is determined by comparing with a predetermined threshold for processing, and the error generated according to the determined dot recording state is diffused to predetermined adjacent pixels. It may be stored in the error buffer.

  According to this configuration, since the common error buffer is used in the edge portion and the non-edge portion, it is possible to prevent the dot recording state from being excessively biased at the boundary between the edge portion and the non-edge portion.

The dot data is data indicating whether or not dots of a plurality of sizes are formed for each print pixel with respect to the same ink,
The halftone processing unit may perform switching between the first and second halftone processes for at least one type of dot.

  According to this configuration, since the halftone process is switched only for the necessary dot size, the overall process can be speeded up.

  Note that the edge portion may be composed of pixels having lower brightness among pixels existing at the boundary between adjacent pixels whose brightness is more than a predetermined amount.

  Further, the edge detection unit may perform edge detection so that the edge portion has a predetermined width equal to or more than two print pixels.

  According to this configuration, it is possible to sufficiently secure the effect of applying the dither process to the edge portion to prevent the outline from being collapsed.

  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.

Next, embodiments of the present invention will be described in the following order based on examples.
A. Example:
B. Variation:

A. Example:
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.

  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.

  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.

  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.

  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 uses the dot recording rate table 242 to obtain the recording rate data for large dots and the recording rate data for small dots for each color of CMYK from multi-tone data for each color of CMYK. Generate.

  FIG. 3 shows an example of the contents of the dot recording rate table 242. The horizontal axis of this graph 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.

  The halftone processing unit 216 further includes a processing method selection unit 250, a dither processing execution unit 252, an error diffusion processing execution unit 254, and an error buffer 256. The processing method selection unit 250 selects a halftone processing method according to the detection result by the edge detection unit 212. Specifically, the dither processing execution unit 252 executes halftone processing at the edge portion, and the error diffusion processing execution unit 254 executes halftone processing at the non-edge portion.

  FIG. 4 is an explanatory diagram illustrating a result of the edge detection process in the embodiment. Here, the image data of FIG. 8A described in the related art is used as the processing target image. Hereinafter, an area composed of dark pixels (areas of columns C1 to C6 on the scanning lines L1 to L6) is referred to as a “dark area DA”. In addition, the areas of the columns C0 and C7 formed of bright pixels are referred to as “bright areas LA1 and LA2”. In this specification, “column” means a pixel position on each scanning line.

  FIG. 4A shows an edge portion detected by the edge detection unit 212. Here, the “edge” means a boundary between adjacent pixels in which values representing color properties such as image saturation, brightness, and hue are separated by a predetermined amount or more. In the present embodiment, a boundary between adjacent pixels whose image brightness is a predetermined amount or more (that is, a boundary between the dark area DA and the bright area LA) is referred to as an “edge”. 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”. In this embodiment, a region having a width of two pixels adjacent to the darker side of the edge (contour) is referred to as an “edge region”. Further, the pixels constituting the edge region are referred to as “edge region pixels”.

  FIG. 4B shows an edge detection filter used in this embodiment. This edge detection filter has a size of 5 × 5 pixels. In edge detection, whether or not the determination target pixel TP corresponds to an edge region pixel is determined according to the value of the central determination target pixel TP and the values of the 24 peripheral pixels PP around it. Specifically, when the brightness of the determination target pixel TP is equal to or less than a predetermined value and at least one of the plurality of surrounding pixels PP is a blank pixel, the determination target pixel TP is an edge region pixel. Determined. By applying this edge detection filter to each pixel in the target image, the edge region pixels shown in FIG. 4A are detected.

  Note that edge detection can also be performed using various known edge detection filters. For example, it is possible to use a filter that detects one pixel adjacent to the darker side of the edge as an edge pixel. In this case, in addition to the edge pixel, if one pixel on the dark area side adjacent to the edge pixel is also recognized as an edge pixel (edge area pixel), the edge area having a width of 2 pixels shown in FIG. It is possible to extract. Note that the edge region may be a region having a width of one pixel, but may be a region having a width of two or more pixels. When two or more pixels are used as the edge region, an effect of applying a dither process to the edge region to prevent the collapse of the contour is enhanced.

  FIG. 5 is an explanatory diagram showing an example of selection of halftone processing in the present embodiment. In FIG. 5, it is assumed that the dot recording rate data value of the dark area DA (also simply referred to as a pixel value) is 110, and the dot recording rate data values of the bright areas LA1 and LA2 on both sides thereof are 0. Yes. Here, it is assumed that the dot recording rate data is 8 bits, and the value “110” of the dot recording rate data corresponds to a dot recording rate of about 43%. The pixel value to be subjected to the halftone process is generally a pixel density value indicating the print density at the pixel, such as dot recording rate data or ink amount data.

  The upper part of FIG. 5 shows whether the pixels of each column on the scanning lines L1 to L6 are processed by the dither method or the error diffusion method. Pixels in the edge area (columns C1, C2, C5, C6) in the dark area DA are processed by the dither method. Further, the pixels in the region (columns C3 and C4) inside the edge region are processed by the error diffusion method. The bright areas LA1 and LA2 are processed by the error diffusion method. For the bright areas LA1 and LA2, other halftone processing methods such as a dither method may be employed. However, if the error diffusion method is employed for the bright areas, there is an advantage that the image quality of the entire image can be improved. .

  FIG. 6 is a flowchart showing the procedure of halftone processing in the present embodiment. In step T1, the processing method selection unit 250 (FIG. 2) determines whether or not the processing target pixel is an edge region pixel according to the detection result of the edge detection unit 212. When the pixel to be processed is an edge region pixel, the dither processing execution unit 252 executes the processing of steps T2 to T4. That is, in step T2, the threshold THd of the target pixel position is acquired from the threshold matrix. In step T3, the threshold value THd is compared with the pixel value DI (dot recording rate data) to determine the dot recording state. In step T4, the error generated according to the determined dot recording state is diffused to adjacent pixels, and the diffused error Derr is stored in the error buffer 256. A method for using the error buffer 256 will be described later.

  On the other hand, if it is determined in step T1 that the processing target pixel is not an edge region pixel, the error diffusion processing execution unit 254 executes the processing in steps T5 to T7. That is, in step T5, the sum of the pixel value DI of the processing target pixel and the error Derr of the processing target pixel stored in the error buffer 256 is calculated. In step T6, the dot recording state is determined by comparing the sum (DI + Derr) with a predetermined threshold value THe for error diffusion processing. In step T7, the error generated according to the determined dot recording state is diffused to adjacent pixels, and the diffused error Derr is stored in the error buffer 256.

  In this embodiment, as shown in FIG. 2, since dot recording rate data for large dots and small dots is obtained, the processing for steps T2 to T4 and the processing for steps T5 to T7 are for large dots. Whether or not a large dot or a small dot is recorded is determined for each pixel using dot recording rate data for small dots. For this determination, for example, the methods described in Patent Documents 1 to 4 cited in the prior art can be used. However, hereinafter, for convenience of explanation, a case where one type of dot on / off is determined in the halftone process will be described.

  FIG. 7 is an explanatory diagram illustrating a specific halftone processing example according to the embodiment. Here, a case where halftone processing is sequentially performed on the pixels in the columns C1 to C3 on the scanning line L1 is shown. FIG. 7A-1 shows the contents of halftone processing of the first pixel in the dark region (line L1 / column C1), and FIG. 7A-2 shows the error buffer 256 at this time. The contents are shown. In FIG. 7, the hatched pixels are the processing target pixels.

  Since the pixels in the line L1 / column C1 are edge region pixels, the dither method is applied. Assuming that the dithering threshold THd for this pixel is 100, since the pixel value DI is equal to or greater than the threshold THd, this pixel is in a dot-on state (recording state). In the dither method of the present embodiment, when the dot is on, the error to be diffused is represented by {[pixel value DI of target pixel] + [error Derr of target pixel] − [on state value]}. . Here, the pixel value DI of the target pixel is 110, and the error Derr of the target pixel stored in the error buffer 256 is zero. Assuming an on-state value of 255, the error to be diffused is -145. In this embodiment, the error to be diffused is evenly diffused to the right adjacent pixel and the pixel immediately below. Therefore, as shown in FIG. 7A-2, the error “−145” generated in the processing target pixel (the pixel in the line L1 / column C1) is the same as that in the line L1 / column C2 and the line L2 / column C1. It is diffused to the pixels and stored in the error buffer 256. The error to be diffused may be calculated by {[pixel value DI of target pixel] − [on state value]} instead of the above equation.

  FIG. 7B-1 shows the contents of the halftone process of the second pixel (line L1 / column C2), and FIG. 7B-2 shows the contents of the error buffer 256 at this time. Show. Since the pixels of the line L1 / column C2 are also edge region pixels, the dither method is applied. Assuming that the dithering threshold THd for this pixel is 181, the pixel value DI is smaller than the threshold THd, so that this pixel is in a dot-off state (non-recording state). In the dither method, since the threshold value changes greatly for each pixel as described above, even if the pixel value is the same, it is usually turned on or off depending on the position of each pixel. In the case of the dot off state in the dither method of the present embodiment, the error to be diffused is represented by {[pixel value DI of target pixel] + [error Derr of target pixel]}. Here, since the pixel value DI of the target pixel is 110 and the error Derr of the target pixel is −73, the error to be diffused is 37. Therefore, as shown in FIG. 7B-2, the error “37” generated in the processing target pixel (the pixel in the line L1 / column C1) is the pixel in the line L1 / column C3 and the pixel in the line L2 / column C2. And stored in the error buffer 256.

  FIG. 7C-1 shows the contents of the halftone process of the third pixel (line L1 / column C3), and FIG. 7C-2 shows the contents of the error buffer 256 at this time. Show. Since the pixels in the line L1 / column C3 are not edge region pixels, the error diffusion method is applied. In the error diffusion method, the threshold value is a predetermined constant value. In this embodiment, it is assumed that the threshold value THe for error diffusion processing is 128. The pixel value DI of the pixel in the line L1 / column C3 is 110, and the error Derr stored in the error buffer 256 is 19. These sums (DI + Derr) are called “pixel values with errors”. In the dot on / off determination, the pixel value with error (DI + Derr) value (= 129) is compared with the error diffusion threshold value THe (= 128). Here, since the pixel value with error (DI + Derr) is equal to or greater than the threshold value THe, this pixel is in a dot-on state (recording state). In the case of the dot on state in the error diffusion method, the error to be diffused is represented by {[pixel value with error (DI + Derr)] − [on state value]}. Therefore, the error to be diffused is −126. As shown in FIG. 7C-2, the error “−126” is diffused to the pixels of the line L1 / column C4 and the pixels of the line L2 / column C3 and stored in the error buffer 256.

  As can be understood from the example of FIG. 7, since the dither method is applied to the pixels in the edge region (columns C1 and C2), dots are also formed in the pixels (column C1 pixel) that are in contact with the edge outline. It is likely to be formed. This is because in the dither method, various threshold values different for each pixel are applied, so that a phenomenon that dots are not formed does not occur until an error is accumulated unlike the error diffusion method. As a result, as in the conventional example shown in FIG. 8B, the phenomenon that dots are hardly formed in the pixels in the dark region adjacent to the boundary BR1 can be prevented, and the contour shape of the edge portion is reproduced beautifully. It is possible. In this embodiment, since the error diffusion method is adopted for pixels other than the edge region, the image quality of the entire image can be improved.

  In the above embodiment, since the common error buffer 256 is used in the edge portion and the non-edge portion, there is an advantage that it is possible to prevent the dots from being excessively biased at the boundary between the edge portion and the non-edge portion.

B. 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.

B1. Modification 1:
In the above embodiment, two types of dots, small dots and large dots, are used. However, the present invention can be applied to the case where only one type of dot is used, and more than two types. This can also be applied when using dots of different sizes. In the above embodiment, the same halftone processing algorithm is applied to small dots and large dots, but different algorithms may be applied to both. For example, the dither method may always be applied to large dots, and the dither method may be applied to edge portions and the error diffusion method may be applied to non-edge portions for small dots. In general, when using dots of a plurality of sizes, it is preferable to switch halftone processing for at least one type of dot. In this way, the halftone process is switched only for the required dot size, so that there is an advantage that the entire process can be speeded up while the collapse of the edge outline is alleviated.

B2. Modification 2:
Although the case where printing is performed using four types of CMYK inks has been described in the above embodiment, the present invention can be applied to the case where any type of ink is used. In this case, it is preferable to perform the above-described edge processing (switching of halftone processing at the edge portion) 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.

1 is an explanatory diagram illustrating a configuration of an image processing system as an embodiment of the present invention. FIG. 3 is a block diagram showing an internal configuration of a color conversion unit 214 and a halftone processing unit 216. The graph which shows an example of the content of the dot recording rate table 242. Explanatory drawing which shows the result of the edge detection process in an Example. Explanatory drawing which shows the example of selection of the halftone process in an Example. The flowchart which shows the procedure of the halftone process in an Example. Explanatory drawing which shows the specific example of a halftone process by an Example. Explanatory drawing which shows an example of the edge part in an image and dot arrangement in a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 200 ... Computer 210 ... Printer driver 212 ... Edge detection part 214 ... Color conversion part 216 ... Halftone processing part 218 ... Output processing part 230 ... Color conversion execution part 232 ... Color conversion lookup table 240 ... Dot recording rate determination part 242 ... Dot recording rate table 250 ... processing method selection unit 252 ... dither processing execution unit 254 ... error diffusion processing execution unit 256 ... error buffer 300 ... printer

Claims (7)

An image processing apparatus for 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 dot recording state for each print pixel of the target image;
With
The dot data generation unit includes a halftone processing unit that performs halftone processing,
The halftone processing unit
An image processing apparatus that executes a first halftone process by a dither method at the edge portion and executes a second halftone process by an error diffusion method at a non-edge portion. The image processing apparatus according to claim 1,
The halftone processing unit
An error buffer commonly used for the first and second halftone processes;
In the first halftone process using the dither method, the recording state of the dots in the processing target pixel is determined by comparing the threshold value read from the predetermined threshold value matrix with the pixel value of the processing target pixel. Stored in the error buffer so as to diffuse the error generated according to the recorded state of the dots to a predetermined adjacent pixel,
In the second halftone process using the error diffusion method, a sum of a pixel value of the processing target pixel and an error for the target pixel stored in the error buffer is calculated, and the sum value and the error diffusion are calculated. The dot recording state in the processing target pixel is determined by comparing with a predetermined threshold for processing, and the error generated according to the determined dot recording state is diffused to predetermined adjacent pixels. An image processing apparatus for storing in an error buffer. The image processing apparatus according to claim 1 or 2,
The dot data is data indicating whether or not dots of a plurality of sizes are formed for each print pixel with respect to the same ink,
The halftone processing unit is an image processing device that performs switching between the first and second halftone processes with respect to at least one type of dot. The image processing apparatus according to any one of claims 1 to 3,
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. The image processing apparatus according to claim 4,
The edge detection unit is an image processing apparatus that performs edge detection so that the edge portion has a predetermined width equal to or more than two print pixels. An image processing method for printing,
(A) detecting an edge portion present in the target image;
(B) generating dot data indicating a dot recording state for each print pixel of the target image;
With
The step (b) includes a halftone processing step for performing halftone processing,
The halftone processing step includes
An image processing method including a step of executing a first halftone process by a dither method at the edge portion and executing a second halftone process by an error diffusion method at a non-edge portion. A computer program for performing image processing for 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 dot recording state for each print pixel of the target image;
A computer program for causing a computer to
The dot data generation function has a halftone processing function for performing halftone processing,
The halftone processing function
A computer program including a function of executing a first halftone process by a dither method at the edge portion and executing a second halftone process by an error diffusion method at a non-edge portion.

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