JP4905403B2 - Image processing apparatus and image processing method - Google Patents

Description

  The present invention relates to an image processing apparatus and an image processing method.

  When an image including a halftone character is screen-processed, dots are generated at regular intervals, resulting in jaggy in which the outline portion of the character appears jagged. To improve jaggy, emphasizing the outline of characters is effective, but as a method for that, avoid degradation of the resolution of the outline by using a screen with a higher number of lines than other areas. There is something.

  For example, the contour part of the image is extracted, and the dithering process is performed using a line screen with a non-contour area and a line screen with an angle different from the normal angle according to the angle formed by the outline part in the outline area. There is a method of adjusting so that the angle formed by the contour portion does not become close to the screen angle (see, for example, Patent Document 1).

In addition, the image is divided into an edge, an area adjacent to the edge and a flat area other than the edge, a dot-dispersed multi-value screen for the edge area, and a dot concentrated type for the flat area. A method of performing multi-value dither processing using a screen in which a dot dispersion type and a dot concentration type are mixed is disclosed in the area adjacent to the screen and the edge (see, for example, Patent Document 2).
JP 2004-40499 A Japanese Patent Laid-Open No. 10-191054

The problem with the above-described conventional method is that density fluctuations occur at portions where different screens are in contact with each other in the vicinity of the contour.
When two or more dots are printed with ink or toner (referred to as printing), the density may differ depending on the positional relationship of the dots depending on the printer. For example, the toner density of the entire two dots may differ between the case where two points are adjacent and the case where they are separated from each other. This is due to the frequency response of the printer. The responsiveness is low with respect to the dot for one dot and the adhesion amount of the toner or the like is suppressed, while the response is improved with respect to the continuous dot and the toner is suppressed. This is due to the fact that adhesion such as the above becomes good.

  In the above-described conventional method, by switching the dither processing in the contour portion, a phenomenon in which dots generated by different screens in the contour portion locally come into contact or leave periodically occurs. In this case, if the output characteristics of the printer are taken into consideration, the density of the area where the dots touch is locally increased, and if this density increase is periodic or disordered, it is jaggy if it is scattered in the contour region. And may cause degradation of image quality. The period of contact of each dot becomes longer when the screen angle of the non-contour portion and the angle formed by the contour portion are close, and jaggy becomes conspicuous accordingly.

  It is also possible to add a contour line by applying contone processing to the contour region without performing screen processing, but the problem that the periodic contact between the contour line and the dot appears as jaggy can be solved. Can not.

  On the other hand, Japanese Patent Application Laid-Open No. 2006-180376 devises a method for predicting the result of screen processing around a pixel of interest and avoiding hitting a pixel actually touching the screen. According to this method, although a hit point can be predicted by providing a plurality of screen processing circuits, a method capable of enhancing the outline with a simpler configuration is desired.

  An object of the present invention is to perform edge enhancement with a simple configuration.

According to the invention of claim 1,
An outline determination unit for determining an image portion constituting the outline of a character or line drawing from an image;
A first screen processing unit that performs screen processing on an image using a first dither matrix;
A second screen processing unit that performs screen processing on an image using a second dither matrix that defines a threshold value so that dots are formed at positions different from the dots formed by the first dither matrix;
In the image portion determined to constitute the contour, the image signal having the larger one of the image screen-processed by the first screen processing unit and the image screen-processed by the second screen processing unit is selected. A selector for selecting and outputting as an image after screen processing;
An image processing apparatus is provided.

According to invention of Claim 2,
2. The image processing apparatus according to claim 1, wherein the threshold value of the second dither matrix is a value obtained by subtracting a threshold value determined at each position of the corresponding first dither matrix for each pixel from a maximum value of the signal value. Provided.

According to invention of Claim 3,
In the density region where the dots of the first dither matrix and the second dither matrix contact, the signal value of the image screen-processed by the second screen processing unit is greater than the signal value of the image screen-processed by the first screen processing unit. The image processing apparatus according to claim 1, further comprising a density adjusting unit that adjusts a density of an image processed by the first screen processing unit and / or an image processed by the second screen processing unit so as to be reduced. The

According to invention of Claim 4,
In the density region where the dots of the first dither matrix and the second dither matrix contact, the signal value of the image screen-processed by the second screen processing unit is greater than the signal value of the image screen-processed by the first screen processing unit. The image processing apparatus according to claim 1 or 2, wherein a threshold value of the first dither matrix and / or the second dither matrix is set so as to be small.

According to the invention of claim 5,
The second screen processing unit uses a conversion table in which a threshold value is set so that dots are formed at positions different from the first dither matrix and the dots with respect to the threshold value of the first dither matrix. The image processing apparatus according to claim 1, wherein a threshold value of a second dither matrix corresponding to a threshold value of the first dither matrix used in the first screen processing unit is obtained and used for screen processing. The

According to the invention of claim 6,
The image processing apparatus according to any one of claims 1 to 5, wherein a size of the second dither matrix is the same size as the first dither matrix.

According to the invention of claim 7,
The image processing apparatus according to claim 1, wherein a size of the second dither matrix is an integral multiple of a size of the first dither matrix.

According to the invention described in claim 8,
Determining an image portion constituting the outline of the character or line drawing from the image;
Applying a screen process to the image using the first dither matrix;
Applying a screen process to the image using a second dither matrix in which a threshold is set so that dots are formed at positions different from the dots formed by the first dither matrix;
In the image portion determined to constitute the contour, the image signal having the larger signal value is selected from the image screened by the first dither matrix and the image screened by the second dither matrix. Selecting and outputting as a processed image; and
An image processing method is provided.

  According to the first and eighth aspects of the invention, by performing the screen processing using the second dither matrix, it is possible to easily form dots in the image portion of the contour, and the contour of the contour can be simplified. Emphasis is possible. As a result, it is possible to eliminate jaggies that are likely to occur in the outline of characters or line drawings, and to reduce image quality deterioration.

  According to the second aspect of the present invention, the dots of the second dither matrix can be formed so that the dots are formed at positions different from the dots of the first dither matrix and the contact is minimized. A local increase in density due to continuous dot formation can be prevented. Therefore, it is possible to perform edge enhancement without degrading the image quality.

  According to the third and fourth aspects of the invention, in the density region where the dots of the first dither matrix and the second dither matrix are in contact, the selector can select the image of the first dither matrix. As a result, contact between the dots of the first dither matrix and the second dither matrix can be prevented, and the image portion of the contour can be prevented from being emphasized more than necessary, thereby preventing image quality deterioration.

  According to the fifth aspect of the present invention, the threshold value of the second dither matrix corresponding to the first dither matrix can be easily obtained, and the condition of the first dither matrix is changed in the first screen processing unit. Even if it exists, it can respond easily.

  According to the sixth aspect of the invention, the threshold value can be easily set.

  According to the seventh aspect of the invention, not only contour enhancement but also gradation can be improved.

  In this embodiment, an example in which the present invention is applied to an MFP (Multi Function Peripheral) will be described. However, the present invention is applicable to any image processing apparatus that performs screen processing on an image.

First, the configuration will be described.
FIG. 1 shows a functional configuration of the MFP 100 according to the present embodiment.
As shown in FIG. 1, the MFP 100 includes an image reading unit 11, a control unit 10, a controller 12, an operation unit 13, a display unit 14, a storage unit 15, an image memory 16, a printer unit 17, and the like.

  The control unit 10 includes a CPU (Central Processing Unit), a RAM (Random Access Memory), and the like. The control unit 10 performs various calculations in cooperation with a control program stored in the storage unit 15 and controls the operation of each unit. For example, in a series of printing processes, image data input / output to each unit is controlled, operation screen display control, and the like are performed.

  The image reading unit 11 includes an ADF (Auto Document Feeder), a scanner, and the like, and performs a process of reading a document and generating an image (analog signal). The image generated here is an image separated into R (red), G (green), and B (blue) colors. The generated image is output to the controller 12.

  When the controller 12 converts data such as a document or photo created in the external PC 200 into PDL (Page Description Language) format by the printer driver software and sends it to the controller 12, the controller 12 rasterizes the PDL format data to generate a pixel. Image data in which the density (signal value) is determined every time is generated. Specifically, a PDL command is analyzed, a pixel is assigned to each image unit (this is called an object) to be drawn, and an image is generated by setting a signal value for each assigned pixel. Here, the following description will be given on the assumption that signal values in the range from the minimum value 0 to the maximum value 255 are assigned to each pixel. In the case of a color image, an image for each color of C (cyan), M (magenta), Y (yellow), and K (black) is generated.

  In the rasterization process, the controller 12 analyzes the PDL command and generates attribute information TAG. The attribute information TAG is information indicating which attribute of a character, a line drawing, or a photographic image corresponds to each pixel of the image. The controller 12 attaches the attribute information TAG to the generated image and outputs it to the image processing unit 1.

  Note that the controller 12 performs necessary processing such as A / D conversion on the image input from the image reading unit 11, then color-converts the RGB image into a CMYK image, and attribute information TAG about each pixel. Is generated.

The operation unit 13 includes operation keys. The operation unit 13 generates an operation signal corresponding to the operation of the operation key or the touch panel and outputs the operation signal to the control unit 10.
The display unit 14 includes a display integrated with the touch panel, and displays various operation screens such as an image forming condition setting screen according to display control of the control unit 10.

The storage unit 15 stores a control program, parameters, and the like. For example, a hard disk or the like can be applied to the storage unit 15.
The image memory 16 is a memory for storing images, and is composed of a large-capacity memory such as a hard disk. The image input from the controller 12 is subjected to necessary image processing by the image processing unit 1 and then stored in the image memory 16. At the time of printing, an image to be printed is read from the image memory 16 under the control of the control unit 10. It is output to the image processing unit 1.

  The printer unit 17 prints an image on a sheet based on the image data input from the image processing unit 1. The printing method of the printer unit 17 may be any one, but here, the case of the electrophotographic method will be described as an example. In the case of the electrophotographic system, the printer unit 17 includes an exposure unit, a developing unit, a paper feeding unit, a fixing unit, and the like. At the time of printing, first, the photosensitive drum is irradiated with laser light from the laser light source of the exposure unit to perform exposure, thereby forming an electrostatic latent image. Thereafter, the electrostatic latent image on the photosensitive drum is developed with toner to form a toner image, which is transferred onto a sheet conveyed from the sheet feeding unit, and a fixing process is performed by the fixing unit.

  The image processing unit 1 performs various image processes on the image. As image processing, A / D conversion processing, shading correction processing for correcting luminance unevenness caused by the image reading unit 2, and I-I for converting scanner-specific luminance characteristics into optimal luminance characteristics according to human visual characteristics. 'Conversion processing, enlargement / reduction, rotation processing, color conversion processing, density correction processing for adjusting the image density in accordance with human visual characteristics, screen processing, and the like.

FIG. 2 is a diagram showing components that mainly function when performing screen processing according to the present embodiment.
As shown in FIG. 2, the image processing unit 1 includes a contour determination unit 2, density adjustment units 3A, 3B, and 3C, a first screen processing unit 4A, a second screen processing unit 4B, a contone processing unit 5, and a selector 6. Configured.

  The contour determination unit 2 determines for each pixel whether or not the image portion constitutes the contour of a character or line drawing. The contour determination unit 2 outputs a flag signal EG indicating a determination result for each pixel with EG = 1 if a contour of a character or line drawing is configured, and EG = 0 if not.

Hereinafter, the determination method will be described.
The attribute information TAG is input to the contour determination unit 2 together with the image. The attribute information TAG classifies the image into the types of photographs, characters, and line drawings, and indicates to which type the pixel belongs. The attribute information TAG indicates a photograph when TAG = 0, a character when TAG = 1, and a line drawing when TAG = 2.

The contour determination unit 2 determines the type for each pixel of the image based on the attribute information TAG, and sets EG = 0 for the pixel with TAG = 0.
On the other hand, for a pixel with TAG = 1 or 2, the contour determination unit 2 inputs an image of 3 pixels × 3 pixels centered on the pixel (hereinafter referred to as a target pixel). FIG. 3 shows an image of 3 pixels × 3 pixels. Here, the signal value of the pixel of interest located at the center is represented by I0, and the signal value of the adjacent pixel is represented by In (n = 1 to 4). Note that when the target pixel is located at the edge of the image, since a portion other than the image is included in the 3 pixel × 3 pixel region, the signal value of this portion is determined to be 0.

Next, differences E0 to E4 between the signal value I0 of the target pixel and the signal values I1 to I4 of the adjacent pixels are obtained by the following formula.
E0 = I0-I1
E2 = I0-I2
E3 = I0-I3
E4 = I0-I4

Next, the maximum value among E0 to E3 is obtained, and this is set as a positive edge signal PEDGE. Similarly, the maximum value is obtained from those obtained by inverting the signs of E0 to E3, and this is set as the negative edge signal REDGE. PEDGE and REDGE are represented by the following formulas. Max () represents a function that outputs the maximum value among the values in parentheses.
PEDGE = Max (E0, E1, E2, E3)
However, when PEDGE <REDGE, PEDGE = 0
REDGE = Max (-E0, -E1, -E2, -E3)
However, when REDGE <PEDGE, REDGE = 0

E0 to E3 and PEDGE and REDGE determined in this way are edge strengths. Next, the obtained PEDGE is compared with a threshold TH set in advance for PEDGE, and it is determined whether or not the pixel of interest is a pixel constituting a contour. If PEDGE> TH, EG = 1 is set as a contour. On the other hand, if PEDGE ≦ TH, no contour is formed and EG = 0.
The flag signal EG is output to the density adjustment units 3A, 3B, 3C and the selector 6.

Next, the density adjustment units 3A, 3B, 3C, the first screen processing unit 4A, and the second screen processing unit 4B will be described. For convenience, the first screen processing unit 4A and the second screen processing unit 4B will be described first. .
Both the first screen processing unit 4A and the second screen processing unit 4B perform screen processing on an image using a dither matrix, but the first screen processing unit 4A has a first dither matrix and a second screen processing unit 4B. Uses a second dither matrix and a different dither matrix.

  The second dither matrix is created by setting a threshold value so that dots are formed at positions different from the dots formed by the first dither matrix. At this time, it is preferable to set a threshold value so as not to contact the dots of the first dither matrix. Contact means that other dots are formed around a certain dot (up, down, left and right, eight pixels located diagonally). As such a second dither matrix, for example, a threshold value obtained by reversing the magnitude relationship of the threshold values of the first dither matrix can be cited.

  The second screen processing unit 4B includes a conversion table in which the threshold value of the second dither matrix is set in advance with respect to the threshold value of the first dither matrix. Then, when performing the screen processing, the second screen processing unit 4B uses this conversion table to obtain a threshold value of the second dither matrix corresponding to the threshold value of the first dither matrix used in the first screen processing unit 4A, and Used for processing. When there are multiple types of first dither matrices used in the first screen processing unit 4A in which conditions such as size and shape are changed, a conversion table is provided for each type, and each type of first dither matrix is supported. The threshold value of the second dither matrix to be obtained can be obtained.

  FIG. 4 shows an example of the first dither matrix, and FIG. 5 shows an example of the second dither matrix. 4 and 5, the circles indicate pixels, and the numbers 1 to 26 assigned to the pixels indicate the order in which the threshold value is small. As shown in FIGS. 4 and 5, both the first dither matrix and the second dither matrix have the same size and the same shape as 26 pixels. The first screen processing unit 4A and the second screen processing unit 4B correspond to the threshold values determined by the first dither matrix and the second dither matrix by associating the first dither matrix and the second dither matrix respectively with the images during the screen processing. Is compared with the signal value of the image, and if it is less than the threshold value, it is binarized with a signal value of 0, and if it is greater than or equal to the threshold value, it is binarized. That is, the order of 1 to 26 in FIGS. 4 and 5 indicates the order in which dots are easily formed.

  The first dither matrix is a general dither matrix that is also used in normal screen processing, and a threshold value is set so that dots grow from the center to the outline in an area corresponding to the first dither matrix. That is, a set of dots formed by a dither matrix is referred to as a halftone dot, but a pixel position where a threshold is set so that the halftone dot is as circular as possible is taken into consideration.

  The 26 threshold values defined by the first dither matrix shown in FIG. 4 are values obtained by equally dividing the minimum value and the maximum value of the signal value of the image by 27. Each threshold value is set at the pixel position corresponding to the center of the 26 pixels of the first dither matrix, and thereafter, the threshold value is set at each pixel position in order from the center in the clockwise direction. As a result, the threshold values set in the order of 1 to 26 shown in FIG.

  Each threshold value of the second dither matrix is a value obtained by subtracting the threshold value determined at each position of the corresponding first dither matrix for 26 pixels from the maximum value of the signal value. For example, when the threshold value 10 is set at the position of number 1 in the first dither matrix, the difference 245 between the maximum value 255 of the signal value and the threshold value 10 is set as the threshold value at the corresponding position in the second dither matrix. The When the threshold values are set for all the pixels in this way, the threshold value 245 corresponds to the 26th as the smallest threshold value in the second dither matrix.

  As a result of setting the threshold value of the second dither matrix as described above, the magnitude relationship between the threshold values of the first dither matrix and the second dither matrix is reversed, and the first dither matrix is contoured from the center of the matrix as shown in FIG. The threshold value increases so as to draw a circle in the clockwise direction toward, whereas the second dither matrix has a threshold value in which a circle is drawn counterclockwise from the outer edge of the matrix toward the center as shown in FIG. Is getting bigger.

  Using the first dither matrix and the second dither matrix, the first screen processing unit 4A and the second screen processing unit 4B perform screen processing, and output the processed binary image to the selector 6. Here, an image screen-processed by the first screen processing unit 4A is represented as S1, and an image screen-processed by the second screen processing unit 4B is represented by S2.

The contone processing unit 5 performs contone processing on the image.
In the contone process, when a multi-value image is output as a multi-value image, the input multi-value image is output as it is. That is, no processing is performed. On the other hand, when a multi-value image is binarized and output, binarization is performed using one threshold value (for example, an intermediate value between the maximum value and the minimum value of signal values) without using a dither matrix. Output an image of values. An image after the contone process is represented by Cn.

  The selector 6 selects a signal value for each pixel. An image composed of the selected signal values is denoted by OUT. The selector 6 determines the image part of the outline of the character or line drawing based on the flag signal EG input from the outline determination unit 2, and the image part other than the outline of the character or line drawing is output from the first screen processing unit 4A. The signal value of the image S1 is selected, and for the image portion of the outline of the character or line drawing, the signal value of the image S1 output from the first screen processing unit 4A or the image S2 output from the second screen processing unit 4B Select the one with larger.

  Here, a case where the image density is around 20% will be described as an example. Since the first and second dither matrices shown in FIGS. 4 and 5 have the same interval between the threshold values, when screen processing is performed, 20% of 26 pixels when the image density is around 20%, that is, The signal value is 1 for the five pixels numbered 1 to 5, and dots are formed. The halftone dot ratio at this time is 5 / 26≈20%. The halftone dot ratio is the ratio of the number of pixels forming dots to the total number of pixels in the dither matrix.

  FIG. 6 is a diagram showing a dot formation pattern in the image S1 output from the first screen processing unit 4A at this time, and FIG. 7 shows a dot formation pattern in the image S2 output from the second screen processing unit 4B. FIG. When the larger signal value of the image S1 shown in FIG. 6 and the image S2 shown in FIG. 7 is selected and output, the dot formation pattern of the output image is as shown in FIG.

  As shown in FIG. 8, the dots formed by the first dither matrix and the dots formed by the second dither matrix are formed at different positions and not in contact with each other. This is because the order in which dots are easily formed is reversed between the first dither matrix and the second dither matrix because the magnitude relationship between the threshold values of the first dither matrix and the second dither matrix is reversed.

9A, 9B, and 9C show the results of selecting an image by the selector 6 when the halftone dot rate is 20%. 9A to 9C, a broken line indicates an edge (boundary) of a character or line drawing.
FIG. 9A shows a dot formation pattern when the selector 6 selects only the signal value of the image S1 of the first screen processing unit 4A. FIG. 9B shows the dot formation pattern when the signal value of the image S1 is selected for pixels other than the outline, and the signal value of the image S2 of the second screen processing unit 4B is selected for the outline pixels. FIG. S1 and outline are dot formation patterns when the larger signal value is selected from the images S1 and S2.

  As shown in FIG. 9B, by selecting the signal value of the image S2 in the outline, it is possible to increase the number of dots formed in the outline portion and emphasize the outline. Further, as shown in FIG. 9C, the dot formed on the contour can be further increased by selecting the larger one of the images S1 and S2 in the contour. Furthermore, since the dots of the image S1 and the image S2 are not in contact with each other, a local density increase due to continuous dot formation can be prevented.

If the halftone dot rate increases, the area where the dots of the image S1 and the dot of the image S2 do not contact decreases, and when the halftone dot rate reaches 50%, the dots of the image S1 and the image S2 completely contact.
Examples when the halftone dot ratio is 30% are shown in FIGS. 10A, 10B, and 10C. The broken lines in the figure indicate the edges of characters or line drawings, as in FIGS. 9A to 9C.
FIG. 10A is a diagram showing a dot formation pattern when only the signal value of the image S1 is selected by the selector 6. FIG. FIG. 10B shows a signal value of the image S1 other than the contour, and the signal value of the image S2 in the contour. FIG. 10C shows the image S1 other than the contour, and the image S1 and the image S2 having the larger signal value in the contour. This is a dot formation pattern.

  As shown in FIGS. 10A to 10C, when the image S2 is output at the contour or the larger one of the images S1 and S2 is output, the contour can be more emphasized than when only the image S1 is output. . However, when the halftone dot ratio is 30%, the number of dots in the image S1 increases, and the rate at which dots are formed in the vicinity of the contour increases, so the diagonal direction between the dots in the image S1 and the dots in the image S2 However, there is a contact part.

Examples when the halftone dot ratio is 46% are FIGS. 11A, 11B, and 11C. FIG. 11A is a diagram showing a dot formation pattern when only the image S1 is selected by the selector 6. FIG. In FIG. 11B, image S1 other than the contour is selected, and image S2 is selected for the contour. FIG. 11C is the image S1 other than the contour. It is a pattern.
As shown in FIG. 11C, when the halftone dot ratio is 46%, dots are formed in almost all the contours, and the degree of contour emphasis increases. Accordingly, the contact between the dots of the image S1 and the image S2 also increases as compared with the case where the halftone dot ratio is 30%. A slight increase in concentration should be inconspicuous.

  Thus, even if the dot ratio is less than 50%, the dots of the image S1 and the image S2 may come into contact with each other, and the contour may be emphasized more than necessary. If the dot ratio is 50% or more, not only the dots come into contact, but the dots of the first dither matrix and the dots of the second dither matrix are formed at the same position, so the image S2 is selected. There is no need. Therefore, the density of the image to be screen-processed by the first screen processing unit 4A and the second screen processing unit 4B is adjusted by the density adjustment units 3A and 3C to prevent image quality deterioration due to excessive edge enhancement.

  The density adjustment unit 3A adjusts the density of the image processed by the first screen processing unit 4A, and the density adjustment unit 3B adjusts the density of the image processed by the second screen processing unit 4B. The adjustment may be performed by both the density adjustment unit 3A and the density adjustment unit 3B, or only one of them may be performed. Further, the object of adjustment is the image portion of the outline of the character or line drawing. The density adjustment units 3A and 3B determine the image portion of the outline of the character or line drawing based on the flag signal EG input from the contour determination unit 2, and adjust the density of the image portion.

An example of the adjustment is shown in FIG.
FIG. 12 shows density conversion curves (or straight lines) a11 and a12 showing the relationship between the density (%) of the image before adjustment and the density (%) of the image after adjustment. The density adjusting unit 3A converts the image signal value (the signal value indicates density) by the density conversion curve a11 and performs density adjustment. Similarly, the density adjusting unit 3B performs density adjustment by the density conversion curve a12.

  In the case of the first dither matrix shown in FIG. 4 and the second dither matrix shown in FIG. 5, the dots are not completely in contact with each other when the dot ratio is less than 25%. Therefore, it is necessary to adjust the density in the density region where the halftone dot ratio is 25% or more. Since the first and second dither matrices correspond to the dot ratio and the density, in the adjustment example shown in FIG. 12, the density conversion curve a11 is a straight line having a slope 1, and the density of the input image is output as it is. That is, the density adjustment unit 3A does not adjust the image density. On the other hand, the density characteristic a12 is a straight line with a slope of 1 up to around 20% density and is not adjusted. However, if the density exceeds 25%, the increase in density output is slowed as the image density increases, and the density is 50% before. Therefore, the output density is adjusted so as to decrease as the image density increases.

  By adjusting the density in this way, the dots of the image S1 and the image S2 are formed as the image S2 in the density region of 25% or more where the dots appear as normal when the dots of the image S1 and the image S2 are not completely in contact with less than 25%. The number of dots to be reduced can be reduced, and the contact between the dots of the image S1 and the image S2 can be avoided.

The adjustment method is not limited to the above, and for example, adjustment as shown in FIG. 13 may be performed.
In the example shown in FIG. 13, the density adjustment unit 3A also performs density adjustment using the density conversion curve a21 to suppress the overall output density. On the other hand, the density adjustment unit 3B is the same as the previous example in that the increase in density output when the density exceeds 20% due to the density conversion curve a22 is the same as the previous example, but in the example of FIG. 13, the density output by the density adjustment unit 3A is In conjunction with the suppression, the output density peak is shifted around 60% density.
In addition, the density conversion curve can be a straight line or the shape of the curve can be changed according to the purpose of adjustment.

  Note that the slope of the density conversion curve or the like needs to be changed depending on the threshold setting method of the dither matrix. Also, since the density area when the dots touch depends on the dither matrix shape, size, threshold, and screen angle, the density area where both dots touch is determined according to the conditions of the first and second dither matrices used. In the density region, the density adjustment may be performed so that the signal value of the image S2 is smaller than the signal value of the image S1.

  Further, instead of performing the density adjustment, the threshold value of the first or second dither matrix may be adjusted. Here, for convenience of explanation, an example of adjustment will be described using the example of a small-size dither matrix shown in FIG. 14A. The dither matrix shown in FIG. 14A has a size of 2 pixels × 2 pixels, and a threshold value for binarizing the signal values of 0 to 255 is set. When the position of each cell of the dither matrix is indicated by e (e = 0, 1, 2, 3), the threshold value at the position of e = 0 is 32, the threshold value of e = 1 is 96, and the threshold value of e = 2 is The threshold for 160, e = 3 is 224.

When the average value of the signal value of the output image after the screen processing when the dither matrix is used is plotted against the signal value of the input image, a characteristic line as indicated by a31 in FIG. 14B is obtained. The characteristic line a31 has a staircase shape, but the slope of the staircase has a shape along a straight line a32 having a slope of 1. A straight line a32 having an inclination of 1 is considered to indicate an average density characteristic of the characteristic line a31.
In the case of adjusting the density by using the threshold value of the dither matrix, first, in FIG. 14B, a straight line a32 having a slope of 1 is deformed in the horizontal axis direction to draw a desired density characteristic curve a33. Then, the characteristic line a31 is moved in the horizontal axis direction and deformed along the drawn density characteristic curve a33. Assuming that the characteristic line after deformation is a34, the average value of the output image at the intersection of the characteristic line a31 before deformation and the straight line a32 having the inclination 1 is the intersection of the characteristic line a34 after deformation and the desired density characteristic curve a33. It becomes the same value as the average value of the output image.

  In the created characteristic line a34, the numerical values 32, 96, 160, and 224 corresponding to the threshold values in the characteristic line a31 before deformation change to 32, 176, 218, and 248 after deformation. Since these numerical values are threshold values used for density adjustment, they are reset as a new threshold value in the dither matrix. By performing screen processing using a dither matrix whose threshold value has been reset, an output image whose density has been adjusted can be obtained. That is, the density characteristic of the output image after the screen processing is the density characteristic indicated by the density characteristic curve a33 shown in FIG. 14B.

  As described above, according to the present embodiment, the first screen processing unit 4A outputs the image S1 obtained by performing screen processing on the image using the first dither matrix, and the second screen processing unit 4B outputs the first dither matrix. The image is subjected to screen processing using a second dither matrix in which threshold values are set so that dots are not formed at the same positions as the dots formed by the above, and an image S2 is output. The selector 6 selects the image S1 for the image portion other than the outline of the character or line drawing, and selects and outputs the image portion of the outline of the character or line drawing, which has the larger signal value between the image S1 and the image S2. Thereby, it is possible to form dots as much as possible in the image portion of the outline of the character or line drawing, and the outline can be emphasized with a simple configuration.

In addition, since the first dither matrix and the second dither matrix are reversed in the magnitude relation of the threshold values, even when images of the same density are processed, the dots of the first dither matrix are formed as the dots of the second dither matrix. It will be formed from a difficult position. Therefore, when the dot ratio is less than 50%, the dots may be formed so that the dots of the first dither matrix and the dots of the second dither matrix are formed at different positions, and the dot contact is minimized. It is possible to avoid continuous dot hits and prevent local density increase.
Further, since the first dither matrix and the second dither matrix have the same size, it is easy to set the threshold value so that dots are not formed at the same position.

  Furthermore, since the contact of dots can be minimized only by providing the second screen processing unit 4B using the second dither matrix, it is possible to confirm whether the formed dots are at different positions or continuously. In order to avoid the formation of dots, it is not necessary to provide a number of circuits for confirming the presence or absence of dots formed in the periphery, and a local density increase can be avoided with a simple configuration.

  Further, in the density region where the dots are in contact, the density adjusting unit 3A adjusts the density of the image processed by the first screen processing unit 4A so that the signal value of the image S2 is smaller than the signal value of the image S1, thereby adjusting the density. The unit 3B adjusts the density of the image processed by the second screen processing unit 4B. As a result, in the density region where the dots come into contact, the first dither matrix can be selected in the contour image portion. Thereby, the contact of the dots of the first dither matrix and the second dither matrix can be prevented, and the contour portion can be prevented from being emphasized more than necessary. In particular, when the dot ratio is 50% or more, the dots of the first dither matrix and the second dither matrix are formed at the same position in addition to the contact of the dots, so there is no point in selecting the dots of the second dither matrix. Therefore, in this case, it is possible to give priority to halftone reproducibility by selecting a dot of the first dither matrix that forms a circular halftone dot shape.

  The second screen processing unit 4B obtains the threshold value of the second dither matrix from the threshold value of the first dither matrix using the conversion table, and uses it for the screen processing. In the first screen processing unit 4A, the conditions of the first dither matrix (the shape and size of the dither matrix, the size of the threshold, etc.) may be changed depending on the color of the image, the purpose of printing, and the like. Even if it exists, the 2nd screen process part 4B can obtain | require easily the 2nd dither matrix according to the 1st dither matrix used by the 1st screen process part 4A, and can respond to a change.

In addition, the said embodiment is a suitable example of this invention, and is not limited to this.
For example, in the above example, the threshold value determined by the first or second dither matrix is compared with the signal value of the image, and if it is less than the threshold value, the signal value is 0 (minimum value of the density range), and if it is equal to or greater than the threshold value. The example of binarization with the signal value 255 (maximum value of the density range) has been described, but in some cases, the signal value 255 is less than the threshold value, and the signal value 0 is binarized when the threshold value is greater than the threshold value. Sometimes it becomes. In this case, the same process as described above may be performed with the threshold relationship reversed. Further, although an example in which an image is binarized by screen processing has been shown, contour enhancement can be performed by the same processing as described above when multi-valued.

  In the case of multilevel conversion, a method of multilevel conversion using two threshold values for each cell of a dither matrix is disclosed in Japanese Patent Laid-Open No. 2003-233811. Referring to the 2 pixel × 2 pixel dither matrix shown in FIG. 14A as an example, the relationship between the signal value of the input image and the output image after screen processing in each cell indicated by e in the dither matrix is shown in FIG. 15A. It will be like that. As shown in FIG. 15A, there are two threshold values of 0 and 63 in the cell of e = 0, and the signal value of the input image that is 0 or more and less than 63 is continuously 0 to 255 according to the signal value. However, if the value is 63 or more, all signal values of 255 are output regardless of the input value. Similarly, thresholds of 63 and 127 are set in the cell of e = 1, 127 and 191 in the cell of e = 2, and 191 and 255 are set in the cell of e = 3, and a signal value of 0 and small is set below the smaller threshold. A signal value corresponding to the input value from 0 to 255 is output when the threshold value is greater than or equal to the larger threshold value, and 255 is output when the threshold value is greater than or equal to the larger threshold value.

  As can be seen from FIG. 15B, in this dematrix, dots are easily output in the order of e = 0, 1, 2, and 3. Therefore, when such a dither matrix is used as the first dither matrix, the second dither matrix may be obtained by reversing the combination of the two threshold values applied to each cell as shown in FIG. 15B. That is, as shown in FIG. 15B, the threshold value applied to e = 0 and e = 3 is switched, and the threshold value applied to e = 1 and e = 2 is replaced as the second dither matrix. As a result, in the first dither matrix and the second dither matrix, the order in which the dots are likely to be output is reversed. Therefore, the dots of the two dither matrices are likely to be generated from different positions that do not contact each other.

  Further, the second dither matrix (see FIG. 5) in which the magnitude relations of all the threshold values are reversed is such that dots are formed at positions different from the first dither matrix (see FIG. 4), and the number of dots in contact is minimum. This is a preferred example in which the threshold value is set to be limited. As long as dots are formed at different positions, threshold values may be determined as shown in FIG.

  As a method for determining the threshold value so that dots are formed at different positions as described above, the order in which dots are likely to be formed is first determined based on the threshold value of the first dither matrix. Based on this order, a position with a dot ratio of less than 50% where the dots of the first dither matrix and the second dither matrix do not contact is obtained, and the order in which the dots of the second dither matrix are likely to be formed is determined at this position. At this time, based on the order in which the dots in the first dither matrix are easily formed, the order in which the dots in the second dither matrix are easily formed is determined so that the dots in the first dither matrix and the dots in the second dither matrix do not contact as much as possible. To do. Once the order is determined, the threshold value of the second dither matrix may be determined in that order. The threshold value may be the same as the threshold value of the first dither matrix.

  Further, the selector 6 has selected the larger one of the image S1 from the first screen processing unit 4A and the image S2 from the second screen processing unit 4B, but the first screen processing unit is not limited thereto. The larger one of the image S1 from 4A and the image Cn from the contone processing unit 5 may be selected. In the case of the contone processing unit 5, when the multi-valued image Cn is used, the input image is output as it is. Therefore, when the larger one is selected, it is possible to easily form dots in the contour, and contour enhancement is possible. Further, since the original image is output as it is, the image reproducibility is good and image quality deterioration can be suppressed.

  In the above embodiment, the size of the second dither matrix is the same as that of the first dither matrix, but may be a size that is an integral multiple of the size of the first dither matrix. For example, in the case of 4 times, a second dither matrix as shown in FIG. 17 can be adopted. The second dither matrix shown in FIG. 17 is a combination of four second dither matrices shown in FIG. 5 and has a size of 26 pixels × 4 = 104 pixels. In this way, by setting the size larger than that of the first dither matrix, not only the contour can be emphasized but also the gradation can be improved.

  In the example shown in FIG. 17, threshold values are set so that dots are formed in order in the four combined dither matrices. However, the threshold values are not limited to such combinations, and as shown in FIG. The threshold values may be set so that dots are formed in units of dither matrices by combining the second dither matrices set with threshold values as they are.

  When the image processing according to the present invention is realized by programming and software processing, the computer-readable medium of the program is a non-volatile memory such as ROM or flash memory, or a portable recording medium such as CD-ROM. It is possible to apply. A carrier wave is also applied to the present invention as a medium for providing program data via a communication line.

FIG. 2 is a diagram illustrating a functional configuration of an MFP according to the present embodiment. It is a figure which shows the component which mainly functions when performing the screen process of an image process part. It is a figure which shows the image unit which performs edge determination. It is a 1st dither matrix example. It is a 2nd dither matrix example. It is a figure which shows the dot formation pattern by the 1st dither matrix when the halftone dot rate is 20%. It is a figure which shows the dot formation pattern by the 2nd dither matrix when a halftone dot rate is 20%. It is a figure which shows a dot formation pattern when the one with a larger signal value is output among the images screen-processed by the 1st dither matrix or the 2nd dither matrix when the halftone dot rate is 20%. It is a figure which shows a dot formation pattern at the time of selecting the image screen-processed by the 1st dither matrix when the dot rate is 20%. FIG. 10 is a diagram showing a dot formation pattern when an image screened with a second dither matrix is selected for an outline when the halftone dot ratio is 20%. When the halftone dot ratio is 20%, dot formation is performed when an image having a larger signal value is selected from the image screened by the first dimatrix and the image screened by the second dither matrix. It is a figure which shows a pattern. It is a figure which shows a dot formation pattern at the time of selecting the image screen-processed by the 1st dither matrix when the halftone dot rate is 30%. FIG. 10 is a diagram showing a dot formation pattern when an image screened with a second dither matrix is selected for an outline when the halftone dot ratio is 30%. When the halftone dot ratio is 30%, dot formation is performed when an image having a larger signal value is selected from the image screened by the first dimatrix and the image screened by the second dither matrix. It is a figure which shows a pattern. It is a figure which shows a dot formation pattern at the time of selecting the image screen-processed by the 1st dither matrix when the dot rate is 46%. When the halftone dot ratio is 46%, the contour is a dot formation pattern when an image screened by the second dither matrix is selected. When the halftone dot ratio is 46%, dot formation is performed when an image having a larger signal value is selected from the image screened by the first dimatrix and the image screened by the second dither matrix. It is a figure which shows a pattern. It is a figure which shows an example of density adjustment. It is a figure which shows an example of density adjustment. This is a dither matrix of 2 pixels × 2 pixels. It is a figure which shows the relationship between the signal value of an input image, and the average value of the signal value of an output image. It is a figure which shows the relationship between the signal value of the input image in the 1st dither matrix in the case of performing multi-value quantization, and the signal value of an output image. It is a figure which shows the relationship between the signal value of the input image in the 2nd dither matrix in the case of performing multi-value quantization, and the signal value of an output image. It is a figure which shows the other example of a 2nd dither matrix. It is a figure which shows the example in case the size of a 2nd dither matrix is an integral multiple of the size of a 1st dither matrix.

Explanation of symbols

100 MFP
DESCRIPTION OF SYMBOLS 10 Control part 12 Controller 15 Memory | storage part 16 Image memory 17 Printer part 1 Image processing part 2 Contour determination part 3A, 3B, 3C Density adjustment part 4A 1st screen process part 4B 2nd screen process part 5 Contone process part 6 Selector

Claims (8)

An outline determination unit for determining an image portion constituting the outline of a character or line drawing from an image;
A first screen processing unit that performs screen processing on an image using a first dither matrix;
A second screen processing unit that performs screen processing on an image using a second dither matrix that defines a threshold value so that dots are formed at positions different from the dots formed by the first dither matrix;
In the image portion determined to constitute the contour, the image signal having the larger one of the image screen-processed by the first screen processing unit and the image screen-processed by the second screen processing unit is selected. A selector for selecting and outputting as an image after screen processing;
An image processing apparatus comprising: The image processing apparatus according to claim 1, wherein the threshold value of the second dither matrix is a value obtained by subtracting a threshold value determined at each position of the corresponding first dither matrix for each pixel from a maximum value of the signal value .   In the density region where the dots of the first dither matrix and the second dither matrix contact, the signal value of the image screen-processed by the second screen processing unit is greater than the signal value of the image screen-processed by the first screen processing unit. The image processing apparatus according to claim 1, further comprising a density adjusting unit that adjusts a density of an image processed by the first screen processing unit and / or an image processed by the second screen processing unit so as to be reduced.   In the density region where the dots of the first dither matrix and the second dither matrix contact, the signal value of the image screen-processed by the second screen processing unit is greater than the signal value of the image screen-processed by the first screen processing unit. The image processing apparatus according to claim 1, wherein a threshold value of the first dither matrix and / or the second dither matrix is determined so as to be small.   The second screen processing unit uses a conversion table in which a threshold value is set so that dots are formed at positions different from the first dither matrix and the dots with respect to the threshold value of the first dither matrix. The image processing apparatus according to claim 1, wherein a threshold value of a second dither matrix corresponding to the threshold value of the first dither matrix used in the first screen processing unit is obtained and used for screen processing.   The image processing apparatus according to claim 1, wherein a size of the second dither matrix is the same size as the first dither matrix.   The image processing apparatus according to claim 1, wherein a size of the second dither matrix is an integer multiple of a size of the first dither matrix. Determining an image portion constituting the outline of the character or line drawing from the image;
Applying a screen process to the image using the first dither matrix;
Applying a screen process to the image using a second dither matrix in which a threshold is set so that dots are formed at positions different from the dots formed by the first dither matrix;
In the image portion determined to constitute the contour, the image signal having the larger signal value is selected from the image screened by the first dither matrix and the image screened by the second dither matrix. Selecting and outputting as a processed image; and
An image processing method including:

Images