JP5521822B2 - Image processing apparatus and image processing method - Google Patents

Description

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

  In recent years, in an image forming apparatus adopting an electrophotographic system, an ink jet system, or the like, the resolution of an image forming unit has been increased, and a high-resolution digital image output almost equivalent to a printing machine such as 2400 dpi or 4800 dpi is becoming possible. .

  Such an image forming apparatus capable of outputting a high-resolution digital image not only outputs a high-resolution digital image, but also uses a digitized printing document to produce a printing plate for printing with the digital plate-making apparatus. In a series of printing processes called CTP (Computer to Plate) that are directly created, interference fringes generated by interference between the periodic pattern of the image of the document to be printed and the screen used in the printing press It is expected to be used for interference fringe proof printing to check the presence or absence in advance.

  As an image processing technique used in such an image forming apparatus capable of outputting a high-resolution digital image, for example, Japanese Patent Laid-Open No. 06-348892, Japanese Patent Laid-Open No. 09-270911, or Japanese Patent Laid-Open No. 2003-219171. What has been disclosed in the official gazette has already been proposed.

  The image data binarization method according to the above-mentioned Japanese Patent Application Laid-Open No. 06-348892 converts a video signal obtained by imaging a processing target into a multi-value image composed of pixels having a density level corresponding to a luminance level, and stores the converted image signal. In response to the horizontal change in the density level of a series of pixels in the horizontal direction of the multi-valued image, for the pixels in which the density level continues to decrease in the horizontal direction and the pixels in the area in which the density level continues to increase, Pixels in a section in which binarization is performed according to the level of the density level with respect to a threshold value determined for each section according to the maximum density level value and the minimum density level value of the pixel, and the lateral change of the density level is within a predetermined range Is configured so as to be binarized according to the level of a predetermined threshold.

  Also, the image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 09-270911 holds binary image data of m × n pixels around the pixel of interest and converts it into multi-value image data using a filter with a specific coefficient. Means, two or more binarization processing units for inputting multivalued image data from the conversion means and performing binarization processing at a plurality of threshold levels, and converting the binarized image to the resolution of the recording apparatus After the resolution conversion unit for conversion and the number of black pixels of m ′ × n ′ pixels around the target pixel of the resolution-converted image are counted, the density of the target pixel is determined from the count result corresponding to each binarization processing unit. A counting / determination unit to be determined, a synthesis unit that synthesizes density results from each counting / determination unit, and a multi-value recording unit that records pixels with density values from the synthesis unit. is there.

  Further, the halftone dot area changing method according to the above-mentioned Japanese Patent Application Laid-Open No. 2003-219171 is a halftone dot area changing method for changing the halftone dot area of the binarized halftone image data. An expansion process for expanding point image data into multiple values, an average mask process for performing average mask processing on the multivalued halftone image data, and multivalued halftone image data subjected to the average mask processing On the other hand, a gradation correction process for performing gradation correction according to predetermined gradation characteristics, and an error diffusion processing process for converting the gradation-corrected multi-value halftone image data into output data by error diffusion processing; The halftone dot area is changed by changing the gradation characteristics.

Japanese Patent No. 06-348892 Japanese Patent No. 09-270911 JP 2003-219171 A

  By the way, the problem to be solved by the present invention is that even when an image is formed by a method other than printing such as an electrophotographic method or an ink jet method, a periodic pattern included in image information of a document and a screen used for printing It is an object to provide an image processing apparatus and an image processing method capable of reproducing interference fringes generated by the interference.

That is, the invention described in claim 1 is a multi-value conversion unit that multi-values binary halftone image data;
A halftone dot area ratio calculating means for calculating a halftone dot area ratio of a pixel region of interest consisting of a plurality of predetermined pixels based on the binary halftone dot image data;
The halftone image data of the target pixel region that has been multi-valued by the multi-value conversion unit is reproduced with a threshold value determined according to the halftone dot area rate of the target pixel region calculated by the halftone dot area rate calculation unit. An image processing apparatus comprising a binarization unit for binarization.

  Further, in the invention described in claim 2, the re-binarization means includes a plurality of look-up tables having different thresholds according to a halftone dot area ratio, and uses the plurality of look-up tables by switching. The image processing apparatus according to claim 1.

  Further, in the invention described in claim 3, the re-binarization unit is configured to respond to a difference between a halftone dot area ratio of the target pixel area and a halftone dot area ratio of a pixel area around the target pixel area. The image processing apparatus according to claim 1, wherein the image processing apparatus is used as a threshold when re-binarizing a value obtained by multiplying the threshold by a predetermined correction coefficient.

According to a fourth aspect of the present invention, there is provided a multi-value conversion process for multi-value binary binary image data;
A halftone dot area ratio calculating step of calculating a halftone dot area ratio of a pixel region of interest consisting of a plurality of predetermined pixels based on the binary halftone dot image data;
The halftone image data of the target pixel region that has been multi-valued by the multi-value conversion step is reproduced with a threshold value determined according to the halftone dot area rate of the target pixel region calculated by the halftone dot area rate calculation step. An image processing method comprising: a binarization step for binarization.

  According to the first aspect of the present invention, compared to the case without this configuration, even when an image is formed by a method other than printing, such as an electrophotographic method or an ink jet method, the period that the image information of the document has Interference fringes generated by interference between a typical pattern and a screen used for printing can be reproduced.

  According to the second aspect of the present invention, an optimum lookup table can be employed in accordance with the characteristics of binary halftone dot image data as compared with the case without this configuration.

  Furthermore, according to the third aspect of the present invention, the larger the density amplitude of the image, the more the image can be corrected with contrast, and the image can be made closer to the printed image.

  According to the invention described in claim 4, even when an image is formed by a method other than printing, such as an electrophotographic method or an ink jet method, the periodic pattern included in the image information of the document is used for printing. The interference fringes generated by the interference with the screen can be reproduced.

A mark to which the image processing apparatus according to Embodiment 1 of the present invention is applied 1 is a block diagram showing an image processing apparatus according to Embodiment 1 of the present invention. It is a block diagram which shows a printing system. It is a block diagram which shows a RIP server. 1 is a configuration diagram illustrating a tandem type full-color printer as an image forming apparatus to which an image processing apparatus according to Embodiment 1 of the present invention is applied. FIG. It is a schematic diagram which shows a printed image and the image by an electrophotographic system. It is a graph which shows the gradation characteristic of the image by a printing image and an electrophotographic system. It is a schematic diagram which shows the input binary image data. It is a graph which shows a floating threshold value. It is a schematic diagram which shows the input binary image data and the processed image data. It is a flowchart which shows operation | movement of the image processing apparatus which concerns on Embodiment 1 of this invention. It is a graph which shows the gradation characteristic of the image by the electrophotographic system correct | amended with the printed image. It is a block diagram which shows the image processing apparatus which concerns on Embodiment 2 of this invention. It is a graph which shows a floating threshold value. It is a block diagram which shows the image processing apparatus which concerns on Embodiment 3 of this invention. It is a schematic diagram which shows image density distribution. It is a graph which shows the number of correction | amendment strips.

  Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1
FIG. 2 shows a printing system in which an electrophotographic image forming apparatus to which an image processing apparatus and an image processing method according to Embodiment 1 of the present invention are applied and a digital plate making apparatus are combined.

  As shown in FIG. 2, the printing system 1 includes a printing RIP (Raster Image Processor) server 2. The print RIP server 2 performs RIP processing on the input image data as a print document or image data as a print document sent from a host computer (not shown) via the communication line 3. As a result, for example, 1-bit Y (yellow), M (magenta), C (cyan), and K (black) image data in a binary Tiff format of 2400 dpi is generated. Of course, the resolution of the image data is not limited to 2400 dpi, and may be 1200 dpi, 4800 dpi, or the like. Also, the format of the image data is not limited to the Tiff format, and may be in another format. The image data generated by the printing RIP server 2 is sent to a digital plate making apparatus 4 using a CTP (Computer to Plate) for creating a printing plate for printing via a communication line 3 including the Internet and the like. A plate for printing is created by the plate making apparatus 4. The printing plate produced by the digital plate making apparatus 4 is used for full-color or monochrome main printing by a printing machine (not shown). The RIP server 2 for printing is positioned as a DFE (Digital Front End) device that performs a wide range of processes in the workflow associated with a series of printing steps, rather than simply a device that performs RIP processing.

  In the printing system 1, as shown in FIG. 2, an image forming apparatus 5 adopting an electrophotographic system capable of outputting a high-resolution digital image of 2400 dpi or the like to a printing RIP server 2 via a communication line 3. Is connected. In this image forming apparatus 5, for example, prior to creation of a printing plate by the digital plate making apparatus 4, proof printing using a halftone print image is performed based on the printing image data output from the printing RIP server 2. Is created.

  The image for proof printing formed by the image forming apparatus 5 is, for example, printed when a printing plate is produced and printed based on printing image data output from the printing RIP server 2. Whether or not an image of a desired color can be formed or whether there is an interference fringe generated by interference between a periodic pattern of image information as a document and a screen used for printing or the like. Used to confirm. Therefore, in the image forming apparatus 5, the same screen as that used for printing is used.

  As shown in FIG. 3, the printing RIP server 2 is basically configured in the same manner as a normal server, personal computer, or the like, and includes a CPU 21, a RAM 22, a ROM 23, a nonvolatile memory 24, an interface, and the like. It is comprised so that the part 25, the output part 26, and the bus part 27 which connects these may be provided.

  In addition, the electrophotographic image forming apparatus 5 includes, for example, an image processing apparatus 6 according to the present embodiment inside an image forming apparatus main body 51, as shown in FIG. The printer main body 51 includes image forming units 52Y, 52M, 52C, and 52K as image forming units corresponding to yellow (Y), magenta (M), cyan (C), and black (K) colors. I have. These four image forming units 52Y, 52M, 52C, and 52K are basically configured in the same manner except for the color of the image to be formed, and are roughly rotated at a predetermined speed along the arrow A direction. A photosensitive drum 53 as an image holding member, a primary charging scorotron 54 for uniformly charging the surface of the photosensitive drum 53, and an image based on image data corresponding to each color on the surface of the photosensitive drum 53. An image exposure device 55 that exposes to form an electrostatic latent image, a developing device 56 that develops the electrostatic latent image formed on the photosensitive drum 53 with toner of a corresponding color, and the surface of the photosensitive drum 53 And a cleaning device 57 for cleaning the toner remaining on the toner.

  As shown in FIG. 4, the image processing apparatus 6 exposes the image forming units 52Y, 52M, 52C, and 52K for each color of yellow (Y), magenta (M), cyan (C), and black (K). Corresponding color image data is sequentially output to the devices 55Y, 55M, 55C, and 55K, and laser light LB emitted according to the image data from these image exposure devices 55Y, 55M, 55C, and 55K corresponds. The surfaces of the photosensitive drums 53Y, 53M, 53C, and 53K are scanned and exposed to form electrostatic latent images. The electrostatic latent images formed on the surfaces of the photosensitive drums 53Y, 53M, 53C, and 53K are respectively yellow (Y), magenta (M), and cyan (C) by the developing devices 56Y, 56M, 56C, and 56K. , And developed as a toner image of each color of black (K).

  The yellow (Y), magenta (M), cyan (C), and black (K) toner images are primarily transferred onto the intermediate transfer belt 58 in a multiplexed manner, and then transferred from the intermediate transfer belt 58 to the recording paper 59. The recording sheet 59 on which a full-color image or a monochrome image is formed is output. The recording paper 59 is supplied with a desired size and material from the paper supply unit.

  By the way, when the image forming apparatus 5 forms an image based on the binary image data for printing output from the printing RIP server 2, for example, the image forming apparatus 5 periodically includes image information to be a document. It is important whether or not the interference fringes generated by the interference between the pattern and the screen used for printing or the like can be faithfully reproduced.

  Usually, when the printing plate for printing is prepared and printed by the digital plate making device 4 based on the binary halftone dot image data output from the printing RIP server 2, the printing plate is output from the printing RIP server 2. In the case where an image is formed by using the image forming apparatus 5 adopting the electrophotographic method based on the binary dot image data for printing, there are the following differences.

  When an image is formed by printing based on halftone dot image data, an image is printed by applying ink to a printing plate created based on halftone dot image data. Therefore, as shown in FIG. A dot image is formed by transferring ink relatively faithfully to the dot image data.

  On the other hand, when an image is formed using the image forming apparatus 5 adopting the electrophotographic method, the image is formed through an electrophotographic process such as formation of an electrostatic latent image by image exposure, development, transfer, and fixing. Therefore, even when an image is formed based on the same halftone dot image data as in printing, as shown in FIG. 5, the halftone dot image is expanded on the recording paper 59 with respect to the halftone dot image data. As shown in FIG. 6, the image area ratio of the print image with respect to the dot area ratio of the image data is shifted to the higher print area ratio particularly in the high-density area image having a high dot area ratio. A halftone dot image having gradation characteristics curved in a convex shape on the upper side is formed. Note that the above-described tendency is not limited to the image forming apparatus 5 that employs the electrophotographic system, but is also observed in the image forming apparatus that employs the ink jet system because ink bleeding or the like occurs.

  For this reason, particularly in a high density region with a high halftone dot area ratio, as shown in FIG. 6, the halftone dot image is crushed. Even in the case where interference fringes are generated due to interference with the screen used, the interference fringes hardly appear in the electrophotographic image forming apparatus 5. In the printing system 1, the interference fringes are corrected in the printing system 1. It has been difficult to use as an image output means.

  Therefore, in this embodiment, even when an image is formed by a method other than printing, such as an electrophotographic method or an inkjet method, the periodic pattern included in the image information of the document interferes with the screen used for printing. The image processing apparatus which can reproduce the interference fringe which arises by this is provided.

  Of course, the image processing apparatus 6 may be configured as an independent apparatus. For example, the image processing apparatus 6 is built in the electrophotographic image forming apparatus 5 in advance or installed as a program in the image forming apparatus 5. ing. Further, as shown in FIG. 2, the image processing apparatus 6 may be built in the printing RIP server 2 in advance or installed as a program.

  FIG. 1 is a block diagram showing an image processing apparatus according to Embodiment 1 of the present invention.

  As shown in FIG. 1, the image processing apparatus 6 is roughly divided into binary halftone dot image data 31 input from the RIP server 2 for printing, and the inputted binary halftone dot image data 31 is stored in various ways. A normalization unit 101 that performs normalization by valuation, and calculates a halftone dot area ratio of a pixel region of interest including a plurality of predetermined pixels based on binary halftone image data input to the normalization unit 101 Based on the floating threshold value held by the floating threshold value holding unit 104, the Cin estimation unit 102 serving as a halftone dot area ratio calculating unit, the smoothing unit 103 that smoothes the image data normalized by the normalizing unit 101, and the like. The floating threshold value determining unit 105 that determines a threshold value when the halftone image data 33 normalized by the normalizing unit 102 and smoothed by the smoothing unit 103 is re-binarized, and the Cin estimating unit 102 A re-binarization unit 106 that re-binarizes the threshold value determined by the floating threshold value determination unit 105 according to the halftone dot area ratio data 35 and outputs the re-binarized image data 36. .

  As shown in FIG. 2, the normalization unit 101 receives binary halftone image data 31 having a halftone component from the RIP server 2 for printing.

  As shown in FIG. 7, the normalization unit 101 converts the inputted binary halftone image data 31 into simple multivalued image data 32, and the binary halftone image data 31 is inputted. Then, each binary pixel value represented by the binary halftone dot image data 31 is converted into a multi-value pixel value for each pixel. Specifically, the normalization unit 101, for example, as shown in FIGS. 5A and 5B, when the pixel value of the input binary halftone image data 31 is “0”. “0” is normalized by performing a simple multi-value conversion that outputs “255” as a new pixel value when the pixel value of the input binary halftone image data 31 is “1”. To do. That is, the input binary dot image data 31 is converted so that the pixel value represented by 1 bit is represented by 8 bits. The normalization unit 101 outputs the simple multilevel image data 32 to the smoothing unit 103 and outputs the input binary halftone image data 31 to the Cin estimation unit 102 as it is.

  The smoothing unit 103 is composed of, for example, a low-pass filter as shown in FIG. 5C, and smoothed image data 33 obtained by smoothing the simple multi-valued image data 32 as shown in FIG. 5D. It is to convert to.

  The Cin estimation unit 102 has a halftone dot of a target pixel region composed of a plurality of (for example, 11 × 11 = 121) pixels determined in advance based on the binary halftone dot image data 31 input to the normalization unit 101. The area ratio is calculated. In the case where the binary halftone dot image data 31 is the image data shown in FIG. 5A, the halftone dot image data 31 includes a halftone dot image data (pixel count window) of a plurality of (for example, 11 × 11 = 121) pixels. The black pixels in which dots are formed are the number of pixels in which the value of the binary halftone image data 31 is “1”, that is, 7 × 7 = 49, and the halftone dot area ratio (Cin) is 49. / 12 = 40% is calculated.

  Further, the floating threshold value determination unit 105 re-processes the halftone image data multi-valued by the normalization unit 102 based on the floating threshold value held by the floating threshold value holding unit 104 including a lookup table (LUT). A threshold value for binarization is determined. For example, as shown in FIG. 8, the floating threshold value holding unit 104 includes a look-up table (LUT) in which the halftone dot area ratio calculated by the Cin estimation unit 102 and the floating threshold value are stored in a one-to-one relationship. . Further, the floating threshold value holding unit 104 is not a lookup table (LUT), but includes a predetermined function for calculating the floating threshold value based on the halftone dot area ratio calculated by the Cin estimation unit 102. May be.

  As described above, when an image is formed using the image forming apparatus 5 adopting the electrophotographic method, the image is formed through an electrophotographic process such as formation, development, transfer, and fixing of an electrostatic latent image by image exposure. Therefore, even when an image is formed based on the same halftone dot image data as in printing, as shown in FIG. 5, the halftone dot image is expanded on the recording paper 59 with respect to the halftone dot image data. As shown in FIG. 6, the image area ratio of the print image with respect to the dot area ratio of the image data is shifted to the higher print area ratio particularly in the high-density area image having a high dot area ratio. A halftone dot image having gradation characteristics curved in a convex shape on the upper side is formed.

  Therefore, even when the floating threshold value holding unit 104 forms an image based on binary halftone dot image data using the image forming apparatus 5, FIG. As shown, the threshold value of the high density region with a high halftone dot area ratio is large, the threshold value gradually decreases as the halftone dot area ratio decreases, and the threshold value decreases rapidly in the low density region with a low halftone dot area ratio. Thus, it is set in a state curved upwards.

  Further, the floating threshold value determination unit 105 performs Cin estimation as a threshold value for re-binarizing the multi-value halftone image data 33 that has been multi-valued by the normalization unit 102 and smoothed by the smoothing unit 103. This is determined according to the halftone dot area ratio calculated by the unit 102.

  In addition, the re-binarization unit 106 is a multi-value network that is multi-valued by the normalization unit 102 and smoothed by the smoothing unit 103 based on the threshold data 34 output from the floating threshold value determination unit 105. The dot image data 33 is compared with the threshold value. When the halftone dot image data 33 is equal to or greater than the threshold value, “1” is output. When the halftone dot image data 33 is less than the threshold value, “0” is output. By doing so, it is binarized again.

  In the above configuration, in the image processing apparatus according to the first embodiment, even when an image is formed by a method other than printing, such as an electrophotographic method or an ink jet method, the period included in the image information of the document is as follows. Interference fringes generated by interference between a typical pattern and a screen used for printing can be reproduced.

  First, as shown in FIG. 2, the user operates the RIP server 2 for printing, for example, 1 bit Y (yellow), M (magenta), C in 2400 dpi binarized Tiff format. (Cyan) and K (Black) image data 31 is output to the image forming apparatus 5 via the communication line 3, and Y (Yellow), M (Magenta), C (Cyan), Based on the image data 31 of K (black), an instruction to print an image for proof printing is given.

  Then, the 1-bit Y (yellow), M (magenta), C (cyan), and K (black) image data 31 output from the printing RIP server 2 in the 2400 dpi Tiff format is shown in FIG. As shown in FIG. 9, once input to the image processing apparatus 6 provided in the image forming apparatus 5 and the dot gain is corrected by the normalization unit 101 of the image processing apparatus 6, the value is once set to “0”. Is converted into “0” and “1” is converted into “255”, thereby normalizing to 8-bit (0 to 255) image data 32 (step 101 in FIG. 10).

  After that, the image data 32 normalized to 8 bits (0 to 255) by the normalization unit 101 substantially gives priority to the dot gain adjustment of the binary image data 32 (image format is 8 bits). In addition, gray level (1 to 254) image data 33 is created by the smoothing unit 103 formed of a low-pass filter (step 102). At this time, as the low-pass filter, for example, a filter having a 3 × 3 matrix is used as shown in FIG. 7C, but the filter size is not limited to 3 × 3, and parameter values are also illustrated. Of course, it is not limited to these.

  In parallel with the processing described above, the Cin calculation unit 102 includes a plurality of predetermined pixels based on the input binary halftone image data 31 as shown in FIG. 9A. The dot area ratio of the target pixel region (pixel count window) is calculated (step 103). In the illustrated example, as shown in FIG. 9A, a black pixel in which a halftone dot is formed is a binary halftone dot in a target pixel region composed of a plurality of (for example, 11 × 11 = 121) pixels. The number of pixels in which the value of the image data 31 is “1”, that is, 7 × 7 = 49, and the dot area ratio (Cin) is calculated as 49/12 = 40%. Note that the pixel region of interest (pixel count window) composed of a plurality of predetermined pixels is not limited to 11 × 11 pixels, and the pixel count window preferably has a size close to the number of screen lines. May be used.

  More specifically, the number of screen lines used in printing, the image forming apparatus 5 and the like is often about 175 lines or 200 lines in Japan, and 15 × 15 or 16 which is close to the number of screen lines. It is desirable to use a size equivalent to × 16 windows. On the other hand, in the United States, those with about 150 lines are often used, and it is desirable to use those having a size equivalent to 11 × 11 or 12 × 12 windows close to the number of screen lines.

  Next, in the image processing apparatus 6, the halftone image data multi-valued by the normalization unit 102 is obtained by referring to the floating threshold value holding unit 104 including a lookup table (LUT) by the floating threshold value determination unit 105. A threshold for re-binarization is determined.

  In the above example, as described above, since the floating dot area ratio (Cin) is calculated as 49/12 = 40%, the floating threshold value determination unit 105 performs the lookup table (LUT) as shown in FIG. ) And the like, the threshold value for re-binarization is determined as “172”.

  Thereafter, the re-binarization unit normalizes by the normalization unit 102 using the floating threshold determined by the floating threshold holding unit 104, and smoothes by the smoothing unit 103, as shown in FIG. 9C. The halftone dot image data is binarized again.

  As a result, as shown in FIG. 9C, the input binary halftone dot image data 31 has a corner portion where the value of the halftone dot image data smoothed by the smoothing unit 103 is “166”. Since the image data was “1”, it was smaller than the floating threshold “172”, so it was converted to “0” and re-binarized, and the edge of the halftone image was cut off. Dots are formed.

  Then, the image data 36 subjected to the image processing as described above in the image processing device 6 is changed into a pulse signal, and the image data is input to the image exposure devices 55Y, 55M, 55C, and 55K of the image forming device 5. Thus, yellow (Y), magenta (M), cyan (C), and black (K) images are formed by the image forming units 52Y, 52M, 52C, and 52K.

  As described above, the image processing apparatus 6 has a high halftone dot area ratio using the floating threshold value that varies according to the halftone dot area ratio of the pixel region of interest (pixel count window) including a plurality of predetermined pixels. The region is configured to rebinarize using a relatively large floating threshold and the region with a low dot area ratio using a relatively small floating threshold, as shown in FIG. In forming an image based on binary halftone dot image data using an electrophotographic image forming apparatus 5 in which an image in which an area having a high halftone dot area ratio is shifted to a higher density side tends to be formed. When the binary halftone image data is rebinarized using the floating threshold value, the threshold value of the high density region having a high halftone dot area ratio is set to a large value, so that the binary halftone image is displayed as shown in FIG. The dot area ratio of dot image data can be made closer to printing. Kill.

  Therefore, even when an image is formed by a method other than printing, such as an electrophotographic method, interference fringes generated by interference between a periodic pattern of image information of a document and a screen used for printing Can be reproduced, and the high-resolution image forming apparatus 5 can be used as an image forming apparatus for calibrating interference fringes and the like.

  The image processing apparatus 6 only changes the size of the halftone image based on the binary halftone image data, and divides the binary halftone image data into a plurality of divided regions for image processing. In addition to the interference fringes generated by the interference between the periodic pattern of the image information of the document and the screen used for printing, the binary halftone image data is divided into a plurality of divided areas. It is possible to suppress the generation of new interference fringes due to the division into two.

  On the other hand, in the halftone dot area changing method according to Japanese Patent Laid-Open No. 2003-219171, since error diffusion processing is performed when converting halftone image data with gradation correction into output data, the screen is changed. The structure is not maintained, and interference fringes generated due to interference between the periodic pattern of the image information of the document and the screen used for printing are difficult to be revealed.

Embodiment 2
FIG. 12 shows a second embodiment of the present invention. The same parts as those in the first embodiment will be described with the same reference numerals. In the second embodiment, the re-binarization means is described. Comprises a plurality of look-up tables having different thresholds according to the dot area ratio, and is configured to switch between the plurality of look-up tables.

  That is, in the second embodiment, as shown in FIG. 12, the floating threshold value determination unit 105 includes a plurality of floating threshold value holding units 104A, 104B, and 104C (loop-up tables) having different threshold values according to the dot area ratio. ing. As shown in FIG. 13, the plurality of floating threshold value holding units 104A, 104B, and 104C have a halftone dot area ratio other than the first look-up table that brings the print area ratio of a region with a high halftone dot area ratio close to printing. A second look-up table for correcting the print area ratio of the low highlight area and a third look-up table for bringing the print area ratio of the halftone area having the halftone dot area ratio in the middle closer to printing are provided. Yes.

  The floating threshold value determination unit 105 is configured to switch between the first to third floating threshold value holding units 104A, 104B, and 104C as appropriate.

  As described above, in the second embodiment, since the first to third floating threshold value holding units 104A, 104B, and 104C are switched as appropriate, the halftone dot area ratio can be further approximated.

  Since other configurations and operations are the same as those of the first embodiment, description thereof is omitted.

Embodiment 3
FIG. 14 shows a third embodiment of the present invention. The same parts as those in the first embodiment will be described with the same reference numerals. In the third embodiment, the re-binarization means is described. Re-binarizes a value obtained by multiplying the threshold by a correction coefficient determined in advance according to a difference between a halftone dot area ratio of the target pixel area and a halftone dot area ratio of a pixel area around the target pixel area It is comprised so that it may use as a threshold value when doing.

  That is, in the third embodiment, as shown in FIG. 14, the halftone dot area ratio is calculated for each pixel area based on the binarized image data, and the halftone dot area of the pixel area around the target pixel area is calculated. A difference from the rate is calculated, and a value obtained by multiplying the threshold by a correction coefficient determined in advance according to the difference is used as a threshold when re-binarizing.

  Therefore, in the third embodiment, as shown in FIG. 14, a Cin holding unit 107 that stores and holds the halftone dot area ratio obtained by the Cin estimation unit 102 over a plurality of target pixel regions (pixel count windows) is provided. Based on the halftone dot area ratios of the plurality of target pixel regions (pixel count windows) held in the Cin holding unit 107, the contrast calculation unit 108 performs a surrounding target pixel region (pixel count) as shown in FIG. Window) contrast is calculated.

  Further, the contrast calculated by the contrast calculation unit 108 is input to the threshold value multiplication unit 110 via the character exception determination unit 109. The character exception determination unit 109 determines whether or not the character part is a character part and removes the contrast of the character part.

  Then, the threshold multiplication unit 110 determines a correction multiplier as shown in FIG. 16 based on the contrast (difference between the maximum Cin and the minimum Cin) calculated by the contrast calculation unit 108, and the determined correction multiplier is determined. Is multiplied by the value of the floating threshold value output from the floating threshold value holding unit 104, and the floating threshold value that is the multiplication result is output to the floating threshold value determining unit 105.

  As described above, in the third embodiment, as shown in FIG. 14, it is determined in advance according to the difference between the halftone dot area ratio of the target pixel area and the halftone dot area ratio of the pixel area around the target pixel area. For example, the halftone dot area ratio of the target pixel region and the pixel region around the target pixel region are used as a threshold when re-binarizing a value obtained by multiplying the threshold value by the correction coefficient. When the difference from the halftone dot area ratio is large, that is, as the density amplitude of the image is larger, it is more effective to correct with contrast, so the Cin of the pixel count window is held in the surrounding pixels, The correction multiplier corresponding to the difference between the maximum Cin and the minimum Cin is multiplied by the value of the floating threshold output from the floating threshold holding unit 104.

  At this time, since the character image or the like tends to be corrected excessively, the character exception determination unit 109 removes the character portion from the maximum Cin and the minimum Cin.

  Since other configurations and operations are the same as those of the first embodiment, description thereof is omitted.

  6: Image processing device, 31: Binary halftone dot image data, 101: Normalization unit, 102: Cin estimation unit (halftone dot area ratio calculating means), 103: Smoothing unit, 104: Floating threshold value holding unit, 105 : Floating threshold value determination unit 106: rebinarization unit

Claims (4)

Multi-value conversion means for converting binary half-tone image data to multi-value;
A halftone dot area ratio calculating means for calculating a halftone dot area ratio of a pixel region of interest consisting of a plurality of predetermined pixels based on the binary halftone dot image data;
The halftone image data of the target pixel region that has been multi-valued by the multi-value conversion unit is reproduced with a threshold value determined according to the halftone dot area rate of the target pixel region calculated by the halftone dot area rate calculation unit. An image processing apparatus comprising: binarization means for binarization.   2. The image processing according to claim 1, wherein the re-binarization unit includes a plurality of lookup tables having different thresholds according to a halftone dot area ratio, and switches and uses the plurality of lookup tables. apparatus.   The re-binarization means multiplies the threshold by a correction coefficient determined in advance according to a difference between a halftone dot area ratio of the target pixel area and a halftone dot area ratio of a pixel area around the target pixel area. The image processing apparatus according to claim 1, wherein the image processing apparatus is used as a threshold value when re-binarizing a value. A multi-value process for multi-value binary image data,
A halftone dot area ratio calculating step of calculating a halftone dot area ratio of a pixel region of interest consisting of a plurality of predetermined pixels based on the binary halftone dot image data;
The halftone image data of the target pixel region that has been multi-valued by the multi-value conversion step is reproduced with a threshold value determined according to the halftone dot area rate of the target pixel region calculated by the halftone dot area rate calculation step. An image processing method comprising: a binarization step for binarization.

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