JP2007174254A - Descreening method of dot image, image forming method, and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a descreening method of a dot image, an image forming method, and an image forming device, capable of obtaining a multiple gradation image which does not have periodic unevenness when transforming dot image data into multi tone image data, without the need for switching a smoothing filter for each piece of dot image data. <P>SOLUTION: The descreening method of this dot image includes a process S04 of dividing the dot image into blocks each having a predetermined size, when transforming the dot image data into the smoothed multiple gradation image data, the process S05 of detecting the boundary pixel of a dot by each block, a process S07 of counting the number of pixels in n multiplied by n-pixel area (n: integer not less than 1) which makes the detected boundary pixel to be the center, and a process S08 of calculating the average of a count value in the n multiplied by n pixel area for each block. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a descreening method for converting halftone image data into smoothed multi-tone image data, an image forming method using the descreening method, and an image forming apparatus.

  Conventionally, in the printing / plate making process, a proof (color proof) is separately created before the printing process, and the final printing finish is confirmed in advance before creating the actual printing plate. The printing machine performs printing based on binary area gradation image data (halftone dot image data) generated by a RIP (raster image processor). On the other hand, a system called DDCP (Direct Digital Color Proof) uses RIP and CMS (Color Management System) to generate binary area gradation image data that can obtain a finished image close to printed matter. An image output device forms an image and outputs a color proof.

  In addition, there is a system in which binary area gradation data generated for printing by RIP is converted into a multi-gradation image by a descreening process, and a color proof is created by an inkjet printer or the like. In these systems, it is necessary to obtain a multi-gradation image having no period unevenness by the descreening process. For this purpose, an appropriate value is obtained depending on the number of screen lines and the screen angle of binary area gradation data (halftone image). It is necessary to select a filter and perform smoothing.

  The following Patent Document 1 describes, as a method of performing color conversion processing on binary area gradation image data used in printing, by binary-multivalue conversion, multivalue-multivalue conversion, multivalue-binary conversion, An image conversion method for increasing or decreasing the dot area ratio of the original binary area gradation image is proposed.

  Patent Document 2 below discloses a device that performs descreening to generate a continuous tone image and changes the resolution of a halftone image, each of which provides an interpolation and screen removal function. A filter device for removing screen information from a halftone image, and a filter for acting on the neighborhood of the pixels as a function of the desired resolution change for each pixel of the continuous tone image And a controller for selecting one of the two.

When the halftone image (binary area gradation image) data is descreened and converted to multi-gradation image data as in Patent Document 1, the image is divided into blocks, and the pixels are counted for each block. A method of converting to a tone image and then performing a smoothing filter process is common. However, the screen line number and screen angle of a halftone dot image are various, and it is necessary to select and optimize a filter for each halftone dot image as disclosed in Patent Document 2. In addition, even when the filter is optimized, there is a problem that periodicity of the halftone image cannot be completely removed and periodic unevenness may occur in the multi-tone image.
JP 2002-290722 A Japanese Patent Laid-Open No. 06-233118

  In view of the above-described problems of the prior art, the present invention can obtain a multi-tone image having no period unevenness when converting halftone image data into multi-tone image data, and for each halftone image data. It is an object of the present invention to provide a halftone image descreening method, an image forming method, and an image forming apparatus that do not require switching of a smoothing filter.

  In order to achieve the above object, a halftone image descreening method according to the present invention is a descreening method for converting halftone image data into smoothed multi-tone image data, wherein the halftone image is a predetermined size. Dividing the block into blocks, detecting a halftone dot boundary pixel for each block, obtaining an area ratio of a region of a predetermined size centered on the boundary pixel for each detected boundary pixel, Calculating an average of the area ratios obtained for each boundary pixel in the block for each block.

  According to this halftone image descreening method, when converting halftone image data into smoothed multi-gradation image data, the area of a predetermined size area centered on the boundary pixel for each detected boundary pixel Since the average of the area ratio obtained for each boundary pixel in the block is calculated for each block in which the halftone image is divided into a predetermined size, the positional relationship between the area for obtaining the area ratio and the halftone dot is periodically calculated. Since no significant change occurs, the obtained multi-tone image has no period unevenness. Further, it is not necessary to switch the smoothing filter finely for each screen line number or angle. In addition, although it is necessary to perform filter processing for boundary pixel detection on the entire image, instead, the processing for obtaining the area ratio needs to be performed only on the region centering on the boundary pixel, which is efficient.

  In the halftone image descreening method, it is preferable that the block size is determined to be larger than the halftone dot period in order to obtain a multi-tone image having no period unevenness. In addition, the area for obtaining the area ratio for each boundary pixel is preferably an n × n pixel area (n is an integer of 1 or more) centered on the boundary pixel. When the necessary number of gradations is 80, n = 9 and the area ratio is obtained in the range of 0 to 81 (n × n = 81).

  An image forming method according to the present invention is characterized in that image formation is performed based on multi-tone image data converted from halftone image data by the above-described halftone image descreening method. According to this image forming method, it is possible to obtain a multi-tone image having no periodic unevenness from a halftone dot image, so that an image having no periodic unevenness can be formed. Since it is not necessary to switch the smoothing filter finely for each screen line number or angle, efficient image formation becomes possible. In addition, although it is necessary to perform filter processing for boundary pixel detection on the entire image, instead, the processing for obtaining the area ratio needs to be performed only on the region centering on the boundary pixel, which is efficient.

  An image forming apparatus according to the present invention forms an image based on multi-tone image data converted from halftone image data by the above-described halftone image descreening method. According to this image forming apparatus, it is possible to obtain a multi-tone image having no period irregularity from a halftone dot image, so that an image having no period irregularity can be formed. Since it is not necessary to switch the smoothing filter finely for each screen line number or angle, efficient image formation becomes possible. In addition, although it is necessary to perform filter processing for boundary pixel detection on the entire image, instead, the processing for obtaining the area ratio needs to be performed only on the region centering on the boundary pixel, which is efficient.

  According to the halftone image descreening method, the image forming method, and the image forming apparatus of the present invention, it is possible to obtain a multi-tone image having no period unevenness when the halftone image data is converted into multi-tone image data. In addition, it is not necessary to switch the smoothing filter for each halftone image data.

The best mode for carrying out the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram schematically showing a configuration example of a printing system including a color proof creation system according to the present embodiment. FIG. 2 is a block diagram schematically showing a halftone image data processing apparatus of the color proof creation system of FIG.

  As shown in FIG. 1, the printing system inputs image data such as PS (PostScript) data from a client terminal 1 of a printing factory to an RIP (Raster Image Processor) 2 that is an image processing apparatus. On the other hand, the data converted into a halftone dot image and converted into a binary halftone dot image at RIP2 is sent to a CTP (Computer To Plate) 3 which is a device for directly creating a printing original plate. It is sent to the halftone image data processing apparatus 100 for color proof creation, and the halftone dot image data processing apparatus 100 converts the binary halftone image data into multi-gradation data, and the printer 10 based on this multi-gradation data. Thus, a color proof is created on a predetermined recording medium S.

  The halftone image data processing apparatus 100 and the printer 10 in FIG. 1 constitute a color proof creating system as an image forming apparatus. With this color proof creating system, a pattern, color tone, text, Create and output a color proof (sample for proofreading) to check characters. A color tone and the like are confirmed by this color proof, an original plate is created by CTP3 in FIG.

  As shown in FIG. 2, the halftone image data processing apparatus 100 divides YMCK halftone dot image data input from the RIP 2 in FIG. 1 into a plurality of blocks, detects boundary pixels for each block, A division / detection / conversion processing unit 11 that performs conversion into gradation image data; a storage unit 12 that includes an HDD (hard disk drive) device that temporarily stores and stores input halftone image data; and a multi-tone image Each unit includes a printer data conversion unit 13 that converts data into printer image data, a printer interface unit 14 that sends image data from the printer data conversion unit 13 to the printer 10, and a central processing unit (CPU). The control part 15 which controls 11 thru | or 14 and the input part 16 with which a user can input various data are provided.

  Descreening is a process of converting halftone dot image data having a resolution of about 1000 to 4000 dpi into a multi-tone image having a resolution of about 120 to 400 dpi. In the division / detection / conversion processing unit 11, The image data is divided into blocks of a predetermined size for smoothing processing, and a boundary pixel of a halftone dot is detected for each of the divided blocks, and then an n × n pixel region centered on the detected boundary pixel ( n: an integer greater than or equal to 1), the average count value of n × n pixel areas is calculated for each block, and the average count value is converted into an area ratio, so that binary image data is binary. -Perform multi-tone image conversion processing (descreening processing).

  The printer 10 may have various known print functions. For example, the printer 10 is an ink jet printer, and the printer data conversion unit 13 converts multi-tone image data into a resolution suitable for the ink jet printer 10. It is like that.

  The operation of the color proof creation system shown in FIGS. 1 and 2 will be described with reference to FIGS.

  FIG. 3 is a flowchart for explaining the operation (steps S01 to S13) of the halftone image data processing apparatus of FIGS. FIG. 4 is a diagram (a) schematically showing a halftone dot image which is an original image input to the halftone dot image data processing apparatus of FIG. 2 and an enlarged view (b) of the halftone dot image. FIG. 5 is a diagram schematically showing a state in which the halftone image of FIG. 4B is divided into blocks. FIG. 6 is a diagram schematically showing a state in which boundary pixels are detected in the divided block of FIG. FIG. 7 is a diagram schematically showing a state in which the number of pixels in a region centering on the detected boundary pixel in FIG. 6 is counted. FIG. 8 is a diagram showing the number of pixels counted for each boundary pixel in FIG.

In the halftone image data processing apparatus 100 of FIG. 2, the resolution of the halftone image and the screen are set so that the block size when the division / detection / conversion processing unit 11 of FIG. It is determined based on the number of lines and input from the input unit 16 (S01). The block size is preferably determined by the following formula (1).
Block size ≥ Resolution of halftone image / Number of screen lines (1)

  Next, the size n of the area for which the area ratio is calculated is determined based on the block size, the resolution of the halftone image, and the number of lines, and is input from the input unit 16 (S02).

The size n of the area for obtaining the area ratio depends on the minimum number of gradations necessary for the converted multi-value data and the minimum resolution required for the converted multi-value data. It is preferable to decide in 3).
n ≧ square root of the minimum required number of gradations (2)
Resolution of halftone image / (block size + n) ≦ minimum required resolution (3)

  Next, the halftone image data of the original image as shown in FIGS. 4A and 4B input from the RIP 2 in FIG. 1 and stored in the storage unit 12 is read (S03).

  For example, the halftone image data of FIGS. 4A and 4B has a resolution of 2400 dpi and a line number of 175 lines, and the block size is 14 × 14 as shown in FIG. The size is 9 × 9 as shown in FIG.

  Next, the halftone image data in FIG. 4B is divided into 14 × 14 blocks as indicated by the bold lines in FIG. 5 (S04). Let M be the number of blocks.

  Next, halftone dot boundary pixels (indicated by gray pixels in FIG. 6) are detected in one block (M = 1) as shown in FIG. 5 (S05). For example, if the number of boundary pixels detected in the central block in FIG. 6 is E, the number of boundary pixels E in the 14 × 14 block is 42 in FIG. The boundary pixel can be detected by a general edge detection process by a 3 × 3 mask operator, for example. Further, when performing edge detection processing on the pixels at the boundary portion of each block, the edge detection processing is performed with reference to pixels outside the block.

  Next, the number of pixels in the area centered on the detected boundary pixel is counted. That is, it is determined whether or not there is a boundary pixel in the block (S06). If it exists, the number of pixels having a value of 1 is counted in the 9 × 9 pixel count area centered on each detected boundary pixel. (S07). In addition, when the pixel number count area | region covers the outer side of a block about the pixel of the peripheral part in each block, the pixel number of the outer side of a block is also counted. The total count value of the number of pixels is N. The total count value N is 1578 in FIG.

Next, the average value A within the 14 × 14 block (M = 1) of the counted number of pixels is calculated by the following equation (2) (S08).
A = N / E (2)
The average value A is, for example, 1578/42 = 37.57 in FIG.

Next, the average value A in the 14 × 14 block (M = 1) is converted into the area ratio D in the block (M = 1) by the following equation (3) (S09).
D = A / (n × n) (3)
In FIG. 8, the area ratio D is, for example, D (%) = {37.57 / (9 × 9)} × 100 = 46.4.

  Next, returning to the above-described step S05 (S10), the boundary pixel of the halftone dot is detected in the next block (M = 2), and the area ratio D is obtained in the same manner. In this way, the area ratio D is obtained for all blocks.

  In step S06, if there is no boundary pixel in the block, it is determined whether or not the pixel value in the block = 1 (S11). If the pixel value in the block = 1, If the area ratio is 100% (S12) and the pixel value in the block = 0, the area ratio in the block is 0% (S13).

  As described above, multi-tone image data can be obtained by performing the descreening process for each block. In this case, if the resolution of the halftone image data that is the original image is, for example, 2400 dpi, the resolution of the converted multi-tone image data is 2400/14 = 171 dpi.

  As described above, according to the present embodiment, a halftone dot image is divided into a plurality of blocks, the number of pixels is counted in an area centering on the boundary pixel for each block, and then the count value is averaged for each block. As a result, it is possible to obtain a multi-tone image having no period unevenness. Further, it is not necessary to switch the smoothing filter finely for each screen line number and angle of the halftone image.

  The size n of the area for which the area ratio is calculated differs depending on the number of gradations and resolution required for the converted multi-value data. As n is larger, the number of gradations can be increased, and as n is smaller, the resolution can be increased.

  In general, in order to convert a binary halftone image into a multilevel (multitone) image, it is necessary to perform a smoothing process in an area having a size close to the period of halftone dots. For example, in the case of a resolution of 2400 dpi and 175 lines, the halftone dot period is about 2400/175 = 13.7, so that smoothing is performed in an area where the block size is 14 × 14 or more. Periodic unevenness that occurs in a multi-tone image due to the periodicity of the halftone image occurs because the halftone dot area ratio and the period of the area (block) to be obtained differ from the period of the halftone image. By obtaining the area ratio in the area centered on the boundary pixel, it is possible to obtain a multi-tone image having no period unevenness without causing a periodic change in the positional relationship between the area for obtaining the area ratio and the halftone dots. It can be done.

  As described above, the best mode for carrying out the present invention has been described. However, the present invention is not limited to these, and various modifications are possible within the scope of the technical idea of the present invention. For example, the printing method is not limited to an ink jet printer, and other electrophotographic methods may be used, and a color silver halide photosensitive material may be used as a recording medium. Further, the image forming apparatus is not limited to the color proof creating apparatus, and may of course form a general image.

  1 and 2, the halftone image data processing apparatus 100 converts halftone image data into multi-tone data and outputs the multi-tone data to the printer 10. The present invention is not limited to this, and the printer 10 may directly receive binary halftone dot image data and convert it into multi-gradation data inside the printer 10.

1 is a diagram schematically illustrating a configuration example of a printing system including a color proof creation system according to an embodiment. FIG. 2 is a block diagram schematically showing the halftone image data processing device of FIG. 1. 3 is a flowchart for explaining the operation (steps S01 to S13) of the halftone image data processing apparatus of FIGS. FIG. 3A is a diagram schematically illustrating a halftone image that is an original image input to the halftone image data processing apparatus of FIG. 2 and an enlarged view of the halftone image. It is a figure which shows typically a mode that the halftone image of FIG.4 (b) was divided | segmented into the block. It is a figure which shows typically a mode that the boundary pixel was detected in the divided | segmented block of FIG. It is a figure which shows typically a mode that the pixel count of the area | region centering on the detected boundary pixel of FIG. 6 is counted. It is a figure which shows the pixel count counted for every boundary pixel in FIG.

Explanation of symbols

10 Printer 11 Division / Detection / Conversion Processing Unit 15 Control Unit 100 Halftone Image Data Processing Device E Number of Boundary Pixels S Recording Medium n Size of Area for Obtaining Area Ratio

Claims (5)

A descreening method for converting halftone image data into smoothed multi-tone image data,
Dividing the halftone image into blocks of a predetermined size;
Detecting a boundary pixel of a halftone dot for each block;
Obtaining an area ratio of a region of a predetermined size centered on the boundary pixel for each detected boundary pixel;
And a step of calculating an average of the area ratios obtained for each boundary pixel in the block for each block.   The halftone image descreening method according to claim 1, wherein the block size is determined to be larger than a period of the halftone dots.   3. The halftone image descreening according to claim 1, wherein the area for obtaining the area ratio for each boundary pixel is an n × n pixel area (n is an integer of 1 or more) centered on the boundary pixel. Method.   4. An image forming method, wherein image formation is performed based on multi-tone image data converted from halftone dot image data by the halftone dot descreening method according to claim 1. An image forming apparatus for performing image formation on the basis of multi-tone image data converted from halftone image data by the halftone image descreening method according to any one of claims 1 to 3.