JP2005333355A - Method and apparatus for forming halftone image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a halftone image forming method and a halftone image forming apparatus which can regulate each correction balancing of a shadow side and a highlight side in the correction of density and a color tone using a pixel existing in a boundary of halftones. <P>SOLUTION: This halftone image forming method includes steps of specifying the boundary pixel of the halftone in the halftone image when the halftone image is formed based on binary halftone image data, detecting the outside edge and the inside edge in the boundary of the specified boundary pixel, correcting the halftone image data so that at least one of the density and the color tone at the detected outside edge and the inside edge, and forming the image according to the corrected halftone image data. Each balance of the highlight side and the shadow side in the halftone image can be regulated by changing at least either of the density and the color tone at the outside edge and the inside edge. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a halftone dot image forming method and a halftone dot image forming apparatus for forming a halftone dot image based on binary halftone dot image data.

  Conventionally, in a color printing process, a proof (color proof) is separately created for proofreading of a color print, and the finished product is checked in advance before creating a production printing plate. For this reason, multi-valued image data is halftoned by a front system such as a halftone image data generating device to generate binary image data, and a color proof creating device based on the halftone image data by a plurality of light sources having different wavelengths. A silver salt color light-sensitive material is exposed to create a color proof (calibration product). In such a color proof creating apparatus, exposure is performed by setting a look-up table (LUT) corresponding to a combination of colors separated into Y, M, C, and K plates (for example, the following patents) References 1 and 2).

  However, in color printing using ink, there is a phenomenon such as dot gain that is thickened by squeezing halftone dots due to the printing pressure of the printing machine. The color proof image created in this way may have a color tone different from that of the actual printed material, and the printed image may not be sufficiently reproduced. For this reason, the dot gain is adjusted by changing the density of the boundary of the halftone dots with respect to the binary halftone image. For example, the present inventor previously calculated the dot gain adjustment in Japanese Patent Application No. 2003-325393 by making the pixel density along the entire circumference of the boundary pixel of the halftone dot lower than that of the halftone dot pixel. It was proposed that it could be realized with processing load.

  However, this adjustment is effective for adjusting the overall dot gain including the shadow part to the highlight part, but by adjusting the dot gain of the shadow part and highlight part independently, It was not possible to correct the balance of the midtone part and the highlight part.

Also, for example, in Patent Document 1 below, the gradation of the halftone image is controlled by inverting the pixels at the boundary of the halftone dots and increasing or decreasing the pixels, but the added pixels are the same as the original halftone dots. Recorded by exposure amount. Further, in Patent Document 2 below, when the halftone image is exposed, the density of the halftone dot is directly controlled by the exposure amount.
JP 2002-290722 A JP 07-156362 A

  In view of the above-described problems of the prior art, the present invention provides a halftone dot in which the correction balance on the shadow side and the highlight side can be adjusted in density / tone correction using pixels existing at the boundary of the halftone dot. An object is to provide an image forming method and a halftone dot image forming apparatus.

  In order to achieve the above object, a halftone image forming method according to the present invention is a method for forming a halftone image based on binary halftone image data, and specifies a boundary pixel of a halftone dot in the halftone image. And detecting the outer edge and the inner edge at the boundary of the specified boundary pixel, and changing the halftone dot image data so as to change at least one of the density and the color tone of the detected outer edge and the inner edge. And a step of forming an image based on the corrected halftone dot image data, and changing at least one of the density and the color tone of the outer edge and the inner edge to change the high level in the halftone dot image. Each correction balance on the light side and the shadow side can be adjusted.

  According to this halftone image forming method, by changing at least one of density and color tone at the outer edge and inner edge at the boundary of the halftone dot boundary pixel, the highlight side is Since the number of pixels on the shadow side tends to be reversed, the correction balance on the highlight side and the shadow side in the halftone image can be adjusted.

  In the halftone image forming method, when creating a color proof based on the binary halftone image data, the dot gain in the color proof can be adjusted based on the corrected halftone image data. Thereby, it is possible to adjust each correction balance on the highlight side and the shadow side in the dot gain adjustment.

  Preferably, an edge width for detecting the outer edge and the inner edge is set based on at least one of a halftone dot size and a pixel size, and at least one of the density and the color tone is changed in the detected edge width. . The halftone dot size varies depending on, for example, the number of screen lines, and an appropriate edge width can be determined accordingly. Also, the pixel size varies depending on, for example, the output resolution, and an appropriate edge width can be determined accordingly.

  A halftone dot image forming apparatus according to the present invention is a device for forming a halftone dot image based on binary halftone dot image data, specifying a boundary pixel of a halftone dot in a halftone dot image, and specifying the specified boundary pixel Means for detecting an outer edge and an inner edge at the boundary of the image, a correcting means for correcting the halftone image data so as to change at least one of density and color tone of the detected outer edge and inner edge, and the correction Means for forming an image with the halftone dot image data, and changing at least one of the density and the color tone of the outer edge and the inner edge, so that each of the highlight side and the shadow side in the halftone image The correction balance can be adjusted.

  According to this halftone dot image forming apparatus, by changing at least one of the density and the color tone at the outer edge and inner edge at the boundary of the halftone dot boundary pixel, the outer edge and the inner edge of the halftone dot are highlighted. Since the number of pixels on the shadow side tends to be reversed, the correction balance on the highlight side and the shadow side in the halftone image can be adjusted.

  When creating a color proof based on the binary halftone dot image data in the halftone dot image forming apparatus, the dot gain in the color proof can be adjusted based on the corrected halftone dot image data. Thereby, it is possible to adjust each correction balance on the highlight side and the shadow side in the dot gain adjustment.

  Preferably, an edge width for detecting the outer edge and the inner edge is set based on at least one of a halftone dot size and a pixel size, and at least one of the density and the color tone is changed in the detected edge width. .

  In the present specification, the outer edge at the boundary of the halftone dot boundary pixel is a non-colored pixel adjacent to the boundary, and similarly, the inner edge is a colored pixel adjacent to the boundary. It is.

  According to the halftone image forming method and halftone image forming apparatus of the present invention, each of the correction on the shadow side and the highlight side is corrected by correcting the density and tone of the outer edge and the inner edge existing at the boundary of the halftone dot. You can adjust the balance.

  Further, in the case of gradation adjustment of halftone dots, since it can be adjusted by a combination of the outer edge and the inner edge, compared to the conventional general method of controlling the correction amount after performing binary-multivalue conversion, 2 Value-multivalue conversion is not required, the calculation load is reduced, and the adjustment of the correction balance can be realized at low cost.

  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 halftone image forming apparatus according to the present embodiment.

  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 a RIP (Raster Image Processor) 2 that is an image processing apparatus, and the RIP 2 rasterizes PS data. Data converted to image and raster-image converted by RIP2 is sent to CTP (Computer Tow Plate) 3 which is a device for creating a digital original directly for printing, while creating a color proof The halftone dot image data is generated, and the halftone dot image forming device 11 generates a color group based on the halftone dot image data.

  A digital image output system is configured by the control device 10 and the halftone image forming device 11 in FIG. Create and output a color proof (calibration sample) to perform. The color proof, moire, unevenness, etc. are confirmed by this color proof, an original plate is created by CTP3 in FIG.

  Next, the halftone image forming apparatus of FIG. 1 will be described with reference to FIGS. FIG. 2 is a block diagram showing a main part of the halftone image forming apparatus of FIG. FIG. 3 is a diagram for explaining edge detection by a 5 × 5 mask in the edge detection unit of FIG. FIG. 4 is a diagram schematically showing an example of RGB light source drive values set by the CPU 23 for the LUT 22 of FIG.

  As shown in FIG. 2, the halftone image forming apparatus 11 includes an edge detection unit 21, a lookup table (LUT) 22, a central processing unit (CPU) 23, an exposure unit 24, a development unit 25, Is provided.

  The edge detection unit 21 detects whether or not a certain pixel is an edge (boundary) between 0 and 1, identifies a boundary pixel, for example, adds a color of an outer edge at the boundary of the identified boundary pixel. Pixels in the first column and pixels in the second column that are not detected are detected, and pixels in the first column and pixels in the second column with the inner edge color at the boundary are detected.

  For example, the edge detection unit 21 detects pixels in the first column and the second column of the outer edge and detects pixels in the first column and the second column of the inner edge. When using such a 5 × 5 mask, for example, when only one pixel outside the boundary of the halftone dot is used as the boundary pixel, the center pixel 50 of the mask is a white pixel, and the width of the two pixels at the four connected positions When any one of the peripheral pixels 51 of the minute is the central pixel of the halftone dot, the central pixel 50 is determined as the boundary pixel. Similarly to FIG. 3, when the central pixel is the central pixel, one pixel inside the boundary of the halftone dot is determined as the boundary pixel.

  The edge detector 21 detects the pixels in the first and second columns of the outer edge and the pixels in the first and second columns of the inner edge as described above, as shown in FIG. Although five masks are used, the mask size is determined according to the edge width of the detection target, and therefore is appropriately set based on the edge width of the detection target pixel.

  The LUT 22 receives the four color (YMCK) binary point image data from the control device 10 of FIG. 1 and the outer edge signal / inner edge signal of the halftone dot detected by the edge detection unit 21, and these input signals. The RGB light source driving values for the exposure unit 24 are output based on the above.

  The CPU 23 is configured to set the RGB light source drive values for the halftone dot portion, the outer edge, and the inner edge for the LUT 22. For example, as shown in FIG. 4, each color of YMCK in the halftone dot portion, each of the two colors, a combination of each of the three colors, each of the one edge, each of the two colors, each of the three colors, each of the inner edges Each light source driving value is set. Each of the light source driving values of the outer edge and the inner edge in FIG. 4 is, for example, a percentage (10%) of Y + E (outside) (Y outer edge) and Y + E (inner) (Y inner edge) with respect to the Y light source driving value a. 90%).

  The exposure unit 24 exposes the silver halide color photosensitive material with each light of RGB from each light source driven based on each light source driving value set in the LUT 22 to form an image. The silver halide color light-sensitive material that has been exposed by the exposure unit 24 and formed with an image is developed by the developing unit 25 on the downstream side of the exposure unit 24, whereby the image is visualized.

  Next, for dot gain adjustment in color proof creation for color printing, gradation adjustment is performed by changing each density of the first and second rows of the outer edge of the halftone dot and the first and second rows of the inner edge. The performed proof image will be described with reference to FIGS. In the following description, an example in which gradation adjustment for dot gain adjustment is performed on a halftone dot image of one color will be described. However, a boundary pixel detection result is similarly applied to a halftone dot where a plurality of colors overlap. Based on this, the same gradation adjustment for dot gain adjustment can be performed.

  FIG. 5 is an enlarged schematic view of a part of a proof image that has not been subjected to gradation adjustment. FIG. 6 is an enlarged schematic view of the proof image in which the densities of the first and second rows of the outer edge of the halftone dot and the first and second rows of the inner edge are changed.

  As shown in FIG. 5, in the case where gradation adjustment is not performed for a single-tone halftone image, a halftone dot 1 constituted by a set of colored pixels 3 is provided in the sea of white pixels 2. .

  Next, as shown in FIG. 6, the boundary pixel 5 is specified for the same image portion as in FIG. 5, the boundary (edge) 6 (indicated by the thick line in FIG. 6) is detected, and the pixel 4 in the first column of the outer edge is detected. The pixel 4a in the second column and the pixel 5 in the first column and the second column pixel 5a at the inner edge are detected. A halftone dot 10 is formed by performing gradation adjustment for dot gain adjustment by changing the densities of the outer edge pixels 4 and 4a and the inner edge pixels 5 and 5a. In the halftone dot 10, the pixels 4, 4 a for the two pixels outside the halftone dot boundary 6 have different densities from the pixel 3 at the center of the halftone dot, and two pixels inside the halftone dot boundary 6 The pixels 5 and 5a have different densities from the pixel 3 at the center of the halftone dot. As shown in FIG. 4, this density can be set by the percentage of the light source driving values of the outer edge and the inner edge. Specifically, the density of the colors of the pixels 4, 4 a and the pixels 5, 5 a is lower than that of the pixel 3.

  Next, the balance adjustment of the dot gain on the shadow side and the highlight side will be described with reference to FIG. FIG. 7A is a schematic diagram in which a part of a proof image is enlarged on the highlight side (part where the gradation density is light) with a halftone dot area ratio of, for example, 1%, and FIG. 7B is a halftone dot area ratio. FIG. 9 is a schematic diagram in which a part of a proof image is enlarged on the shadow side (the dark gradation portion) of 99%, for example.

  As shown in FIG. 7A, when considering a case where the halftone dot is 1 pixel g on the highlight side where the dot area ratio is 1% (part where the gradation density is light), the pixel g of the inner edge is 1 As shown by the solid line in the figure, the number of pixels on the outer edge is eight considering the second column.

  Further, as shown in FIG. 7B, the solid area Gb in which one pixel g ′ having no color on the shadow side (the portion having a high gradation density) having a halftone dot area ratio of 99%, for example, is colored. Considering the case where the pixel is present inside, there are eight pixels on the inner edge and one pixel on the outer edge in consideration of the second column as shown by the broken line in the figure.

  7A and 7B, it can be seen that the number of pixels of the outer edge generally tends to be larger than the inner edge on the highlight side, and the number of pixels of the inner edge tends to be larger than the outer edge on the shadow side. . Therefore, when gradation adjustment for dot gain adjustment is performed by changing the densities of the pixels in the first and second columns of the outer edge and the pixels in the first and second columns of the inner edge, respectively. As shown in FIG. 7 (a), on the highlight side, there is a characteristic that the outer edge of the halftone dot has a stronger influence on the highlight side (the portion where the gradation density is lighter), and as shown in FIG. There is a characteristic that the inner edge has a stronger influence on the shadow side (the darker part). Therefore, the balance of correction on the highlight side and the shadow side can be adjusted by utilizing the difference between these characteristics.

  FIG. 8 is a diagram schematically showing the relationship between the adjustment rate and the dot area rate when dot gain adjustment is performed. A curve indicated by a broken line in FIG. 8 shows a case where the dot gain is adjusted by making the density of the pixels along the entire circumference of the halftone dot boundary pixels lower than that of the halftone dots as in the conventional case. While the dot gain is adjusted most in the halftone range where the halftone dot area ratio is around 50%, the adjustment rate approaches 0% on the highlight side and the shadow side and approaches 0% as it approaches 100%. On the other hand, in the case of the present embodiment, the number of pixels on the outer edge and the inner edge arranged along the halftone dot boundary 6 in FIG. 6 in the halftone range where the halftone dot area ratio is around 50%. As described above, the correction on the highlight side where the dot area ratio is smaller than 50% and the shadow side where the dot area ratio is larger than 50% are achieved. By adjusting the balance, for example, as shown by the solid line in FIG. 8, it is possible to perform a dod gain adjustment in which the adjustment rate on the highlight side is made larger than that on the broken line and the adjustment rate on the shadow side is made smaller.

  Next, the operation of the halftone image forming apparatus 11 shown in FIGS. 1 to 4 will be described with reference to the flowchart of FIG.

  First, when the binary dot image data generated by the control device 10 of FIG. 1 is input to the dot image forming device 11 of FIG. 2 via an interface such as a data buffer (S01), the edge detection unit 21 The binary halftone image is scanned for each bit plane to identify the center pixel and the boundary pixel of the halftone dot (S02), and the first and second columns of the outer edge at the boundary of the specified boundary pixel. The pixels in the first column and the pixels in the second column with the inner edge color are detected (S03).

  Next, the data of the binary halftone dot image is corrected for each specified boundary pixel. Specifically, the CPU 23 in FIG. 2 applies to the LUT 22, for example, as shown in FIG. Each RGB light source driving value of the inner edge is set (S04). Thereby, the density is set for each pixel including the pixels of the outer edge and the inner edge. By repeating this operation, the density is set for all the pixels.

  Next, the set image data is output to the exposure unit 24 in FIG. 2 (S05), and based on the image data, the exposure unit 24 exposes the recording paper made of a silver halide photosensitive material from each of the RGB light sources. (S06). Immediately after the exposure is completed, the exposed recording paper is conveyed to the developing unit 25, and after the exposed recording paper is developed by the developing unit 25 (S07), it is output as a color proof (S08).

  As described above, according to the present embodiment, the density of up to two columns outside (colorless) and the density of up to two columns (colored) inside the edge (boundary) portion of the halftone dot are obtained. When performing dot gain adjustment or color adjustment by changing the halftone dot outer edge and inner edge, the number of edge pixels on the highlight side and shadow side tends to be reversed. The correction balance on the shadow side and highlight side can be adjusted by combining the respective densities on the shadow side and highlight side.

  Conventionally, when halftone adjustment is performed, a binary-multilevel conversion is performed to obtain a halftone dot area ratio%, and then the correction amount is controlled based on the obtained halftone dot area ratio% value. However, according to this conventional method, the calculation load is large, but in the present embodiment, by adjusting the combination of the outer edge and the inner edge, binary-multivalued Conversion is not necessary, and correction balance adjustment can be realized at low cost.

  As described above, in the present embodiment, in order to adjust the correction balance on the shadow side and the highlight side, the density of the edge width corresponding to two pixels from the first row to the second row at the outer edge and the inner edge. However, for example, when a halftone dot having a screen line number of 175 lpi is output at an output resolution of 2400 dpi, the edge width is preferably equivalent to one pixel or two pixels.

  However, since the appropriate edge width varies depending on the number of screen lines and the output resolution, it is preferable to set the edge width at the outer edge and the inner edge so as to be an appropriate edge width according to them. Therefore, in the present invention, it is preferable to appropriately set the number of columns (number of pixels) of the edge width including the case where the edge width is one column.

  That is, in the halftone image, the size of the halftone dot varies depending on the number of screen lines, and the smaller the screen line number, the larger the halftone dot size. Therefore, it is preferable to increase the edge width.

  Also, when the output resolution changes, the number of pixels constituting the halftone dot changes even with the same screen line number, and as the output resolution increases, the size of each pixel decreases, so the width of one row of edges also decreases. Become. Therefore, it is preferable to increase the edge width as the output resolution increases.

  The screen line number indicates the printing accuracy and is expressed by how many lines of halftone dots are arranged in 1 inch. The unit is “line” or “lpi” (lines per inch). Each one becomes smaller. The output resolution indicates how finely the output is performed by printing, and the unit is dpi (dots per inch). Printing accuracy is almost determined by the combination of the number of screen lines and the output resolution.

  In addition, when the edge width detected for the outer edge and the inner edge is one column, the edge detection unit 22 in FIG. 2 detects up to the first column using a 3 × 3 mask. Tone adjustment is performed by changing each density of the pixel 4 in the column and the pixel 5 at the inner edge, thereby adjusting the dot gain. Also in this case, as in FIG. 7, the highlight side has a characteristic that the outer edge of the halftone dot has a stronger influence on the highlight side, and the inner edge has a characteristic that the influence on the shadow side is stronger. The balance of correction on the highlight side and the shadow side can be adjusted by using the difference in the characteristics.

  Further, even if the edge width for detecting and changing the density of the outer edge and the inner edge is three columns or more, similarly, the gradation adjustment is performed by changing the density of each pixel, and the highlight side and the shadow side are adjusted. The balance of correction can be adjusted.

  Further, the outer edge at the boundary of the halftone dot boundary pixel is a non-colored pixel adjacent to the boundary, the first column is a non-colored pixel next to the boundary, and the second column is the non-colored pixel. The adjacent pixels are not colored. Similarly, the inner edge is a colored pixel adjacent to the boundary, the first column is a colored pixel next to the boundary, and the second column is a colored pixel next to the boundary pixel. It is.

  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, in the present embodiment, the density is changed to be low for each pixel of the detected outer edge and inner edge, but it goes without saying that the color tone may be changed.

  Further, as indicated by a broken line in FIG. 1, PS data may be directly input from the client terminal 1 to the control device 10, and PS image raster image conversion processing may be performed in the control device 10.

  The image data input to the control device 10 in FIG. 1 may be image data in various formats. For example, the vector system may be PDF, EPS, etc. in addition to PS (PostScript) data. Further, the raster system may be TIFF, TIFF / IT, and other dedicated formats, and the binary data may be TIFF bitmap, other dedicated formats, and the like.

1 is a diagram schematically illustrating a configuration example of a printing system including a halftone dot image forming apparatus according to the present embodiment. FIG. 2 is a block diagram illustrating a main part of the halftone image forming apparatus of FIG. 1. It is a figure for demonstrating the edge detection by a 5x5 mask in the edge detection part of FIG. It is a figure which shows typically the example of each light source drive value of RGB set by CPU23 with respect to LUT22 of FIG. FIG. 4 is a schematic diagram for enlarging a part of a proof image that has not been subjected to gradation adjustment for comparison. FIG. 6 is an enlarged schematic diagram of a proof image in which the densities of the first and second rows of the outer edge and the first and second rows of the inner edge are changed with respect to the halftone image of FIG. 5 in the present embodiment. . FIG. 7A is a schematic diagram enlarging a part of the proof image on the highlight side (part where the gradation density is light) with a halftone dot area ratio of, for example, 1% in the present embodiment. FIG. 4 is an enlarged schematic view of a part of a proof image on the shadow side (the dark gradation portion) where the dot area ratio is 99%, for example. It is a figure which shows the relationship between the adjustment rate at the time of dot gain adjustment in this Embodiment, and a halftone dot area rate roughly compared with a prior art example (broken line). 6 is a flowchart for explaining the operation of the halftone dot image forming apparatus 11 of FIGS. 1 to 4;

Explanation of symbols

4 pixel of the first column of the outer edge 4a pixel of the second column of the outer edge 5 boundary pixel, pixel of the first column of the inner edge 5a pixel of the second column of the inner edge 6 dot boundary 10 dot 11 dot Image forming apparatus 21 Edge detection unit 24 Exposure unit 25 Development unit 50 Central pixel 51 Peripheral pixel g One pixel with highlight side color g 'One pixel without color on shadow side

Claims (6)

A method of forming a halftone image based on binary halftone image data,
Identifying a boundary pixel of a halftone dot in a halftone image and detecting an outer edge and an inner edge at the boundary of the identified boundary pixel;
Correcting the halftone image data so as to change at least one of density and color tone of the detected outer edge and inner edge;
Performing image formation with the corrected halftone dot image data,
A halftone image forming method, wherein the correction balance on the highlight side and the shadow side in the halftone image can be adjusted by changing at least one of density and color tone of the outer edge and the inner edge. . 2. The halftone dot according to claim 1, wherein when creating a color proof based on the binary halftone image data, a dot gain in the color proof is adjusted by the corrected halftone image data. Image forming method. An edge width for detecting the outer edge and the inner edge is set based on at least one of a halftone dot size and a pixel size, and at least one of the density and the color tone is changed in the detected edge width. The halftone image forming method according to claim 1 or 2. An apparatus for forming a halftone dot image based on binary halftone dot image data,
Means for identifying a boundary pixel of a halftone dot in a halftone image, and detecting an outer edge and an inner edge at the boundary of the identified boundary pixel;
Correction means for correcting the halftone image data so as to change at least one of the density and tone of the detected outer edge and inner edge;
Means for forming an image with the corrected halftone image data,
A halftone image forming apparatus characterized in that the correction balance on the highlight side and the shadow side in the halftone image can be adjusted by changing at least one of the density and color tone of the outer edge and the inner edge. . 5. The halftone dot according to claim 4, wherein when creating a color proof based on the binary halftone dot image data, a dot gain in the color proof is adjusted by the corrected halftone dot image data. Image forming apparatus. An edge width for detecting the outer edge and the inner edge is set based on at least one of a halftone dot size and a pixel size, and at least one of the density and the color tone is changed in the detected edge width. The halftone dot image forming apparatus according to claim 4 or 5.

Images