JP2006134162A - Method and device for halftone dot gradation image formation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image formation method which can commonly use halftone gradation images used for the creation of a printed matter, when creating proofs and has the same halftone structure with a printed matter and densities of a monotone color and moreover, can create proofs, capable of compensation, corresponding to the differences in color tones for many colors of color materials and also of reproducing suitable image quality. <P>SOLUTION: A method for obtaining a second image from a first dot image comprises a step of estimating the dot gain, corresponding to a printed matter using the first image to each pixel of a dot boundary of the first image, a step of changing the color tone of an inner edge pixel in the boundary by increase probability determined with the dot gain, and a step of changing at least a part of the outer edge pixels in the boundary into dot pixels, on the basis of the increase probability fixed based on the dot gain. The step of changing color tone is carried out prior to a step of increasing the pixels. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention mainly relates to an image forming method and an image forming apparatus for forming a halftone image as a color calibration of a printed material. More specifically, an image forming method for obtaining a color proof that approximates a printed material based on a binary image for the printing press that has been subjected to color separation and binarization for each pixel by a RIP (Raster Image Processor). About.

  It has become common practice to produce a print original as digital data on a computer and to form a halftone image for printing through RIP. In normal color printing, at least four halftone dot gradation images for printing of cyan (C), magenta (M), yellow (Y), and black (K) are created from a printed document through RIP and printed. used.

  This halftone image is configured as a collection of pixels, and the gradation of the image is expressed by the size of the halftone dot area. To each pixel, 1-bit (binary) data is pasted, which means either a halftone dot pixel where ink is placed in the corresponding printed matter or a missing pixel pixel without ink. ing.

  By the way, it is wasteful to make corrections after creating and trial printing a printing plate. For this reason, before creating a printing plate, it is usual to produce a proof that outputs the same halftone image as the printed material and checks the finished characters and color tone of the printed material in advance.

  Proof creation is performed using an image forming apparatus called a proofer having an output method different from that of the printing press. At that time, the halftone image used for the creation is an image obtained through another RIP adjusted for proof different from that for printing. This is because dot gain with thick dots occurs in this printing machine, but the same dot gain does not necessarily occur in image forming apparatuses with different output methods, and the coloring characteristics of the two differ depending on the color material. by. Therefore, a print document created by a computer is sent to both the print RIP and the proof RIP, and a halftone image is generated in each.

  However, as a result, when the overall image quality of the proof obtained from the image forming apparatus and the printed matter are substantially matched, the halftone dot shape and size of the details may differ. In addition, when the color tone (solid color) of the 100% solid portion of the proof is matched, the color tone of the portion where the dot area ratio is about 50% is shifted, so that the color tone of the solid portion is shifted and balanced. Things were also done. Furthermore, due to the difference in RIP, human error such as an error in setting a designated font and an error in setting a halftone dot shape may occur.

  Here, there has been disclosed an image calibration method that can express dot gain in printing with another output device using the same halftone image as that of a printing press (see, for example, Patent Document 1). Specifically, a step of providing a halftone bitmap image, a step of estimating a pixel halftone dot area ratio, a step of calculating a target pixel halftone dot area ratio by a predetermined color calibration function, A method for calibrating a halftone bitmap image comprising a step of calculating the number N of pixels to be converted to an on state and an off state to form an image and a step of converting N pixels to the on state and the off state is disclosed. . According to this method, it is possible to obtain a proof image that matches the halftone dot area ratio of the printed matter by appropriately setting the color calibration function.

However, this method simply adjusts the size of the halftone dots, and cannot cope with a color tone shift due to a difference in color materials. For example, when a silver halide light-sensitive material is used as a color material, the density of the solid portion of the proof is low when the mechanical size of the halftone dot and the halftone dot area ratio measured from the density are matched to the printed matter by this method. As a result, only a low-contrast image quality proof can be obtained. In addition, if the number of pixels that are colored around a halftone dot is increased randomly, there is a problem that the image has a feeling of roughness and the effect of tone jump becomes large. Furthermore, since the degree of color tone deviation resulting from the difference in color material varies from color to color, there is a problem in that it is difficult to perform fine correction on a wide range of color tones.
JP 2004-40781 A

  The present invention also uses the halftone image used for creating the printed material in common for creating the proof, has the same halftone dot structure and solid color density as the printed material, and has a difference in color tone in many colors depending on the color material. It is an object of the present invention to provide an image forming method and the like that can create a proof that can be corrected and can reproduce good image quality.

  A first aspect of the present invention is an image forming method for obtaining a second halftone image from a first halftone image, and a pixel located at a halftone dot boundary of the first halftone image. For each position, the estimation step for estimating the dot gain corresponding to the printed matter using the first halftone image, and the change probability determined based on the dot gain is positioned at the inner edge of the halftone dot boundary. A halftone dot color changing step for changing the color tone of the boundary pixel and an increase probability determined based on the dot gain, and at least a part of the boundary pixels located at the outer edge of the halftone dot boundary is a halftone pixel A halftone pixel increasing step for changing to halftone dots, and the halftone tone changing step is executed in preference to the halftone pixel increasing step.

  Here, it is preferable that the halftone dot increasing step is executed after the color tone of almost all of the boundary pixels located at the inner edge of the halftone dot boundary is changed by the halftone dot color changing step. In addition, for each pixel located at the halftone dot boundary, the estimating step calculates a partial area ratio that the halftone dot occupies in the image, and the dot area is calculated from the partial area ratio and the dot gain curve of the printed matter. Preferably the step of estimating the gain. Moreover, it is preferable that the partial area ratio is calculated using a mask having a predetermined size centering on a pixel located at the halftone dot boundary.

  A second aspect of the invention is an image forming apparatus for obtaining a second halftone image from a first halftone image, and each pixel located at a halftone dot boundary of the first halftone image. And an estimation means for estimating the dot gain corresponding to the printed matter using the first halftone gradation image, and the change probability determined based on the dot gain is located at the inner edge of the halftone dot boundary. Halftone dot color changing means for changing the color tone of the boundary pixel and an increase probability determined based on the dot gain, at least a part of the boundary pixels located on the outer edge of the halftone dot boundary is changed to a halftone dot pixel. An image forming apparatus comprising: a halftone pixel increasing unit for changing; and the halftone color changing unit being executed in preference to the halftone pixel increasing unit.

  According to a third aspect of the invention, a printing step of outputting a printed matter using the first halftone image, and a second halftone image formed from the first halftone image, A proof output method for outputting a proof of the printed matter, wherein the proof output step applies the printed matter to each pixel located at a halftone dot boundary of the first halftone image. An estimation step for estimating a corresponding dot gain; a halftone color tone changing step for changing a color tone of a boundary pixel located at an inner edge of the halftone dot boundary according to a change probability determined based on the dot gain; and the dot A halftone pixel increasing step of changing at least some of the boundary pixels located at the outer edge of the halftone dot boundary to halftone pixels with an increase probability determined based on the gain, and the halftone color tone Change The image forming method characterized in that the flop is executed prior to the halftone pixel increment step.

  In the proof, it is possible to correct a color shift caused by a difference in color material in a wide color range, and the proof image reproducibility is improved. In addition, with the adjustment of the image, it is possible to obtain a high quality proof that is less susceptible to the effects of roughness and tone jumping.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing the entire processing flow when the image forming method of the present invention is embodied. Here, as a first halftone image, a halftone image 21 that is a set of 1-bit data obtained by separating and binarizing from DTP document data 11 of a printed matter by RIP 20 is used as an example. An example of the first step is a step of obtaining the printed matter 31 from the print original 11, and an example of the second step corresponds to a step of obtaining the proof 41 from the print original 11.

  In the processing flow of FIG. 1, first, a print document 11 is created by a computer. As a computer, a DTP (Desk Top Publishing) system 10 using a description language called Postscript is often used. The printed document 11 is created as a file such as PDF (Portable Document Format), for example, and the text portion is composed of a combination of text data and font data. This is combined with graphic part data for graphic display and format data to form the whole.

  The RIP 20 separates a print document including vector data constituting a figure into element colors and develops it into a halftone image that is a set of 1-bit data that can be printed and displayed. Since the halftone image obtained by RIP is expressed by the size of a halftone dot (or cell) that is a set of pixels, a halftone image can be formed. In RIP processing, since it is necessary to process an enormous amount of data, RIP is often configured with dedicated hardware, but may be configured with software on a computer. Even if the same printed document is used, the same result is not always obtained when different RIPs are used. Unexpected errors may occur due to different fonts installed in each RIP or different software versions.

  The halftone image 21 that is an aggregate of 1-bit data obtained through the RIP process 20 is sent to the printing machine 30 and the image forming apparatus 40 as they are, and a printed matter 31 and a proof 41 are obtained from them. Here, since the processing in the RIP 20 is common to the printing press 30 and the image forming apparatus 40, the basic halftone dot structure such as the halftone dot shape and the number of lines is the same in both. However, on the other hand, the image forming apparatus 40 performs processing described below with reference to FIGS. 5 to 16 in order to cope with a change in color tone due to a difference in dot gain or color material generated during printing.

  The printing machine 30 uses the YMCK plates that are process colors and the special color plates that are used as necessary, and prints the inks on the printing paper in a predetermined order. Various colors are expressed by overprinting ink. For example, in the combination of YMCK, four colors of each one, 11 colors by a combination of a plurality of plates, and a total of 16 colors including a white background are expressed.

  FIG. 2 shows that when a YMCK plate, which is a process color, is used in a printed material, a solid color with a dot area ratio of 100% on the printed material indicated in the color name column is indicated by a white circle in the same row. It means that it is expressed by overprinting the printing plate. Here, “+” means that colors are overprinted. The display of the color of the plate used in the following description is as shown in FIG. It is optional to add a feature plate to this.

  The image forming apparatus 40 generates a second halftone image suitable for proof formation from the first halftone image 21 and outputs the second halftone image to the output device 50. The image forming apparatus 40 is provided on a general-purpose computer, and an output device 50 is connected to its control output. As the output device 50, in the following example, a silver halide color photosensitive material is used which develops colors in the column of the color name in FIG. 2 by a combination of Y, M, and C coloring layers that are the element colors of the photosensitive material. This will be described with reference to an example in which this is exposed while being two-dimensionally scanned with an LED or the like and further developed with a predetermined developer or the like.

  In such an output device, for each of the 15 colors excluding white (W) in FIG. 2, the exposure amounts of the R, G, and B LEDs for each element color of the silver halide photosensitive material are changed in multiple stages. Thus, it is possible to change the concentration almost continuously. As a result, it is possible to cope with various color differences associated with differences in ink and printing paper grades.

  However, the output device is not particularly limited as long as it can express various colors. For example, the output device may express various colors by combining pixels having different colors. In order to clearly reproduce the halftone dot shape and improve the plate inspection property, it is preferable to use a silver halide photosensitive material.

  FIG. 3 is a conceptual diagram showing the data structure of a halftone image, which is an aggregate of 1-bit data obtained by RIP processing. For each pixel, 1-bit data is pasted to indicate whether ink is placed (printed) on each plate, “1 (put ink)” or “0 (do not put ink)” ing. In order to simplify the description, any one of the plates (for example, the Y plate) will be described below, and a plurality of plates will be referred to if necessary.

  FIG. 4 is a diagram schematically showing a part of the halftone image of any one of the plates. In FIG. 4, an image substantially corresponding to the standard number of lines is shown as an example. The image is configured as an aggregate of pixels, and the halftone dot 5 which is a portion where ink is placed is composed of a set of pixels 2 which are shaded to mean that ink is placed. The portion where no ink is placed is composed of pixels 1 that mean a white background. Here, the bold, substantially square frame 6 means a region having a size corresponding to one period in which halftone dots appear. Based on the size of the area in the frame, a process for obtaining a halftone dot area ratio described later is performed.

  A schematic flow of processing performed in the image forming apparatus 40 is shown in FIG. First, when the halftone image 21 through the RIP 20 is input to the image forming apparatus 40, the dot gain data at the time of printing measured using a standard printed material is also input (S10). Here, the dot gain of the printed material is based on the optically measured density of the white background and the solid area of 100% of the standard printed material. For example, the density of a plurality of standard images in 5% increments or 10% increments is optically measured, and the dot thickness at each halftone dot area ratio is measured with respect to the density of the white background and solid background previously measured. It is obtained in advance by calculating the amount (dot gain). The measured dot gain data of the printed matter is stored in the dot gain table of FIG. In the table of FIG. 6, data is stored in halftone dot area ratios in 5% increments in the highlight and shadow portions and in 10% increments in the intermediate portion.

  At the same time, characteristic data for identifying a difference in characteristics derived from printing conditions such as paper quality and ink type is also input to the printed matter (step S10). As a result, the conditions for color development are specified during proof formation. Corresponding to the printing conditions input here, subsequently, a lookup table (LUT) used for proof formation is specified (step S20). The LUT is a table that stores a so-called device profile of the image forming apparatus 40, and is necessary for reproducing a solid color (standard color) at a halftone dot area rate of 100%, which is used as a standard in a printed material, by proofing. 3 is a table for specifying combinations of color densities of YMC that are element colors of a silver halide photosensitive material. An example of this table is shown in FIG. The adjustment color will be described later.

  This table is obtained by combining the color of each layer of Y, M, and C in advance in the output device in a multi-stage condition, and for the color patch created by exposure / development under those conditions in the CIELAB color space. By storing Y, M, C density measured under *, a *, b * coordinates and status T, the data of the appropriate YMC density corresponding to the standard color of the printed matter is linked and stored It is. In order to simplify the description, it is assumed that the color of the printed matter determined by the printing conditions has already been specified using values in the L * a * b * coordinate system. Further, it is assumed that a halftone image is required for all the color plates used in the printed material.

  The 100% solid color of the proof mesh matches the solid color (standard color) of the printed material as much as possible. Since this image forming method includes a plurality of color tone adjusting means as will be described later, even in this way, a difference in image quality between the proof and the printed matter is unlikely to occur. Matching both the solid color of the proof and the halftone dot structure to the printed material makes it difficult to adjust the difference in image quality due to the difference in color material. The color tone is changed from the standard color to the adjustment color, and the halftone pixels are further increased outside the halftone dots. Details will be described later.

  Next, a boundary pixel located at the boundary of the halftone dot is specified (step S30). In this example, the halftone dot boundary pixel is a pixel that is included in the halftone dot and is located at the inner edge of the halftone dot, that is, a pixel that is in contact with the halftone dot boundary from the inside. FIG. 8 shows a mask for specifying such boundary pixels. In this mask, when the pixel of interest 42 at the center is binary data “1 (place ink)”, the binary data of the inspection pixel 43 located in the vicinity of the four is examined, and any binary data is obtained. Is “0 (no ink is placed)”, the target pixel 42 is determined as a boundary pixel. The determination result is stored in the pixel type table shown in FIG. In this example, “1” data is stored in pixels determined to be boundary pixels, and “0” data is stored in pixels that are not boundary pixels.

  FIG. 10 shows the result of specifying the boundary pixels as described above for the halftone image shown in FIG. In FIG. 10, the boundary pixel 3 specified by the mask is indicated by a bold line. This boundary pixel 3 is preferentially subjected to color tone adjustment. The priority color adjustment target here is primarily a white background pixel adjacent to the boundary pixel 3 (such as this) only when the color adjustment of the boundary pixel 3 alone is not sufficient. In this case, the tone adjustment is performed by changing a pixel such as a pixel of an outer edge) to a halftone dot pixel. That is, the color tone adjustment is performed as much as possible at the boundary pixel 3, and the color tone adjustment using the pixels of the outer edge is performed only when the color tone adjustment range is still insufficient. However, they may not be completely separated, and some of them may overlap. That is, the color tone adjustment by changing the color tone of the boundary pixel 3 reaches most of the boundary pixels 3, but the color tone adjustment using the outer edge pixels is started at a stage where not all the boundary pixels 3 are reached. May be. As a method of prioritizing the boundary pixels, a table described later may be used. In a time series, first, the tone of the boundary pixel 3 is adjusted, and then the tone of the outer edge pixel is adjusted as necessary. May be.

  Next, using the halftone image 21, the partial area ratio occupied by the halftone image in the vicinity of each boundary pixel is measured (step S40). This is because the dot gain in the printed material changes according to the halftone dot area ratio, and it is necessary to measure the halftone dot area ratio in the first halftone image. In order to directly measure the halftone dot area rate, the halftone dot image is divided into regions 6 having a constant size as shown in FIG. Can also be measured. However, in this method, regularity occurs in the measurement value, and an unexpected pattern may occur in the second halftone image.

  In this example, the halftone dot area ratio is not directly measured, but the mask 8 having the size of 9 pixels × 9 pixels illustrated in FIG. 11 is used, and the boundary pixels shown in FIG. The area ratio of the halftone dot pixel to the area of the mask 8 is detected, and this is used as the partial area ratio. In the following example, assuming that the partial area ratio correlates with the halftone dot area ratio, the following processing is performed by assuming the halftone dot area ratio as it is.

  The mask 8 shown in FIG. 11 includes a target pixel 7 and inspection pixels of 9 pixels × 9 pixels−1 pixel = 80 pixels with the target pixel 7 as a central pixel, and forms a square as a whole. In the mask as shown in FIG. 11, the target pixel 7 is the center pixel of the mask, but in the case of a mask using an even number of pixels such as 8 pixels × 8 pixels or 10 pixels × 10 pixels, the center pixel Does not exist. However, in such a case, any one of the 2 × 2 4 pixels located at the center may be set as the target pixel. This case is referred to as substantially the center, and the pixel of interest may be located approximately at the center in the mask for measuring the partial area ratio.

  When any one of the boundary pixels 3 is set as the target pixel 7 using such a mask 8, the number of pixels in which the binary data of the inspection pixel and the target pixel 7 is “1 (color development)” is calculated. The partial area ratio is counted for each boundary pixel. The partial area ratio is a numerical value obtained by dividing the total number of pixels whose binary data is “1 (color development)” by the total number of pixels of the mask. The count number may be used as the partial area ratio as it is. Such a partial area ratio has a measured value for each boundary pixel, and the measured value varies depending on the position of the halftone dot even if the boundary pixel of the same halftone dot is measured. Become. The measured partial area ratio data is stored for each boundary pixel corresponding to the partial area ratio table illustrated in FIG.

  As in this example, the partial area ratio may be estimated as it is as the halftone dot area ratio for each boundary pixel, or some kind of averaging is performed between the boundary pixels to more accurately determine the halftone dot area ratio. It may be estimated. Alternatively, a halftone dot area ratio corresponding to each partial area ratio value may be determined using a separately prepared table. Thus, by using a mask for each boundary pixel, the dot area ratio can be estimated without causing an unexpected pattern in the second halftone image.

  Incidentally, the size of the mask is preferably the same as or smaller than the size of one cycle in which halftone dots are generated in the first halftone image. The size of the mask is the processing load (processing speed for software processing, circuit scale for hardware processing), the number of lines of the image, the resolution of the image, the number of steps of the dot area ratio required when correcting the image, the image It may be determined in consideration of the sharpness of the image.

  First, the size of the mask affects the resolution of the process for estimating the dot area ratio. For example, when the area ratio is actually measured using an image having 175 lines, when the mask size is 3 × 3, the halftone dot area ratio is approximately 0 to 50% and 50 to 100%. It can be seen that the dot area ratio can be determined at each stage. When the size of the mask is 5 × 5, the halftone dot area ratio can be determined in three stages of approximately 0 to 30%, 30 to 70%, and 70 to 100%. With 7 × 7, the halftone dot area ratio can be determined in 5 steps with 20% increments, and with 9 × 9, the halftone dot area ratio can be determined with 10 steps in 10% increments. Accordingly, the preferable mask size is preferably larger from the viewpoint of resolution regarding the dot area ratio, and the lower limit is preferably 3 × 3 = 9 pixels or more.

  On the other hand, since the processing load increases as the mask becomes larger, it is preferable that the mask size is smaller from the viewpoint of processing load. Further, it is preferable that the size of the mask is set to be slightly smaller than the period of the halftone dots because it is less susceptible to extraneous influences from the peripheral portions of the halftone dots when calculating the area ratio.

  Therefore, it is preferable that the size of the mask is approximately the same as or slightly lower than one halftone dot period. Specifically, the upper limit of the mask size is more preferably 85% or less, and even more preferably 75% or less, with respect to the maximum size of one halftone dot period. Further, the lower limit of the mask size is preferably 13% or more, more preferably 25% or more, with respect to the maximum size of one halftone dot period. After all, from these viewpoints, in this example, an image having a screen line number of 175 lines, a halftone dot period of about 13 to 15 pixels, and a mask size of about 9 × 9 = 81 pixels is illustrated. is doing.

  Next, using these partial area ratios and the table of FIG. 6, assuming that the partial area ratio is a halftone dot area ratio, a dot gain (hereinafter referred to as a partial dot gain) that is considered to correspond to a dot gain of a printed material is set for each boundary pixel. (S50 step). That is, in order to match the dot gain of the printed material, the dot gain (partial dot gain) to be increased in the proof image is obtained for each boundary pixel. The partial dot gain data is stored in the partial dot gain table illustrated in FIG. 13 for each boundary pixel.

  Next, using the partial dot gain, it is specified how to form the second halftone image. Specifically, using the probability table illustrated in FIG. 14, the tone change probability of the inner edge pixel and the increase probability of the outer edge pixel are specified (step S60). In the table of FIG. 14, what measures are taken corresponding to the dot thickening according to the size (the unit is “%” in the same way as the dot gain of the printed matter) that increases the partial dot gain in the first row of the table. It is a table that stipulates.

  The tone change probability of the inner edge pixel in the table of FIG. 14 is a pixel in which the tone of the inner edge pixel is changed from the standard color at the original halftone dot area rate of 100% to an adjustment color determined in advance corresponding to the printing conditions. Say the probability of each. The adjustment color is exemplified in the lower part of the LUT shown in FIG. 7, but the color tone is determined in advance so that the image quality approximates the printed material for each specific printing condition. In principle, the adjustment color has a higher density of the standard color, but it is not always necessary to do so, and the adjustment color may be completely different from the corresponding standard color. What is necessary is just to determine suitably according to the characteristic of a color material. The change probability of the color tone of the inner edge pixel to the adjustment color is determined in advance according to the value of the partial dot gain obtained previously, and the change probability increases as the partial dot gain increases. Thereby, the difference in color tone due to the difference between the color material of the printed matter and the proof is first adjusted by the change probability with respect to the color tone of the pixel at the inner edge.

  However, with this alone, it is difficult to cover all adjustments for various colors. This is because the number of inner edge pixels is limited, and more delicate color tone adjustment may be required. For this reason, when the color adjustment of the inner edge pixel is not enough or the variety of adjustment is insufficient, the white pixel located on the outer edge is changed to a halftone pixel to increase the halftone dot area. The image quality can be further adjusted. This is the increase probability of the outer edge pixel. In other words, the image quality adjustment by adjusting the color tone of the inner edge pixel is performed first, and the insufficient portion is compensated by the increase of the outer edge pixel. In other words, the color adjustment of the inner edge pixel is executed with priority over the increase of the outer edge pixel.

  This is because if priority is given to color tone adjustment by increasing the number of outer edge pixels, the resulting image tends to have a feeling of roughness or tone jumps that are prone to occur. . Therefore, the inner edge pixel is first adjusted, and the missing part is compensated by adjusting the outer edge pixel. In the table of FIG. 14, as the partial dot gain increases, first, the tone change probability of the inner edge pixel increases, reaches 100%, and the tone of all the inner edge pixels is changed from the standard color to the adjustment color. If this is not enough, it can be seen that the increase probability of the outer edge pixel is increased from 0% to 100%. Note that the table of FIG. 14 is determined in advance for each printing condition, and the value of the partial dot gain for switching from the inner edge to the outer edge is also appropriately determined according to the printing condition.

  In the table of FIG. 14, the outer edge pixels are not increased at all before the color change of the inner edge pixels reaches 100%. However, if the inner edge pixels are given priority, the inner edge pixels are not increased. The adjustment may be performed so that a part of the adjustment of the edge pixel and the adjustment of the outer edge pixel overlap. Alternatively, it may be set so that almost all overlap, and the adjustment weight is changed between the inner edge and the outer edge, so that the tone adjustment of the inner edge is substantially prioritized over the outer edge.

  By the way, in this example, the color tone of the outer edge pixel increased as the halftone dot pixel is set to the standard color. The color of the outer edge pixel can be set to the same adjustment color as that of the inner edge pixel. However, in this case, the influence on the image quality due to the increase of the outer edge pixel tends to be too large. Therefore, the color tone of the outer edge pixel is set to the standard color. However, the adjustment color may be used, or another adjustment color may be used.

  In this way, it is possible to adjust the image quality for a wide range of color tones, and an unexpected pattern is produced with the adjustment of the image quality, the resulting image is gritty, and tone jump is emphasized. The problem of being done does not occur.

  Next, when the increase probability of the outer edge pixel read for each boundary pixel is not 0%, according to the increase probability, any binary data of the outer edge pixel adjacent to the boundary pixel is “0 ( Non-colored) "to" 1 (colored) ". This is performed for all boundary pixels (step S70). FIG. 15 shows an example of a halftone image in which outer edge pixels that increase as halftone pixels are specified. In FIG. 15, the outer edge pixels 4 that increase as halftone pixels are indicated by thick lines. Thus, a halftone image having an increased number of outer edge pixels is obtained.

  A similar adjustment is performed for all printing plates, and a second halftone image is obtained. For each pixel of the second halftone image, the density of the color to be developed in each YMC coloring layer of the photosensitive material is specified by the LUT in FIG. 7 (step S80). At that time, it is determined as described above whether each pixel should be a standard color or an adjustment color, and the result is stored in the image density table shown in FIG. A schematic diagram of an image obtained by this image density table is shown in FIG. In FIG. 17, the halftone dot 9 has a color tone of the inner edge pixel changed to an adjustment color as compared with the halftone dot 5 of FIG. Randomly added. The color of the outer edge pixel is a standard color.

  By doing in this way, the shift | offset | difference derived from the difference in a coloring material can be correct | amended with respect to the color tone of a wide range, and it becomes possible to improve the closeness to printed matter. At this time, since the solid color having a halftone dot area ratio of 100% remains as a standard color in most of the halftone dots, there is no problem that the solid color is shifted. In addition, an unexpected pattern does not occur, and the degree of roughness and tone jump emphasis that tends to occur with image correction can be minimized.

  The data in the image density table in FIG. 16 is output to the output device 50 (step S90), converted into LED drive current data for the output device for each pixel via the exposure amount code table shown in FIG. The material is exposed and developed. As a result, a proof approximating the printed matter can be obtained.

  The proof obtained by such an image forming method uses the first halftone image having undergone the same RIP as that of the printing press as it is in the image forming apparatus 40 as it is. In addition, although the color materials are different, the image quality is also excellent and the proof is excellent.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the specific aspect of the invention shown above. For example, the proof output means may be a printer that can reproduce a color tone of almost continuous tone, such as an ink jet method. The pixels for the adjustment color are preferably inner edge pixels, but may be arranged in the center of the halftone dot, for example. In short, by dividing the adjustment color portion and the solid color portion, it is only necessary to arrange the adjustment color pixels so that the solid color tone can be confirmed and moire or the like does not occur. The screening method is not limited to AM screening, and may be FM screening. The processing flow in FIG. 5 is not limited to this, and the order of the steps can be changed as necessary.

  In the above description, the function of the control device of the image forming apparatus 40 is realized by a computer program executed on a general-purpose computer, but it goes without saying that it may be realized by dedicated hardware. Further, this program may be stored in a computer-readable recording medium. When storing a program in a recording medium, it is also possible to divide the program into a plurality of parts and store the divided parts in a storage medium. Here, the recording medium means a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, a hard disk built in a computer system, and the like.

1 is a block diagram illustrating an example of a configuration and a flow of an entire system including an image forming method. It is the figure which showed the printing overprint of the color plate for every color which can be expressed with the printing press. It is the conceptual diagram which showed the example of the data structure of a halftone image. It is the schematic diagram which showed the example of the halftone image with respect to any one plate. 3 is a flowchart showing an outline of a processing flow in the image forming apparatus. It is the conceptual diagram which showed the example of the dot gain table. It is the conceptual diagram which showed the example of the lookup table. It is the conceptual diagram which showed the example of the mask. It is the conceptual diagram which showed the example of the pixel classification table. It is a schematic diagram of a halftone image showing a state where boundary pixels are specified. It is a figure explaining the measuring method of a partial area rate. It is the conceptual diagram which showed the example of the partial area rate table. It is the conceptual diagram which showed the example of the partial dot gain table. It is the conceptual diagram which showed the example of the probability table. It is the schematic diagram which showed the state by which the outer edge pixel to increase was specified. It is the conceptual diagram which showed the example of the image density table. It is a schematic diagram of a proof image with image quality adjusted. It is the conceptual diagram which showed the example of the exposure amount code table.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 White background pixel 2 Halftone pixel 3 Boundary pixel 4 Outer edge pixel 5 Halftone dot 6 Repeat area | region 7 Attention pixel 8 Mask 9 The corrected halftone dot 42 Attention pixel 43 Inspection pixel

Claims (6)

  An image forming method for obtaining a second halftone image from a first halftone image, wherein for each pixel located at a halftone dot boundary of the first halftone image, the first halftone image The estimation step for estimating the dot gain corresponding to the printed matter using the halftone image and the change probability determined based on the dot gain change the color tone of the boundary pixel located at the inner edge of the halftone boundary. A halftone dot increase step of changing at least some of the boundary pixels located at the outer edge of the halftone dot boundary to a halftone dot pixel by the halftone color changing step to be performed and an increase probability determined based on the dot gain And the halftone dot color changing step is executed in preference to the halftone pixel increasing step.   The halftone dot increasing step is performed after the halftone dot color changing step has changed the color tone of almost all of the boundary pixels located at the inner edge of the halftone dot boundary. 2. The image forming method according to 1.   For each pixel located at the halftone dot boundary, the estimating step calculates the partial area ratio that the halftone dot occupies in the image, and the dot gain is calculated from the partial area ratio and the dot gain curve of the printed matter. The image forming method according to claim 1, further comprising an estimating step.   4. The image forming method according to claim 3, wherein the partial area ratio is calculated using a mask having a predetermined size centering on a pixel located at the halftone dot boundary.   An image forming apparatus for obtaining a second halftone image from a first halftone image, wherein each of the pixels located at a halftone dot boundary of the first halftone image is a first halftone image. The color tone of the boundary pixel located at the inner edge of the halftone dot boundary is changed by the estimation means for estimating the dot gain corresponding to the printed matter using the halftone image and the change probability determined based on the dot gain. A halftone dot increase means for changing at least some of the boundary pixels located at the outer edge of the halftone dot boundary to a halftone pixel by an increase probability determined based on the dot gain; And the halftone color tone changing means is executed in preference to the halftone pixel increasing means. A printing process for outputting a printed matter using the first halftone image, and a proof of the printed matter using the second halftone image formed from the first halftone image A proof output step, wherein the proof output step estimates a dot gain corresponding to the printed matter for each pixel located at a halftone dot boundary of the first halftone image. And a halftone color tone changing step for changing the color tone of a boundary pixel located at an inner edge of the halftone dot boundary according to a change probability determined based on the dot gain, and a dot color tone changing step. A halftone pixel increasing step for changing at least some of the boundary pixels located at the outer edge of the halftone dot boundary to halftone pixels according to the increased probability, and the halftone color changing step includes the halftone dot changing step. Image forming method characterized in that it is performed in preference to oxygen increases step.

Images