JP2005111946A - Method for nonlinear calibration of halftone screen - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for the nonlinear calibration of a halftone screen which does not require the setting of a permanent program and installation of an additional apparatus. <P>SOLUTION: The method for the nonlinear calibration of a halftone screen comprises a phase 10 in which an original halftone screen table is formed, a phase 20 in which a plurality of parameters are established and the plurality of parameters are applied to a gamma function, a phase 30 in which the gamma function is used to recalculate the density level of the original halftone screen table to form a new halftone screen table, and a phase 40 in which printing is performed by the new halftone screen table and the optical density of a printed matter is measured. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a halftone screen calibration method for an image forming apparatus, and more specifically, the present invention is used in a halftone screen generation process.

  Generally, a printer needs to perform color conversion from an RGB color space to a CMYK color space. Typically, 3-byte RGB electronic data is converted to 4-byte CMYK data. Here, R, G, B, C, M, Y, and K are each represented by 1-byte data having a value between 0 and 255. In a color rendering process, the CMYK values of the original image are compared with corresponding values included in the halftone screen table. As a result, whether or not to print each dot (whether or not to print) is determined, and the density level (gray level) is expressed according to the dot dot condition of each dot.

  FIG. 1 is a schematic view illustrating a conventional halftone screen table. A 16 × 16 matrix with 256 grids is formed so that each represents a density level between 0 and 255. In the figure, only the values of 224 to 255 are shown, but values of other grids can be similarly shown. The density level in each grid of the halftone screen table represents a reference value to be compared with the raw data to determine whether or not to hit the corresponding dot.

  The method for printing dots depends on the setting of a command program such as PCL or PS3. For example, when the raw data value is larger (or smaller) than the halftone screen table value, a dot is hit.

  FIG. 2 shows a case where a dot is hit when the value of the halftone screen table is larger than the value of the raw data. Here, it is assumed that it is desired to print a rectangular area that maintains the density level 247. In this case, each dot printed as shown in FIG. 2 can be obtained by comparing the screen table shown in FIG.

  In the case of not only monochrome printing but also a color printing system, the CMYK halftone screen tables can be adjusted using the same concept.

  Generally, each printing apparatus has different characteristics. The different characteristics include, for example, dot gain and toner characteristics that are different for each printing apparatus. Here, the dot gain means a phenomenon in which the half screen dot size changes during repeated reproduction. If the white portion is filled with the dot gain, the image may become dark, and the dark portion may be completely filled with the dot gain. In this case, it may become black instead of the original gray.

  Therefore, even a halftone screen made for a certain device behaves differently depending on different characteristics such as dot gain and toner characteristics. For this reason, in order to compensate for such a difference in the characteristics of each device, screen calibration is required as a result, and screen calibration is executed according to the model of the printer engine in order to output optimal images / data. The

Patent Document 1 discloses a nonlinear calibration method for an output device used to solve a problem related to the color intensity of an image (see Patent Document 1). In this calibration method, 8-bit density level data of an image is converted into 16-bit density level data to which compensation information used for data calibration is added. However, this calibration method typically requires the installation of a permanent program, or in other cases, the installation of additional equipment. Therefore, the burden is heavy and practically inconvenient.
US Pat. No. 5,953,498

  The present invention can be used in different types of printing devices to adjust the inevitably occurring characteristics of the printer engine, and does not require the setting of a permanent program or the installation of additional devices. It aims to provide a calibration method.

  (1) A non-linear calibration method for a halftone screen according to the present invention is a non-linear calibration method for a halftone screen, the step of generating an original halftone screen table, a plurality of parameters set, and the plurality of parameters Applying to the gamma function, re-calculating the density level of the original halftone screen table using the gamma function to generate a new halftone screen table, and a new halftone screen table Performing printing and measuring the optical density of the printed matter.

  (2) The above parameters are parameters for the printing apparatus.

  (3) The above parameters are for adjusting the halftone screen table of the monochrome printing apparatus.

  (4) The original halftone screen table is a cluster-dot-order (AM) halftone screen table.

  According to the halftone screen table nonlinear calibration method of the present invention, a plurality of parameters are set, the plurality of parameters are applied to the gamma function, and the density level of the original halftone screen table is determined using the gamma function. Is recalculated to generate a new halftone screen table. Then, printing is performed using a new halftone screen table, and the optical density of the printed matter is measured. Therefore, effective calibration can be realized even when there is no setting of a permanent program or installation of an additional device. In particular, only one non-linear gamma function is required and the necessary adjustments are performed according to the characteristics of the printing device. Therefore, no additional adjustment of the density level in other stages of the printing process is required. In particular, the present invention is effective when the printing apparatus is not so complicated, and it is not necessary for the designer to apply adjustment processing to the software driver.

  In particular, the method of the present invention can effectively simplify the calibration of printing devices and does not require additional devices to achieve non-linear calibration. This is advantageous in providing an economical solution.

  To ensure the highest level of color quality during the color rendering process, calibration can be performed in several processes. One of the effective mounting locations where calibration can be performed is the halftone screen table itself. In order to implement calibration on the halftone screen table itself, the halftone screen table needs to be intelligent enough to adjust factors arising from output device characteristics such as dot gain and toner characteristics. There is. For this purpose, it is necessary to generate a screen table using a normalized gradation curve (normalized gradation curve), and the method of the present invention described below is introduced. Some color balance adjustment functions are also considered.

  The nonlinear calibration method of an embodiment of the present invention is implemented on a cluster-dot-order (AM) halftone screen table. Here, the cluster is a connected component between dots, and the AM halftone screen table is a halftone screen table used in the case of a dot concentration type image forming apparatus. On the other hand, what is shown in FIG. 3 is a schematic diagram for explaining a dot diffusion type FM halftone screen growth order.

  Also in the present embodiment, first, the original halftone screen table (hereinafter referred to as “original halftone screen table”) is created. The method for creating the original halftone screen table is the same as that conventionally shown as a Holladay algorithm, for example. Therefore, detailed description of this point is omitted. This method is not affected by paper quality problems.

  When a halftone screen table with 256 density levels is used for data representation, the raw data of the input image is compared with the values of the halftone screen table to determine whether the corresponding dot should be printed. It is determined.

  For different printing devices, the present invention uses a predetermined non-linear gamma function with different function parameters for adjusting the density level. Different function parameter values included in the nonlinear gamma function are adjusted according to the characteristics of the printing apparatus. Thus, once the halftone screen table calculated using the adjusted nonlinear gamma function is applied to image / photo printing for a particular printing device, reliable screen quality can be obtained.

  In the present invention, only one nonlinear gamma function is required, and necessary adjustments are performed according to the characteristics of the printing apparatus. Therefore, no additional adjustment of the density level in other stages of the printing process is required. In particular, the present invention is effective when the printing apparatus is not so complicated, and it is not necessary for the designer to apply adjustment processing to the software driver.

  The method of the present invention can effectively simplify the calibration of the printing device and does not require additional equipment to achieve non-linear calibration. This is advantageous in providing an economical solution.

  The scope of application of the present invention will be apparent from the following description. However, it should be understood that the detailed description and specific examples are given for purposes of illustration. Therefore, it is apparent that various changes and modifications can be made by those skilled in the art within the scope of the present invention.

  FIG. 1 is a schematic diagram for explaining a conventional AM screen spot growth order.

  FIG. 2 is another schematic diagram for explaining an AM screen spot growth order as a prior art.

  FIG. 3 is a schematic diagram for explaining the FM screen spot growth order.

  FIG. 4 is a diagram showing the relationship between the original density level value and the new density level value. FIG. 4 shows the case where no calibration is performed and the inclination is 1.

  FIG. 5 is a diagram showing the relationship between the original density level value and the new density level value. FIG. 5 shows the case where all the updated density levels are increased compared to the original level by linear (brightness) adjustment. In this case, the tilt is not changed and is maintained. However, the entire histogram moves in the right direction of the figure. Again, some high concentration level portions are lost.

  FIG. 6 is a diagram showing the relationship between the original density level value and the new density level value. Here, the new density level value is calculated by multiplying the original density level value by a predetermined ratio greater than 1. This is the result of contrast adjustment for the original density level.

  FIG. 7 is the same as FIG. 6 except that the ratio to be multiplied is smaller than 1. This is a result of flattening the change in density level.

FIG. 8 is a diagram illustrating a function of Y = e ^X used to construct the concept of nonlinear calibration.

FIG. 9 is a diagram illustrating Y = log ₁₀ X, which is another function used for nonlinear calibration.

  FIG. 10 is a diagram illustrating Y = lnX, which is a third function used for nonlinear calibration.

FIG. 11 is a diagram illustrating Y = e− ^X , which is a fourth function used for nonlinear calibration.

FIG. 12 is a diagram illustrating Y = X ^1.1 , which is an example function used for nonlinear calibration.

  FIG. 13 is a flowchart for explaining the non-linear calibration method of the halftone screen table in the embodiment of the present invention.

FIG. 14 is a diagram showing an original halftone screen table before calibration. FIG. 15 is a diagram showing an example of image output when nonlinear gamma function calibration is not executed according to the embodiment of the present invention.

  FIG. 16 is a diagram showing a halftone screen table after applying the first nonlinear gamma function calibration according to the embodiment of the present invention.

  FIG. 17 is a diagram showing an image output example obtained by applying the halftone screen table calibrated by the first nonlinear gamma function calibration according to the embodiment of the present invention.

  FIG. 18 is a diagram showing a halftone screen table after applying the second nonlinear gamma function calibration according to the embodiment of the present invention.

  FIG. 19 is a diagram showing an image output example obtained by applying the halftone screen table calibrated by the second nonlinear gamma function calibration according to the embodiment of the present invention.

  According to the present invention, a non-linear calibration method for a halftone screen is implemented in the data printing process. The present invention uses a non-linear gamma function to adjust the original density level in the halftone screen table. This is useful for making quick and consistent screen data changes. In a typical application, a calibrated screen table is used to adjust the density level of the shadow area. In this case, some printer characteristics make it look exactly the same, i.e. dark, despite having different density levels in the shadow area.

  The embodiment of the present invention will be described more specifically below.

  As shown in FIG. 14, the original halftone screen is formed using a cluster-dot-order approach. A method for handling this screen formation is shown, for example, as a Holladay algorithm. This is the same as in the prior art. This method is not limited by paper quality problems. In FIG. 14, the original density level value (0-255) is linearly mapped to the new density level value. In this case, the new density level value is the same as the original density level value.

  As already known, compensation by calibration is mainly used to reduce the effect of dot gain in the printing device. In order to eliminate the influence of dot gain, adjustments in the printing device itself, such as adjustment of the development bias and drum charging voltage in the engine, are attempted, but adjustments in software such as halftone screen table adjustments are made. It can also be done. Here, there are several factors that affect the result of the halftone screen adjustment. For example, these factors include color balance factors (contrast, brightness, saturation, and color density), toner characteristics, and dot gain. These factors are treated uniformly using a predetermined gamma function. In this case, it is only necessary to adjust the halftone screen once. This means that throughout the printing process, only one calibration is required on the halftone screen and no other calibration is required. Thus, the processing steps required for color calibration in the printing process can be advantageously simplified.

  The present invention can be described using several mathematical expressions, which can be combined into one form called a gamma function. The gamma function is used to normalize the screen gradation. By using the gamma function, the range in which the difference in gradation is visible in the shadow and highlight areas is increased, and favorable results can be obtained even in the middle tone area.

  Each time an adjustment is made, the selected target file is printed quickly. The printed color quality is then perceived by using an optical densitometer or other optical measuring device. Also, depending on the measured optical density or visual observation, the color engineer can decide whether to perform the next adjustment and continue the adjustment process.

  The color engineer can apply different values as parameters in a given gamma function before entering the next adjustment process. The gamma function is used to adjust the original density level. In the following gamma function, “x” indicates the original density level, and “F (x)” indicates the calibrated new density level. According to the present invention, the representation of the gamma function is described as follows: That is, in this specification, the gamma function is defined as a function expressed in the following format.

F (x) = min {max [(x + ( ^xp_val / div_val) −shift) * exp (e_val) * log (l_val), 0], u_bound},
Here, p_val is an output value that affects the curve bend, div_val is a divisor that affects the contrast, shift is a shift value that affects the luminance, and e_val is an index that affects the curve bend. 1_val is a logarithmic parameter that affects the bending of the curve, and u_bound is the upper limit of the dot density level at which dots are shot.

  In one embodiment of the present invention, it is as follows.

F (x) = min {max ^{[(x + x 1.1 / 10-42)} * exp (0) * log (10), 0 ], 249}
A set including all adjusted density levels can be obtained. All such density levels are used to print the target file. When the target file is printed out, the optical density measurement is performed again. Depending on the adjusted quality, it is determined whether to continue the non-linear calibration. Such a process can be repeated until an acceptable quality is achieved.

  FIG. 4 shows the relationship between the original density level value and the new density level value. FIG. 4 shows a case where no calibration is performed and the inclination is 1. In the case shown in FIG. 5, the constant is subtracted from the initial density level value. That is, each concentration level is reduced by the same amount. This corresponds to “shjft” set in the above gamma function. This is a linear adjustment and affects the brightness of the final image. Depending on this application design, the resulting image will be lighter or darker. On the other hand, in the cases shown in FIGS. 6 and 7, the given level value is multiplied by a constant, or the level value is divided by the constant, thereby adjusting the contrast of the original density level value. This is also a linear adjustment.

Figures 8, 9, 10, 11 and 12 represent non-linear calibration functions. Figure 8 is a diagram illustrating a function of Y = e ^x, serve to configure the concept of non-linear calibration. By using an exponential function, a new non-linear change in concentration level is obtained.

FIG. 9 shows Y = log ₁₀ X, the second function for use in non-linear calibration. FIG. 10 shows Y = lnX, a third function for use in nonlinear calibration. FIG. 11 shows Y = e− ^x , which is a fourth function for use in non-linear calibration. FIG. 12 shows Y = X ^1.1, which is an exemplary function used for nonlinear calibration.

  The flow chart shown in FIG. 13 schematically illustrates the non-linear calibration process of the present invention, and particularly shows the process steps to be implemented in the printing apparatus. Non-linear calibration methods can be implemented, for example, in a printing device to calibrate each density level of the halftone screen table, thereby improving the rendering characteristics of the image output.

  First, the original halftone screen table is generated (step 10). A target image (as shown in FIG. 15) is then printed out from the printing device to be calibrated. This is done based on the original halftone screen table (as shown in FIG. 14). In the printed image as shown in FIG. 15, the grids No. 4 and No. 5 in the highlight area have an excessively white color. In addition, the grids from 86 to 100 in the shadow area have excessively dark / dark colors, making it impossible to distinguish between light black and dark black.

  In such a case, it is determined from the printout quality that further calibration of the screen table is necessary. In this example, looking at the printed out, the brightness and contrast are insufficient. In addition, some non-linear curves may be required to adjust the color density. To summarize the above, a set of function parameters are set and applied to the non-linear gamma function (step 20). That is, in this step, one nonlinear gamma function is generated by setting several parameters.

  Then, the density level of the original halftone screen table is recalculated by the gamma function to generate a new halftone screen table (step 30). FIG. 16 shows a new halftone screen table obtained by calibrating the original density level shown in FIG. Such processing can be performed, for example, by applying a nonlinear gamma function as follows in EXCEL (registered trademark) of Microsoft Corporation.

MIN (ROUNDDOWN (MAX (x + POWER (x, 1.1) / 10-20,0), 0), 249)
Here, MIN is a minimum operator. In this case, the calibrated concentration level will not be higher than 249. ROUNDDOWN is a truncation operator. This is because the concentration level needs to be obtained as an integer. MAX is the maximum operator. The calibrated concentration level will not be lower than zero. x is a variable representing the original density level of the halftone screen table. POWER is an exponent operator. Used for non-linear curves.

  Finally, a new halftone screen table is printed out and the optical density of the printed product is measured (step 40). FIG. 17 shows a printout obtained from a halftone screen table subjected to a first calibration process. In this example, the color of the entire image is dark.

  Thus, the user returns to step 20 to apply a new non-linear gamma function to set new function parameters and recalibrate the halftone screen table density levels. This process is repeated until the image is of acceptable quality.

  FIG. 18 shows another halftone screen table. This halftone screen table is calibrated with a second non-linear gamma function as implemented in Excel® below.

MIN (ROUNDDOWN (MAX (x + POWER (x, 1.1) / 10-42,0), 0), 249)
FIG. 19 illustrates a printout obtained from a calibrated halftone screen table. As shown in the figure, a sufficiently distinguishable gradation is maintained in all shadow areas and highlight areas. As can be seen by comparing FIG. 15 and FIG. 17, this process provides a significant improvement.

  As described above, the present invention is not limited to a monochrome printing apparatus, and is suitable for use in a color printing apparatus. For color printing devices, calibration must be applied independently for each of the four colors of CMYK.

  Although the present invention has been described above, the present invention is not limited to the above description, and it is obvious that various modifications can be made by many methods. All modifications apparent to those skilled in the art should be included within the scope of the invention.

It is the schematic for demonstrating AM screen spot growth order. It is another schematic diagram for demonstrating AM screen spot growth order. It is the schematic for demonstrating FM screen spot growth order. It is a figure which shows the relationship between the original density | concentration level value and new density | concentration level value when not calibrated and the inclination is 1. It is a figure which shows the relationship between the original density level value and a new density level value in case all the updated density levels are raised compared with the original level by linear (luminance) adjustment. It is a figure which shows the relationship between the original density level value and a new density level value in the case where a new density level value is calculated by multiplying the original density level value by a predetermined ratio greater than 1. . It is a figure which shows the relationship between the original density level value and a new density level value in case a new density level value is calculated by multiplying the original density level value by a predetermined ratio smaller than 1. . Is a diagram illustrating a Y = e ^X is the first function used to construct the concept of non-linear calibration. Is a diagram illustrating a Y = log 10 _X is a second function to be used for the non-linear calibration. It is a figure which shows Y = lnX which is the 3rd function used for nonlinear calibration. It is a figure which shows Y = e- ^X which is the 4th function used for nonlinear calibration. FIG. 6 is a diagram illustrating Y = X ^1.1 as an example function used for nonlinear calibration. It is a flowchart for demonstrating the nonlinear calibration method of the halftone screen table in embodiment of this invention. It is a figure which shows the original halftone screen table before a calibration. It is a figure which shows the example of an image output when not performing nonlinear gamma function calibration by embodiment of this invention. It is a figure which shows the halftone screen table after applying the 1st nonlinear gamma function calibration by embodiment of this invention. It is a figure which shows the image output example obtained by applying the halftone screen table calibrated by the 1st nonlinear gamma function calibration by embodiment of this invention. It is a figure which shows the halftone screen table after applying the 2nd nonlinear gamma function calibration by embodiment of this invention. It is a figure which shows the image output example obtained by applying the halftone screen table calibrated by the 2nd nonlinear gamma function calibration by embodiment of this invention.

Explanation of symbols

10 Generation process of original halftone screen table,
20 Parameter setting and application processing to gamma function,
30 New halftone screen table generation process,
40 Printout and optical density measurement process.

Claims (4)

A non-linear calibration method for a halftone screen,
(A) generating an original halftone screen table;
(B) setting a plurality of parameters and applying the plurality of parameters to the gamma function;
(C) re-calculating the density level of the original halftone screen table using the gamma function to generate a new halftone screen table;
(D) performing printing with a new halftone screen table and measuring the optical density of the printed matter, and a non-linear calibration method for a halftone screen, comprising:   The nonlinear calibration method according to claim 1, wherein the parameter is a parameter for a printing apparatus.   The nonlinear calibration method according to claim 1, wherein the parameter is for adjusting a halftone screen table of a monochrome printing apparatus.   The non-linear calibration method of claim 1, wherein the original halftone screen table is a cluster-dot-order (AM) halftone screen table.