JP2008092349A - Image processor, image forming apparatus, and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To properly set the content of an image data conversion table based on the kind of an object of image data to be processed. <P>SOLUTION: The image processor comprises a receiving part 11 which receives image data, and an image data conversion part 14 in which the image data is converted into output image data formed on a recording medium by an image forming apparatus 1. It also comprises a gradation correction part 15 in which the gradation of an output image data is corrected according to characteristics of the image forming apparatus 1, and if the gradation of the image output data is 0, it is corrected to a value smaller than the corrected value when the gradation of output image data is 1, and larger than 0. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image processing apparatus, an image forming apparatus, and a program for controlling them.

  2. Description of the Related Art Conventionally, techniques for forming various images in an image forming apparatus such as a printer are known. For example, when printing image object data (image data) of a business document created with an application such as Word (trademark) or PowerPoint (trademark) on paper, etc. This is the case of printing on a medium such as. In that case, in the image processing apparatus, processing corresponding to the printing characteristics of the image forming apparatus (print engine) is performed on the image data to be printed. (For example, refer to Patent Document 1 and Patent Document 2).

  The image forming apparatus disclosed in Patent Document 1 includes a correction step of correcting the density gradation of the output image data by correcting the image data of the input image based on the correction data obtained by forming the reference pattern. It features an image processing method. This method includes a correction data generation step for generating a plurality of correction data used in the correction step according to the type of the input image. Further, the correction data generation step includes a classification step of classifying a plurality of adjacent pixels in the input image data as a set. Then, based on the density values of a plurality of pixels constituting a set obtained in the classification step, correction data is selected for each type of input image from among characteristic data prepared for each type of input image. It is characterized by including a process.

  In the image processing apparatus of Patent Document 2, the density of the output gradation test pattern is read by an image reading unit, and the relationship between the laser output level of the image forming apparatus and the read density is stored in a memory. Then, a γ lookup table is created so that the read density matches the ideal target table of the image forming apparatus. Further, in such an image processing apparatus or the like, the γ look-up table is automatically inspected, and when it is determined that the gradation of the high density portion is destroyed, the gradation is maintained in the photo mode. The first correction means for correcting the γ lookup table and the first holding means for holding the corrected first γ lookup table. In the character mode, the gradation is slightly maintained, but the second correction means for correcting the γ look-up table so that the density is firmly obtained, and the second correction means for holding the corrected second γ look-up table. And holding means.

Japanese Patent Laid-Open No. 11-98357 (page 3-7, FIG. 2) Japanese Patent Laying-Open No. 2006-166327 (page 2-7, FIG. 4)

  As described above, setting the contents of the image data conversion table used when processing the image data to be printed according to the image data to be processed is an important technical problem that greatly affects the print quality. . That is, it is necessary to appropriately set the contents of the image data conversion table based on the type of object of the image data to be processed in order to form a high-quality image on the medium.

  The present invention has been made to cope with the above-described situation, and an object thereof is to form an image of a predetermined object with high quality.

  For this purpose, in the image processing apparatus of the present invention, a receiving means for receiving image data, a conversion means for converting the image data into output image data formed on a recording medium by the image forming apparatus, and the image formation When the gradation of the output image data is corrected according to the characteristics of the apparatus, and the gradation of the output image data to be corrected is 0, the correction value is smaller than the correction value when the gradation of the output image data to be corrected is 1. And gradation correction means for correcting the gradation of the output image data to be corrected to a value larger than 0. In an actual apparatus, the image data conversion table in which the correction value when the gradation of the output image data is 0 is set as described above is held in a predetermined storage device, and the gradation correction means uses this image data. The gradation of the output image data is corrected using the conversion table.

  As such an image processing apparatus, the converting unit converts the image data received by the receiving unit into output image data for each color formed on the recording medium, and the gradation correcting unit outputs the output for each color. The gradation of the image data can be corrected to a predetermined value according to the characteristics of each color of the image forming apparatus.

In addition, as such an image processing apparatus, the image processing apparatus further includes a recognition unit that recognizes that the image data received by the reception unit includes a photographic object, and the gradation correction unit is determined to be a photographic object by the recognition unit. The gradation of the output image data for the object can be corrected.
Here, there are a case where the entire image data is a photographic object and a case where a part of the image data includes a photographic object. In the former case, the tone correction means corrects the tone for the entire image data using the image data conversion table in which the correction value when the tone of the output image data is 0 is set as described above. Just do it. On the other hand, in the latter case, the entire image data can be corrected using the image data conversion table, or only the photo object can be corrected using the image data conversion table, It is also possible to perform correction using other image data conversion tables for parts other than objects.

  On the other hand, the present invention can also be understood as an image forming apparatus. In this case, in the image forming apparatus of the present invention, the receiving unit that receives the image data, the image processing unit that performs image processing on the image data received by the receiving unit, and the image data that has been subjected to image processing by the image processing unit And image forming means for forming an image on the recording medium, and the image processing means converts the image data received by the receiving means into output image data formed on the recording medium by the image forming means. The gradation of the output image data is corrected according to the characteristics of the conversion unit and the image forming means. When the gradation of the output image data is 0, the correction value when the gradation of the output image data is 1 A gradation correction unit that corrects the gradation of the output image data to a value that is smaller and greater than 0, and the image forming means adjusts the gradation of the output image data corrected by the gradation correction unit. Forming an image on a recording medium Zui can feature.

  Further, as such an image forming apparatus, the image processing unit further includes a recognition unit for recognizing that the image data received by the reception unit includes a photographic object, and the gradation correction unit uses the recognition unit to The gradation of the output image data with respect to the object determined as the object can be corrected.

  Furthermore, as such an image forming apparatus, in the image processing unit, the conversion unit converts the image data received by the receiving unit into output image data for each color formed on the recording medium, and the recognition unit recognizes it. It is possible to perform color conversion processing for each color formed on a recording medium corresponding to the photographic object.

  On the other hand, the present invention can also be understood as a program for controlling the image processing apparatus and the image forming apparatus. In this case, the computer has an acquisition function for acquiring image data, a conversion function for converting the image data into output image data formed on a recording medium by the image forming apparatus, and an output image according to the characteristics of the image forming apparatus. When the gradation of the data is corrected and the gradation of the output image data is 0, the output image data is reduced to a value smaller than the correction value when the gradation of the output image data is 1 and larger than 0. And a gradation correction function for correcting gradation.

  Further, as such a program, the computer further realizes a recognition function for recognizing that the image data acquired by the acquisition function includes a photographic object, and the gradation correction function is determined as a photographic object by the recognition function. The gradation of the output image data for the selected object can be corrected.

According to the first aspect of the present invention, it is possible to process the output characteristics of an image that is greatly affected by the gradation as compared with the case where the invention according to the first aspect is not adopted, so as to have higher quality. it can.
According to the second aspect of the present invention, the output characteristics for each color to be printed on the medium can be processed so as to have higher quality compared to the case where the invention of the second aspect is not adopted. .
Further, according to the invention of claim 3, it is possible to process the output characteristics of the photographic object so as to have a higher quality as compared with the case where the invention of claim 3 is not adopted.

According to the invention of claim 4, compared to the case where the invention of claim 4 is not adopted, the print output relating to the density of the image greatly affected by the gradation is formed so as to have higher quality. be able to.
According to the invention of claim 5, compared to the case where the invention of claim 5 is not adopted, the print output relating to the density of the object of the photograph can be formed so as to have higher quality.
Further, according to the invention of claim 6, compared to the case where the invention of claim 6 is not adopted, the print output of the object of the photograph can be formed so as to more preferably express the color. it can.

According to the seventh aspect of the invention, the output characteristics of the image that are greatly affected by the gradation are processed by the computer so as to have a higher quality than when the invention of the seventh aspect is not adopted. Can do.
According to the invention of claim 8, compared with the case where the invention of claim 8 is not adopted, the output characteristics of the photographic object can be processed by the computer so as to be of high quality.

(Detailed description of the image forming apparatus)
DESCRIPTION OF EMBODIMENTS Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “embodiment”) will be described in detail with reference to the accompanying drawings.
FIG. 1 is a block diagram illustrating an example of a configuration of an image forming apparatus including an image processing apparatus to which the exemplary embodiment is applied. An image forming apparatus 1 illustrated in FIG. 1 is, for example, a digital color printer, and an image processing unit (image processing apparatus) 10 serving as an image processing unit that performs predetermined image processing on image data input from an external device. It has. A secondary storage unit 20 realized by, for example, a hard disk (Hard Disk Drive) in which a processing program or the like is recorded, and a control unit 30 that controls the operation of the entire image forming apparatus 1 are provided. In addition, an image forming unit 40 that forms an image corresponding to the image data of each color component is provided. The image forming unit 40 can use various types of engines such as an electrophotographic system and an inkjet system.

Next, the image processing unit 10 includes a receiving unit 11 as an image data receiving unit that receives image data from an external device such as a personal computer (PC) 3 or an image reading device 4 such as a scanner. A PDL analysis unit 12 that analyzes image data in a PDL (Page Description Language) format received by the reception unit 11 is provided. The image processing unit 10 also includes a rendering processing unit 13 that develops (renders) the image data analyzed by the PDL analysis unit 12 into raster image data. Here, the raster image data refers to image data for printing (output image data) expressed by an array of pixels.
Furthermore, the image processing unit 10 includes an image data conversion unit 14 that performs color conversion of raster image data to color system image data (YMCK) suitable for printing processing. Here, YMCK means a process color composed of Y (Yellow), M (Magenta), C (Cyan), and K (blacK). Furthermore, the image processing unit 10 includes a gradation correction unit 15 that corrects the gradation value of the raster image data color-converted by the image data conversion unit 14. An LUT holding unit 16 that holds an image data conversion table called an LUT (Look Up Table) used by the tone correction unit 15 for tone correction of image data, and screen processing for the raster image data subjected to tone correction And an image output unit 17 for performing the above.

  The receiving unit 11 receives image data and a drawing command from an image reading device 4 such as a user's personal computer (PC) 3 or a scanner. Then, the image data and the drawing command are output to the PDL analysis unit 12. Here, the image data includes, for example, pixel data and tag data. Here, the pixel data is, for example, data belonging to, for example, the sRGB color space expressed by 256 gradations (0 to 255) of 8 bits (1 byte) for each RGB. The tag data is data including, for example, information on the type of image object (characters, graphics, photographs, etc.). Here, RGB means three primary colors of light composed of R (red), G (green), and B (blue).

The PDL analysis unit 12 creates image data for one print page, for example, according to the analysis result of the image data acquired according to the drawing command. Then, the PDL analysis unit 12 outputs the created image data to the rendering processing unit 13.
The rendering processing unit 13 performs a rendering process on the image data acquired from the PDL analysis unit 12 according to the drawing command. Then, the rendering processing unit 13 outputs raster image data composed of the pixel data subjected to the rendering processing and tag data to the image data conversion unit 14.

  The image data conversion unit 14 performs color conversion processing on the received raster image data into color system image data (YMCK) suitable for printing processing in the image forming unit 40, and outputs it to the gradation correction unit 15. . Here, the image data conversion unit 14 performs color conversion processing using a color conversion table called a direct look-up table (DLUT) that differs for each type of image object. The DLUT is held in the secondary storage unit 20, for example. Then, the image object can be recognized based on the tag data included in the raster image data from the rendering processing unit 13, and the optimum color conversion corresponding to the type can be performed.

The gradation correction unit 15 receives the raster image data color-converted by the image data conversion unit 14, reads the LUT from the LUT holding unit 16, and corrects the gradation of the raster image data. This correction is performed according to the characteristic that the image forming unit 40 converts the gradation into the density of the print image. Further, this correction is performed on the gradation of each color using an LUT corresponding to each color converted by the image data conversion unit 14.
The LUT holding unit 16 included in the secondary storage unit 20 holds an LUT corresponding to each color converted by the image data conversion unit 14. Also, an LUT corresponding to a predetermined object, for example, a photo object is held.

  The image output unit 17 performs screen processing on the raster image data subjected to multi-value (8 bits each) color conversion processing for each color component (YMCK) input from the gradation correction unit 15. Thereby, based on the color-converted raster image data which is multi-value image information having density gradation, binarized image data representing the density of a halftone image in a pseudo manner by the size of colored dots called halftone dots (1-bit image data) is generated.

For example, the image output unit 17 recognizes an image object based on the tag data included in the raster image data subjected to the color conversion processing from the gradation correction unit 15, and uses the screen parameters set for each type, Optimal screen processing can be performed for each image. The screen parameter is a parameter for creating a screen, and includes a screen pattern, a screen line width, a screen pitch, a screen angle, and the like. The screen parameters are stored in the secondary storage unit 20 for each type of image object, for example.
Then, the image output unit 17 outputs the generated binarized image data to a laser exposure device (not shown) of the image forming unit 40.

(Detailed description of the image processing apparatus)
FIG. 2 is a block diagram illustrating the internal configuration of the image processing unit 10 of this embodiment. As shown in FIG. 2, the image processing unit 10 includes a CPU 101 that executes digital arithmetic processing according to a predetermined processing program when processing image data. A RAM 102 used as a working memory of the CPU 101 and a ROM 103 that stores processing programs executed by the CPU 101 are provided. When the CPU 101 reads this processing program when the image forming apparatus 1 is started up, each function in the image processing unit 10 of the present embodiment is realized. These functions include the above-described image data reception function, image data creation function, rendering processing function, color conversion processing function, gradation correction function, and screen processing function. In addition, a non-volatile memory 104 such as an SRAM or a flash memory backed up by a battery, which can be rewritten and can retain data even when power supply is interrupted, is provided. Further, an interface unit 105 that controls input / output of signals to / from each unit such as the PC 3 connected to the image processing unit 10, the secondary storage unit 20, and the image forming unit 40 is provided. The secondary storage unit 20 (see FIG. 1) holds, for example, a DLUT, a screen parameter, and the like in addition to the LUT.

(Description of engine characteristics and tone correction table for converting tone values to print density)
FIG. 3 is a diagram illustrating an example of engine characteristics for converting tone values into print density and an example of a tone correction table. FIG. 3A shows the relationship between the gradation value received by the engine and the measured density when the engine actually prints an image on a medium (paper) based on the value (hereinafter referred to as “printing”). Example of “characteristic”). FIG. 3B shows an example of a gradation correction table (hereinafter referred to as “normal LUT”) for correcting the gradation value of raster image data subjected to color conversion processing to be printed in accordance with the printing characteristics. . The normal LUT is set individually for each color of YMCK. In the following, the content of the present embodiment will be described using the case of Cyan color, but other colors (Yellow color, Magenta color, and Black color) may be the same content embodiment.

The example of FIG. 3A shows print characteristics indicating the relationship between the gradation value of the raster image data for the accepted cyan color and the measured density as a result of printing the cyan color on a paper medium. It should be noted that the measured density is “0.09” even though the gradation value received by the engine is “0” (nothing is printed) because the density of the cyan color of the paper medium itself is measured. It has been done.
In the example of FIG. 3A, this engine has a print characteristic in which the print density becomes low when the tone value of the raster image data subjected to the color conversion process accepted by the engine is about 0 to 40 (so-called highlight portion). Have In particular, when the gradation value of raster image data subjected to color conversion processing received by the engine is about 0 to 26, the cyan color has a printing characteristic that nothing is actually printed.

  In the example of FIG. 3B, the engine having the printing characteristics of the example of FIG. 3A uses a normal LUT used for correcting the value before accepting the gradation value of the cyan raster image data. Content. The normal LUT is a relationship between the gradation value of cyan raster image data received by the engine having the printing characteristics shown in FIG. 3A and the density at which the engine prints the cyan color on a paper medium. Is as close to linear as possible (the relationship between the input and the output is represented by a straight line). Therefore, since the shape of the graph in the example of FIG. 3A is S-shaped, the shape of the graph in the example of FIG.

(Description of the result of converting the gradation value corrected by the normal LUT into the print density)
In the example of FIG. 4, the engine having the gradation value of cyan raster image data before correction based on the normal LUT of the example of FIG. 3B and the printing characteristics of the example of FIG. This is a relationship with the measured density as a result of printing the cyan color on the paper medium according to the gradation value corrected based on the normal LUT. Since the example of FIG. 4 is the measured density, the density of the cyan color of the paper medium is measured.
In the example of FIG. 4, the gradation value of the raster image data subjected to the color conversion process is corrected based on the normal LUT of the example of FIG. As a result, the relationship between the gradation value of the raster image data subjected to color conversion processing before correction and the measured density of the printing result is closer to the linear relationship than the relationship shown in FIG. It can be seen that the improvement is significant.

(Explanation of design printing characteristics and variable printing characteristics)
FIG. 5 is a diagram illustrating examples of different printing characteristics in the highlight portion.
The example of FIG. 5A shows the relationship between the gradation value (0 to 40) of the raster image data in the highlight portion for the accepted cyan color and the measured density as a result of printing the cyan color on a paper medium. . An example of the solid line 50 is the printing characteristics (hereinafter referred to as “design printing characteristics”) in the example of FIG. An example of the dotted line 51 is a printing characteristic that has deviated from the design printing characteristic (hereinafter referred to as “variable printing characteristic”). Note that, according to FIG. 5A, when the gradation value according to the printing characteristics indicated by the solid line 50 and the dotted line 51 is about 0 to 16, the result of printing on a paper medium based on cyan raster image data. The measured concentrations of are the same. The difference between the solid line 50 and the dotted line 51 is that the printing of the highlight portion has a higher density in the printing with the variable printing characteristics than the printing with the design printing characteristics.
Here, as shown in FIG. 3A, the measured density is “0.09” even though the gradation value accepted by the engine is “0” (nothing is printed). The Cyan color density of the paper medium itself is measured.

  The example of FIG. 5B shows the relationship between the gradation value of the raster image data in the highlight portion for the accepted cyan color and the print density of the cyan color. This corresponds to the example of FIG. 5A excluding the influence of the density (Cyan color) of the paper medium itself. Therefore, by using FIG. 5B, when the gradation value of the raster image data for the same Cyan color is received, it is possible to grasp each print density for the Cyan color printed by the design print characteristic or the variable print characteristic. it can.

(Explanation of printing result by normal LUT and design printing characteristics of highlight part)
FIG. 6A shows an example of a normal LUT. FIG. 6B shows the design print characteristics according to the gradation value (0 to 20) of the raster image data for the cyan color before correction and the gradation value corrected based on the normal LUT. An example of the relationship with the print density in which Cyan color is actually printed on a paper medium is shown.

The example of the solid line 60 in FIG. 6A is a part of the normal LUT shown in FIG. Although the portion where the gradation value is larger than 20 is not shown, it is the same as the normal LUT shown in FIG.
This normal LUT sets the gradation value of the raster image data for the cyan color after correction to 0 when the gradation value of the raster image data for the cyan color before correction is 0. This is to prevent any Cyan color from being printed in an area where there is no Cyan color raster image data. In this normal LUT, when the gradation value of the raster image data for the cyan color before correction is 1, the gradation value of the raster image data for the corrected cyan color is set to 28. This is to make the relationship between the gradation value of the raster image data for the cyan color before correction and the print density close to linear, particularly in the highlight portion. If the relationship between the two can be made linear in this way, the print density can be made faithful to the gradation value of the raster image data before correction, and a print result with no problem in image quality can be obtained.

The example of the solid line 61 in FIG. 6B shows the gradation value of the raster image data before correction and the print density in the highlight portion where the gradation value of the raster image data before correction is from 0 to 20. It shows that the relationship with is approaching linearity. As a result, it can be seen that the result of printing the cyan color by combining the design print characteristics and the normal LUT is a print density faithful to the gradation value of the raster image data before correction.
Printing combining such design printing characteristics and normal LUT is performed by the existing image forming apparatus 1 (see FIG. 1), and a print result with no problem in image quality can be obtained.

(Explanation of the printing result by the modified LUT of highlight section and variable printing characteristics)
FIG. 7A shows an example of a modified LUT. Further, FIG. 7B shows a change according to the gradation value (0 to 20) of the raster image data with respect to the cyan color before correction and the gradation value corrected based on the modified LUT or the normal LUT. The example of the relationship with the printing density in which Cyan color was actually printed on the paper medium by the printing characteristics is shown.
Here, the correction LUT is a gradation correction table that corrects the gradation value of raster image data subjected to color conversion processing to be printed in accordance with the variable printing characteristics. Similarly to the normal LUT, the correction LUT is set individually for each color of YMCK.
In addition, the variable print characteristics are print characteristics that have been changed from the design print characteristics due to variations in engine manufacture, despite being manufactured by a manufacturing method (process) aimed at reproducing the design print characteristics. Furthermore, the variable print characteristic is a print characteristic that has changed from the design print characteristic due to a change with time (usage period, number of output sheets, etc.) although the designed print characteristic was initially exhibited.

The example of the dotted line 70 in FIG. 7A is a part of the correction LUT. The difference from the normal LUT in the example of the solid line 60 (see FIG. 6A) is only the gradation value corrected when the gradation value of the raster image data for the cyan color before correction is 0. Therefore, the contents of the modified LUT when the gradation value of the raster image data for the cyan color before correction is 1 or more are the same as the normal LUT shown in FIG.
This modified LUT is 0 when the gradation value of the cyan color after correction is 0 based on the normal LUT, whereas the gradation value of the cyan color after correction is 0 based on the normal LUT. Large value. The Cyan color gradation value before correction is smaller than 28, which is the Cyan color gradation value after correction based on the normal LUT for the case where the value is 1. Here, when the gradation value of the raster image data for the cyan color before correction is 0, the gradation value of the raster image data for the cyan color after correction is 24. This is to intentionally print the cyan color in order to suppress the occurrence of tone jump (pseudo contour) even in an area where there is no raster image data of cyan color, as will be described later.

  Here, the tone jump refers to, for example, a sudden change in print density that deteriorates the image quality of a print result in a highlight portion, which is a problem particularly when a photographic object is printed due to a variable print characteristic. As shown in FIG. 6B, when the print density can be faithful to the gradation value of the raster image data before correction, tone jump does not occur. This is because a rapid change in printing density does not occur.

The example of the dotted line 71 in FIG. 7B shows the relationship with the print density at which the cyan color is actually printed on the paper medium by the variable print characteristic according to the gradation value corrected based on the modified LUT. When the gradation value of raster image data before gradation correction by the modified LUT is 1 or more, this relationship is close to linear. Therefore, there is no sudden change in print density and tone jump does not occur.
Further, the example of the alternate long and short dash line 72 is corrected with the print density at which the cyan color is actually printed by the variable print characteristic according to the gradation value of the raster image data with respect to the cyan color corrected based on the normal LUT. The relationship with the gradation value of the previous cyan color is shown. The difference between the example of the dotted line 71 and the example of the alternate long and short dash line 72 is only the print density where the gradation value of the raster image data with respect to the cyan color before correction is in the range of 0 to 1, and the other parts are the same. Therefore, even if the gradation value of the raster image data before gradation correction by the normal LUT is 1 or more, a tone jump does not occur.

Next, the cause of the occurrence of a tone jump in printing combining the variable printing characteristic and the normal LUT will be discussed with reference to FIG. First, pay attention to the difference 73.
Here, the difference 73 is the density at which the cyan color is printed by the variable printing characteristic according to the gradation correction based on the normal LUT when the gradation value of the raster image data with respect to the cyan color before correction is 0, When the gradation value of the raster image data with respect to the cyan color before correction is 1, the difference from the density at which the cyan color is printed by the variable printing characteristic according to the gradation correction based on the normal LUT is shown.
When the gradation value of the raster image data for the Cyan color before correction is 0, the gradation value of the Cyan color after correction based on the normal LUT is 0 (see FIG. 6A). Therefore, no cyan color is printed due to the variable printing characteristics (see FIG. 5B). When the gradation value of the raster image data for the Cyan color before correction is 1, the gradation value of the Cyan color after correction based on the normal LUT is 28 (see FIG. 6A). Therefore, the cyan color is printed on the paper medium with a density of “0.05” due to the variable printing characteristics (see FIG. 5B).
Therefore, when the normal LUT is used, the density difference “0.05” is generated due to the variable printing characteristics only by changing the gradation value of the raster image data for the cyan color before correction from 0 to 1. It will be. This indicates that the density difference (density fluctuation rate) per unit gradation is “0.05”.

Next, the density variation rate of the dotted line 71 when the gradation value of the raster image data with respect to the cyan color before correction is 1 or more will be examined. Referring to FIG. 7B, the density fluctuation rate is “0.007”.
As a result, it can be seen that the density fluctuation rate when the gradation value of the raster image data with respect to the cyan color before correction is 1 or more rapidly decreases to 14% of the density fluctuation rate in the case of 0 to 1. Then, it can be understood that this sudden change in the density fluctuation rate is the cause of the tone jump.

Next, the reason why the tone jump that has occurred in the printing combining the variable printing characteristic and the normal LUT can be suppressed by using the modified LUT will be described with reference to FIG. 7B.
First, when the gradation value of the raster image data with respect to the cyan color before correction is 0 or 1, the density at which the cyan color is printed by the variable printing characteristics according to the gradation correction based on the correction LUT will be examined. When the gradation value of the raster image data for the Cyan color before correction is 0, the gradation value of the Cyan color after correction based on the modified LUT is 24 (see FIG. 7A). Therefore, the cyan color is printed on the paper medium with the density “0.03” due to the variable printing characteristics (see FIG. 5B). When the gradation value of the raster image data for the Cyan color before correction is 1, the gradation value of the Cyan color after correction based on the modified LUT is 28 (see FIG. 7A). Therefore, the cyan color is printed on the paper medium with a density of “0.05” due to the variable printing characteristics (see FIG. 5B).
Therefore, when the modified LUT is used, the density difference “0.02” is generated due to the variable printing characteristics only by changing the gradation value of the raster image data for the cyan color before correction from 0 to 1. It will be. This indicates that the density difference (density fluctuation rate) per unit gradation is “0.02”.

Next, considering the density fluctuation rate of the dotted line 71 when the gradation value of the raster image data for the cyan color before correction is 1 or more, as described above, the density fluctuation rate is “0.007”. . Thus, it can be seen that the density fluctuation rate when the gradation value of the raster image data for the cyan color before correction is 1 or more is 35% of the density fluctuation rate when 0 to 1. This change in the density fluctuation rate is moderated from 14% when the normal LUT is used. Therefore, the occurrence of tone jump can be suppressed.
In order to suppress the occurrence of tone jump in this way, the Cyan color gradation value after correction based on the correction LUT for the case where the Cyan color gradation value before correction is 0 is set to the value before correction. It has been found that it is desirable to set the value to about 80% to 90% of the Cyan color gradation value after correction based on the normal LUT when the Cyan color gradation value is 1. Accordingly, in FIG. 7A, when the gradation value of the raster image data for the cyan color before correction is 0, the gradation value of the cyan color after correction based on the modified LUT is 24.

Further, when the change in density variation rate is further reduced to suppress the occurrence of tone jump, the contents of the correction LUT may be changed. For example, when the gradation value of the raster image data for the cyan color before correction is 0, the gradation value of the corrected cyan color based on the modified LUT may be set to 25 or 26. Thereby, the change of the density fluctuation rate is further alleviated.
In this case, the cyan color is intentionally printed in an area where nothing should be printed. However, when the occurrence of a tone jump that greatly affects the image quality is a problem, such as a photographic object, it is most important to solve this problem. In addition, there is a situation in which a photographic object has almost no area where nothing is printed. Therefore, it can be said that there is no problem even if a photograph object is printed in an area where nothing is originally printed.

Next, printing that combines the design printing characteristics and the modified LUT will be considered. Here, when the gradation value of the raster image data with respect to the cyan color before correction is 0 or 1, the density at which the cyan color is printed according to the design print characteristics according to the gradation correction based on the correction LUT is examined. .
When the gradation value of the raster image data for the Cyan color before correction is 0, the gradation value of the Cyan color after correction based on the modified LUT is 24 (see FIG. 7A). Therefore, no cyan color is printed due to the design printing characteristics (see FIG. 5B). When the gradation value of the raster image data for the Cyan color before correction is 1, the gradation value of the Cyan color after correction based on the modified LUT is 28 (see FIG. 7A). Therefore, the cyan color is printed on the paper medium with a density of “0.004” according to the design printing characteristics (see FIG. 5B). Therefore, the density variation rate is “0.004”.

Also, in the example of the solid line 61 (see FIG. 6B), the cyan print density due to the design print characteristics changes almost linearly in accordance with the change in the gradation value of the cyan raster image data before correction. is doing. Here, the difference between the normal LUT and the modified LUT in the example of the solid line 60 (see FIG. 6A) is that the gradation of the corrected gradation is 0 when the gradation value of the raster image data for the cyan color before correction is 0. It is only a value. Therefore, even when the modified LUT is used, it can be said that the cyan print density due to the design print characteristics changes as indicated by the solid line 61 in accordance with the change in the gradation value of the raster image data subjected to the color conversion process before correction. Here, the density fluctuation rate represented by the solid line 61 is “0.004”.
As a result, the density fluctuation rate when the gradation value of the raster image data with respect to the cyan color before correction is 1 or more is the same as the density fluctuation rate in the case of 0 to 1. It can be said that either the normal LUT or the modified LUT is not changed.
Therefore, when printing an object of a photograph by correcting the gradation value of cyan raster image data using a modified LUT, regardless of whether the engine print characteristics are designed print characteristics or variable print characteristics. Therefore, it is possible to suppress the occurrence of tone jump, which has been particularly problematic.

(Example in which gradation correction is performed on a photo object by a new LUT)
FIG. 8 is a flowchart showing a flow of processing for correcting the gradation value of raster image data subjected to color conversion processing to the color system (YMCK) according to a new LUT or a normal LUT. In the example of FIG. 8, when the raster image data is a photographic object, the gradation value of the raster image data subjected to color conversion processing for the photographic object is corrected according to the correction LUT. If the raster image data is other than a photographic object, the gradation value of the raster image data subjected to color conversion processing for the non-photographic object is corrected according to the normal LUT.

  First, the receiving unit 11 (see FIG. 1) receives image data from the PC 3 or the like (see FIG. 1) (step 101). Next, the PDL analysis unit 12 (see FIG. 1) analyzes the image data received by the reception unit 11 and creates image data to be printed on a print medium such as paper (step 102). Subsequently, the rendering processing unit 13 (see FIG. 1) creates raster image data using the image data created by the PDL analysis unit 12 (step 103). Then, the image data conversion unit 14 performs color conversion processing on the raster image data expressed in RGB to the color system (YMCK) (step 104).

Next, the gradation correction unit 15 (see FIG. 1) determines whether or not the color-converted raster image data is raster image data of a photographic object (step 105). If it is determined in step 105 that the image is raster image data of a photographic object, a modified LUT corresponding to the color to be corrected (for example, cyan color) is read from the LUT holding unit 16 (see FIG. 1). (Step 106). Then, the process proceeds to Step 108. If it is determined in step 105 that the raster image data is not color-converted raster image data of the photograph object, a normal LUT corresponding to the tone correction color (for example, cyan color) is read from the LUT holding unit 16 ( Step 107).
Subsequently, the gradation correction unit 15 corrects the gradation value of the raster image data for the color whose gradation is corrected by the read LUT (step 108). In this case, the gradation value of the raster image data subjected to the color conversion process for the photo object is corrected according to the correction LUT, and the gradation value of the raster image data subjected to the color conversion process for the object other than the photograph depends on the normal LUT. Corrected.

Next, the gradation correction unit 15 determines whether or not the gradation correction process of the raster image data has been performed for all the YMCK colors subjected to the color conversion process in Step 104 (Step 109). If it is determined that the process has been performed, the process proceeds to step 110. If it is determined that the process has not been performed, the process returns to step 105.
If it is determined in step 109 that the gradation correction processing of raster image data has been performed for all YMCK colors, the image output unit 17 (see FIG. 1) performs raster correction for each YMCK color. Screen processing is performed on the image data in accordance with the type of object of the image. Subsequently, after the processing is completed, the raster image data subjected to the screen processing is sent to the image forming unit 40 (see FIG. 1) (step 110). Then, a series of processing ends.

As described above, the process of correcting the gradation value of the raster image data color-converted to the color system (YMCK) according to the modified LUT (see FIG. 7A) or the normal LUT (see FIG. 6A). Explained the flow. However, these are only examples.
For example, it may be determined whether to replace the normal LUT with the modified LUT for each color of YMCK. There may be a case where the printing characteristics of the Cyan color are variable printing characteristics and the printing characteristics of other colors are design printing characteristics. In this case, after determining whether or not the object is a photograph in step 105 (see FIG. 8), it is determined whether the print characteristic of the color to be corrected for gradation is the designed print characteristic or the variable print characteristic.

  In step 105 (see FIG. 8), it is determined whether or not the color-converted raster image data is raster image data of a photographic object. However, the present invention is not limited to this. It may be determined whether a part of the raster image data subjected to the color conversion processing is raster image data of a photographic object. When a part of the raster image data subjected to the color conversion processing is raster image data of a photographic object, the gradation value of the raster image data of the photographic object is corrected by reading the correction LUT. Then, the gradation value of raster image data of an object other than a photograph is corrected by reading the normal LUT so that nothing is printed in an area where nothing is originally printed.

When the variable print characteristic is a print characteristic with a poor print responsiveness in the highlight portion than the design print characteristic indicated by the solid line 52 (see FIG. 5B), it is a case where a photo object is printed. However, if a normal LUT is used, there is no possibility that a tone jump will occur. In the print result obtained by combining such print characteristics and the normal LUT, the density variation rate fluctuates abruptly because printing is always performed when the gradation value of the raster image data subjected to color conversion processing before correction is 1 or more. Because there is nothing to do.
In this case, a modified LUT may be used. When the print responsiveness is poor, printing is started with the gradation value of the raster image data corrected at the portion where the modified LUT and the normal LUT are common. Therefore, the density fluctuation rate does not fluctuate abruptly and there is no possibility of tone jump.

In FIG. 1, the normal LUT and the modified LUT are held in the LUT holding unit 16 (see FIG. 1) included in the secondary storage unit 20 (see FIG. 1). However, the present invention is not limited to this. . For example, these LUTs may be held in the ROM 103 (see FIG. 2), the nonvolatile memory 104 (see FIG. 2), or the like. In addition, a printer driver held in the image processing apparatus 10 (see FIG. 1) or the PC 3 (see FIG. 1) may have a function of setting correction contents of these LUTs.
Further, in the present specification, a specific form for providing the program is not described. However, for example, the program may be provided by a bidirectional communication means such as the Internet, or a recording such as a CD-ROM may be used. The embodiment may be provided by being held in a medium.

By using the modified LUT in this way, the occurrence of a tone jump, which becomes a problem when printing an image object (for example, a photographic object) in which there is almost no solid area where nothing is printed, is caused in advance. Can be prevented.
In addition, by using the modified LUT only for such an image object, even when printing an image object (for example, a document object) in which many plain areas exist, the plain LUT is used to print a plain area. Can be secured.

1 is a block diagram illustrating an example of a configuration of an image forming apparatus including an image processing apparatus of the present invention. It is a block diagram which shows the internal structure of an image process part. It is a figure which shows the example of the characteristic of the engine which converts the value of a gradation into printing density, and an example (normal LUT) of a gradation correction table. It is a figure which shows an example of the result of having converted the gradation value correct | amended by normal LUT into the printing density. It is a figure which shows a design printing characteristic and a variable printing characteristic. It is a figure which shows an example of the printing result by the normal LUT and design printing characteristic of a highlight part. It is a figure which shows an example of the printing result by correction LUT of a highlight part, and a variable printing characteristic. It is the flowchart which showed the flow which carries out gradation correction | amendment with the new LUT with respect to the image data of the object of a photograph.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 3 ... Personal computer (PC), 4 ... Image reading apparatus, 10 ... Image processing part (image processing apparatus), 11 ... Reception part, 12 ... PDL analysis part, 13 ... Rendering process part, 14 ... Image data conversion unit, 15 ... gradation correction unit, 16 ... LUT holding unit, 17 ... image output unit, 40 ... image forming unit, 101 ... CPU, 104 ... non-volatile memory, 105 ... interface unit

Claims (8)

Receiving means for receiving image data;
Conversion means for converting the image data into output image data formed on a recording medium by an image forming apparatus;
The gradation of the output image data is corrected according to the characteristics of the image forming apparatus. When the gradation of the output image data is 0, the correction value is smaller than the correction value when the gradation of the output image data is 1. Gradation correction means for correcting the gradation of the output image data to a value greater than 0;
An image processing apparatus comprising: The converting means converts the image data received by the receiving means into output image data for each color formed on the recording medium,
The image processing apparatus according to claim 1, wherein the gradation correction unit corrects the gradation of the output image data for each color to a predetermined value according to the characteristics of the image forming apparatus for each color. . Recognizing means for recognizing that the image data received by the receiving means includes a photo object;
The image processing apparatus according to claim 1, wherein the gradation correcting unit corrects a gradation of the output image data with respect to an object determined to be the object of the photograph by the recognition unit. Receiving means for receiving image data;
Image processing means for performing image processing on the image data received by the receiving means;
Image forming means for forming an image on a recording medium based on the image data image-processed by the image processing means,
The image processing means includes
A conversion unit that converts the image data into output image data formed on the recording medium by the image forming unit;
The gradation of the output image data is corrected in accordance with the characteristics of the image forming means. When the gradation of the output image data is 0, the correction value is smaller than the correction value when the gradation of the output image data is 1. And a gradation correction unit for correcting the gradation of the output image data to a value larger than 0,
The image forming unit includes:
An image forming apparatus, wherein an image is formed on the recording medium based on the gradation of the output image data corrected by the gradation correction unit. The image processing means includes
A recognition unit for recognizing that the image data received by the receiving unit includes a photo object;
The image forming apparatus according to claim 4, wherein the gradation correction unit corrects a gradation of the output image data with respect to an object determined to be an object of the photograph by the recognition unit.   In the image processing unit, the conversion unit converts the image data received by the receiving unit into the output image data for each color, and the recognition unit recognizes the photo object as the photo object. 6. The image forming apparatus according to claim 5, wherein a corresponding color conversion process is performed for each color. On the computer,
An acquisition function to acquire image data;
A conversion function for converting the image data into output image data formed on a recording medium by an image forming apparatus;
The gradation of the output image data is corrected according to the characteristics of the image forming apparatus. When the gradation of the output image data is 0, the correction value is smaller than the correction value when the gradation of the output image data is 1. And a gradation correction function for correcting the gradation of the output image data to a value larger than 0,
A program to realize In the computer,
Further realizing a recognition function for recognizing that the image data acquired by the acquisition function includes a photo object,
8. The program according to claim 7, wherein the tone correction function corrects the tone of the output image data for an object that is determined to be an object of the photograph by the recognition function.

Images