Patent Application
20100328691
This application is based on Japanese Patent Application No. 2009-154156, filed on Jun. 29, 2009, the contents of which are incorporated herein by reference.
1. Technical Field
The present invention relates to a computer readable recording medium storing a control program of an image processing device using color profiles, and a evaluation method color adjustment results by an image processing device using color profiles.
2. Description of Related Art
Nowadays, many print shops are equipped not only with large-scale printing machines like an offset printing machine, but also with small-scale printing machines like a laser printer, which generally serve to proof larger scale printing machines, or to print a small amount of copies. However, commercial printing machines and general-purpose printers have different color reproducibility with each other due to the difference in their printing methods, and therefore their print outputs may have different colors even if they print the same image data. In this context, various techniques have been proposed for adjusting color profiles in order to match printed colors between devices.
Japanese Unexamined Publication No. 2004-153667 proposes an exemplary color proof system for adjusting printed colors by a low-end color printer to those by an large-scale printing machine (hereinafter called a target device). This system is provided with a color measuring device for measuring color patches within color charts printed by the color printer and the target device, respectively, and is designed to calculate correction values for correcting the color profiles, from the difference between the measured color values. This system is intended to lower the color difference below a certain level through repetitions of feed-back correction using the calculated correction values.
However, the aforementioned method including calculation of the difference in measured color values between the devices does not necessarily accomplish asymptotic approximation of the measure values for the printer into their target values (i.e. measured values for the target device) no matter how many times the correction is repeated. The is because color range of the color printer may be narrower than that of the target printer, and this narrower color range would causes inability of the color printer to reproduce some of the colors contained in a color chart, and thereby entailing color differences in these colors.
A similar problem may occur when the color printer and the target device use different types of printing paper from each other. Concretely speaking, while the color printer and the target device both reproduce the stark white color (C=M=Y=K=0%) by leaving blank the relevant portions of the paper surface, these blank portions may be erroneously recognized as different colors if these devices use different types of printing paper with different colors. In other words, the color measuring device may give a color difference even when there is no color adjustment error.
Thus, the system in the prior art will even attempt to correct the uncorrectable color difference if there is a mismatch between the reproducible color ranges by the devices or a difference between the types of printing paper used by the devices. This results in a difficulty in properly evaluating the color adjustment results, nor making corrections with a high accuracy.
The present invention is intended to solve the above-mentioned problem in the prior art, and its objective is to provide a control program of, and an evaluation method of color adjustment results by an image processing device capable of making the color adjustment of an object device to a target device through a color conversion process by means of color profiles, even in the presence of a mismatch between the reproducible color ranges of the devices or a difference between the types of printing medium used by the devices.
To achieve at least one of the above-mentioned objects, the computer readable recording medium reflecting one aspect of the present invention stores a control program of an image processing device capable of making color adjustment of an object device to a target device through a color conversion process which includes converting color values in a device-dependent color space in to those in a device-independent color space by means of an input conversion table of the color profile for said target device, and further converting the color values in said device-independent color space after the conversion into color values in said device-dependent color space by means of an output conversion table of the color profile for said object device.
Said program causes said image processing device to execute the steps of: (A) executing said color conversion process on image data containing various color patches; (B) causing said object device to output said image data after said color conversion process executed in said step (A), and causing a color measuring device to measure the color values in said device-independent color space corresponding to the individual color patches contained in the output of said image data; (C) calculating color values in said device-independent color space, defined as conversion-output values, by converting the color values in said device-dependent color space corresponding to the individual color patches contained in said image data after said color conversion process executed in said step (A) by means of said input conversion table for said object device; and (D) calculating the differences between the color values in said device-independent color space measured in said step (B) and the conversion-output values calculated in said step (C).
Preferably, the control program should further cause said image processing device to execute the step of (E) displaying information on the differences calculated in said step (D) on a display device.
Preferably, the program should further cause said image processing device to execute the steps of: (F) either calculating or causing said color measuring device to measure the color values in said device-independent color space, defined as target values, which correspond to the individual color patches contained in the output of said image data by said target device; and (G) calculating the differences between the target values calculated in said step (F) and the conversion-output values calculated in said step (C) as well as the differences between the target values calculated in said step (F) and the color values in said device-independent color space measured in said step (B).
Preferably, in said step (E), information on the differences calculated in said step (G) should be displayed, together with, or instead of information on the differences calculated in said step (D).
Preferably, the information should include at least one of the mean and maximum values of the difference for each of the color patches.
Preferably, the program should further cause said image processing device to execute the step of (H) of correcting said input conversion table for said target device based on the differences calculated in said step (D).
Preferably, in said step (H), said input conversion table for said target device should be corrected so that the color values in said device-independent color space after the conversion by means of said input conversion table will be replaced by the results of subtracting the differences calculated in said step (D) from the conversion-output values calculated in said step (C).
Preferably, said step (H) should be repeated more than one time.
Preferably, in the second round of said step (H) onward, said program should further cause said image program to execute the steps of: (I) causing said object device to output said image data after said color conversion process by means of said input conversion table for said target device after the most recent correction, and causing said color measuring device to measure the color values in said device-independent color space corresponding the individual color patches contained in the output of said image data by said target device; and (J) calculating the differences between the color values in said device-independent color space measured in said step (I) and the conversion-output values calculated in said step (C).
Preferably, said input conversion table for said target device should further be corrected so that the color values in said device-independent color space after the conversion by means of said input conversion table for said target device will be replaced by the results of subtracting the differences calculated in said procedure (J) from the color values in said device-independent color space after the conversion by means of said input conversion table after the most recent correction.
Preferably, said device-dependent color space is CMYK color space.
Preferably, said device-independent color space is L*a*b* color space.
The objects, features, and characteristics of this invention other than those set forth above will become apparent from the description given below with reference to preferred embodiments illustrated in the accompanying drawings.
The embodiments of this invention will be described below with reference to the accompanying drawings.
As shown in
The control unit 11 is a CPU for controlling various units according to control programs. The storage unit 12 contains a ROM for storing various programs, a RAM for temporarily storing various data to serve as a work area, and a hard disk for temporarily storing print data sent from the PC 2. The operating panel unit 13 is an operation panel with a touch panel capable not only of displaying various kinds of information but also of receiving user's instructions, and various fixed keys.
The printing unit 14 is an print engine for printing images based on the image data sent from the PC 2 on a recording medium, using the electronic photography method including the charging, exposing, developing, transferring, and fixing steps. The printing unit 14 can also use other printing methods such as the impact method, the thermal transfer method, the ink jet method, etc. The input/output interface 15 is an interface for communication with the PC 2, and is typically IEEE.1394 or USB. The printer 1 and the PC 2 can be connected via the network N as well, and in this case the input/output interface 15 can be an NIC (Network Interface Card) complying with standards as Ethernet®, Token ring, FDDI, etc.
The control unit 21 is a CPU for controlling various units and performing various calculations according to control programs. In particular, the control unit 11 in the present embodiment conducts the image processing on the print data sent from the outside. The storage unit 22 comprises a ROM for storing various programs and parameters for PC 2's basic operations, a RAM for temporarily storing various data to serve as a work area, and a hard disk for storing various programs including the OS. In particular, the hard disk of the storage unit 22 stores various programs for the image processing, together with color profiles used for the color conversion process.
The display unit 23 is a display device like a CRT display, a liquid crystal display, etc., and displays various kinds of information to user. The input unit 24 is a combination of a keyboard, a mouse, and other input devices, and is used by user to giving the PC 2 various instructions. The input/output network interface 25 is an interface for establishing connection with network devices on the network N, and is typically a NIC complying with standards like Ethernet®, Token Ring, FDDI, etc. The PC 2 is also designed to generate the color profiles used for the color conversion process, based on the data sent from the color measuring device 3. (See also
The control unit 31 performs various calculations in addition to controlling various units according to control programs. The storage unit 32 stores various programs and parameters. The storage unit 32 also retains temporarily the measurement data acquired by the color measuring unit 34. In particular, the storage unit 32 stores a program for converting the measurement data acquired by the color measuring unit 34 into L*a*b* color values. The operating panel unit 33 is a combination of fixed keys for receiving user's instructions.
The color measuring unit 34 makes measurement of each color patch by moving a spectrophotometer over a color chart, and transmits the measurement results to the storage unit 32. The color measurement results will be converted into spectral reflectance values, XYZ color values, or L*a*b* color values after being received by the storage unit 32.
The following is an outline of the operation by the color measuring system A in the present embodiment.
Firstly, the PC 2 causes the color measuring device 3 to make measurement of the color chart printed by the target device in order to acquire L*a*b* color values for individual color patches contained in the color chart (S101). The PC 2 generates the color profile for the target device based on the L*a*b* color values acquired in S101 (S102), and stores the color profile into the storing unit 22. The color chart in the present embodiment is a color chart complying with ISO 12642.
Next, the PC 2 causes the printer 1 to print the color chart without executing the color conversion process, and then causes the color measuring device 3 to measure the image printed by the printer 1 in order to acquire L*a*b* color values for individual color patches (S103). The PC 2 then generates the color profile for the printer 1 based on the L*a*b* color values acquired in S103 (S104), and stores the color profile into the storing unit 22.
The second look-up table L12 is a conversion table for converting each L*a*b* color value into a CMYK color value, and it contains CMYK color values corresponding to 35973 L*a*b* color values wherein the number of L*a*b* color values is derived from the multiplication of L*a*b*: 33×33×33 as shown in
A color profile P2 for the printer 1 is a combination of the first and second look-up tables L21 and L22, and is similarly generated in S104. The first look-up table is also called as input conversion table as this table is used at the input side of the color conversion, and the second look-up table is also called as output conversion table as this table is used at the output side of the color conversion. These synonyms for the tables will also be used in the following descriptions. In S102 and S104, the PC 2 can generate more than one type of different color profiles with different rendering intents like “colorimetric”, “perceptual”, “saturation”, etc.
The following is a detailed description of the generating method of the color profile P1 in S102 shown in
(I) Causing the color measuring device to measure the L*a*b* color values corresponding to the following CMYK colors contained in the printed color chart:
(a) C×M×Y: 6×6×6 with K=0% (0%, 10%, 20%, 40%, 70%, and 100% for each of CMY)
(b) C×M×Y: 5×5×5 with K=40% (0%, 20%, 40%, 70%, and 100% for each of CMY)
(c) C×M×Y: 5×5×5 with K=60% (0%, 20%, 40%, 70%, and 100% for each of CMY)
(d) C×M×Y: 4×4×4 with K=80% (0%, 40%, 70%, and 100% for each of CMY)
(e) C×M×Y: 2×2×2 with K=100% (0% and 100% for each of CMY)
(f) Monochromatic gradations for each of CMYK (13 steps: 3%, 7%, 10%, 15%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% for each color)
(II) Calculating L*a*b* color values corresponding to the CMYK colors shown in the paragraphs (g) to (k) using the measured L*a*b* color values in step (I) corresponding to the CMYK colors show in the paragraphs (a) to (e) above. Also calculating L*a*b* values for non-measured CMYK colors in the step (I), by means of an interpolation method based on the measurements for their adjacent colors as well as those for the monochromatic gradations shown in the paragraph (f) above.
(g) C×M×Y: 9×9×9 with K=0% (0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100% for each of CMY)
(h) C×M×Y: 9×9×9 with K=40% (0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100% for each of CMY)
(i) C×M×Y: 9×9×9 with K=60% (0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100% for each of CMY)
(j) C×M×Y: 9×9×9 with K=80% (0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100% for each of CMY)
(k) C×M×Y: 9×9×9 with K=100% (0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100% for each of CMY)
(III) Calculating L*a*b* color values for the CMYK colors shown in the paragraphs (l) to (n) below, by means of an interpolation based on the L*a*b* color values obtained in the step (II) for the CMYK colors shown in the paragraphs (g) to (h) above and the L*a*b* color values obtained in the step (I) for the K monochromatic gradations shown in the paragraph (f) above, and further calculating L*a*b* color values for the CMYK colors shown in the paragraph (o) below by means of an interpolation based on the L*a*b* color values obtained in the step (II) for the CMYK colors shown in the paragraphs (h) to (i) above and the L*a*b* color values obtained in the step (I) for the K monochromatic gradations shown in the paragraph (f) above.
(l) C×M×Y: 9×9×9 with K=10% (0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100% for each of CMY)
(m) C×M×Y: 9×9×9 with K=20% (0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100% for each of CMY)
(n) C×M×Y: 9×9×9 with K=30% (0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100% for each of CMY)
(o) C×M×Y: 9×9×9 with K=50% (0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100% for each of CMY)
This is how the PC 2 in the present embodiment calculates a L*a*b* color value for each of C×M×Y×K: 9×9×9×9 in order to generate the first look-up table (input color conversion table) L11 for converting a CMYK color values into a L*a*b* color value.
The following is a description of the generating method of the second look-up table shown in
As shown in
The method for this correction typically includes correcting the L*a*b* color values in such a way that the correction amount will decrease in proportion to the distance from the coordinate origin with C=M=Y=K=0%, or replacing the all L*a*b* color values with the media-relative L*a*b* color values which are calculated based on XYZ color values of stark white of each paper medium. In case of the media-relative L*a*b* values, C=M=Y=K=0% always becomes L*=100, a*=0 and b*=0 in each profile.
Instead of the two-step process illustrated in
Next, the PC 2 calculates conversion-output values of the color conversion process in S105 (S106). A conversion-output value herein means a L*a*b* color value corresponding to each of the CMYK color values after the color conversion process in S105, and is calculated from the first look-up table for the printer 1.
Next, the PC 2 causes the color measuring device 3 to measure an output of the color chart C printed by the printer 1 having executed the color conversion in S105, and acquires L*a*b* color values for the color patches (S107). The PC 2 then calculates the color difference ΔE between the measured color value in S107 and the conversion-output value calculated in S106 for each of the 590 color patches (S108).
ΔE={(L*0−L*1)2+(a*o−a*1)2+(b*0−b*1)2}0.5 Formula (1)
After calculating the color difference (ΔE) for each of the 590 color patches using the formula (1), the PC 2 further calculates the mean and maximum values of the color differences to display these values on the display unit 23 (S109).
The following is an observation on the color adjustment results of the colors outside printer 1's color gamut. As for 100% magenta (CMYK: 0/100/0/0), the measured color value (L*a*b* value) of the color chart C printed by the target device is 48.0/74.1/−8.1, the conversion-output value (L*a*b* value) is 46.4/66.5/−6.7, and the measured value (L*a*b* value) of the color chart printed by the printer 1 is 46.8/67.1/−7.9, which means that the color differences (ΔEO) between the measured color values turn out to be relatively high: ΔEO=7.1. On the other hand, the color differences (ΔE) between the conversion-output values and the measured color values turn out to be much smaller: ΔE=1.4. This suggests that comparisons between the conversion-output values calculated in the steps shown in
Observations on the color adjustment results of the stark white background (C=M=Y=K=0%) reveals that the measured value (in L*a*b*) of the color chart C printed by the target device is 94.0/0.7/−1.3, the conversion-output value (in L*a*b*) is 92.8/0.7/−0.2, and the measured value (in L*a*b*) of the color chart C printed by the printer 1 is 92.7/0.7/−0.5, meaning that the color difference (ΔEO) between the measured values turns out to be 1.5 while the color difference (ΔE) between the conversion-output value and the measured value of the output printed by the printer 2 turns out to be no more than 0.2.
Observations on the adjustment results of the 10% cyan (CMYK: 10/0/0/0), which is similar in color to the stark white background, reveals that the measured value (in L*a*b*) of the color chart C printed by the target device is 90.7/−2.8/−5.5, the conversion-output target value (in L*a*b*) is 87.6/−2.3/−4.5, and the measured value (in L*a*b*) of the color chart C printed by the printer 1 is 88.8/−3.0/−3.7. This means that the color difference (ΔE) between the conversion-output value and the measured value of the output turns out to be no more than 1.6 while the color difference (ΔEO) between the measured values turns out to be 2.5. This suggests that a comparison between the conversion-output value as shown in
As shown in
In the latter case, the PC 2 corrects the first look-up table (i.e. input color conversion table) L11 for the target device based on the color differences (ΔE) calculated in S108. More specifically, the PC 2 calculates corrected values (hereinafter called as “corrected conversion-output values”) for the conversion-output values calculated in S106, based on the color differences (ΔE) calculated in S108 (S111), and replaces the L*a*b* color values contained in the first look-up table L11 by these corrected conversion-output values.
The 2nd corrected conversion-output value (t2) is calculated by subtracting the difference (d0) between the measured color value (m1) of the output after the 1st correction and the conversion-output value (c0), from the 1st corrected conversion-output value (t1). Similarly, the Nth corrected conversion-output value (tN) is calculated by subtracting the difference (dN-1) between the measured value (mN-1) of the output after the (N−1)th conversion and the conversion-output value (c0), from the (N−1)th corrected conversion-output value (tN-1).
The PC 2 performs the calculation of the corrected conversion-output value for each of the 590 color patches, and then generates the corrected look-up table L11′ storing a corrected CMYK value for each of C×M×Y×K: 9×9×9×9 by means of the interpolation method described in
After that, the PC 2 further executes the steps from S107 to S109 on the image data after the color conversion process by the corrected profiles. The PC 2 further repeats the steps from S111 to S112 and the steps from S107 to S109 until user becomes satisfied with the calculation results of the color differences (ΔE) displayed in S109.
Exemplary calculation results of the mean and maximum values of the color differences (ΔE) after 2 times repetitions of the correction are shown below.
Initially (without correction): 1.5/4.2
After 1st correction: 1.0/4.8
After 2nd correction: 1.1/4.1
These figures suggest that the color differences (ΔE) will generally reduce through repetitions of the profile correction. They also suggest that the color differences (ΔE) of the color patches outside printer 1's color range reduces asymptotically through repetitions of the correction.
As seen from the above, the PC 2 serving as the image processing device in the present embodiment, makes the color adjustment of the printer 1 through the color conversion process using the color profiles P1 and P2, calculates the differences between the L*a*b* measurements of the image data output after the color conversion and the L*a*b* values obtained by converting the image data after the color conversion process using printer 1's first look-up table L11, and evaluates the color adjustment results based on the calculated color differences. Therefore, the present embodiment allows for proper evaluation of the color adjustment results even if there is a mismatch between the reproducible color ranges of the target device and the printer 1, or a difference between the paper types for printing used by the target device and the printer 1.
The invention is not limited to the embodiment described above, and hence it can be modified in various ways within the scope of the appended claims. For example, both the target and object devices in the above embodiment are printing devices, but at least one of them can be any other type of output device such as a display device. If one of them is a display device, the device-dependent color space will typically be RGB color space.
The image processing device according to the invention can be implemented by a dedicated hardware circuit for executing the abovementioned steps, or a program run by a CPU for executing these steps. If the present invention is implemented by the latter, the programs for driving the image processing device can take the form of a computer-readable recording medium such as a Floppy® disk and CD-ROM, or a downloadable file via a network like the Internet. The program stored in the computer readable recording medium is normally transported to a memory device such as a ROM and a hard disk. The program can also take the form of an independent application software, or a built-in function of the image processing device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-154156 | Jun 2009 | JP | national |
