Method for calibrating color printer and color printer

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention makes it possible to edit a look-up table that represents the characteristics of a printer and convert the colors defined in the first color space into the colors defined in the color space of the printer. More specifically, it relates to a method of mixing or combining the properties represented by such a look-up table.

【0002】

[Conventional technology]

Creating a color document can be thought of as a two-step process. In the first stage, the original is scanned by a color image input terminal or a scanner to generate an image, or a color image is generated by a workstation operating according to a color image generation program. The images are then output by a color printer according to the colors defined by the images read by the scanner or generated by the computer. The scanner output is usually converted into a tristimulus value, i.e. an RGB (red-green-blue) value color space. These values are usually linearly transformed with respect to the standard XYZ coordinates in the CIE color space, or else they are modified and transformed. For computer-generated images, the colors that the user defines on the user interface on the workstation can first be defined using tristimulus values. These colors are defined independently of any particular model and can be referred to as "device independent" information.

【0003】

Printers usually have an output that can be defined as being in a color space called CMYK (cyan-magenta-yellow-key or black), which is the performance and colorant of each printer. Is uniquely defined by. The printer operates by adding a plurality of layers of ink or colorants to the page in layers. Therefore, the response characteristics of the printer tend to be relatively non-linear. Since these colors are defined for each particular printer, this information is referred to as "device dependent". Thus, when a printer receives non-device dependent color information, it must convert that information to print in a device-dependent color space that reflects the printer's color reproduction range, or color reproducibility range. The printer may also print colors other than CMYK for various special purposes and to extend the color reproduction range of the device.

【0004】

The need for manipulation in converting a non-device-dependent color space to a device-dependent color space is described in Schreiber's US Pat. No. 4,500,919, Neugebower's US Pat. No. 2,790,844, and Sakamoto's US Pat. It is well known as shown in Nos. 4,275,413. There are many ways to convert between color spaces, but the first thing they all do is measure the response characteristics of the printer to a given input value. Generally, a printer is driven by a set of color inputs that reflect a color swatch over the entire color reproduction range of the printer, and this color swatch is printed by the normal operation of the printer. As already mentioned, the response characteristics of most printers are non-linear.

【0005】

In Sakamoto's US Pat. No. 4,275,413, the color information obtained is placed in a look-up table stored in a storage device, probably a ROM storage device or a ram RAM storage device. Associate the input color space with the output color space. This look-up table is generally a three-dimensional table because colors are defined by three variables. In RGB space, in a scanner or computer, space is the origin of the 3D coordinate system 0,0,0, and white is the maximum value of the 3D coordinate system that will be located at 255,255,255 in the 8-bit system. It can be defined as 3D. Therefore, that of the three axes extending radially from the origin defines red, green, and blue. However, in an 8-bit system as proposed, there are 16 million (256) possible colors.³) There are more. Obviously, there are too many values when mapping RGB to CMYK 1: 1. For this reason, the values in the look-up table are a set of several values, which can be said to be the values of the joints when the cubes are stacked. The colors contained within each cube use a number of interpolation methods, including tri-linear interpolation, tetrahedral interpolation, polynomial interpolation, and linear interpolation, depending on the required accuracy. Then, it is possible to interpolate from the measured value.

【0006】

It is extremely easy to create an index of device-dependent color values or specifications for non-device-dependent color specifications, but there is no need for it. Instead, the non-device-dependent specification must be mapped to the device-dependent specification. In this case, some problems arise. Of course, the first problem is that the response characteristics of the printer are not linear response characteristics. The second problem is that the color space, and therefore the coordinates defined within the color space, must be maintained as a uniform grid for maximum efficiency with some interpolation method.

【0007】

Therefore, a multidimensional look-up table (LUT) can be configured to arrange non-device-dependent input values in a predictable grid pattern. One way to achieve this is by an interpolation procedure that derives the value at the desired location as a function of all (or most) measured values. This interpolation procedure is known as the Shepard method (eg, in Mathematics of Computation, Vol.32, No.141, January 1978, pp.253-264). "Shepard's Method of'Metric Interpolation' to Bivariate and Multivariate" co-authored by W. Gordon and J. Wixom. See Interpolation). According to the shepherd method, a vector is interpreted as defining the difference between the printed color and the color instructed by the printer to print. Therefore, since the vector quantity is observed at any other point in the target color space, the average value of the vectors in the entire target space is obtained. The individual vectors are weighted by a function that has the effect of diminishing its effect as the distance from the color station increases. One of the useful functions applied here is 1 / d⁴Then, each vector is weighted using this value as a function.

【0008】

On the other hand, Po-Chie Hung's method is based on the paper "Scanners and Media" published in SPIE, Vol.1448, "Camera and Input Scanner System" (1991). Published by "Colorimetric Calibration for Scanners and Media", it describes the inverse tetrahedron interpolation method and states that the same effect as the shepherd method described above can be obtained.

【0009】

As a result of actual color calibration experiments, when using the color lookup table generated by the weighted average value method, good color reproducibility is obtained for a certain area part (light color part) of the color space. However, good results were not obtained in other areas (dark areas). Further, in the inverted tetrahedron method, the opposite of this, that is, good color reproducibility is obtained for the color of the dark region where good color reproducibility was not obtained by the weighted averaging method, and the weighted averaging method is used. The color reproducibility was inferior in the portion where good color reproducibility was obtained (the bright color region portion).

【0010】

In addition to the above-mentioned problems, the following problems often occur. That is, it is necessary to change the calibration contents after changing the processing parameters due to the elapsed time, changing the material, replenishing the toner, etc., but the change is necessary only in a part of the entire color reproduction range of the printer. It means that there is. Performing recalibration across the entire color space imposes a heavy burden on execution time. Therefore, it is desirable to calibrate only part of the color space or instead use only the best part of the color space mapping.

【0011】

The present invention has been made in view of the above, and an object of the present invention is to generate a single lookup table from individual conversion tables at the time of color calibration measurement of a printer, and to provide a color calibration device using a color mixing lookup table. A color printer and a color calibration method thereof are provided.

【0012】

[Means for solving problems]

According to the present invention, a color printer that adheres a printer colorant onto a medium according to a received printer signal in response to a printer signal is color-matched by a printer signal selected to cause the printer to print a color swatch on the medium. Operate the printer, measure the color swatches to determine the response characteristics of the printer's first color system to the printer signal, and use the first measured color system response characteristics to determine the color system values for the printer signal. And map the color system values to the printer signal at least once using the first measured response characteristics of the color system or the subsequently measured response characteristics of the color system. , The first mapped and the additionally mapped are stored in the color conversion storage device, and as a function of the first mapped and the additionally mapped one stored in the color conversion storage device. Color definitions are calibrated by converting the initial color space into a printer signal suitable for producing the corresponding response characteristics in a color printer.

【0013】

Another feature of the invention is that selected areas or parts of two (or more) look-up tables that map the values of the color system to the response characteristics of the color printer are mixed. The output response characteristics for which description in which color system from this mixed table are linearly combined with the output response characteristics of the individual tables, and this linear function is a table in the color space of the color system. Adjusted as a function of some of the color system descriptions.

【0014】

To describe yet another feature of the present invention, even if the same set of color swatches is used, the second look-up table may and may be different from the first look-up table due to the method of generation. The second look-up table may differ from the first look-up table due to the use of a set of color swatches.

【0015】

[Example]

Both FIGS. 1 and 2 are explanatory views of a block diagram showing the overall configuration of a color printing apparatus for which the use of the present invention can be found, and FIG. 3 shows the mixed mapping described.

【0016】

All of the drawings posted herein are for the purpose of specifically explaining the present invention, and the present invention is not limited thereto. FIG. 1 shows a basic configuration for realizing the present invention. In such a device configuration, the scanner 10 is almost equivalent to the color scanner used in the Xerox 5775 digital color copier, and is a set of digitized color systems showing the original image 12. A color scanner that can be calibrated to generate and output data, i.e. device-independent data, and when the original image 12 is read, the color signal Ro, defined in the form of the rgb spatial color system. Disassemble into Go and Bo and output. When the scanner is operated, a set of scanner image signals Rs, Gs, Bs are output, and these signals are scanner-specific device-dependent. A scanner post-processing device 14 is provided for these signals integrally with the scanner or in another signal path, and the scanner post-processing device 14 modifies the image signals Rs, Gs, and Bs from the scanner. It becomes an expression of the Rc, Gc, Bc color system generally used in the digital system. These numbers are in CIE color space (rgb) format or L^*a^*b^*Format or Brightness-Chrominance Color Space (LC)₁C₂It may be described in the) format. The color space converter 20 is similar to that described in Sakamoto's US Pat. No. 4,275,413, and is a device that converts non-device dependent data into device dependent data. The output of the color space converter 20 is converted into a device-dependent format, that is, the colorant signals Cp, Mp, Yp, Kp of the printer, and the printer 30 is driven. To give a possible example, the colorant signal should also adhere to a given area in an electrophotographic printer such as the Xerox 5775 digital color copier, cyan, magenta, yellow, and Represents the relative distribution of black toner. The printed output image has been converted to Rp, Gp, Bp, but associated with Ro, Go, Bo so that the printed output image has a color similar to the original image in color. It is desirable, but how close it is will ultimately depend on the color reproduction range of the printer.

【0017】

Next, the color space conversion and color corrector 20 of FIG. 2 will be described. First, the color signals of Rc, Gc, Bc are guided to the look-up table and the interpolator 40. This device is equipped with a storage device in which RAM or addressable memory elements are arranged three-dimensionally, and these storage devices have high speed and memory function corresponding to a specific element. There is. The color signals Rc, Gc, Bc are led to the specified address in the table. The location contains a set of conversion coefficients, where the signals Rc, Gc, Bc are Cx, Mx, Yx colorant signals or other multidimensional color space signals (including, of course, CMYK and spectral data). Can be converted to). The unmapped value can be determined by the interpolation method. Rolleston US Pat. No. 5,305,119, "Color Printer Calibration" As explained in the title of Architecture), it is possible to add a functional part that adds black to the color space conversion element for undercolor removal and gray balance. These functions are not indispensable for the present invention, but are desirable, and are therefore added in the drawings. When referring to the color space, it is based on the CIE XYZ (1931) conversion display method. However, since the color space dealt with here is a device-dependent space, it is merely a definition of a color that is valid only inside the device that uses it. Many color spaces are displayed in three dimensions (three primary colors), but they can be displayed in less than three dimensions or in more dimensions than three dimensions, as well as in printers. It is also possible to use a colorant with less than three colors, or a colorant with four or more colors.

【0018】

There is no doubt that there are innumerable ways to convert non-device-dependent data into device-dependent data. Sakamoto's US Pat. No. 4,275,413 describes one method, but the method itself can be modified. Once the conversion table is created, you can use any interpolation method after that, and apply the interpolation method operation from a limited number of input values regardless of whether it is a tri-linear interpolation method or a cube method. Then, the value to be output should be generated.

【0019】

A set of color patches is created to create the table, at which time linearity correction and black addition are performed as needed. At this time, 1000 to 4000 patches distributed over the entire color space are printed and measured. That is, a large number of sets of printer drive signals are generated and used to drive the printer while changing the combination of C, M, Y, K, or other printer color densities. The color of each patch is measured using a spectrophotometer and decomposed into Rc, Gc, and Bc. The measured color values in these patches are used to create a multidimensional look-up table (LUT) that associates the colors defined by Rc, Gc, Bc with the colors defined by CxMxYx. Points that cannot be measured without mapping are interpolated or extrapolated. The reference here is Laureston's US Patent Application No. 08 / 144,987, filed October 29, 1993, entitled "Color Printer Calibration Test Pattern" for this purpose. The calibration test pattern conforming to the above is shown.

【0020】

Returning to the description of FIG. 1 again, the calibration image data is stored in a storage device such as a color calibration ROM 60, RAM, a floppy disk, or an equivalent device for convenience, or is transiently stored by a predetermined generation function or the like. May be generated (on the fly). These stored signals are input to the printer under the control of the printer control device 65. A densitometer, spectrophotometer, or color-corrected scanner 70 scanned the calibration target to generate signal values Rc, Gc, Bc as a function of the read density, obtained by scanning the color patch. Represents an individual color. The interpatch correlation processor 80 reads the response characteristic signal for each point of the calibration target supplied by the densitometer 70 and associates this response characteristic with the input colorant signal, and as a result, from Rc, Gc, Bc. Mapping to the colorant signal is performed.

【0021】

It is probably possible to map non-device dependent values to device dependent spaces, according to the following literature. The document is U.S. Patent Application No. 08 / 223,494, filed April 5, 1994, entitled "Color Printer Calibration with Improved Color Mapping Linearity." ) . In addition, Po Chi Han's method is "Colorimetric Calibration for Scanners and Media", SPIE, Vol.1448, "Camera and Input Scanner System". (1991), a method of obtaining the same effect as the result of the shepherd method by the inverse tetrahedron interpolation method is explained.

【0022】

Once the look-up table is created, it is stored in the LUT and interpolator 40 and used to convert the device-dependent values received from the image generator into non-device-dependent printer signals.

【0023】

According to the present invention, for whatever reason, as soon as the second set of mappings from the colorimetric values to the printer signal is performed, the effect appears, and one set of values is the other one. It turns out that the color is better than the pair. But what is more certain is that even if one set of colors is better than the other, it is only a very limited part of the color space. That would be the case. Thus, using both of these two sets of data is a requirement to generate the optimal state.

【0024】

Therefore, as described herein with reference to FIG. 3, the LUT and interpolator 40 is a composite look-up table that includes a weighting table 100 and a look-up table group LUT1 to LUTN numbered 102. ing. For convenience of explanation, these tables will be described as being stored in RAM or a storage device having a similar function. These tables are formatted to provide descriptive Rc, Gc, B in colorimetric colors as an index or input value for each table. Output coefficients A1 to AN are written in the coefficient table 100. Each value of the coefficients A1 to AN can be derived as a function of the position where the color measurement value in the color space is described, and each LUT value is related to a region or a part in the color space. Because of the relative weighting, they are different. To give a concrete example, it is assumed that there is a set of coefficient groups, A1 to AN, corresponding to LUT1 to LUTN. These values are transmitted from the LUT processor 100, and are written and stored in the coefficient table 100 and the lookup tables LUT1 to LUTN, respectively. The coefficient group stored in the coefficient table 100 can also be independently generated by the operator or the user and input to the coefficient table 100 via the user interface.

【0025】

The look-up tables LUT1 to LUTN are created by the above procedure, and are generated by the conversion from the first color space to the second color space. These are also formatted so that you can enter the values of the colorimetric values Rc, Gc, Bc to index the table and enter the values, but the output is For example, a CMYK signal. Each output signal from the LUT has a corresponding coefficient multiplier 120₁~120_NIs entered in. Each of these coefficient multipliers also has a second input terminal, and the corresponding values of the coefficients A1 to AN are obtained from Table 100. Each output of the coefficient multipliers 1201 to 120N is introduced into the adder or adder 130, the addition is performed, the output is output, and the LUT is placed elsewhere in the storage device._NEWStored as 134. The interpolator 140 reads the stored information from this location in the storage device and generates the color conversion value thereof. The interpolator 140 employs, for example, the tri-linear interpolation method or the tetrahedron interpolation method, and utilizes the stored LUT value (as taught by Sakamoto's US Pat. No. 4,275,413). And the interpolated output value is obtained. (Fig. 3 shows the block 40 in Fig. 1, and the label of the block is "LUT and interpolation." Therefore, the function of the tetrahedral interpolation method or the tri-linear interpolation method is the "interpolator" in the figure. It is represented by "140".

【0026】

Therefore, the characteristic formulas that give the color mixing results for a plurality of tables are expressed as the following formulas at each location in the color space. That is, LUT_NEW(r, g, b) = A1 (r, g, b) x LUT1 (r, g, b) + A2 (r, g, b) x LUT2 (r, g, b) + A3 (r, g, b) × LUT3 (r, g, b) + + AN (r, g, b) × LUTN (r, g, b) However, A1 (r, g, b) + A2 (r, g, b) + A3 (r, g, b) + + AN (r, g, b) = 1 And.

【0027】

In the above equation, an interesting result can be obtained by using only two tables. In this case, only LUT1 and LUT2 remain (in normal use, this would be the case), but the formula can be transformed into a special form. That is, if the weight for LUT1 is set to A1 = A, the weight for LUT2 is A2 = 1-A, and it is not necessary to store A1 and A2 separately. Therefore, in the special case where there are two LUTs, the above equation can be rewritten as the following equation. That is, LUT_NEW(r, g, b) = A (r, g, b) x LUT1 (r, g, b) + [1-A (r, g, b)] x LUT2 (r, g, b) Here, LUT1 () and LUT2 () show the values at a certain place (that is, indicating any of r, g, and b) in the first and second tables, respectively. , And A () is a function for each location in the table. However, the value of A is within the range of 0 <= A <= 1. Of course, this result can be generalized by setting the weight to N-1 even if the number of tables is N.

【0028】

We use the term location function here, but this function does not require a linear mix of LUTs, so it can be used conveniently. Of particular value is the fact that for every position in the color space, all can be expressed with just one LUT. Thus, with N LUTs, the best state can be achieved with AN = 1, at which time the value in all other LUTs is A = 0.

【0029】

So far, LUT_NEWI have explained concretely how to generate a new LUT named. However, the method of generating a color-mixed value transiently from the look-up table group, LUT1 to LUTN, that is, without storing in a storage device or creating a new LUT is within the claims of the present invention. It belongs to. Therefore, even if the output of the adder 130 for each value of RcGcBc, which was derived from the coefficient table 100 and the LUT table 102, is directly guided to the interpolator 140, the same result can be obtained. There may be an opinion to do.

【0030】

But that said, for discussions that color mixing is feasible in any color space and in any coordinate system, or for the idea of transiently using LUTs for linear color mixing. The idea that is not restricted is meaningless. The novelty of the present invention lies in its excellent functionality of aggregating a plurality of different LUT groups into only one LUT. The technique of the present invention has already been put into practical use, and as seen in the above example, the values of R, G, B, C, M, Y and W (all colors including white) are set with the weighting factor A = 1. Is determined, and the value of K (black) is obtained with A = 0. Further, the intermediate value of the coefficient A is calculated by the interpolator and written in this cube. Therefore, LUT1 is a LUT generated from a weighted conversion table, and LUT2 is obtained by applying an interpolation method based on a tetrahedral interpolation method.

[Simple explanation of drawings]

[Figure 1]

It is explanatory drawing of the block diagram which shows the whole structure of a color printer printing apparatus.

[Figure 2]

It is explanatory drawing which extracted the part of the color space conversion and the color corrector in the block diagram which shows the whole structure of a color printer printing apparatus.

[Fig. 3]

It is explanatory drawing of the mapped state after color mixing.

[Explanation of symbols]

10 ... Scanner, 12 ... Original Image, 14 ... Scanner Corrector, 20 ... Color Space Converter and Color Corrector, 30 ... Printer, 40 ... Lookup Table and Interpolator, 60 ... Color calibration ROM, 65 ... Printer controller, 70 ... Densitometer, 80 ... Interpatch correlation processor, 100 ... Coefficient table, 101 ... Lookup table processor, 102 ... Lookup Table (LUT), 120 ... Multiplier, 130 ... Adder, 134 ... LUT_NEW, 140 ... Interpolator