JPH06225132A - Method and apparatus for color printing based on table of color out of color gamut - Google Patents

Abstract

PURPOSE: To make colors outside the color gamut of a printer highly smoothly printable by selecting a transition color outside the color gamut of a printer listed on a printer table from colors at a fixed angle from the colors on the edge of the color gamut. CONSTITUTION: A processing circuit 21 supplies a color picture to a printer driver 26 and the driver 26 generates a CMY value for each picture element of the picture based on a color value. A printer table 26a provides CMY values for the printable colors of a printer 40 and a boundary table 26b provide CMY values for the unprintable colors of the printer 40. A transition color is selected from colors at a fixed angle from colors on the edge of the color gamut of the printer 40 so that the colors on the table 26a may smoothly change on the outside of the color gamut of the printer 40. In addition, the lightness of colors are set so that the lightness may monotonously increase. Therefore, the colors on the outside of the color gamut of the printer 40 are printed in such a manner that the colors smoothly change and the lightness of the colors smoothly increase.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and method for forming and using a look-up table for determining a color so that a color printer prints according to a print request of a specific color. is there. Here, the specific color requested to be printed includes a color that cannot be printed by the printer. For these colors in particular, the colors in the look-up table are designed to change very smoothly, so that the desired slight change in color results in large non-contiguous jumps in the printed result. It doesn't appear. Furthermore, when the lightness of these colors changes monotonously, that is, when a shadow that makes the colors gradually brighter is printed, the actual printed color also gradually becomes brighter and becomes darker. There is no undesired color drop.

[0002]

2. Description of the Related Art In recent years, the usefulness of color monitors and color printers has increased, and it has become commonplace for computer users to view full-color images on a color monitor and instruct full-color printing by a color printer. However, the color images formed by the color printer and the color monitor are different. In particular, a color monitor is a light-emitting device, and color formation is generally red,
This is performed by additively mixing the lights of the three basic colors of green and blue. On the other hand, the printed image is simply a reflection of the ambient light, and the colors of the printed image perceived through the ambient light are generally the three basic colors of the subtractive methods of cyan, magenta, and yellow (sometimes including black). Affected by color.

[0003]

These color mixing methods are basically different, and as a result, the range of colors that can be displayed on a monitor is different from the range of colors that can be printed by a printer. FIG. 1 is a CIE1931 chromaticity diagram, which shows a range of colors that can be displayed on a monitor (ie, “color gamut”) (area A),
The range of colors that can be printed by the printer (that is, the "color gamut") (area B) is shown. As shown, the range of colors that a monitor can display is generally wider than the range of colors that a printer can print. This is because the monitor is a device that emits light and displays colors with a greater range of saturation. However, there is also a low saturation region such as the region 10 in which the printed image has a wider color range than the monitor although it is based on the subtractive color method.

Due to such a difference between the printable range and the displayable range, it was previously impossible to print a color image that would be perceived as a faithful color reproduction of the displayed color image. In particular, it has been completely impossible to print the color of a region such as the out-of-gamut region 11 located outside the printable color region B. Therefore, although these colors can be seen on the color monitor, they cannot be printed by the color printer.

In US Pat. No. 4,941,038, when correcting out-of-gamut colors to colors on the outer perimeter of the printer gamut,
A printable color that has the shortest vector distance to the out-of-gamut color and retains the out-of-gamut color and the hue is selected. However, because each out-of-gamut color was corrected separately, the out-of-gamut colors were printed with poor color smoothness, so a slight color difference in the requested color would make a big difference. It was printed. In particular, in some cases, poor color smoothness may appear as a non-monotonic change in luminance, or as a result, the lightness of the out-of-gamut color does not change smoothly and monotonically from dark to light, On the contrary, it sometimes went down from light to dark. This means that a color that should appear smoothly and monotonically from dark to light will appear as a dark part that is actually undesired.

FIG. 2 shows an example of a non-smooth color change in which a change in lightness appears non-monotonically. Figure 2 shows CIELAB
The cross-sectional view projected along the arbitrary hue planes on the a * and b * axes of the space and projected on the L * brightness axis is shown. In these two figures, 24 indicates the edge of the color gamut of the color printer. Edge 2
The colors inside 4 are printable, the colors outside them are out of gamut and therefore unprintable. 26 designates the boundary within which it is converted to the edge 24 of the printer gamut. Each row 27 shows a table giving CMY values to be printed in response to a command to print a color in the corresponding color space. For the purpose of illustration, Figure 2 shows
Only the luminance value L * when the CMY value is printed is shown.

It is usually considered that the luminance smoothly increases in parallel with the L * axis. Thus, row 2 parallel to the L * axis
For 8, the value of luminance L * smoothly increases from L * = 50 to 56. However, in some areas (eg, 29 columns) the luminance starts to rise from L * = 50 to 52, however, it falls to L * = 51 before rising to the final value of L * = 52. Thus, changes in brightness appear in the resulting printed color as unwanted dark dips and non-monotonicity.

It should be noted that the non-monotonic change in lightness is only one example of a non-smooth color change for colors outside the gamut.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a printer table to address the above problems, the colors of which change very smoothly in the out-of-gamut region, Colors outside the color gamut exhibit a monotonous increase in lightness. According to one aspect of the invention, the transition color is selected from the colors on the edges of the gamut of the printer, the selected color being at a constant angle from the transition color without taking into account changes in lightness. Is the color. Since each of the transition colors is a color at a certain angle from the edge of the printer color gamut, the state shown in FIG. 2 does not occur.
Therefore, the color outside the color gamut changes smoothly, and the lightness shows a monotonous increase with respect to the color outside the color gamut that increases in the brighter direction.

The above summary of the invention is provided for a quick understanding of the essence of the invention. The invention will be more fully understood by reference to the following detailed description of the preferred embodiments and the accompanying drawings, which form a part of this specification.

[0011]

3 is a block diagram showing a printing apparatus according to an embodiment of the present invention. As shown, the printing device includes a host CPU 20, a color monitor 30, and a color printer 4.
It consists of zero. The host CPU 20 includes a processing circuit 21 such as an 80286 microprocessor and a random access memory (“RA
M ”) 22, a read-only memory (“ ROM ”) 24 that is a static storage area of the processing circuit 21, a monitor driver 25, and a printer driver 26. The operator accesses the host CPU 20 via the keyboard 27. The keyboard 27 is connected to the processing circuit 21 by the interface 29. Using the keyboard, the operator executes the program instructions stored in the processing circuit 21, displays the color image on the monitor 30, and causes the color printer 40 to print the corresponding color image.

The host CPU 20 is also connected to other peripheral devices such as a disk drive, a tape drive, a color video interface, a color scanner interface, etc., but these devices are not shown here for the sake of simplicity of description. Such a device cooperates with stored program instructions executed by processing circuitry 21 to scan, for example, a color image for storage in RAM 22 or monitor 30.
, The color of the image is processed, and the image processed as a result is printed by the printer 40.

The processing circuit 21 forms a color image on the monitor 30 in accordance with the stored program instructions. The processing circuit 21 provides the color image to the monitor driver 25,
The monitor driver 25 uses R for each pixel of the monitor 30.
Generate a GB value. The RGB values are provided to the monitor 30 via the interface 31, and those values are displayed on the monitor 30.

Upon request, processing circuitry 21 also provides color images to printer driver 26 for printing by color printer 40. The printer driver 26 generates a CMY value for each pixel of the color image based on the color value from the processing circuit 21. The CMY value is determined according to the printer table 26a or the boundary table 26b. The printer table 26a is a table that provides the CMY values for all the printable colors to the printer 40. The boundary table 26b is a table that provides CMY values for colors that cannot be printed by the printer 40. still,
The printer table may also include CMY values for some non-printable colors to smooth the transition from printable to non-printable colors. Further, a black (hereinafter “K”) value may be included. The CMYK values are provided to the printer 40 via the interface 41 and stored in the bitmap memory 42 in the printer 40. The bitmap memory 42 may store the full bitmap image of the image to be printed, or may store the bitmap image of a certain area or part. When sufficient color data is stored in the bit map memory 42, the color printer head 44 reciprocates on the platen close to the recording paper. In this embodiment, the print head 44 has 32 inkjet nozzles arranged in 4 rows and 8 columns. The first row of nozzles all eject cyan ink drops. The second row nozzles all eject magenta ink drops, and the third row nozzles all eject yellow ink drops. The nozzles in the fourth row all discharge black ink droplets. These nozzles are independently controlled according to the color data in the bitmap memory 42 so that one row of print head 44 travels across the platen to print eight rows of pixels.

FIG. 4 is a flow chart for explaining the operation of the printer driver 26 selecting the CMYK values from the color data provided to the processing circuit 21. In step S401, the printer driver 26 obtains the RGB value at a certain position (x, y) in the bitmap 42. In step S402, the printer driver 26 sets R
From the GB value, a color coordinate value that does not depend on the device (hereinafter, referred to as “device independent”) is formed. Preferably, this device-independent color coordinate is C
IELAB color coordinates. This is because the CIELAB color space is perceptually uniform, and equally sized intervals in the CIELAB color space are consistent with equally sized changes in perceived color. Furthermore, CIEL
Since the AB color space can be viewed as columnar coordinates regarding hue and luminance, it is an intuitive color coordinate that makes it easy to define a color gamut map.

In step S403, the luminance coordinate is CIE.
Compressed in the extreme luminance parts on the L * axis of LAB space. The compression step S403 may be executed directly by mathematically operating the L * value from step S402, or indirectly by storing the corrected CMY value in the printer table or the boundary table. You may make it carry out. In some cases, which is preferable, in both cases, both the printer table and the boundary table store precompressed values. That is, in the printer table and the boundary table, for example, the value at the brightness L * = 99 is actually adjusted to correspond to the brightness L * = 94. Similarly, the value of the brightness L * = 7 actually corresponds to the brightness L * = 26. The values in the central part of the luminance range, eg L * = 38-90, remain uncorrected. As a result, luminance can be compressed without the need for direct compression by data manipulation.

The compression step S403 is an optional step. However, it is preferable to carry out this step because it guarantees that even a color with extreme brightness will be perceived to perceive changes in brightness. That is, since the monitor 30 displays a color by a light emitting body, it is possible to display a color having a higher luminance value than the printer 40, while the printer 40 can display a color.
This is because the maximum value of the luminance of is limited by the whiteness of the paper on which the color image is formed. Furthermore, the monitor 30 can completely turn off the light of the issuer, and thus can display colors having lower brightness values than those printed by the printer 40. This is because even black ink reflects some ambient light. Therefore, in order to reliably print a certain color, even if printing is performed with the highest and lowest luminance values, the luminance value determined in step S402 is used by the printer 40.
It is desirable to compress to a printable range.

In step S404, step S40
2. It is checked whether the L *, a *, b * coordinates generated in S403 are within the range covered by the printer table 26a. If the L *, a *, b * coordinates are within the range of the printer table 26a, the process proceeds to step S405, and the L *, a *, b * coordinate positions (the L *, a *, Since only the discrete value is stored in the b * coordinate position, the CMY value corresponding to the position closest to the L *, a *, b * is actually referred to (lookup). On the other hand, if the L *, a *, and b * coordinates are outside the range of the printer table 26a, the process proceeds to step S406, and the hue angle θ is obtained from the a * and b * values according to the following formula.

Θ = arctan (b * / a *) Then, step S of looking up the boundary table
Proceeding to step 407, the closest position C in the boundary table corresponding to the luminance L * and the hue angle θ obtained in step S306.
Look up the MY value. In either case, step S
Proceeding to 408, those CMY values are stored at the (x, y) location in the bitmap memory 42. C if necessary
The MY values may be modified prior to storage, eg, interpolation of the difference between the actual L *, a *, b * values stored in these tables and the desired values calculated as above. You may make it adjust by.

In step S409, the printer driver 26 determines whether the bitmap memory is completed. If the bitmap memory is not completed, the process returns to step S401 to obtain the next RGB value for the next position (x, y) in the bitmap memory. On the other hand, when the entire bitmap memory is completed, or in the bitmap memory, a sufficient area (head 44
When a band having a length of 8 rows corresponding to 8 rows of the inkjet nozzles) is completed, the process proceeds to step S410, and gamma correction is performed. The CM of the bitmap memory so that the brightness is evenly distributed by gamma correction.
The Y value is adjusted. In step S411, undercolor removal is performed to obtain a black value for position (x, y) in the bitmap memory. The under color removal in this embodiment is performed by a simple method of selecting the minimum value among the CMY values and assigning that value to the black value. After that, each of the CMY values is adjusted by subtracting the black value.

The order of steps S410 and S411 is not fixed, and the order may be changed to use a specific color printing technique such as continuous tone or dithering or error diffusion. In step S412, color printing is started using the CMY values obtained as a result of the above processing.

FIG. 5 is a flow chart for explaining a method of forming the printer table 26a and the boundary table 26b. The illustrated flow procedure may be performed once for each printer, or may be performed when readjustment becomes necessary. More preferably, the flow procedure of FIG. 5 is performed only once for a set of printers, such as printers of the same model number, and provided to the operator in the form of software as part of factory adjustment of the printer.

In step S501, the color gamut or range of colors printable by the printer 40 is measured. Preferably,
This is done by printing a very large subset or complete set of all colors printable by printer 40. For example, in the printer used in this embodiment, each of CMY and K values is 6 which is a numerical value of 0-64.
It is printed in 5 gradations. Thus, for example, 17 C
The values, ie the numbers 0, 4, 8, 12, ... 64, are printed,
Then, 17 M values and 17 Y values are printed in the same manner. Each possible 17 possible combinations of CMY values are printed, resulting in 17 × 17 × 17 = 4,913
You can make individual color patches.

In addition to the above hued colors, all possible gray values, in this case 48 achromatic values, are printed on the 17 already printed colors.
It will be appreciated that sampling the printer gamut as described above will print pure achromatic colors along with chromatic colors. Whatever the sampling method used, the properties of this purely achromatic print should be retained, since proper achromatic reproduction is a desirable property in color reproduction.

For each of the 4,913 color patches and the added 48 gray patches, the color is the above-mentioned CIE.
It is measured in a device-independent color space such as the LAB color space. Thus, at the end of step S501, L *, a *, for each of the 9,913 + 48 = 4,961 unique CMY color combinations.
The b * coordinate is measured, which defines the printer gamut.

In step S502, a mathematical smoothing function for converting CIELAB coordinates into CMY coordinates is obtained. In the present embodiment, a three-dimensional least squares fit from CIELAB space to CMY space was selected. That is, the coefficients C ₀ to C _19, the coefficients m ₀ to m ₁₉ , and the coefficients y ₀ to y ₁₉ are compared with the gamut measured in step S501 by using the known least square fitting technique. Obtained to give the best fit in the least squares sense.

C = c ₀ + c ₁ L * + c ₂ a * + c ₃ b * + c ₄ L * ² + c ₅ a * ² + c ₆ b * ² + c ₇ L * a * + c ₈ L * b * + c ₉ a * b * + c ₁₀ L * ³ + c ₁₁ a * ³ + c ₁₂ b * ³ + c ₁₃ L * ² a * + c ₁₄ L * a * ² + c ₁₅ L * ² b * + c ₁₆ L * b * ² + c ₁₇ a * ² b * + c ₁₈ a * b * ² + c ₁₉ L * a * b *… (1) M = m ₀ + m ₁ L * + m ₂ a * + m ₃ b * + m ₄ L * ² + m ₅ a * ² + m ₆ b * ² + m ₇ L * a * + m ₈ L * b * + m ₉ a * b * + m ₁₀ L * ³ + m ₁₁ a * ³ + m ₁₂ b * ³ + m ₁₃ L * ² a * + m ₁₄ L * a * ² + m ₁₅ L * ² b * + m ₁₆ L * b * ² + m ₁₇ a * ² b * + m ₁₈ a * b * ² + m ₁₉ L * a * b *… (2) Y = y ₀ + y ₁ L * + y ₂ a * + y ₃ b * + y ₄ L * ² + y ₅ a * ² + y ₆ b * ² + y ₇ L * a * + y ₈ L * b * + y ₉ a * b * + y ₁₀ L * ³ + y ₁₁ a * ³ + y ₁₂ b * ³ + y ₁₃ L * ² a * + y ₁₄ L * a * ² + y ₁₅ L * ² b * + y ₁₆ L * b * ² + y ₁₇ a * ² b * + y ₁₈ a * b * in _{^{2 + y 19 L * a *}} b * ... (3) step S502, to use some mathematical function that converts (fit) the measurement of step S401 from the device-independent color coordinate space to CMY coordinate space It may be. However, it is preferable that the mapping function includes a smoothing process to remove the measurement error that may have occurred in step S501.

Furthermore, it is desirable to weight some points measured in step S501 before performing the conversion in step S502. For example, reproduction of proper skin tone is an important characteristic of color printing. Therefore, in some cases, it is desirable to perform processing by weighting the color corresponding to the flesh color area more than other colors.

In step S503, the device-independent color space, that is, the CIELAB color space, is divided into sections of equal size. One such section includes the L * axis, such as centered around the L * axis. Such a partition provides a blank printer table. The size of the printer table preferably includes the printer gamut in addition to the typical color monitor gamut. For example, according to FIG. 1, the printer table preferably includes a color region, generally indicated at 12. The size of the printer table section must be as small as possible, with due consideration of the printer table storage limit. For example, it has been clarified that a fine gradation change in luminance is more important than a fine gradation change in hue or saturation. The brightness axis is ΔL
It is also determined that sufficient gradation change of luminance can be obtained by dividing into a section of * = 1 (the range of luminance L * is 0 to 100). On the other hand, such a minute gradation change is not usually required for the hue, and it has been known that the section of Δa * = Δb * = 3 gives an appropriate gradation change of the hue (a *, b *).
Is about -100 to +100 near the center of the luminance axis, that is, near L * = 50).

In addition to the above considerations, it should also be noted that the printer gamut is not the same for each luminance value. In particular, the color gamut is relatively small in the extreme brightness range and relatively large at the center of the brightness axis. 6 is CIEL
Figure 6 illustrates a typical division of AB space into printer tables. However, here, gradation changes for all luminances and hues are not shown for simplification of description. L
At relatively low luminance values such as * = 10, a relatively small rectangular grid of a *, b * axes is suitable for expanding the printer gamut. Similarly, for relatively high luminance values such as L * = 90, a relatively small rectangular grid of a *, b * axes is suitable for storing the printer gamut. But L *
For intermediate luminance values such as = 50, a relatively large rectangular grid of a * and b * axes is needed to expand the printer gamut.

Further, as shown in FIG. 6, the rectangular grid of each luminance level includes the L * axis (in FIG. 5, it is concentrated on the L * axis). That is, exactly a in the rectangular grid
There is a cell that matches * = b * = 0. Its center point,
That is, a * = b * = 0 corresponds to a pure achromatic color, and is appropriately reproduced as a pure achromatic color for proper color reproduction as described above.

In practice, the printer table preferably contains more colors than the printer gamut, and most preferably the colors found in a typical monitor gamut. As a result, the printer table includes a transition value that smoothly transitions the color of the edge portion of the printer gamut to the color of the border table and stores the color difference in the area outside the printer gamut.

In step S504, step S502
The C, M, and Y values are inserted around the L * axis of the printer table by using the conversion function obtained in. Corresponding to digital color printing as opposed to continuous tone printing, the C, M and Y values that are fractional values are rounded down or rounded down to integer values. The entire rectangular grid for each brightness level is not completely filled with numbers, only those known as within the printer gamut are filled with numbers. Furthermore, the cell just on the L * axis, that is, the point of a * = b * = 0, is also not converted by the conversion function. Rather, the CMY values for these points are inserted in step S505 by determining the achromatic color of the printer measured in step S501 using the corresponding L * value. This ensures that the hue value is not brought into a pure achromatic value by the smoothing process introduced by the conversion function.

In steps S506 and S507, the CMY values in the printer table are corrected for unprintable colors. The non-printable color is caused by the artifacts due to the conversion function selected in step S502. For example, in the conversion function used, the area 45 in FIG.
Inappropriate areas that do not exist in the printer color gamut occur in the printer table. Such defects are removed in step S506 by removing all regions that are not connected to the region around the L * axis.

Non-printable colors also result from the situation shown in FIG. In FIG. 8, reference numeral 47 is an arbitrary luminance value L
Represents the edge of the printer gamut for *. The printer gamut shown is not radially convex, since all radial lines from the L * axis do not intersect edge 47 at only one point. In particular, the radial line 48 has edges 47 and 49a, 49b,
They intersect at 3 points of 49c. The area between 49a and 49b forms a radial recess, which is a factor in generating an improper CMY value of the printer table. Therefore, in step S507, the values in the printer table are corrected so as to be radially convex.

FIG. 9 illustrates this process. FIG. 9 shows a rectangular grid in the a * and b * axes for an arbitrary luminance value L *. Cells 51-55 all contain printable values within the printer gamut. However, cell 59
Is the two cells (5
3, 56) to be radially concave. Therefore,
The table is made radially convex by assigning a certain CMY value to cell 59. The value is selected by retaining the hue of the color (angle θ in the figure) as much as possible and selecting the saturation value closest to the desired value. Thus
In FIG. 9, the value of cell 59 depends on one of cells 51 to 55 depending on the cells having closer hue and saturation values.
You can assign the value of one cell. In Figure 9, C
= 1, M = 18, Y = 14 values are selected.

In step S508, the transition color of each printer table is obtained. FIG. 10 shows how CMY values for transition colors are selected. In the figure, reference numeral 60 is the edge of the printer gamut, and 61 is the edge of the printer table gamut that substantially matches the edge of a typical color monitor, as described above. For each transition color in the region between the edges 60, 61, the color on the edge 60 of the printer gamut at a position at a constant angle α from the transition color is selected for the transition color. still,
Any change in brightness is acceptable, which means that the range of brightness changes is not limited to any threshold. For example, for the transition color 62, the point 64 on the edge 60 of the printer gamut is selected, which is the point 64.
Is located at an angle α from the point 62. Similarly,
For each point in the transition region, such as point 65, the edge 6 of the printer gamut at a position at a constant angle α, such as point 66.
The relevant point on 0 is selected. For transition colors that exceed the maximum saturation point 67 in the printer gamut, the angle α is downward. Conversely, for transition colors less than the maximum saturation point 67, such as color 69, the angle α is upward. Maximum saturation point 6
The maximum saturation point 67 is selected for the colors in the wedge region 70 that form 7 to 2α. In this way, all the colors in the wedge 70 are converted into the color 67.

In order to ensure that the transition color between the edges 60, 61 results in the smoothest color change, the saturation on the edge 60 of the printer gamut expands by a constant angle α. Before performing, it is desirable to confirm whether or not the minimum saturation monotonously increases to reach the maximum saturation point, and the minimum saturation monotonically decreases. If the change in saturation is not monotonic, the color saturation at the edges of the printer gamut is modified to eliminate the change prior to the development of a constant angle.

It has been found that when the angle α is set to 15 °, the saturation is sufficiently increased, and a satisfactory result is produced without causing an unreasonably large change in the brightness. Other values of α, such as 10 ° and 20 ° are also applicable. Step S5
In 09, the boundary table 26b is created. The printer table 26a is formed as a rectangular grid of a * and b * axes for each brightness value, while the boundary table is formed as one wheel for each brightness value of the printer table. Thus, as shown in FIG. 11, one wheel-shaped boundary table is provided for each of the brightness values in which the printer table exists, and one boundary table corresponds to each printer table. The boundary table has a plurality of cells, which are accessed as a function of the a *, b * coordinates with the hue angle θ calculated as follows.

Θ = arctan (b * / a *) FIG. 12 shows the correspondence between the boundary table and the printer table. The printer table 26a is a rectangular grid table for an arbitrary luminance value L *, while the boundary table 26b is a wheel table centered on a * = b * = 0. Individual cells of the boundary table are accessed at an angle θ in the a *, b * axes that corresponds to the hue.
Experimentally, it has been found that a one-time increase, which corresponds to 360 cells in each boundary table, gives an appropriate hue gradation change. However, this can be modified as described below with reference to FIG.

In step S510, the CMY values for each cell in the border table are selected in a manner similar to selecting CMY values for transition colors in the printer table. In this way, the border table color is selected from the edge colors of the printer gamut at a certain angle α (α same as that used in the printer table) from the border table. As mentioned above, the angle α should be taken upwards or downwards depending on whether the boundary table color is above or below the maximum printer saturation point, and the maximum when the boundary color table is in the wedge 70. Limited to printer saturation points.

In step S510, the boundary table value is examined to confirm that the saturation of the color of the boundary table changes smoothly. This situation is explained in FIG. 13 which shows the printer gamut 70 at arbitrary luminance values L * on the a * axis and the b * axis. As described above, colors located outside the printer gamut, such as color 71, are converted to printer gamut boundary colors 72 while preserving the hue angle. In particular, the area 74
In such a region where the printable saturation changes rapidly, a slight change in hue causes a rapid change in the saturation of the boundary table. For example, when the hue angle changes from θ ₁ to θ ₂ , even a small change in hue causes a relatively large change in saturation. Such a large change in saturation looks unnatural when printed.

In order to avoid such an unnatural printing result, the size of the boundary table is increased until a fine enough increase in hue appears to ensure a smooth change in saturation. When the boundary table is incremented, the calculation of step S509 is repeated to fill the CMY values of the new boundary table. In step S511, the CMY values in the printer table are checked and corrected to ensure that CMY is smoothly mixed with achromatic colors (L * axis). Specifically, at discrete brightness levels, the printer table colors are redetermined so that the printer table colors near the L * axis are smoothly mixed in achromatic colors.

In step S512, the printer table is completed into a rectangle. More specifically, up to this step, the CMY value is the printer gamut 6 of the printer table.
It was inserted only in the area within 0 (steps S504 and S505) and the transition area 61 between the printer gamut and the boundary table (step S509). Step S50
In 2, the hue angle is calculated for the remaining cells of the printer table such as 69 of FIG. 12, and the color of the boundary table of the hue angle as indicated by 68 of FIG. 12 is inserted.

In step S513, the hue angles of the printer table and the boundary table are warped to compensate for the Abney effect. Specifically, the CMY values for the colors outside the color gamut (the values are stored in the printer table and the boundary table at this point) are calculated in step S508.
As alluded to above, are all based on a constant angular expansion. However, for highly saturated colors, the development of returning to the edges of the printer gamut while holding a constant angle causes a change in the perceived hue due to the Abney effect. For example, as discussed above, the development of a constant angle causes a highly saturated (but unprintable) purple blue to be converted to a less saturated purple at the edge of the printer gamut. To compensate for this effect, the hue angles of the printer and boundary tables are warped (distorted). More specifically, for both the printer table and the border table, the CMY values for one hue angle are transferred to another different hue angle to preserve the perceived hue of the print color. Figures 14 and 15 show θ =
This warp is shown for a blue / purple color space region at a hue angle θ between 255 ° and θ = 333 °.

FIG. 14 shows the printer table 80 before warping.
And the printer table 81 after warping. These printer tables in Fig. 14 are a * and b *.
For any printer table on the surface and for any luminance value L *. Obviously, the warp shown in FIG. 14 is performed for each of the printer tables for the L * value selected in step S503. As shown in FIG. 14, the blue area 8
2a has been expanded into the corresponding warped blue region 82b. This ensures that the CMY values printed for an instruction to print a highly saturated blue color outside the gamut will be a hue of a blue hue rather than a purple blue. For example, a highly saturated out-of-gamut blue color 84, when printed according to the non-warped table 80, becomes a purplish blue color,
When printed according to the warped table 81, it becomes blue.

Due to the additional hue angle warp, the area 85
The CMY value of a is converted (mapped) into the area 85b, and
The area 86a is converted to the area 86b. The details of the conversion are as follows. Hue angle 255 ° to 305 ° : Warp angle [255 + ang] = 255 + 0.5 * an
g However, 0 <ang <50 °. Hue angle 305 ° to 309 ° : Warp angle [305 + ang] = 280 + 1.25 * a
However, 0 <ang <4 °. Hue angle 309 ° to 333 ° : Warp angle [309 + ang] = 285 + 2 * ang However, 0 <ang <24 °. Thus, the unwarped region 255 ° to 280 ° is the region 25.
Warped by expanding from 5 ° to 305 °,
The unwarped region 280 ° to 285 ° is the region 305 ° to 30
Warped by compressing to 9 °. Further, the unwarped regions 285 ° to 333 ° are regions 309 ° to 333 °.
Warped by compressing to. These warped regions remain continuous but have the same end points (here 255 ° and 333 °) as the unwarped regions.

The same warp is performed for red and cyan. The warping process for red is as follows. Hue angle 10 ° to 40 ° : Warp angle [10 + ang] = 10 + 0.5 * ang, where 0 <ang <30 °. Hue angle 40 ° to 53 ° : Warp angle [40 + ang] = 25 + 1.25 * ang, where 0 <ang <12 °. Hue angle 52 ° to 76 ° : Warp angle [52 + ang] = 40 + 1.5 * ang, where 0 <ang <24 °. Thus, the unwarped region 10 ° to 25 ° is the region 10 °
Warped by expanding to ~ 40 ° and unwarped regions 25 ° -40 ° are warped by compressing to regions 40 ° -52 °. Also, unwarped area 40 °
~ 76 ° is warped by compressing into the region 52 ° -76 °.

The warping process for cyan is as follows. Hue angle 170 ° to 195 ° : Warp angle [170 + ang] = 170 + 2.0 * an
g However, 0 <ang <25 °. Hue angle 195 ° to 245 ° : Warp angle [195 + ang] = 220 + 0.5 * an
g However, 0 <ang <50 °. Thus, the unwarped region 170 ° -220 ° is region 1
Warped by compressing from 70 ° to 195 °,
The unwarped region 220 ° to 245 ° is the region 195 ° to 24
Warped by expanding to 5 °.

The same warp processing that was performed on the printer table is also performed on the boundary table, as shown in FIG. Similar to the above, this process causes a blue print instruction such as the color 87 outside the color gamut to be printed as a purple-blue hue when printing according to the unwarped boundary table. However, the warped boundary is printed. When printed according to the table, the color of blue hue is surely obtained.

By the above warp, both the color within the color gamut and the color outside the color gamut are warped, so that the smoothness can be maintained in the colors in the printer table and the boundary table. Although it is possible to warp only the out-of-gamut colors, this causes poor color continuity at the edges of the printer gamut.
Furthermore, even if the colors are displaced due to warping of the color within the color gamut, the lines of the hue angle become closer to each other for the color within the color gamut, and far from each other for the color outside the color gamut of high saturation, It has been experimentally found that the amount of deviation does not matter much.

In the warping technique described above, all colors at the same hue angle are warped equally regardless of saturation, but it is possible to introduce a factor based on saturation into the warp. In that case, colors with higher saturation are warped to colors with lower saturation. In step S514, the colors in the yellow area of the printer table and the boundary table are corrected so that the yellow area expands. In detail,
As shown in FIG. 2, the pure yellow color of the printer falls into a very narrow printer color gamut, which is difficult for the user to find (because the yellow color range of the monitor is wider). Since the printable range for pure yellow is so narrow, users often choose greenish yellow over the desired pure yellow. Therefore,
In step S514, the yellow area is enlarged. The expansion of the yellow region can be performed by the following warp of the hue angle. Hue angle 87 ° to 91 ° : Warp angle [87 + ang] = 87 + 1.25 * ang where 0 <ang <4 °. Hue angle 91 ° to 97 ° : Warp angle [91 + ang] = 92, where 0 <ang <6 °. Hue angle 97 ° to 112 ° : Warp angle [97 + ang] = 92 + 0.5 * ang, where 0 <ang <15 °. Hue angle 112 ° to 132 ° : Warp angle [112 + ang] = 99.5 + 1.25 *
However, 0 <ang <20 °. Hue angle 132 ° to 147 ° : Warp angle [112 + ang] = 124.5 + 1.5 *
However, 0 <ang <15 °. A computer program has been developed that automatically performs steps S501 to S514 described above and will be submitted in the form of a microfiche in the Appendix.

[0053]

As described above, according to the printing method and apparatus of the present invention, in the color printing, it is possible to print the color outside the color gamut with high smoothness.

[Brief description of drawings]

FIG. 1 is a chromaticity diagram showing a relationship between a color gamut printable by a printer and a color gamut displayable on a monitor.

FIG. 2 is a diagram showing a printer table in a sectional view of a CIELAB space.

FIG. 3 is a block diagram showing a printing apparatus according to this embodiment.

4 is a flowchart illustrating selection of CMYK values of a color printer by a printer driver of the apparatus shown in FIG.

FIG. 5a is a flowchart illustrating a method of forming a printer table and a boundary table.

FIG. 5b is a flowchart illustrating a method of forming a printer table and a boundary table.

FIG. 6 is a diagram showing a typical division from the CIELAB color space into a printer table.

FIG. 7 is a diagram showing how an unconnected area is removed from a printer table.

FIG. 8 is a diagram showing radially concave regions in the printer color gamut.

FIG. 9 is a diagram showing a manner in which a concave area is removed by forming a printer table in a radially convex shape.

FIG. 10 is a diagram showing how CMY values are selected for each cell in the transition area of the printer table.

FIG. 11 is a diagram showing an arrangement of a boundary table.

FIG. 12 is a diagram showing a relationship between a printer table and a boundary table in the CIELAB color space with respect to an arbitrary luminance value L *.

FIG. 13 is a diagram showing a printer gamut of an arbitrary luminance value L * on a * and b * axes.

FIG. 14 is a diagram showing how the hue angle is warped with respect to the printer table.

FIG. 15 is a diagram showing how the hue angle is warped with respect to the boundary table.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI Technical display location G06F 15/66 N 8420-5L 310 8420-5L 15/68 310 9191-5L A 9191-5L 15 / 72 310 9192-5L

Claims (22)

[Claims] 1. A color printer lookup table that provides basic color values corresponding to colors in a device-independent color space, the basic color values being smooth for colors outside the color gamut of the color printer. A color printer look-up table having a color value that changes to. 2. The color according to claim 1, wherein the color out of the color gamut of the color printer has a lightness that monotonically increases from the darkest value to the brightest value in the printer table. Printer lookup table. 3. The color printer lookup table according to claim 1, wherein the basic color value includes a basic color value for a color within the color printer gamut and a color outside the color gamut. 4. The color printer lookup table of claim 3, wherein the basic color values include basic color values for colors within a color gamut of a color monitor. 5. The basic color value of a color inside the color printer gamut is an associated color value.
The color printer look-up table according to claim 3, wherein the color printer look-up table is set so as to correspond exactly to (s). 6. The basic color value for a color outside the color gamut of the color printer is set so as to increase the saturation and change the lightness while retaining the hue. 5. The color printer lookup table described in 5. 7. A driver for a color printer, the printer table providing basic color values corresponding to colors within the color printer gamut and basic color values for colors outside the color printer gamut, the printer comprising: A printer table configured to smoothly change the basic color value of a color outside the color gamut, a means for receiving a print command of a designated color, and a basic color value corresponding to the designated color from the printer table are stored. A color printer driver including a control unit for extracting. 8. The color according to claim 7, wherein the color outside the color gamut of the color printer has a lightness that monotonically increases from the darkest value to the brightest value in the printer table. Printer driver. 9. The color printer driver according to claim 7, wherein the printer table includes basic color values for colors within a color gamut of a color monitor. 10. The basic color values in the printer table for colors inside the color printer gamut are:
The color printer driver according to claim 7, wherein the color printer driver is set so as to accurately correspond to the system color. 11. The basic color values in the printer table for colors outside the color printer gamut are set to retain hue but increase saturation and change lightness. The color printer driver according to claim 7, wherein the color printer driver is a color printer driver. 12. A boundary table for providing basic color values to colors outside the printer table is further provided, and the control means selects one of the printer table and the boundary table based on an instruction color. The color printer driver according to claim 7, wherein basic color values are extracted from a selected one of the printer table or the boundary table. 13. The color printer driver according to claim 12, wherein the boundary table includes wheel-shaped cells centered on a lightness axis. 14. The basic color values in the boundary table for colors outside the printer gamut are set to retain hue but increase saturation and change lightness. The color printer driver according to claim 12, wherein the color printer driver is a color printer driver. 15. A method of forming a printer table, the method comprising: determining an edge of a color printer gamut; converting a basic color value for a color within the edge of the color printer gamut into the printer table. Extending a transition color between the edge of the color printer gamut and the edge of the printer table by an angle to the edge of the printer gamut edge to increase color saturation. And a step of converting into a printer table. 16. All points in the wedge area extending by the certain angle from the maximum saturation point at the edge of the printer gamut are defined by:
The image is mapped onto the maximum saturation point.
5. A method of forming a printer table according to item 5. 17. The method of forming a printer table according to claim 15, wherein the constant angle is approximately 15 degrees. 18. The printer table forming method according to claim 15, wherein the basic color value of the transition color in the printer table is set for a color in a color gamut of a color monitor. 19. The method of forming a printer table according to claim 15, wherein the basic color values of the colors within the color printer color gamut are set so as to correspond exactly to their system colors. 20. The basic color values for colors outside the printer gamut retain hue but increase saturation,
20. The printer table forming method according to claim 19, wherein the setting is made so as to change the lightness. 21. The printer table forming method according to claim 15, further comprising the step of creating a boundary table. 22. The basic color value for a color in the boundary table outside the printer color gamut is set to retain hue but increase saturation and change lightness. The method for forming a printer table according to claim 21, wherein.