JP5429457B2 - Color conversion apparatus and color conversion program - Google Patents

Description

  The present invention relates to a color conversion device and a color conversion program.

  There is a technique for simulating color reproduction of a color output device (target output device) that varies depending on a color output device including black, such as CMYK. This technique is called ink simulation, and is used for applications such as simulating color reproduction of a printing press with a printer to realize proofing at low cost. In addition, the color reproduction texture, sharpness, and the like are held by a technique called K plate holding that holds the K (black) plate of the printing image on the printer side.

  However, since the color reproduction characteristics of the target output device and the actual output device (actual output device) are generally different, the black plate in the target output device is used as it is in the actual output device. When trying to simulate color reproduction, the output is in a color reproduction range narrower than the color reproduction range in the actual output device. In addition, xerographic printers and inkjet printers used for simulation often have a limit on the total amount of color material used, and this total color material amount limit causes a difference in color reproduction characteristics from the target output device. Make it bigger.

  As described above, a technique for determining an appropriate black amount in the output device is indispensable in order to realize a highly accurate ink simulation while maintaining the black plate of the target output device. As one of such techniques, there is a method described in Patent Document 1.

  The method described in Patent Document 1 uses a color signal (hereinafter referred to as a target device color signal) in a device-dependent color space of a target output device as a target, and a color signal (hereinafter referred to as a conversion target) in a color space independent of the device. After conversion to an independent color signal, a minimum required black amount that satisfies the total color material amount limit in the output device that actually outputs the conversion-target independent color signal is calculated. At this time, a model that calculates the minimum required black amount from the given conversion target independent color signal is constructed by calculating a pair of the color signal independent of the device and the minimum required black amount. By using this model, the minimum required black amount is calculated. Then, an appropriate black amount is determined according to the saturation from the minimum required black amount and the black amount of the target target device color signal. Further, the color component of the output device color signal other than black is calculated from the conversion-target independent color signal to be converted and the calculated appropriate black amount, and the output device color signal is set together with the appropriate black amount.

JP 2007-43250 A

  The present invention provides a color conversion device and a color conversion program for reproducing a color output by a target output device on an actual output device in which the color material total amount limit is set by controlling graininess and gradation. It is for the purpose.

  The invention according to claim 1 of the present application uses the limit value of the total color material amount set in the actual output device that outputs an image using M (M ≧ 4) color material including black as it is, and Control for controlling the appropriate total amount limit value within the range of the case where the value obtained by subtracting the upper limit value of black from the limit value of the total amount of color material set in the actual output device is used as the limit value of the total amount of color material other than black Based on the control coefficient setting means for setting the coefficient, the control coefficient set by the control coefficient setting means and the limit value of the total amount of color material set in the actual output device, an appropriate total amount limit value is set for each black amount. A lower outer surface color signal satisfying the appropriate total amount limit value is calculated and generated from the independent color signal to be converted in the device independent color space based on the independent color signal in the device independent color space corresponding to the lower outer surface color signal. Having an appropriate black amount determining means for determining an appropriate black amount A color conversion device according to symptoms.

  The invention according to claim 2 of the present application is such that the appropriate black amount determination means in the invention according to claim 1 determines the appropriate black amount from the independent color signal and the black amount of the lower outer surface to which the independent color signal belongs. A color conversion apparatus is characterized in that a model for calculating the color is generated, and the appropriate black amount is calculated from the independent color signal to be converted using the model.

  The invention according to claim 3 of the present application is N-dimensional in a target output device that uses N (N ≧ 4) color materials including black in addition to the configuration of the invention of claim 1 or claim 2 of the present application. Forward conversion means for converting the target device color signal in the color space into the conversion-independent color signal in the device independent color space, the appropriate black amount determined by the appropriate black amount determination means, and the black amount of the target device color signal The optimum black amount calculating means for calculating the optimum black amount from the above, the optimum black amount calculated by the optimal black amount calculating means and the color material color used by the actual output device from the conversion target independent color signal as components. A color having an inverse conversion unit that calculates a color component other than black in the three-dimensional output color signal, and using the color component calculated by the reverse conversion unit and the optimum black amount as an output color signal It is a conversion device.

  According to the fourth aspect of the present invention, an independent color signal to be converted from a target device color signal in an N-dimensional color space to a device independent color space in a target output device using N (N ≧ 4) color materials including black. A forward conversion means for converting to an appropriate black quantity determination means for determining an appropriate black quantity from the conversion-target independent color signal converted by the forward conversion means; an appropriate black quantity determined by the appropriate black quantity determination means; and An optimum black amount calculating means for calculating an optimum black amount from the black amount of the target device color signal, and an actual output device for actually outputting the optimum black amount calculated by the optimum black amount calculating means and the conversion target independent color signal And a reverse conversion means for calculating a color component other than black in the M-dimensional output color signal whose component is the color of M (M ≧ 4) color material including black Output color signal based on the color component calculated in step 1 and the optimum black amount, and the appropriate color The amount determining means determines an appropriate black amount from the conversion target independent color signal using a model for calculating the appropriate black amount, and the model is a color material set in the actual output device. When the total amount limit value is used as it is, and when the value obtained by subtracting the black upper limit value from the color material total amount limit value set in the actual output device is used as the limit value of the total color material amount other than black Calculate the appropriate total amount limit value for each black amount based on the control coefficient that controls the appropriate total amount limit value in the range and the color material total amount limit value set in the actual output device, and calculate the appropriate total amount limit value A lower outer surface color signal that satisfies the following condition, an independent color signal in the corresponding device independent color space is calculated from the lower outer surface color signal, and the lower outer surface black ink to which the independent color signal belongs. Characterized by being created from the quantity and That is a color conversion apparatus.

  In the invention according to claim 5 of the present application, the appropriate total amount limit value for each black amount in the invention according to any one of claims 1 to 4 is obtained by subtracting the black amount from the upper limit value of the black amount. The color conversion device is characterized in that a value obtained by reducing the calculated value according to the control coefficient is subtracted from the limit value of the total amount of color material used in the actual output device.

  In the invention according to claim 6 of the present application, the appropriate total amount limit value for each black amount in the invention according to claim 5 is further reduced as the black amount decreases to a minimum value when the black amount is an upper limit value. The color conversion apparatus is characterized in that a value that is monotonically increased and the maximum value when the black amount is 0% is further subtracted.

  In the invention described in claim 7 of the present application, the appropriate total amount limit value in the invention described in claim 5 of the present invention further presets a linear degree of increase in the black amount from the start point toward the upper limit value of the black amount. A value obtained by subtracting a value obtained by subtracting the changed black amount from the upper limit value of the black amount according to the control coefficient, and subtracting from a limit value of the total color material amount used in the actual output device. Color conversion device.

  In the invention according to claim 8 of the present application, the appropriate total amount limit value in the invention according to any one of claims 1 to 7 is an apparatus independent color space indicating an achromatic color axis in the actual output device. A value based on the control coefficient for the achromatic color reference line and the limit value for the total amount of color material used in the actual output device is calculated for each black amount, and the appropriate total amount limit value for each black amount on the achromatic color reference line As a reference, a value for each black amount is calculated for colors other than the achromatic reference line so as to approach the limit value of the total amount of color material used in the actual output device as the outline of the color gamut in the corresponding black amount approaches The color conversion device characterized by the above.

  In the invention according to claim 9 of the present application, the appropriate total amount limit value in the invention of claim 8 is the color used in the actual output device as a value when the black amount of the achromatic color reference line is the maximum value. The color conversion device is characterized in that a value smaller than the limit value of the total material amount is set.

  The invention described in claim 10 of the present application is a color conversion program for causing a computer to execute the function of the color conversion apparatus described in any one of claims 1 to 9.

  According to the first aspect of the present invention, the converted image using the limit value of the total amount of color material used in the actual output device and the limit value of the total amount of color material used in the actual output device, and the converted image using the limit value of the total amount of color material used in the actual output device. Appropriate black amount that gives a converted image with controlled graininess and gradation within the range of the converted image that emphasizes gradation by using the value obtained by subtracting the upper limit of the value as the limit value of the total amount of color material other than black There is an effect that can be calculated.

  According to the second aspect of the present invention, the appropriate black amount in which the graininess and the gradation are controlled is calculated from the given conversion target independent color signal with higher accuracy than in the case where the present configuration is not provided. be able to.

  According to the third aspect of the present invention, it is possible to obtain an output color signal from which a converted image with controlled graininess and gradation can be obtained.

  According to the invention described in claim 4 of the present application, from the converted image using the limit value of the total amount of color material used in the actual output device and the limit value of the total amount of color material used in the actual output device, the black ink is obtained. An output color signal that provides a converted image with controlled graininess and gradation within the range of the converted image that emphasizes gradation by using the value obtained by subtracting the upper limit of the value as the limit value of the total amount of color material other than black Can be obtained with a simple configuration as compared with the case without this configuration.

  According to the fifth aspect of the present invention, the graininess and gradation can be controlled by controlling the appropriate total amount limit value.

  According to the sixth aspect of the present invention, the gradation can be further improved as compared with the fifth aspect of the present invention.

  According to the invention described in claim 7 of the present application, the graininess can be further improved as compared with the invention described in claim 5 of the present application.

  According to the eighth aspect of the present invention, it is possible to effectively use the color gamut together with the control of the graininess and gradation in the achromatic reference line.

  According to the invention described in claim 9 of the present application, it is possible to suppress the occurrence of glossiness at least in an achromatic color as compared with the case where the present configuration is not provided.

  According to the invention of claim 10 of the present application, the effect of the invention of any one of claims 1 to 9 can be obtained.

It is a block diagram which shows one Embodiment of this invention. It is a block diagram which shows an example of the appropriate black amount determination part. It is explanatory drawing of an example of the color gamut outline and lower side outline of a CMYK output device when the color material total amount limitation value in the apparatus independent color space is uniformly imposed. It is a graph which shows an example of the relationship between the gradation of the process black of a target output device, and the coverage of each color component in an actual output device when the color material total amount limit value is uniformly imposed. It is explanatory drawing of an example of the appropriate total amount limiting value according to the amount of black, and a lower outer surface. It is a graph which shows an example of the relationship between the gradation of the process black of a target output device, and the coverage of each color component in the actual output device when the appropriate black amount limit value corresponding to the black amount is set. It is a graph which shows an example of the relationship between the gradation of the process black of a target output device, and the coverage of each color component in the actual output device at the time of controlling the starting point of black. It is a graph which shows another example of the relationship between the gradation of the process black of a target output device, and the coverage of each color component in the actual output device at the time of controlling the starting point of black. It is a graph which shows an example of the relationship between the gradation of the process black of a target output device, and the coverage of each color component in the actual output device at the time of using a gamma coefficient. It is a graph which shows an example of the relationship of the black amount in the actual output device at the time of changing the gradation of the process black of the target output device and each coefficient. FIG. 3 is an explanatory diagram of an example of a computer program, a storage medium storing the computer program, and a computer when the function described in the embodiment of the present invention is realized by a computer program.

  First, in order to facilitate understanding of the embodiment of the present invention, a technique to which the embodiment of the present invention is applied will be described. In order to perform the above-described K (black) plate storage, it is necessary to calculate a minimum black amount necessary for colors reproduced by an output device (actual output device) that actually outputs. Preserving the black amount below the minimum required black amount and performing color reproduction may cause image deterioration and device failure. Of course, the minimum black amount required for various color conversions is calculated in addition to storing the K version.

  As a method of calculating the minimum required black amount from the color signal of the device independent color space, the device independent operation is performed using a pair of the minimum required black amount corresponding to the color signal and the color signal of several device independent color spaces. There is a method of constructing a model that calculates the minimum required black amount from the color signal in the color space, and using this model to calculate the minimum required black amount from the color signal in the given device independent color space . Here, the pair of the color signal in the device independent color space and the minimum required black amount is an output color signal whose upper limit is any color material other than black, or the total amount of color material in the actual output device is limited It is created from the color signal of the device independent color space corresponding to the output color signal equal to the value (hereinafter referred to as the total color material amount limit value) and the black amount corresponding to the color signal of the device independent color space. The color signal of the device-independent color space that created this pair is an outline composed of lightness colors below the lightness of the maximum saturation color in each hue, among the outlines of the color gamut when black is fixed, Hereinafter, this outline is referred to as a lower outline.

  FIG. 3 is an explanatory diagram of an example of the color gamut outline and the lower outline of the CMYK output apparatus when the total color material amount limit value in the apparatus independent color space is uniformly imposed. In FIG. 3, the color gamut outline when the black amount is 0% and the lower outline when the black amount is 22%, 40%, 61%, and 80% are indicated by solid lines. In addition, an achromatic color reference line assuming a color signal in the device independent color space corresponding to the gradation of process black in the target output device is represented by a broken line. The black circle on the broken line is the intersection with the lower outer surface for each black amount. The example of the CMYK value in the black circle color is an example when the total color material amount limit value is 160%.

  FIG. 4 is a graph showing an example of the relationship between the process black gradation of the target output device and the coverage of each color component in the actual output device when the total color material amount limit value is uniformly imposed. As an example, when a color signal indicating process black that does not use black is given in the target output device, the color is on the achromatic reference line indicated by a broken line in FIG. As an example, the color is changed from the high brightness side to the low brightness side along the achromatic color reference line. Then, since the black amount is stored, C, M, and Y increase monotonously in the color gamut of K = 0%, but if the total of CMY exceeds the color material total amount limit value, the black amount must be increased. Accordingly, CMY starts to decrease accordingly. FIG. 4 shows an outline of such behavior of CMY and K (black amount). The decrease in CMY is also indicated by the value of CMY at the black dots shown in FIG. FIG. 4 shows CMY values and K values when a color signal obtained by converting the process black gradation of the target output device into the device independent color space is reproduced by the actual output device. In other words, the process black gradation of the target output device does not match the process black gradation of the actual output device, and C, M, and Y of the actual output device are not necessarily equal. In FIG. 4 and the following graphs, for convenience of explanation, it is assumed that C, M, and Y of the color signal of the actual output device that reproduces the black circle are equal.

  As described above, when the total color material amount limit value set in the actual output device is exceeded, the tendency of increase in CMY is reversed and decreased. The gradation is deteriorated by such a change in increase and decrease. For this reason, there is a method of setting a total amount limit value (appropriate total amount limit value) according to, for example, the black amount so that CMY does not start to decrease. Here, an example is shown in which a total amount limit value (appropriate total amount limit value) is set according to the black amount on the achromatic color reference line.

  FIG. 5 is an explanatory diagram of an example of the appropriate total amount limit value and the lower outer surface according to the black amount. In FIG. 5 as well, the color gamut outline when the black amount is 0% and the lower outline when the black amount is 22%, 40%, 61%, and 80% are indicated by solid lines. In addition, an achromatic color reference line assuming a color signal in the device independent color space corresponding to the gradation of process black in the target output device is represented by a broken line. The black circle on the broken line is the intersection with the lower outer surface for each black amount. In this example, the total color material amount limit value is set to 160%.

  In the example shown in FIG. 5, the appropriate total amount limit value GCT on the achromatic color reference line is GL− (Kmax−K). Here, GL is the total color material amount limit value set in the actual output device, and Kmax is the upper limit value of black. Here, Kmax = 80%. In addition, the appropriate total amount limit value at the boundary portion (shown by a black square in FIG. 5) that is output with the uniform color material total amount limit value set in the actual output device on the lower outer surface is GL ( The limit value of the total amount of color material set in the actual output device). Then, the appropriate total amount limit value is determined so as to change monotonously for the colors from the black circle to the black square.

  FIG. 6 is a graph showing an example of the relationship between the process black gradation of the target output device and the coverage of each color component in the actual output device when an appropriate black amount limit value corresponding to the black amount is set. In the graph shown in FIG. 6, none of CMYK starts to decrease, and the deterioration of gradation is improved. On the other hand, compared to the case of FIG. 4, the starting point at which black is used moves to a side where coverage is small, that is, lightness is high. For this reason, even if the color is lighter than the starting point of black in FIG. 4, black that is conspicuous is used, and the graininess deteriorates.

  As described above, when the total color material amount limit value set in the actual output device is used as it is, the gradation is deteriorated as compared with the case where the appropriate total amount limit value in which the total color material amount is limited is used. Excellent in terms of sex. Conversely, when the appropriate total amount limit value is used, the gradation is superior to the case where the color material total amount limit value is used as it is, but the graininess is deteriorated. In the embodiment of the present invention described below, a configuration is shown in which both the graininess and the gradation that are contradictory as described above are compatible or controlled according to the purpose and preference of the user.

  FIG. 1 is a configuration diagram showing an embodiment of the present invention. In the figure, 11 is a forward conversion unit, 12 is a color reproduction area mapping unit, 13 is an appropriate black amount determination unit, 14 is an optimal black amount calculation unit, 15 is an inverse conversion unit, and 16 is a coefficient setting unit. In the following description, the target output device is a CMYK printer, the actual output device is a CMYK printer, and the target device color signal (CMYK color signal) is output color signal (C′M′Y′K ′ color signal). It is assumed that it is converted to Further, unless restricted by the appropriate total amount limit value, the black value of the target device color signal (hereinafter referred to as black amount) is held and the target device color signal is converted into an output color signal. Note that the CIELAB color space is used here as the device-independent color space used during the processing. Not limited to this example, the color of the color material used by the target output device and the color component of the target device color signal may be N (N ≧ 4) including black, and the color of the color material used by the actual output device The color component of the output color signal may be M (M ≧ 4) colors including black. The device independent color space may also be another color space that does not depend on the device, such as CIEUV.

The forward conversion unit 11 converts the target device color signal into a color signal in the device independent color space. Here, the CMYK color signal is converted into an L ^* a ^* b ^* color signal. This conversion method may be performed by conversion using a model such as matrix conversion or a neural network. Further, for example, a regression model described in JP-A-10-262157 may be used.

The color gamut mapping unit 12 converts the color signal (L ^* a ^* b ^* color signal) of the device independent color space converted by the forward conversion unit 11 into the color signal (Color Gamut) of the actual output device (color gamut) L ^* 'a ^* ' b ^* 'color signal). As this mapping method, various known methods may be used. The mapped L ^* 'a ^* ' b ^* 'color signal is assumed as an independent color signal to be converted here. If the color signal in the device independent color space converted by the forward conversion unit 11 is a color signal in the color gamut of the actual output device, the color gamut mapping unit 12 need not be provided. The color signal in the device independent color space converted by the forward conversion unit 11 is set as a conversion target independent color signal.

The appropriate black amount determining unit 13 calculates an appropriate total amount limit value for each black amount based on the control coefficient set by the coefficient setting unit 16 and the color material total amount limit value in the actual output device, and for each black amount. Based on the appropriate total amount limit value, the appropriate black amount (Kopt) is determined from the independent color signal to be converted (L ^* 'a ^* ' b ^* 'color signal) in the device independent color space. Details of the appropriate black amount determination unit 13 will be described later.

  The optimal black amount calculation unit 14 calculates the optimal black amount (K ′) from the black amount (K) of the appropriate black amount (Kopt) determined by the appropriate black amount determination unit 13 and the target device color signal (CMYK color signal). Is calculated. For example, from the appropriate black amount (Kopt) and the black amount (K) of the target device color signal, the degree of black plate holding is controlled according to the saturation and the optimal black amount (K ′) is calculated. Good. For example, the technique described in Japanese Patent Application Laid-Open No. 2004-112269 may be applied to the optimum black amount calculation unit 14. The optimum black amount calculating unit 14 may be omitted, and in this case, the appropriate black amount (Kopt) calculated by the appropriate black amount determining unit 13 may be output as the optimal black amount (K ′). .

The inverse conversion unit 15 uses the optimal black amount (K ′) calculated by the optimal black amount calculation unit 14 and the conversion-target independent color signal (L ^* ′ a ^* ′ b ^* ′ color signal) to use the color used by the actual output device. Color components (C ′, M ′, Y ′) other than black in the M-dimensional output color signal (C′M′Y′K ′ color signal) having the material color as a component are calculated. An output color signal (C′M′Y′K ′ color signal) is output based on the color components (C ′, M ′, Y ′) calculated by the inverse conversion unit 15 and the optimum black amount (K ′).

  The coefficient setting unit 16 sets a control coefficient for controlling a starting point at which the use of black is started when color reproduction is performed by the actual output device. As one of the control coefficients to be set, the starting point is controlled in the range of the change in black amount shown in FIG. 4 and the change in black amount shown in FIG. That is, the starting point of the minimum black amount when the total color material amount limit value set in the actual output device is used as it is and the upper limit value of black is subtracted from the total color material amount limit value set in the actual output device. The starting point is controlled within the range of the starting point of the minimum black amount when the value is used as the limit value of the total amount of color material other than black. In an example to be described later, the appropriate total amount limit value is controlled by a control coefficient to be set, and the starting point of the black amount is controlled by controlling the appropriate total amount limit value. As a control coefficient setting method, for example, a user specifies a value, and a specific control coefficient is determined in advance for each type of the actual output device or for each output mode of the actual output device, and used. The control coefficient may be set according to the type and output mode. Of course, it may be given in advance as a fixed value. Further, as described later, other coefficients may be set as control coefficients. In addition to the control coefficient, various settings may be performed. For example, the total color material amount limit value of the actual output device may be set. In this case, the control coefficient may be set to be changed according to the total color material amount limit value. Further, as described later, a coefficient indicating a linear degree of increase in the black amount from the start point of the black amount toward the upper limit value may be set.

FIG. 2 is a configuration diagram illustrating an example of an appropriate black amount determination unit. In the figure, 21 is a proper virtual surface color signal group creation unit, 22 is a black amount calculation model creation unit, and 23 is a proper black amount calculation unit. In the configuration shown in FIG. 1, the appropriate black amount determination unit 13 has the control coefficient set by the coefficient setting unit 16 and the independent color signal to be converted (L ^* ′ a ^* ′ b ^*) from the color gamut mapping unit 12 ^. 'Color signal' is given.

  The appropriate virtual surface color signal group creating unit 21 calculates an appropriate total amount limit value for each black amount based on the control coefficient set by the coefficient setting unit 16 and the color material total amount limit value set in the actual output device, A lower outline color signal that satisfies the calculated appropriate total amount limit value is created. This lower outer surface color signal is a color signal in a device-dependent color space in which the color of the color material used by the actual output device is a color component. For example, it is a CMYK color signal. Note that the lower outer surface is an outer surface composed of lightness colors equal to or lower than the lightness of the maximum saturation color in each hue, out of the color reproduction range when black is fixed as described above.

First, the black amount calculation model creation unit 22 first creates an independent color signal (L ^* ) in the corresponding device independent color space for each of the lower outer surface color signals (CMYK color signals) created by the appropriate virtual surface color signal group creation unit 21 ^. a ^* b ^* color signal). Based on the calculated independent color signal (L ^* a ^* b ^* color signal) and the black amount of the lower outline to which the independent color signal belongs (black component (K) of the lower outline color signal) Create a model for calculating the amount of ink. For example, using the method described in Japanese Patent Application Laid-Open No. 2005-64774, a model may be created for the lower outline color signal group created by the appropriate virtual color signal group creation unit 21. The present invention is not limited to this method, and the black amount (K) corresponding to the color signal in the given device independent color space is calculated from the discretely distributed device independent color signal and the corresponding black amount. Any method may be used as long as it is a method to be used.

The appropriate black amount calculation unit 23 uses the model created by the black amount calculation model creation unit 22 to obtain the appropriate black amount (L ^* 'a ^* ' b ^* 'color signal) from the given conversion target independent color signal (L ^* ' a ^* 'b ^* ' color signal). Kopt) is calculated. This appropriate black amount (Kopt) is passed to the optimum black amount calculation unit 14, and the optimal black amount (K) is calculated from the appropriate black amount (Kopt) and the black amount (K) of the target device color signal (CMYK color signal). ') Is calculated.

  If a model is created in advance by the appropriate virtual surface color signal group creation unit 21 and the black amount calculation model creation unit 22, the appropriate virtual surface color signal group is created when the appropriate black amount calculation unit 23 calculates the appropriate black amount. The unit 21 and the black amount calculation model creation unit 22 may be omitted.

  The appropriate black amount determination unit 13 will be further described using a specific example. In the following specific example, the total color material amount limit value of the actual output device is TL, and the control coefficient set in the coefficient setting unit 16 is KS. In this example, it is assumed that the control coefficient KS takes a value of 0 ≦ KS ≦ 1. When KS = 0, color conversion is performed with emphasis on graininess using the color material total amount limit value set in the actual output device as it is. In addition, when KS = 1, the value obtained by subtracting the upper limit value of black from the total color material amount limit value set in the actual output device is used as the limit value of the total color material amount other than black, and color with an emphasis on gradation is used. Conversion shall be performed.

First, the appropriate virtual surface color signal group creation unit 21 uses the control coefficient (KS) set by the coefficient setting unit 16 and the total color material amount limit value (TL) set in the actual output device in the device independent color space. The appropriate total amount limit value (GTC) on the achromatic color reference line is calculated for each black amount (K). For example, when the upper limit value of the black amount is Kmax, it is calculated using the following formula.
GTC (K) = TL−KS × (Kmax−K) (1)

  For example, if the total color material amount limit value (TL) is 160% and Kmax = 80%, when KS = 1, K = 0% and GTC = 80%, and K = Kmax and GTC = 160%. When KS = 1, GTC (K) = TL−Kmax + K, and since the total color material amount limit value (TL) and the upper limit value (Kmax) of the black amount are predetermined values, black (K) The upper limit of the total amount of color materials other than is (TL-Kmax). Therefore, as shown in FIG. 6, the appropriate total amount limit value (GTC) on the achromatic color reference line is calculated so that there is no change in the trend such as the amount of the color material other than black changes from increase to decrease and the gradation is good. Is done.

  On the other hand, when KS = 0, GTC = 160% when K = 0%, and GTC = 160% even when K = Kmax. When KS = 0, the appropriate total amount limit value (GTC) on the achromatic color reference line becomes the color material total amount limit value (TL) regardless of the black amount. Therefore, the appropriate total amount limit value (GTC) on the achromatic color reference line when K = 0 is higher than when KS = 1. As a result, the black amount start point moves to the low lightness side (high coverage side in the process black gradation of the target output device), so the appropriate total amount limit value (GTC) on the achromatic reference line where the graininess is good is calculated. Is done.

In calculating the appropriate total amount limit value (GTC), if the minimum black amount of the actual output device required to reproduce the color signal when the black amount of the target output device is a certain value is KB, An expression may be used.
GTC (K) = TL−KS × (KB−K) (2)
The black amount KB may be obtained from, for example, a model that takes into account the total color material amount limit value of the actual output device shown in FIG.

  For example, when the minimum black amount KB required for the actual output device to reproduce a color signal with a black amount of 0% in the target output device is 60%, the total color material amount limit value (TL) on the achromatic reference line It is not necessary to reduce Kmax from, but only 60 should be reduced. For example, assuming that the total color material amount limit value (TL) is 160% and the upper limit value (Kmax) of the black ink amount is 80%, the appropriate value on the achromatic color reference line corresponding to K = 0% when the equation (1) is used. The total amount limit value (GTC) is 80%. On the other hand, when the expression (2) is used, the appropriate total amount limit value (GTC) on the achromatic reference line corresponding to K = 0% is 100%, and color reproduction at K = 0% correspondingly. The range will expand. In this way, when the expression (2) is used, the appropriate total amount limit value (GTC) on the achromatic color reference line is increased, and the range in which the black amount is stored is expanded.

Furthermore, the amount of change of the color material other than black does not become zero, and the amount of ink other than black increases so that the amount of ink decreases when the ink amount is the upper limit value so that the amount of ink other than black increases. Therefore, the difference value that is monotonously increased and becomes the maximum when the black amount is 0% may be a value obtained by further subtracting the appropriate total amount limit value. For example, the expressions (1) and (2) are expressed as follows: GTC (K) = TL−KS × {(Kmax−K) + α (1−K / Kmax) ^β } (3)
GTC (K) = TL−KS × {(KB−K) + α (1−K / Kmax) ^β } (4)
It is good. Here, α (1−K / Kmax) ^β is a difference value, and when K = 0%, the appropriate total amount limit value is reduced by α. As described above, the appropriate total amount limit value is reduced by 0 when K = Kmax and by α when K = 0%, but a gamma coefficient for designating what changes to K during that time. Is β. Note that the coefficient α and the gamma coefficient β may be set in the coefficient setting unit 16.

  FIG. 7 is a graph showing an example of the relationship between the process black gradation of the target output device and the coverage of each color component in the actual output device when the starting point of black is controlled. As described above, the appropriate total amount limit value is changed by changing the control coefficient KS, thereby controlling the starting point of black. For example, when the color material total amount limit value TL = 160% and α = 0 in Equation (1) or (3) to obtain the appropriate total amount limit value GTC, the control coefficient KS = 0 is shown in FIG. Thus, the black amount is controlled, and the start point of black is the point where C, M, and Y in the actual output device are 54%. In this actual output device, if the control coefficient KS = 1, the black amount is controlled as shown in FIG. 6, and the start point of black is a point where C, M, and Y in the actual output device are 27%.

  Further, FIG. 7 shows a case where the control coefficient KS = 0.5. In this case, the black starting point is a point where C, M, and Y in the actual output device are 37%. In this case as well, the amount of the color material other than black changes from an increasing tendency to a decreasing tendency, but the amount of change is small compared to the case where the control coefficient KS = 0 shown in FIG. Deterioration is suppressed. Compared with the example of the control coefficient KS = 1 shown in FIG. 6, since the black starting point moves to the side where C, M, and Y are smaller in the actual output device, the graininess is also improved. By changing the control coefficient KS in this way, color conversion emphasizing granularity, color conversion emphasizing gradation, or color conversion balancing graininess and gradation is performed. . In the example shown in FIG. 7, equation (3) is used, and α = 20 and β = 1. Further, the total color material amount limit value (TL) = 160% and the upper limit value (Kmax) of black = 80%.

  FIG. 8 is a graph showing another example of the relationship between the process black gradation of the target output device and the coverage of each color component in the actual output device when the black start point is controlled. In this example, the control coefficients KS = 1, β = 1, and α = 40 are shown. The total color material amount limit value (TL) = 160% and the upper limit value of black (Kmax) = 80%. Compared with the example shown in FIG. 6, the change tendency of CMY becomes gentler, and the start point of black moves to the higher brightness side. As in this example, the black start point may be controlled by changing the coefficient α together with the control coefficient KS. In addition, compared to the case where the coefficient α is not used, the range of the black start point to be controlled is widened.

By changing the gamma coefficient β in the equations (3) and (4), the tendency of the black amount to increase is controlled without changing the black start point and the upper limit value. Such a gamma coefficient may be used for the formula (1) or the formula (2), for example. (1)
GTC (K) = TL−KS × (Kmax−K) × (1−K / Kmax) ^β (5)
And it is sufficient.

  FIG. 9 is a graph showing an example of the relationship between the process black gradation of the target output device and the coverage of each color component in the actual output device when the gamma coefficient is used. In this example, the total color material amount limit value (TL) = 160%, the black limit value (Kmax) = 80%, the control coefficient KS = 1, and the gamma coefficient β = 2. . Even if black is used from the start point of the brighter black than in the case of KS = 0 shown in FIG. 4, the increase in the black amount accompanying the increase in the gradation of the process black is suppressed, and the deterioration of the graininess is reduced. .

  FIG. 10 is a graph showing an example of the relationship between the black of the target output device and the black amount in the actual output device when each coefficient is changed. The changes in the black amount in the examples described so far are summarized as shown in FIG. The dotted line shows the case of the control coefficient KS = 0 shown in FIG. 4, the broken line shows the case of the control coefficient KS = 1 shown in FIG. 6, and the alternate long and short dash line shows the case of the control coefficient KS = 0.5 shown in FIG. These are shown, for which the gamma coefficient β = 1. The solid line shows the case where the control coefficient KS = 1 and the gamma coefficient β = 2 shown in FIG. Here, the total color material amount limit value (TL) = 160% and the upper limit value of black (Kmax) = 80%. In this way, the starting point of black is controlled by changing the control coefficient KS. Further, even if the coefficient α is changed, the starting point of black is controlled. Further, the increasing amount (linear degree) of the black amount is controlled by the gamma coefficient β.

  In the above-described example, the appropriate total amount limit value (GTC) is set so as to be the color material total amount limit value (TL) when the black amount reaches the upper limit value (Kmax). Not limited to this, the appropriate total amount limit value (GTC) when the black amount is the upper limit value (Kmax) is set to be a limit value smaller than the total color material amount limit value (TL), and the limit value is exceeded. An appropriate total amount limit value (GTC) for each black amount may be set so that there is no such amount. When the total color material amount limit value (TL) is about 160% as in the above example, the glossiness can be suppressed by the set total color material amount limit value. For example, the total color material amount limit value is 240% or the like. When a large amount of color material is used, glossiness may occur. In such a case, the occurrence of glossiness is suppressed by limiting the color material amount by providing a limit value smaller than the total color material amount limit value (TL). For example, if a limit value is set for an achromatic color that has a larger amount of color material than other colors, the occurrence of glossiness in the achromatic color can be suppressed. Of course, for other colors, the occurrence of glossiness in the color area where the limit value is set can be suppressed.

  Next, with the appropriate total amount limit value (GTC) on the achromatic color reference line set as described above as a reference, the appropriate total amount limit value becomes a color material as it approaches the outline of the color gamut when limited to the corresponding black amount. An appropriate total amount limit value (TC) other than the achromatic reference line is calculated so as to approach the total amount limit value (TL). Calculation of the appropriate total amount limit value (TC) other than the achromatic color reference line is performed for each black amount (K).

For example, it calculates using the following formula | equation.
TC (C, M, Y, K) = (GTC (K) −TL) × (1−Ch (C, M, Y)) ^γ + TL (6)
Here, Ch (C, M, Y) is the saturation determined from the color signals C, M, Y of the actual output device corresponding to the color signals in the device independent color space for which the appropriate total amount limit value is calculated. It is an index representing Ch (C, M, Y) = 0 when C = M = Y, and 1 when C or M or Y is 0% or 100% or the single color limit value of each color material. Also, γ is a gamma coefficient for designating how to change the appropriate total color material amount limit value from GTC (K) to SL in response to a change in saturation index.

  Furthermore, the appropriate virtual surface color signal group creation unit 21 is a color signal in a device-dependent color space that constitutes the lower outer surface corresponding to each black amount that satisfies the appropriate total amount limit value (TC) for each calculated black amount. A plurality of lower outer surface color signals are created. As a method for creating the lower outline color signal, here, a CMYK color signal group on the lower outline for each black amount that satisfies the total color material amount limit value (TL) in the actual output device is calculated. An example in which each of the calculated CMYK color signals is modified to a CMYK color signal satisfying the above-described appropriate total amount limit value (TC) to generate a lower outline color signal. Of course, it is not limited to this method.

  First, the lower outer surface for each black amount that satisfies the total color material amount limit value (TL) of the actual output device is either 100% of C, M, or Y, or the single color limit value of each color material, or none. It is composed of CMYK color signals that are the color material total amount limit value (TL) of the chromatic reference line. For a certain black amount (K), C, M, and Y satisfying the above conditions are created within the CMY total amount value obtained by subtracting the black amount (K) from the color material total amount limit value (TL). From (K), the CMYK color signals of the lower outer surface satisfying the total color material amount limit value (TL) at a certain black amount (K) may be used. Such CMYK color signals of the lower outer surface are acquired by changing the black amount (K). For example, the ink amount may be changed at a preset interval such as 10% increments, and the CMYK color signal of the lower outer surface may be obtained.

  Next, the appropriate total amount limit value (TC) described above is calculated for the generated CMYK color signal satisfying the total color material amount limit value (TL) so that the total amount value becomes this appropriate total amount limit value (TC). The original CMYK color signal is corrected. For example, the ratio of the original C, M, Y color components is preserved, and the CMYK color signal is corrected so that the sum of C, M, Y, K becomes an appropriate total amount limit value (TC) without changing K. . Alternatively, the sum of C, M, Y, and K is appropriate by subtracting an equal amount from the color component having a positive value among the original C, M, and Y color components, not the ratio of the C, M, and Y color components. The CMYK color signal may be modified to be equal to the total amount limit value (TC). The CMYK color signal obtained by the correction is used as the lower outline color signal.

  The black amount calculation model creation unit 22 uses a color signal in the device-independent color space for each group of lower outer surface color signals that are color signals constituting the lower outer surface created by the appropriate virtual surface color signal group creation unit 21. Convert to an independent color signal. Then, a model for calculating an appropriate black amount corresponding to the color signal of the device independent color space is obtained from the converted independent color signal and the black amount (K) of the lower outline color signal corresponding to the independent color signal. create. As a model creation method, for example, a method described in Japanese Patent Application Laid-Open No. 2005-64774 is used, and a model is created for a group of lower outer surface color signals created by the appropriate virtual surface color signal group creation unit 21. That's fine. Of course, the present invention is not limited to this method, and any other method can be used as long as it is a method for calculating an appropriate black amount when a conversion target independent color signal is given from discretely distributed independent color signals and corresponding black amounts. It may be used.

  In this way, a model for calculating the appropriate black amount from the color signal of the given device independent color space is created in advance and passed to the appropriate black amount calculation unit 23. It is not necessary to provide the appropriate black amount calculation unit 23 when creating the model, and when creating the model and calculating the appropriate black amount using the appropriate black amount calculation unit 23, the appropriate virtual surface color signal The group creation unit 21 and the black amount calculation model creation unit 22 may not be provided.

When a target device color signal to be output by the target output device is given, the forward conversion unit 11 changes the target device color signal (CMYK color signal) to a color signal (L ^* a ^* b ^* color signal) in the device independent color space. Convert. For the converted color signal of the device independent color space (L ^* a ^* b ^* color signal), the color gamut mapping unit 12 uses the color signal of the color gamut of the actual output device (L ^* 'a ^* ' b ^* 'color). Then, the color signal after mapping (L ^* 'a ^* ' b ^* 'color signal) is passed to the appropriate black amount determination unit 13 as the conversion-independent color signal.

The appropriate black amount calculation unit 23 of the appropriate black amount determination unit 13 uses the model for calculating the appropriate black amount created by the black amount calculation model creation unit 22 as described above, and is given a conversion target independent color. An appropriate black amount (Kopt) is calculated from the signal (L ^* 'a ^* ' b ^* 'color signal). This appropriate black amount (Kopt) becomes the output of the appropriate black amount determination unit 13 and is passed to the optimum black amount calculation unit 14.

  The optimal black amount calculation unit 14 calculates the optimal black amount (K) from the appropriate black amount (Kopt) passed from the appropriate black amount determination unit 13 and the black amount (K) of the target device color signal (CMYK color signal). ') Is calculated. The calculation method is as described above.

In the inverse conversion unit 15, the optimum black amount (K ′) calculated by the optimal black amount calculation unit 14 and the conversion target independent color signal (L ^* ′ a ^* ′ b ^* ′) after being mapped by the color reproduction area mapping unit 12. Color components other than black, here, C, M, and Y are calculated from the color signal. In general, the conversion from the LAB color space to the CMYK color space is not uniquely determined because of the increase in dimension, but CMY is uniquely determined by fixing the optimum black amount (K ′). For this conversion, a color conversion model in an actual output device may be used. An output color signal (C′M′Y) based on the calculated C, M, Y color components (C ′, M ′, Y ′) and the optimum black amount (K ′) calculated by the optimum black amount calculation unit 14. 'K').

  In this way, the target device color signal including black in the target output device is converted into the output color signal including black in the actual output device. At that time, the control coefficient KS or further the coefficient α and the gamma coefficient β are set, and based on these set values and the total color material amount limit value (TL) set in the actual output device, the ink in the actual output device is determined. Calculate the appropriate total amount limit value for each amount, calculate the appropriate black amount from the conversion target independent color signal of the target output device based on the appropriate total amount limit value, and output color from the appropriate black amount and black amount of the target device color signal The optimum black amount that is the black amount of the signal is calculated. Thereby, color reproduction according to the importance of graininess and gradation is performed.

  Note that the above-described configuration may be used to directly perform color conversion from the target device color signal to the output color signal, or CMYK color signals corresponding to input values (grid points) in the multidimensional table may be used. It is also possible to create a multidimensional table that realizes color conversion by associating CMYK values obtained by color conversion as output values, and to perform color conversion of CMYK images using the multidimensional table. Good.

  FIG. 11 is an explanatory diagram of an example of a computer program, a storage medium storing the computer program, and a computer when the functions described in the embodiment of the present invention are realized by a computer program. In the figure, 31 is a program, 32 is a computer, 41 is a magneto-optical disk, 42 is an optical disk, 43 is a magnetic disk, 44 is a memory, 51 is a CPU, 52 is an internal memory, 53 is a reading unit, 54 is a hard disk, and 55 is An interface 56 is a communication unit.

  You may implement | achieve all or a part of function of each part demonstrated by one Embodiment of the above-mentioned this invention with the program 31 executable by a computer. In this case, the program 31 and data used by the program may be stored in a storage medium that can be read by a computer. A storage medium is a signal of a corresponding signal that causes a change state of energy such as magnetism, light, electricity, etc., depending on the description content of the program, to the reading unit 53 provided in the hardware resource of the computer. The program description is transmitted to the reading unit 53 in a format. For example, there are a magneto-optical disk 41, an optical disk 42 (including a CD and a DVD), a magnetic disk 43, a memory 44 (including an IC card and a memory card), and the like. Of course, these storage media are not limited to portable types.

  The program 31 is stored in these storage media, and the program 31 is read from the computer and stored in the internal memory 52 or the hard disk 54, for example, by mounting these storage media on the reading unit 53 or the interface 55 of the computer 32. By executing the program 31 by the CPU 51, the functions described in the above-described embodiment of the present invention are realized in whole or in part. Alternatively, the program 31 is transferred to the computer 32 via the communication path, and the computer 32 receives the program 31 by the communication unit 56 and stores it in the internal memory 52 or the hard disk 54, and the CPU 31 executes the program 31. May be.

  In addition, the computer 32 may be connected to various devices via the interface 55. For example, display means for displaying information, reception means for receiving information from the user, and the like may be connected. For example, the coefficient setting unit 16 may be configured to receive a setting of a limit value by the user from the reception unit. Further, for example, an image forming apparatus as an output apparatus may be connected via the interface 55, and an image may be formed by the image forming apparatus using the converted output color signal. In this case, the connected image forming apparatus is the actual output apparatus.

  Note that each component does not have to operate on one computer, and the processing may be executed by another computer depending on the processing stage. For example, the processing for creating a model for calculating the appropriate black amount and the processing for color conversion from the target device color signal including the color conversion using the model to the output color signal are performed by separate computers. You may go.

  DESCRIPTION OF SYMBOLS 11 ... Forward conversion part, 12 ... Color reproduction area mapping part, 13 ... Appropriate black amount determination part, 14 ... Optimum black amount calculation part, 15 ... Inverse conversion part, 16 ... Coefficient setting part, 21 ... Preparation of appropriate virtual surface color signal group , 22 ... black amount calculation model creation unit, 23 ... appropriate black amount calculation unit, 31 ... program, 32 ... computer, 41 ... magneto-optical disk, 42 ... optical disk, 43 ... magnetic disk, 44 ... memory, 51 ... CPU, 52 ... Internal memory, 53 ... Reading unit, 54 ... Hard disk, 55 ... Interface, 56 ... Communication unit.

Claims (10)

  When the limit value of the total color material amount set in the actual output device that outputs an image using M (M ≧ 4) color materials including black is used as it is, and the color set in the actual output device A control coefficient setting means for setting a control coefficient for controlling an appropriate total amount limit value in a range with a value obtained by subtracting the upper limit value of black from the limit value of the total amount of material as a limit value of the total amount of color material other than black; Based on the control coefficient set by the control coefficient setting means and the limit value of the total amount of color material set in the actual output device, an appropriate total amount limit value is calculated for each black amount to satisfy the appropriate total amount limit value Determining an appropriate black amount by creating a lower outline color signal and determining an appropriate black amount from the independent color signal to be converted in the device independent color space based on the independent color signal in the device independent color space corresponding to the lower outline color signal A color conversion device comprising means.   The appropriate black amount determination means creates a model for calculating an appropriate black amount from the independent color signal and the black amount of the lower outer surface to which the independent color signal belongs, and uses the model to convert the conversion target The color conversion apparatus according to claim 1, wherein the appropriate black amount is calculated from an independent color signal.   And a forward conversion means for converting the target device color signal in the N-dimensional color space into the conversion target independent color signal in the device independent color space in the target output device using N (N ≧ 4) color materials including black. An optimum black amount calculating means for calculating an optimum black amount from the appropriate black amount determined by the appropriate black amount determining means and the black amount of the target device color signal; and the optimum black amount calculated by the optimum black amount calculating means. And an inverse conversion means for calculating a color component other than black in the M-dimensional output color signal having the color of the color material used by the actual output device as a component from the independent color signal to be converted. The color conversion apparatus according to claim 1, wherein an output color signal is generated based on the color component calculated by the means and the optimum black amount.   A forward conversion means for converting a target device color signal in an N-dimensional color space into a conversion target independent color signal in a device independent color space in a target output device using N (N ≧ 4) color materials including black; From an appropriate black amount determining means for determining an appropriate black amount from the conversion target independent color signal converted by the converting means, an appropriate black amount determined by the appropriate black amount determining means, and a black amount of the target device color signal M (M ≧ M) including an optimum black amount calculating means for calculating an optimum black amount, and an ink used by an actual output device that actually outputs the optimum black amount calculated by the optimum black amount calculating means and the independent color signal to be converted. 4) Inverse conversion means for calculating a color component other than black in the M-dimensional output color signal having the color of the color material as a component, and the color component calculated by the reverse conversion means and the optimum black color The appropriate black amount determination means calculates the appropriate black amount. Using the model for determining the appropriate black amount from the independent color signal to be converted, the model using the limit value of the total amount of color material set in the actual output device as it is Control for controlling the appropriate total amount limit value within the range of the case where the value obtained by subtracting the upper limit value of black from the limit value of the total amount of color material set in the actual output device is used as the limit value of the total amount of color material other than black The appropriate total amount limit value is calculated for each black amount based on the coefficient and the color material total amount limit value set in the actual output device, and a lower outline color signal that satisfies the calculated appropriate total amount limit value is created. The independent color signal in the corresponding device independent color space is calculated from the lower outline color signal and is created from the independent color signal and the black amount of the lower outline to which the independent color signal belongs. A color conversion device characterized by the above.   The appropriate total amount limit value for each black amount is obtained by subtracting the value obtained by subtracting the black amount from the upper limit value of the black amount according to the control coefficient from the limit value of the total color material amount used in the actual output device. The color conversion device according to claim 1, wherein the color conversion device is obtained.   The appropriate total amount limit value for each black amount is further subtracted from the minimum value when the black amount is the upper limit value, monotonically increasing as the black amount decreases, and the maximum value when the black amount is 0%. The color conversion apparatus according to claim 5, wherein the color conversion apparatus is a value.   The appropriate total amount limit value is a value obtained by further changing a linear degree of increase of the black amount from the start point toward the upper limit value of the black amount according to a preset coefficient, and subtracting the changed black amount from the upper limit value of the black amount 6. The color conversion apparatus according to claim 5, wherein a value obtained by reducing the value according to the control coefficient is subtracted from a limit value of a total amount of color material used in the actual output device.   The appropriate total amount limit value is a value based on the control coefficient and the limit value of the total amount of color material used in the actual output device for the achromatic color reference line of the device independent color space indicating the achromatic color axis in the actual output device. The total amount of color material to be used in the actual output device as it approaches the outline of the color gamut in the corresponding black amount based on the appropriate total amount limit value for each black amount on the achromatic color reference line as calculated for each black amount 8. The color conversion apparatus according to claim 1, wherein a value for each black amount is calculated for colors other than the achromatic reference line so as to approach the value.   9. The appropriate total amount limit value is set to a value smaller than a limit value of a total color material amount used in the actual output device as a value when the black amount of the achromatic color reference line is a maximum value. The color conversion device described in 1.   A color conversion program for causing a computer to execute the function of the color conversion device according to any one of claims 1 to 9.

Images