Source: https://patents.google.com/patent/US20050264837?oq=7%2C173%2C247
Timestamp: 2018-05-22 16:34:08
Document Index: 636840869

Matched Legal Cases: ['art 702', 'art 701', 'art 3004', 'art 703', 'art 701', 'art 704', 'art 702', 'art 705', 'art 707', 'art 704', 'art 708', 'art 707', 'art 702', 'art 705', 'art 703', 'art 709', 'art 709', 'art 706', 'art 703', 'art 702', 'art 705', 'art 709', 'art 709', 'art 3004', 'art 702', 'art 701', 'art 703', 'art 701', 'art 902', 'art 902', 'art 902', 'art 902', 'art 703', 'art 901', 'art 902', 'art 703']

US20050264837A1 - Color conversion method - Google Patents
Color conversion method Download PDF
US20050264837A1
US20050264837A1 US11138330 US13833005A US2005264837A1 US 20050264837 A1 US20050264837 A1 US 20050264837A1 US 11138330 US11138330 US 11138330 US 13833005 A US13833005 A US 13833005A US 2005264837 A1 US2005264837 A1 US 2005264837A1
US11138330
US7453602B2 (en )
To suitably reproduce an image reproduced on an input system on an output system, when a color signal A acquired from an image processing device is contained in a low saturation area on the color space containing a color signal P of gray for an image output device at a lightness of the color signal A and a color signal O of calorimetrically achromatic color, the color signal A is converted in accordance with a conversion function of converting the color signal on a line segment BP connecting a color signal B that is an intersection between a line segment connecting the color signal A and the color signal P and the outermost contour of the low saturation area and the color signal P into the color signal on a line segment BO connecting the color signal B and the color signal O. On the other hand, when the color signal A is not contained in the low saturation area, the color signal is not converted.
The present invention relates to a color conversion method for converting a color signal regenerated by an image input/output device into a color signal in a color space for color matching.
Since the color characteristics of the image input/output device are varied depending on the device, when certain color image data is employed between different devices, there is a problem that the color combination reproduced by each device is unmatched even when the color image data is the same. To solve this problem, a color management system (hereinafter referred to as a “CMS”) is utilized to unify the color appearance of color image between different devices.
In the CMS, to excellently reproduce the same color image on plural image input/output devices (e.g., color copy, color monitor, digital camera, color printer and so on), a color signal of each device is converted into a color signal in a color space for color matching, in which the color signal of an input system is converted into the color signal of an output system so that they may be matched in this color space, if possible. Herein, the input system designates a device aimed at the color matching. For example, when the color of a color printer B is matched with the color of a color printer A, the color printer A is the input system, and the color printer B is the output system. Also, the color space for color matching is generally a colorimetric color space, such as CIE/XYZ, CIE/LAB and CIE/LCh (e.g., refer to Japanese Patent Laid-Open No. 7-200814 (patent document 1)).
However, especially in a low saturation area, an excellent image may not be reproduced by simply matching the calorimetric values. For example, if the ink or toner is employed as a while color signal of the color printer, it is perceived as the “color cast” to make a bad impression, even if the colorimetric values are matched. Therefore, for the white color typically reproduced on the color printer, the color of an image recording medium (printing paper) itself is employed, without depending on the calorimetric value of the input system.
Also, the black color is often reproduced in the maximal density color intrinsic to the device to utilize a dynamic range of the device to the maximum, and the gray of intermediate lightness is typically a value dependent on the device, because the chromaticity is continuously changed from while to black. That is, it is required in some cases that the image is not reproduced to match the calorimetric value, but reproduced in the color intrinsic to the device, depending on the kind of color. And in such cases, in the color space for color matching, for example, it is desired that the color signal corresponding to gray of the input system and the color signal corresponding to gray of the output system, which are different calorimetrically, are matched.
The color space for color matching with the corrected gray line to meet the above requirement (hereinafter referred to as a “color space S”) is employed in which the equal lightness plane is translated so that the color signal of gray containing white and black may become the color signal having the same lightness and representing the achromatic color (saturation=0) in the calorimetric color space based on three attributes of color perception of lightness, saturation and hue.
FIG. 7 is a diagram showing the relationship between calorimetric color space and color space S using the equal lightness plane in the CIE/LAB space. In FIG. 7, the abscissa is a*, the ordinate is b*, point P is a point indicating the gray, point P′ is a point indicating calorimetrically achromatic color, 71 is a gamut of the device in the calorimetric color space, and 72 is a gamut of the device in the color space S. As shown in FIG. 7, the color signal in the color space S is translated from the color signal in the calorimetric color space along a vector PP′.
By this translation (parallel displacement), the gray of each device is represented by the point on the same straight line (axis of lightness), and treated as the color of the same chromaticity point. Also, in the color space S, a line segment indicated by the gray of the input system and a line segment indicated by the gray of the output system can be easily matched by compression/extension on the same straight line, and due to this compression/extension, the color signals indicating the while color and the black color of the input system can be matched respectively with the color signals indicating the white color and the black color of the output system. A technique for preventing color cast by moving the gray line was described in Japanese Patent Laid-Open No. 2001-326826 (patent document 2).
However, in the color space S, the color conversion for converting the achromatic color signal has influence not only on the achromatic color but also the chromatic color, resulting in a new problem that the color signal of color indicating the same colorimetric value for the chromatic color is unmatched. For example, in the color space S, the flesh color of each device is reproduced by a different color signal, whereby the flesh color of the input system can not be suitably reproduced by the output system. FIG. 8 is a diagram showing the equal lightness plane in the CIE/LAB space to explain the conventional color conversion method. In FIG. 8, the abscissa is a*, the ordinate is b*, point Pi is a point indicating the colorimetric value of color on the gray line in the input system, point Po is a point indicating the colorimetric value of color on the gray line in the output system, point P′ is a point indicating colorimetrically achromatic color, point C is a point indicating the calorimetric value of flesh color, point Ci′ is a point indicating the color signal in the color space S for the input system corresponding to the color at point C, and point Co′ is a point indicating the color signal in the color space S for the output system corresponding to the color at point C indicating the calorimetric value of flesh color.
With the conventional method as shown in FIG. 8, the color signal in the color space S corresponding to the color on the gray line of each device can be represented by point P′ without regard to the device, but the color signal in the color space S corresponding to the flesh color of each device is different depending on the device, and indicated at point Ci′ in the input system and point Co′ in the output system. Accordingly, even if the color signal in the input system is converted into the color signal in the output system so that they may be matched in the color space S, the flesh color of the input system and the flesh color of the output system may be different, resulting in less excellent reproduced image.
In order to achieve the above object, the present invention provides a color conversion method for converting a color signal for an image output device into a color signal in a color space for color matching, comprising:
an acquisition step of acquiring a color signal A of an image processing device;
a conversion step of converting the color signal A into the color signal D in accordance with a conversion function of converting the color signal on a line segment BP connecting a color signal B that is an intersection between a line segment connecting the color signal A and the color signal P and the outermost contour of a low saturation area and the color signal P into the color signal on a line segment BO connecting the color signal B and the color signal O, when the color signal A is contained in the low saturation area on the color space containing the color signal P of gray for the image output device at a lightness of the color signal A and the color signal O that is calorimetrically achromatic color;
wherein when the color signal A is not contained in the low saturation area, the color signal is not converted at the conversion step.
determining whether or not the uniform color space signal is inside a low saturation area; and correction processing the uniform color space signal according to the gray signal if it is inside the low saturation area as a result of determination.
FIG. 2 is a flowchart for explaining a processing procedure of the color conversion method according to the embodiment of the invention;
FIG. 3 is a typical view for explaining one example of a chromaticity plane (equal lightness section) in a low saturation area R in a CIE/LAB color space according to this embodiment;
FIG. 4 is a diagram showing one example of a gray line table acquired at step S201;
FIG. 5 is a flowchart for explaining the details of a calculation procedure of color signal B at step S207;
FIG. 7 is a diagram showing the relationship between colorimetric color space and color space S using the equal lightness plane in the CIE/LAB space;
FIG. 1 is a typical view for explaining a color conversion method according to the first embodiment of the present invention, and shows an equal lightness plane in a CIELAB color space as one example of a uniform color space. In FIG. 1, point P is a point indicating the gray intrinsic to an image output device, point O is a point indicating the achromatic color, 101 designates the outermost contour of a low saturation area R containing point P and point O, and point B is any point on the outermost contour 101 of the area R. At this time, the color conversion method of this embodiment converts the color indicated by point A on the line segment BP into the color indicated by point D on the line segment BO so that the ratio of PB to PA may be equal to the ratio of OB to OD, for example. Also, the color outside the area R is not converted (input color signals are directly outputted).
Due to these features, with the color conversion method of this embodiment, the gray P that has different colorimetric value depending on the image output device is converted to be the same color signal indicated by point O, and the color outside the area R such as flesh color or memory color is converted so that the color of the same colorimetric value may be the same color signal. Of course, if a different processing than in the area R is done, green of grass and leaf and blue of sky among memory colors may be corrected as vivid and flesh color may be subjected to such a preferable color correction as to add to red.
A color conversion procedure for the color conversion method according to this embodiment will be described below. FIG. 2 is a flowchart for explaining a processing procedure for the color conversion method according to the embodiment of the invention.
First of all, various parameters, a color conversion table, and a gray line table are acquired (step S201). The color signals RGB of the image output device that are converted color signals are acquired (step S202). And the color signals RGB acquired at step S202 are converted into the color signals Lab ((L*,a*,b*)=(L,a,b)) in the calorimetric color space (step S203). This conversion is performed by a well-known interpolation method, employing the color conversion table acquired at step S201.
The color conversion table stores the calorimetric values Lab corresponding to the lattice points ({R,G,B}, {0,0,0}, {0,0,32}, {0,0,64}, . . . , {0,0,255}, {0,32,0}, . . . , {255,255,255}) of nine slices for 8-bit RGB color signals, for example. The color signals Lab converted at step S203 are defined as the color signal A in the following.
The color signal P of gray for the image output device at the lightness of the color signal A is acquired (step S204). The color signal P is calculated employing the gray line table acquired at step S201. The calculation of the color signal P will be described later in detail.
Supposing that the low saturation area including the color signal P and the color signal O that is calorimetrically achromatic at the lightness of the color signal A is denoted as area R, it is determined whether the color signal A is inside or outside the area R (step S205). As a result, if the color signal A is inside the area R (Yes), the procedure proceeds to step S206. On the other hand, if it is outside the area (No), the procedure goes to step S210.
At step S206, if the color signal A is inside the area R, the color signal P and the color signal A are compared. And if both are the same color signal (Yes), the procedure goes to step S211. On the contrary, if both are not the same color signal (No), the procedure goes to step S207.
At step S207, a color signal B at an intersection between the line segment connecting the color signal P and the color signal A and the outermost contour of the area R is acquired. If two intersections exist, the intersection closer to the color signal A is made the color signal B.
And at step S208, the color signal A is converted into an output color signal D ((L*,a*,b*)=(L′,a′,b′)) in accordance with a conversion function for converting the color signal on the line segment BP connecting the color signal B and the color signal P into the color signal on the line segment BO connecting the color signal B and the color signal O. Then, the procedure proceeds to step S209.
On the other hand, if the color signal A is outside the area R at step S205 (No), the output color signal D is set to the color signal A (step S210), and the procedure goes to step S209. Also, if the color signal P and the color signal A are identical at step S206 (Yes), the output color signal D is set to the color signal O (step S211), and the procedure goes to step S209.
And at step S209, the output color signal D set at step S208, S210 or S211 is outputted. The processing contents of each step will be further described in detail.
From the coordinates (Pa,Pb) of the point P and the coordinates (0,0) of the point O, the coordinates (Ga,Gb) of the point G are obtained in accordance with the following formulas.
The distance E between point O and point P is obtained in accordance with the following formula.
E={square root}{square root over (Pa 2 +Pb 2 )} (3)
Supposing that α and β are parameters acquired at step S201, the lengths L1 and L2 for the principal axis of the ellipse are obtained in accordance with the following formulas.
L 1 =E×α (4)
L 2 =L 2×β (5)
The parameter α is a value of one or greater (e.g., α=1.8), in which if the value is greater, the low saturation area is corrected more gently, but the range affected by correction is extended. On the other hand, if the value is smaller, the range affected by correction is narrowed, but the influence of correction for the low saturation area is more significant, possibly causing a gradation failure. Also, the parameter β decides the shape of ellipse, in which if β=1, the circle is obtained. For the parameter β, if the value is greater, the range affected by correction is extended, while if the value is smaller, the influence of correction for the low saturation area is more significant, possibly causing a gradation failure.
Also, the st coordinate system which has the point G as the origin, and which is composed of the vector OP and the vector obtained by rotating the vector OP by 90 degrees counterclockwise is considered, as shown in FIG. 3. In FIG. 3, the conversion from (a*,b*) coordinates to (s,t) coordinates is conducted in accordance with the following formula. ( s t ) = M ⁡ ( a * - Ga b * - Gb ) ( 6 )
Where M is the matrix given by the following formula. M = ( cos ⁡ ( θ ) sin ⁡ ( θ ) - sin ⁡ ( θ ) cos ⁡ ( θ ) ) ( 7 )
Where θ is the angle as shown in FIG. 3, and given by the following formula.
Also, the conversion from (s,t) coordinates to (a*,b*) coordinates is conducted in accordance with the following formula. ( a * b * ) = M - 1 ⁡ ( s t ) + ( Ga Gb ) ( 9 )
A calculation method of the color signal P at step S204 will be described below in detail. FIG. 4 is a diagram showing one example of the gray line table acquired at step S201. As shown in FIG. 4, the gray line table of this embodiment is the table storing the colorimetric value CIE/LAB pertaining to the discrete gray level GL. The gray level GL pertains to the color signal in which each of the R, G and B signals is identical. For example, the gray level 32 stores the colorimetric value when (R,G,B)=(32,32,32) is outputted by the image output device.
The values of chromaticity coordinates a*,b* at any lightness L are obtained employing the gray line table by the well-known interpolation method. That is, the color signal P is generated from the chromaticity coordinates and the lightness value L by calculating the chromaticity coordinates of the gray corresponding to the lightness value L of the color signal A by the well-known interpolation method, based on the gray line table acquired at step S201.
An inside/outside determination method at step S205 will be described below in detail. In this example, the following determination process is performed to determine whether or not the color signal A is inside the low saturation area R. That is, first of all, the chromaticity coordinates (a,b) of the color signal A are converted into the coordinate value (S,T) in the (s,t) coordinate system, employing the formula (6). Then, an evaluation function Q is computed as follows.
={square root}{square root over ((2S/L1) 2 +(2T/L2) 2 )} (10)
Where L1 and L2 are the lengths of the principal axis of the ellipse given by the formulas (4) and (5). Also, when the value of the evaluation function Q in the formula (10) is 1, the color signal A is located on the ellipse. Also, if Q is less than 1, the color signal A is located within the ellipse, namely, inside the low saturation area R. Furthermore, if Q is greater than or equal to 1, the color signal A is outside the ellipse, namely, out of the low saturation area R.
A calculation method of color signal B at step S207 will be described below in detail. For the color signal (L,U,V) on the line segment Lpa connecting the color signal P(L,Pa,Pb) and the color signal A(L,a,b), L of the color signal is a fixed value, and V is decided from the value of U employing the following formula (11).
V(U)=(b−Pb/a−Pa)×(u−Pa)+Pb (11)
FIG. 5 is a flowchart for explaining the details of a calculation procedure of color signal B at step S207. In this embodiment, the color signal B is calculated in accordance with the following procedure. First of all, as the initialization, the U value Umin corresponding to the color signal inside the low saturation area R and the U value Umax corresponding to the color signal conceivably outside the low saturation area R are set at the initial values (step S501). For example, Umin is set at Pa, and Umax is set at 10 if Pa<a, or −10 if Pa>a.
Then, it is determined whether or not the color signal corresponding to Umax is inside the low saturation area R by the above method (step S502). Consequently, if the color signal is inside the area (Yes), the procedure goes to step S503, or if it is not (No), the procedure proceeds to step S504. Herein, the color signal corresponding to Umax is given by (L,Umax,V(Umax)), employing the formula (11).
At step S503, which takes place when it is determined that the color signal is inside the area at step S502, Umax is modified so that the color signal corresponding to Umax may be outside the low saturation area R. After modification, the procedure returns to step S502. Herein, Umax is modified in accordance with the following formula.
Umax=Umax×2 (12)
On the other hand, at step S504, which takes place when it is determined that the color signal is outside the area at step S502, the average value Uave of Umax and Umin is obtained in accordance with the following formula.
At step S505, it is determined whether or not the chromaticity coordinates corresponding to Uave are located inside the low saturation area R by the above method. Consequently, if they are inside the area R (Yes), the procedure goes to step S507, or if they are outside the area R (No), the procedure proceeds to step S506. If it is determined at step S505 that the color signal is outside the area, Umax is updated to Uave at step S506, and the procedure goes to step S508. On the contrary, if it is determined that the color signal is inside the area at step S505, Umin is updated to Uave at step S507, and the procedure goes to step S508.
At step S508, a difference between Umax and Umin is computed, and it is determined whether or not its absolute value is less than a predetermined value SH. Consequently, if the absolute value is less than SH (Yes), the procedure proceeds to step S509, or if the absolute value is greater than or equal to SH, the procedure returns to step S504. And at step S508, if the absolute value of the difference between Umax and Umin is less than the predetermined value SH, the color signal B is given by (L,Umin,V(Umin)), for example, and outputted (step S509).
On the other hand, the color signal B may be also calculated by the following method. First of all, the chromaticity coordinates of the color signal P and the chromaticity coordinates of the color signal A are converted into the (s,t) coordinate system, employing the formula (6). Then, the straight line passing through two chromaticity coordinates after conversion is obtained. Supposing that the chromaticity coordinates in the (s,t) coordinate system corresponding to the chromaticity coordinates (Pa,Pb) of the color signal P are Pst(Ps,Pt), and the chromaticity coordinates in the (s,t) coordinate system corresponding to the chromaticity coordinates (a,b) of the color signal A are Ast(As,At), the straight line passing through the chromaticity coordinates Pst and the chromaticity coordinates Ast is given by the following formula (14). s - Ps As - Ps = t - Pt At - Pt ( 14 )
Then, the ellipse making up the outermost contour of the low saturation area R is obtained. Employing the lengths L1 and L2 of the principal axis given by the formulas (4) and (5), this ellipse is given in accordance with the following formula (15). ( s L1 / 2 ) 2 + ( t L2 / 2 ) 2 = 1 ( 15 )
By solving the simultaneous equations of formulas (14) and (15), the intersection Bst (Bs,Bt) between the straight line and the ellipse is obtained. When the simultaneous equations have multiple solutions, the intersection Bst (Bs,Bt) is obtained as the solution in which La is smaller than Lp, where the distance between the chromaticity coordinates Pst and the chromaticity coordinates of the solution is Lp, and the distance between the chromaticity coordinates Ast and the chromaticity coordinates of the solution is La. Finally, the intersection Bst(Bs,Bt) is converted into the chromaticity coordinates (Ba,Bb) in the (a*,b*) coordinate system in accordance with the following formula (16). ( Ba Bb ) = M - 1 ⁡ ( Bs Bt ) + ( Ga Gb ) ( 16 )
A conversion function for converting the color signal A at step S208 will be described below in detail. The conversion function converts the color signal on the line segment BP into the color signal on the line segment BO, and converts the color signal A(L,a,b) that is the color signal on the line segment BP into the color signal D(L,a′,b′) on the line segment BO. FIG. 6 is a diagram showing one example of the conversion function for use in this embodiment. In FIG. 6, the abscissa is the color signal on the line segment BP that is the input value, and the ordinate is the color signal on the line segment BO that is the output value. The conversion function of this embodiment is a linear transformation, in which the color signal is converted so that the ratio of the distance of line segment BP to the distance of line segment BA may be equal to the ratio of the distance of line segment BO to the distance of line segment BD. That is, the color signal D is obtained in accordance with the following formulas (17) and (18).
a′=(a−Ba)/(Pa−Ba)×(Oa−Ba)+Ba (17)
b′=(b−Bb)/(Pb−Bb)×(Ob−Bb)+Bb (18)
As described above, with the color conversion method according to this embodiment, supposing that the color signal of gray dependent on the device is the color signal P, the color signal of achromatic color is the color signal O, and the predetermined low saturation area including the color signal P and the color signal O is area R, the color signal that is not included in the area R is directly outputted. Also, for the color signal included in the area R, when arbitrary point on the outermost contour of the area R is point B, the color signal on the line segment BP connecting the color signal B and the color signal P is converted into the color signal on the line segment BO connecting the color signal B and the color signal O.
Due to this conversion, in the low saturation area containing the gray dependent on the device, the gray is moved to the achromatic axis, and the color near the gray is smoothly transformed with gradations, whereby the dynamic range of the device can be utilized to the maximum while the “color cast” is suppressed. Furthermore, the chromatic color such as flesh color outside the low saturation area can be reproduced in the calorimetrically equivalent color because the color signal is not converted. Consequently, if the color matching is performed in the color space for color matching based on this color conversion method, the image of the input system is excellently reproduced on the output system.
Though in this embodiment the CIELAB is employed as the color space, other uniform color spaces may be employed. More suitably, the color space of CAM may be employed.
The calorimetric value conversion table storage part 702 inputs and stores a calorimetric value conversion table of the image output device via the data input part 701. The profile creation device of this embodiment creates a profile (stored in the input system profile storage part 3004) for converting the color signal of the image output device into the color mapping color signal in the CMS as shown in FIG. 11.
The colorimetric value conversion table is the LUT of colorimetric values indicating the color reproduction characteristics of the image output device, as described in the first embodiment. For example, a color chart image composed of the 8-bit RGB color signals at lattice points of nine slices ({R,G,B}=(0,0,0), {0,0,32}, {0,0,64}, . . . , {0,0,255}, {0,32,0}, . . . , {255,255,255}) by the image output device, and the color of this output image is measured to acquire the calorimetric value conversion table. The parameter storage part 703 stores beforehand the parameters (α in the formula (4) and β in the formula (5)) for deciding the shape of the low saturation area R, or acquires and stores them via the data input part 701. The interpolation computation part 704 computes the output color of the image output device corresponding to any input RGB color signals by a well-known interpolation method, employing the colorimetric value conversion table of the image output device stored in the colorimetic value conversion table storage part 702.
The gray line table storage part 705 stores the result of generating the discrete color signal of gray line in the color signal generation part 707 and computing the output color of the image output device corresponding to the color signal of gray line in the interpolation computation part 704. The gray line table is the table as shown in FIG. 4 and described in the first embodiment, for example. The low saturation area conversion part 708 converts the discrete RGB color signals making up the LUT, which are generated by the color signal generation part 707, by the color conversion method as described in the first embodiment, employing the colorimetic value conversion table of the image output device stored in the colorimetric value conversion table storage part 702, the gray line table stored in the gray line table storage part 705, and the parameters stored in the parameter storage part 703, and stores the converted signals in the output profile storage part 709. The discrete RGB color signals making up the LUT are the 8-bit color signals at lattice points of nine slices ({R,G,B}={0,0,0}, {0,0,32}, {0,0,64}, . . . , {0,0,255}, {0,32,0}, {255,255,255}), for example. The color signals stored in the output profile storage part 709 are outputted via the data output part 706.
First of all, at step S801, an initialization process is performed. More specifically, the parameters for deciding the shape of the low saturation area R and the colorimetric value conversion table of the image output device are acquired and stored respectively in the parameter storage part 703 and the colorimetric value conversion table storage part 702. Then, at step S802, a gray line table is created, employing the colorimetric value conversion table acquired at step S801, and stored in the gray line table storage part 705. At step S803, the 8-bit RGB color signals at lattice points of nine slices making up the LUT of profile are generated.
Subsequently, to acquire the color signal in the color mapping color space corresponding to the RGB color signals by the method according to the first embodiment, the flowchart of FIG. 2 is performed from step S203, whereby the color signal D in the color mapping color space corresponding to the RGB color signals are obtained at step S209. Then, the procedure proceeds to step S804.
At step S804, the color signal in the color mapping color space obtained at step S209 is stored in the output profile storage part 709. At step S805, it is determined whether or not for all the color signals making up the LUT of profile, the color signal in the corresponding color mapping color space is stored. If storing all the color signals is ended, the procedure proceeds to step S806, or if not, the procedure returns to step S803. Finally, at step S806, the output profile stored in the output profile storage part 709 is outputted.
As described above, the profile creation device of this embodiment, the profile creation device of this embodiment creates a profile of the image output device for use in the CMS using the color conversion method as described in the first embodiment. The CMS stores the profile created by the profile creation device of this embodiment in the input system profile storage part 3004, converts the gray line of the input system into the gray line of the output system, which is calorimetrically different, employing this profile, and for the color such as flesh color outside the predetermined low saturation area, converts the color signal of the input system into the color signal of the output system to be reproduced in the color at the same colorimetric value as the input system.
In this embodiment, the gray line table is created, employing the colorimetric value conversion table stored in the colorimetric value conversion table storage part 702, but may be inputted via the data input part 701 separately.
In a third embodiment, a function of adjusting the size of the low saturation area R interactively is added to the profile creation device as described in the second embodiment. A functional configuration of the profile creation device according to this embodiment and a profile creation procedure are identical to those of the second embodiment, and the characteristic function is only described in this embodiment.
The profile creation device of this embodiment provides a UI (User Interface) for setting the parameters stored in the parameter storage part 703 in FIG. 11 interactively. This function is implemented as a part of the function of the data input part 701 in FIG. 11, and presented to the user at step of acquiring the parameters at step S801 in FIG. 8.
The slide bar 905 is employed to set the parameter a of the formula (4), in which if the bar is moved to the right, the low saturation area (area R) is increased, or if the bar is moved to the left, the low saturation area is decreased. The size of the outermost contour 907 in the low saturation area displayed on the low saturation area checking part 902 is changed corresponding to the movement of the slide bar 905. The slide bar 906 is employed to set the lightness of the equal lightness plane displayed on the low saturation area checking part 902, in which if the bar is moved to the right, the lightness is increased, or if the bar is moved to the left, the lightness is decreased. The lightness displayed on the low saturation area checking part 902 is changed corresponding to the movement of the slide bar 906.
The low saturation area checking part 902 illustrates a graph representing the equal lightness plane in the color space, having the outermost contour 907 in the low saturation area, a chromaticity point 908 on the gray line of the device, and a point 909 indicating the achromatic color. If the OK button 903 is depressed, the value of the parameter dependent on the position of the slide bar is set, and stored in the parameter storage part 703. If the cancel button 904 is depressed, the setting is ignored.
FIG. 10 is a diagram showing another UI example of the parameter setting part 901, in which the UI comprises a text box 1001 for specifying the parameter a of the formula (4) in numerical value directly, a text box 1002 for specifying the parameter β of the formula (5), and a text box 1003 for specifying the lightness displayed on the low saturation area checking part 902.
As described above, the parameter α is a value of one or greater, in which if the value is greater, the color signal in the low saturation area is changed more gently, but the range affected by conversion is extended. On the contrary, if the value is smaller, the range affected by conversion is narrowed, but the color signal in the low saturation area is rapidly changed, possibly causing a gradation failure. Also, the parameter β decides the shape of ellipse, in which if β=1, a circle is obtained. For the parameter β, if the value is greater, the range affected by conversion is extended, while if the value is smaller, the color signal in the low saturation area is rapidly changed, possibly causing a gradation failure. If the OK button 903 is depressed, the value of the parameter dependent on the position of the slide bar is set, and stored in the parameter storage part 703. If the cancel button 904 is depressed, the setting is ignored.
With the profile created by the profile creation device of this third embodiment, like the profile created by the profile creation device of the second embodiment, the color conversion method of the first embodiment may be employed in which the color signal inside the low saturation area and the color signal outside the low saturation area are differently converted. And with the profile creation device of this embodiment, the user can set the size of the low saturation area interactively.
As described above, with the present invention, in conversion of color signals between the image output devices having different color reproduction characteristics, both a conversion required for the low saturation area and a conversion required for the chromatic color can be compatible, whereby an image reproduced on an input system is suitably reproduced on an output system. That is, in order to convert the color signal of the image output device into the color signal in the color space for color matching, the white color (gray) of a colorimetric value dependent on the device in the low saturation area is converted into the same color signal in the color space for color matching, and the chromatic color such as flesh color not dependent on the device is converted into the same colorimetric value color signal in the color space for color matching. With this configuration, in the color matching, the “color cast” is suppressed, and the dynamic range of the device is utilized to the maximum, whereby the chromatic color such as flesh color can be reproduced in the colorimetrically equal color.
a conversion step of converting said color signal A into the color signal D in accordance with a conversion function of converting the color signal on a line segment BP connecting a color signal B that is an intersection between a line segment connecting the color signal A and the color signal P and the outermost contour of a low saturation area and said color signal P into the color signal on a line segment BO connecting the color signal B and the color signal O, when said color signal A is contained in said low saturation area on the color space containing the color signal P of gray for said image output device at a lightness of said color signal A and the color signal O that is calorimetrically achromatic color;
wherein when said color signal A is not contained in said low saturation area, said color signal is not converted at said conversion step.
2. The color conversion method according to claim 1, wherein said color space is a uniform color space such as CIELAB.
3. The color conversion method according to claim 1, wherein said conversion step comprises setting said color signal D at a position on said line segment BO corresponding to said color signal A located on said line segment BP, so that a ratio of the length of said line segment BP to the length of said line segment BA may be equal to a ratio of the length of said line segment BO to the length of said line segment BD.
4. The color conversion method according to claim 1, wherein said color signal P, said color signal O and said color signal B are located on the equal lightness plane.
a first conversion step of converting said color signal into a color signal A on a calorimetric color space;
a second conversion step of acquiring a color signal P of gray for said image output device at a lightness of said color signal A;
a determination step of determining whether or not said color signal A is contained in a low saturation area containing the color signal P and a color signal O of calorimetrically achromatic color at a lightness of said color signal A;
a third acquisition step of acquiring a color signal B that is an intersection between a line segment connecting said color signal P and said color signal A and the outermost contour of said low saturation area, when it is determined that said color signal A is contained in said low saturation area at said determination step;
a second conversion step of converting said color signal A into the color signal D in accordance with a conversion function of converting the color signal on the line segment connecting said color signal B and said color signal P into the color signal on the line segment connecting said color signal B and said color signal O; and
an output step of outputting said color signal D.
6. The color conversion method according to claim 5, wherein said third acquisition step comprises acquiring an intersection closer to said color signal A as said color signal B, when there are a plurality of intersections between the line segment connecting said color signal P and said color signal A and the outermost contour of said low saturation area.
7. The color conversion method according to claim 5, further comprising a setting step of setting said color signal A as said color signal D, when it is determined at said determination step that said color signal A is not contained in said low saturation area.
8. The color conversion method according to claim 5, further comprising a setting step of setting said color signal O as said color signal D, when it is determined at said determination step that said color signal A is contained in said low saturation area and said color signal P is the same as said color signal A.
9. The color conversion method according to claim 5, wherein said second conversion step comprises converting said color signal A into said color signal D, so that a ratio BP/BA of the distance BP between said color signal B and said color signal P to the distance BA between said color signal B and said color signal A may be equal to a ratio BO/BD of the distance BO between said color signal B and said color signal O to the distance BD between said color signal B and said color signal D.
10. A program for enabling a computer to perform the color conversion method according to claim 1.
11. A computer readable recording medium storing the program according to claim 10.
determining whether or not said uniform color space signal is inside a low saturation area;
calculating a gray signal of an image forming apparatus; and
correction processing said uniform color space signal according to said gray signal if it is inside the low saturation area as a result of determination.
13. A color conversion method for converting a color signal of a predetermined image output device into a color signal in a color mapping color space that is a common color space for mutually converting the color signals of plural kinds of image output devices, comprising:
a first conversion step of converting the color signal of said predetermined image output device into a color signal A in a uniform color space; and
a second conversion step of converting the color signal A in said uniform color space into the color signal in said color mapping color space;
wherein said second conversion step comprises converting the color signal inside the low saturation area R that is set in said uniform color space and the color signal outside said low saturation area R into the color signal in said color mapping color space in different ways.
14. The color conversion method according to claim 13, wherein supposing that a color signal having the same lightness as said color signal A on the gray line of said predetermined image output device is the color signal P, and a color signal indicating achromatic color in said uniform color space is a color signal O, said low saturation area R is defined as the area containing said signal P and said signal O.
15. The color conversion method according to claim 14, wherein said second conversion step comprises outputting directly said color signal A as the color signal after conversion, when said color signal A exists outside said low saturation area R, or when said color signal A exists inside said low saturation area R, outputting a color signal D as the color signal after conversion so that a ratio of the distance BO between said color signal B and said color signal O to the distance BD between said color signal B and said color signal D may be equal to a ratio of the distance BP between said color signal B and said color signal P to the distance BA between said color signal B and said color signal A, supposing that an intersection between the straight line G passing through said color signal A and said color signal P and said low saturation area R is the color signal B in said uniform color space and the color signal on the straight line passing through said color signal B and said color signal O is the color signal D.
16. The color conversion method according to claim 15, further comprising acquiring an intersection closer to said color signal A among the intersections as said color signal B, when there are a plurality of intersections between said straight line G and said low saturation area R.
17. A profile creation device for creating the profile data for mutually converting a color signal of a predetermined image output device into a color signal in a color mapping color space that is a common color space for mutually converting the color signals of plural kinds of image output devices, comprising:
first conversion means for converting a discrete color signal C of said predetermined image output device into a color signal A in a uniform color space;
second conversion means for converting said color signal A into a color signal D in said color mapping color space; and
profile creation means for creating a profile from the correspondent information between said discrete color signal C and said color signal D;
wherein said second conversion step comprises converting said color signal A into the color signal in said color mapping color space in different ways, depending on whether said color signal A exists inside the low saturation area R that is preset in said uniform color space or outside said low saturation area R.
18. The profile creation device according to claim 17, wherein supposing that a color signal having the same lightness as said color signal A on the gray line of said predetermined image output device is a color signal P, and a color signal indicating achromatic color in said uniform color space is a color signal O, said low saturation area R is defined as an area containing said signal P and said signal O.
19. The profile creation device according to claim 18, wherein said second conversion means outputs directly said color signal A as the color signal after conversion, when said color signal A exists outside said low saturation area R, or when said color signal A exists inside said low saturation area R, outputs a color signal on a straight line passing through said color signal B and said color signal O, as the color signal D, at which the distance BO between said color signal B and said color signal O is divided at a ratio of the distance BP between said color signal B and said color signal P to the distance BA between said color signal B and said color signal A supposing that an intersection between the straight line G passing through said color signal A and said color signal P and said low saturation area R is the color signal B in said uniform color space.
20. The profile creation device according to claim 19, wherein an intersection closer to said color signal A among the intersections is made said color signal B, when there are a plurality of intersections between said straight line G and said low saturation area R.
21. The profile creation device according to claim 17, further comprising UI presentation means for presenting a user interface for enabling the user to set the parameters for controlling the size of said low saturation area R, and area decision means for deciding said low saturation area R based on said parameters set via said user interface.
22. A profile creation method for creating the profile data for mutually converting a color signal of a predetermined image output device and a color signal in a color mapping color space that is a common color space for mutually converting the color signals of plural kinds of image output devices, comprising:
a first conversion step of converting a discrete color signal C of said predetermined image output device into a color signal A in a uniform color space;
a second conversion step of converting said color signal A into a color signal D in said color mapping color space; and
a profile creation step of creating a profile from the correspondent information between said discrete color signal C and said color signal D;
23. A program for enabling a computer to perform the color conversion method according to claim 13.
24. A program for enabling a computer to operate as the profile creation device according to claim 17.
25. A computer readable recording medium storing the program according to claim 23.
subjecting color signals in a low saturation area to conversion processing according to a relationship between a gray and an achromatic color specific to the image forming apparatus; and
subjecting color signals of memory colors to conversion processing for memory color;
wherein the color signals in a low saturation area and the color signals of memory colors are generated from an image.
27. The image processing method according to claim 26, wherein the low saturation area is set to be a circle or an ellipse.
US11138330 2004-06-01 2005-05-27 Color conversion method Expired - Fee Related US7453602B2 (en)
JP2004163751A JP2005347972A (en) 2004-06-01 2004-06-01 Color converting method
JP2004-163751 2004-06-01
JP2004175998A JP4541772B2 (en) 2004-06-14 2004-06-14 Color conversion method and a profile creation method and apparatus
JP2004-175998 2004-06-14
US20050264837A1 true true US20050264837A1 (en) 2005-12-01
US7453602B2 US7453602B2 (en) 2008-11-18
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