JP2005184394A - Color gamut conversion method and image processing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To control at a certain level reproducibility such as suppressing of falling of saturation when reproducing an image after conversion, in a color gamut conversion for compressing a color reproduction range. <P>SOLUTION: When setting a point B on the outermost periphery of a color reproduction range of an output system device which corresponds to a color represented by a point A on the outermost periphery of color reproduction range of an input system device, the point B is so set that the color difference from the color of the point A is minimum. Since the color on the outermost periphery of the color reproduction range of the input system device is converted to the color of minimum color difference within the color reproduction range of the output system device in the same hue, falling of saturation is generally suppressed, allowing the image whose impression is close to the image reproduced by the input system device to be reproduced by the output system device. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a color gamut conversion method and an image processing apparatus, and more particularly to color gamut compression.

  The color reproduction characteristics of devices that input and output images, such as monitors, digital cameras, printers, and copiers, differ depending on the devices. For this reason, when image data is used between different apparatuses, a color management system (hereinafter referred to as CMS) is used to unify the color reproduction characteristics of the image between the apparatuses.

  In this CMS, in order to reproduce the same image satisfactorily by a plurality of input / output devices, the color signal of the input system device is changed to the color signal of the output system device so that the colorimetric values of the colors reproduced by each device are matched. Convert. Here, the input system device indicates a device that is a target of color matching. For example, when the color of a color printer is matched with a color monitor, the color monitor is an input system device and the color printer is an output system device.

  However, even if CMS is used, it is physically impossible to achieve the same color in all devices. For example, the color reproduction range of a CRT monitor is generally different from the color reproduction range of a printer, and the color reproduction range of a printer is smaller than the color reproduction range of a CRT monitor in the green and blue color gamuts. Depending on the recording medium (printing paper) used, the color reproduction range in the cyan color gamut is larger than that of the CRT monitor. In this way, colors that are within the color reproduction range of the input system device but are outside the color reproduction range of the output system device cannot be reproduced by the output system device as they are.

  On the other hand, it is known to perform color gamut compression. In this color gamut compression, it is desirable to convert within the color gamut of the output system device while keeping the impression of the image expressed by the input system device as much as possible. As a method of such color gamut compression, as described in Patent Document 1, a method of compressing lightness and saturation toward a predetermined point on the achromatic color axis on a uniform hue plane is common.

  FIG. 23 is a schematic diagram for explaining a conventional color gamut compression method, in which the horizontal axis represents saturation and the vertical axis represents a uniform hue plane. In this figure, 5001 is the outermost contour of the color reproduction range in the input system device, 5002 is the outermost contour of the color reproduction range in the output system device, point O is a predetermined point indicating the compression direction, and point A is the outermost color reproduction range of the input system device. A point on the outline, point B, indicates a point on the outermost outline of the color reproduction range of the output device and on the straight line OA. Here, the color on the line B-A is within the color reproduction range of the input system device, but is outside the color reproduction range of the output system device, and cannot be reproduced by the output system device. In the color gamut compression in the conventional method, the lightness and saturation are compressed with a constant hue, and for example, the color on the line segment OA in the input system device is converted to the color on the line segment OB in the output system device. .

  As a method of converting the color on the line segment OA of the input system device to the color on the line segment OB of the output system device, there are three methods described with reference to FIGS. It has been done. In these figures, each point on the horizontal axis indicates a color on the straight line OA of the input system device, and each point on the vertical axis indicates a color on the straight line OB of the output system device.

  The conversion shown in FIG. 24 is a conversion method generally called clipping, and for colors within the color reproduction range of the input system device, the colors within the color reproduction range of the output system device are output as they are, and are outside the color reproduction range of the output system device. This color is converted to the outermost outline of the color reproduction range of the output system device. In this method, colors within the color reproduction range of the output system device can be accurately reproduced, but colors outside the color reproduction range of the output system device are greatly compressed, and there is a problem such as loss of gradation in highly saturated colors. is there. The conversion method shown in FIG. 25 is a method in which the entire color reproduction range of the input system device is linearly compressed and converted to a color within the color reproduction range of the output system device. In this method, gradation of an image can be saved, but accurate color reproduction cannot be performed for colors within the color reproduction range of the output system device, resulting in a dull image with low saturation. In the conversion method shown in FIG. 26, the colors in the color reproduction range of the input system device are output as they are in the low saturation portion, and the colors in the high saturation portion are compressed while maintaining a certain level of gradation. A method of converting to a color within the color reproduction range. In this method, gradation preservation and saturation preservation can be performed to some extent, and is the most preferable compression method among the above three methods.

JP 09-098298 A

  However, even if the compression method shown in FIG. 26, which is most preferable among the above methods, is used, the image reproduced by the output system device has lower saturation than the image reproduced by the input system device. May have a dull impression. This is considered to be one of the reasons that the saturation compression is large in the intermediate saturation color. For example, in FIG. 23, the color of the output system device corresponding to the color indicated by the point A of the input system device is the color indicated by the point B when based on the above conventional compression method. The color of the closest output system device (the color with the smallest color difference) is the color indicated by the point C. Point B has a much smaller saturation than point C, and it can be seen that the conventional compression method compresses the saturation more than necessary.

  That is, in the conventional compression method, as shown in FIG. 23, the point B on the outermost contour of the color reproduction range of the output system device corresponding to the point A on the outermost contour of the color reproduction range of the input system device is changed to the above point. Since it is determined on the straight line OA connecting A and the compression fixed point O, the amount of compression of saturation and lightness is determined according to the point A (and the fixed point O) to be compressed. For this reason, in the color reproduction of the output system device, the saturation is low as described above, causing problems such as a dull impression and lightness that is lower than necessary.

  The present invention has been made to solve the above problems, and the object of the present invention is to determine the color of the input system device when the color reproduction range of the input system device is wider than the color reproduction range of the output system device. In the conversion to a color within the color reproduction range of the output system device, the reproducibility is controlled to some extent when the output system device reproduces the image reproduced by the input system device, such as suppressing the decrease in saturation. An object of the present invention is to provide a color gamut conversion method and an image processing apparatus that can be used.

  To this end, the present invention provides a color gamut conversion method for performing color gamut conversion based on a relationship in which the color of the first area is associated with the color of the second area smaller than the area in the color space, The color on the outermost contour on the second region corresponding to the color on the outermost contour, the color on the outermost contour representing the predetermined fixed point in the color space and the color on the outermost contour of the first region It is determined on the basis of a predetermined correspondence not associated with a straight line connecting points in the space, and the color on the outermost contour on the determined second region and the corresponding outermost contour on the first region A step of preparing the relationship that associates the color of the first region with the color of the second region, which is determined based on the color of the first region, and performing color gamut conversion according to the relationship. .

  An image processing apparatus that performs color gamut conversion based on a relationship in which a color of a first area is associated with a color of a second area that is smaller than the area in the color space, and converts the color signal. The color on the outermost contour on the second area corresponding to the color on the outermost contour of the image, the color on the outermost contour indicating the predetermined fixed point in the color space and the color on the outermost contour of the first region. It is determined on the basis of a predetermined correspondence not associated with a straight line connecting points in the color space, and the color on the outermost contour on the determined second region and the outermost contour of the first region corresponding thereto The relationship between the color of the first area and the color of the second area determined based on the upper color is provided as a table, and color gamut conversion is performed using the table.

  According to the above configuration, when obtaining the relationship in which the color of the first region is associated with the color of the second region smaller than that region, the outermost contour on the second region corresponding to the color on the outermost contour of the first region. A predetermined correspondence in which the upper color is not associated with a straight line connecting a predetermined fixed point in the color space and a point in the color space indicating a color on the outermost contour of the first region. Since it is determined on the basis of the relationship, the color on the outermost contour on the second region corresponding to the color on the outermost contour of the first region is defined as the point indicating the color on the outermost contour of the first region and the predetermined fixed point. It can be determined regardless of the relationship (the relationship of the straight line).

  As a result, for example, the predetermined correspondence relationship may be a relationship that minimizes the color difference between the color on the outermost contour of the first region and the color on the outermost contour on the second region, or the outermost contour of the first region. When the point indicating the upper color moves in a predetermined direction along the outermost contour and moves to a point indicating the outermost color of the first region, the point indicating the outermost color of the second region is It is possible to make a relationship that moves in the same predetermined direction along the outermost contour and moves to a point indicating a color on the outermost contour of the different second region, and suppresses a decrease in saturation, etc. It is possible to control the reproduction characteristics when the reproduced image is reproduced by the output system device.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
<First Embodiment>
Color space for color conversion Human perception has three attributes: lightness representing color brightness, saturation representing color vividness, and hue representing color system. Examples of the color space based on these three attributes include CIE / L ^* C ^* h color space. This L ^* C ^* h color space is obtained by converting the CIE / L ^* a ^* b ^* color space or the CIE / L ^* u ^* v ^* color space into polar coordinates, where L ^* is lightness and C ^* is saturation. H represents the hue angle. In the following description, the CIE / L * C * h color space is used as a color space corresponding to human perception, but the color gamut conversion method of the present invention uses another color space based on the above three attributes. It is clear from the following explanation that there is no problem even if it is used.

Color Gamut Conversion Method FIG. 1 is a schematic diagram for explaining a color gamut conversion method according to the first embodiment of the present invention, where the horizontal axis represents chroma C ^* and the vertical axis represents lightness L ^*. ) Shows the surface.
In the figure, 101 is the outermost contour of the color reproduction range of the input system device, 102 is the outermost contour of the color reproduction range of the output system device, and point 0 is a predetermined compression fixed point on the vertical axis. In this embodiment, in the area where the color reproduction range of the output system device is larger than the color reproduction range of the input system device, the color reproduction range of the output system device is converted to match the color reproduction range of the input system device. In the following description, it is assumed that the color reproduction range 101 of the input system device includes the outermost color reproduction range 102 of the output system device.

  In FIG. 1, first, a point B on the outermost color reproduction range of the output system device corresponding to the color indicated by the point A on the outermost color reproduction range of the input system device is determined. That is, the position of the point B corresponding to the point A is set in consideration of the compression ratio in the lightness direction and the saturation direction. For example, in the setting of the point B with respect to the point A as shown in FIG. 7, the lightness is not compressed much (ΔL is small), and the saturation can be greatly compressed (ΔC is large). On the other hand, in the setting of the point B with respect to the point A shown in FIG. 8, the lightness can be greatly compressed (ΔL is large) so that the saturation is not so compressed (ΔC is small).

  In addition to the setting example of the point B described with reference to FIGS. 7 and 8, the point B can be predicated so that the color difference between the color of the point B and the color of the point A is minimized. In this case, since the color in the outermost color reproduction range of the input system device is converted to a color with the smallest color difference within the color reproduction range of the output system device in the same hue, in general, a decrease in saturation is suppressed. Thus, an image having an impression close to that of the image reproduced by the input system device can be reproduced by the output system device.

FIG. 2 is a diagram for explaining a method of converting the color corresponding to the point on the line segment OBA shown in FIG. 1 into the color corresponding to the point on the line segment OB.
In FIG. 2, L (OB) indicates the distance of the line segment OB, and L (OBA) indicates the sum of the distance of the line segment OB and the distance of the line segment BA. Also, the horizontal axis Vi in FIG. 2 indicates the path from the point O to the point on the line segment OBA corresponding to the color of the input system device. Similarly, the vertical axis Vo indicates the path from the point O to the point on the line segment OB corresponding to the color of the output system device. In the color gamut conversion method of the present embodiment, a predetermined point C is defined on the line segment OB as will be described later, and a point Vi corresponding to the color of the input system device is set as the color of the output system device by the following equation. Convert to the corresponding point Vo.
When 0 ≦ Vi ≦ L (OC),
Vo = Vi (1)
When L (OC) ≦ Vi ≦ L (OBA),
Vo = L (CB) / L (CBA) × (Vi−L (OC)) + L (OC) (2)
That is, in FIG. 1, the color corresponding to the point on the line segment OC of the input system device is output as it is, and the color corresponding to the point on the line segment CB and the line segment BA is the point on the line segment CB of the output system device. Is converted to a color corresponding to. In equation (2), L (CB) is the distance of the line segment CB, L (CBA) is the sum of the distance of the line segment CB and the distance of the line segment BA, and L (OC) is the distance of the line segment OC. Show.

  Further, the color gamut conversion method of the present embodiment is a method for easily determining the position of the point C regardless of the size of the operation area of the output system device (distance of the line segment OB in FIG. 1). A method of making the distance of the line segment OC constant, 2) a method of making the distance of the line segment CB constant, 3) a method of making the ratio of the distance of the line segment OB and the distance of the line segment OC, 1) to above Any of 3) can be used. In particular, the method 3) described above is applicable to the input system device even when the operating range of the output system device is large (the distance of the line segment OB in FIG. 1 is long) or small (the distance of the line segment OB is short). A region (the line segment OC in FIG. 1) in which the color of the color is directly output can be secured.

Color Gamut Conversion Procedure FIG. 3 is a flowchart showing the color gamut conversion procedure of the present embodiment.
The color conversion of this embodiment is performed according to the following procedure. First, in step S301, color reproduction range data of an input system device and an output system device is acquired. Next, in step S302, a color reproduction range outermost conversion table is created. As will be described in detail later, this conversion table includes discrete colors on the outermost contour of the color reproduction range of the input device, and colors on the outermost contour of the color reproduction range of the output device corresponding to these colors. Is a table storing the correspondence relationship.

  Next, in step S303, the color signal P (Lp, Cp, hp) of the input system device is input. In step S304, it is determined whether the color signal P input in step S303 is within the color reproduction range or outside the color reproduction range of the output system device. If it is determined that the color signal P is within the color reproduction range of the output device, the process proceeds to step S305. If it is determined that the color signal P is outside the color reproduction range, the process proceeds to step S308.

  If the color signal P is within the color reproduction range of the output system device, in step S305, the color B on the outermost color reproduction range of the output system device relating to the color signal P is obtained.

FIG. 4 is a diagram for explaining a method for obtaining the color B based on the color signal P, where the horizontal axis indicates a uniform hue (h = hp) surface with saturation C ^* and the vertical axis with lightness L ^* . . In this figure, 401 is the outermost contour of the color reproduction range of the input system device, 402 is the outermost contour of the color reproduction range of the output system device, point O is the predetermined compression fixed point, and point P is the color signal P input in step S303. Show. In the figure, the color B is obtained as the color B at the intersection of the straight line connecting the point O and the point P and the color reproduction range outermost contour 402 of the output system device.

  Next, in step S306, the color A on the color reproduction range outermost contour of the input system device corresponding to the color B is obtained using the color B obtained above and the color reproduction range outermost contour conversion table created in step S302. Ask. This color A is indicated by point A in FIG.

  Next, in step S307, the point P on the line segment OBA is converted to the point P ′ on the line segment OB by the equations (1) and (2) described with reference to FIG. The color signal P ′ of the output system device to be obtained is obtained.

On the other hand, if it is determined in step S304 that the color signal P is outside the color reproduction range of the output system device, the color signal P and the color reproduction range outermost conversion table created in step S302 are obtained in step S308. The point A and the point B shown in FIG. 5 are obtained, and the process proceeds to step S307. That is, in the correspondence between the points A and B in the outermost color reproduction range table, the line segment connecting the points A and B is obtained on L ^* C ^* and the point P is added. A set of point A and point B in the equation is obtained as point A and point B shown in FIG.

Hereinafter, some processing contents of the above-described steps will be described in more detail.
Color Reproduction Range Data FIG. 6 is a diagram for explaining the coordinate system used in this data regarding the color reproduction range data acquired in step S301. In the figure, the horizontal axis represents the saturation hue C ^* , and the vertical axis represents the lightness L ^* . Further, a point O in the figure is a predetermined compression fixed point (L ^* , C ^* ) = (L (O), 0), and a straight line G is a straight line passing through the point O and parallel to the saturation axis (horizontal axis). Here, the arbitrary point P can be expressed using the lightness L, the saturation C, and the hue angle h, and at the same time, the angle θ formed by the straight line G and the straight line OP and the distance S of the line segment OP. It can also be expressed. Conversion from the LCh coordinate system to the hθS coordinate system is performed by the following equation.
θ = tan ⁻¹ ((L−L (O)) / C) (3)
S = (C ² + (L−L (O)) ² ) ^1/2 (4)
Conversely, conversion from the hθS coordinate system to the L * C * h coordinate system is performed by the following equation.
L = Ssin (θ) + L (O) (5)
C = Scos (θ) (6)
The hue angle h is common to both coordinate systems.

  FIG. 9 is a diagram illustrating an example of the color reproduction range data acquired in step S301. As shown in the figure, the color reproduction range data is related to discrete h (hue angle) and θ, the S value So of the outermost color of the color reproduction range of the output system device at h and θ, and the input system. This is in the form of a table storing S values Si of colors in the outermost color reproduction range of the apparatus.

  FIG. 11 is a diagram showing an equi-hue surface for explaining the relationship between θ, Si, and So. 1101 is the outermost color reproduction range of the input system device, 1102 is the outermost color reproduction range of the output system device, and the point O. Is a predetermined compression fixed point, point Mi is a point indicating the outermost color of the color reproduction range of the input system device at θ, and point Mo is a point indicating the outermost color of the color reproduction range of the output system device at θ. Here, the above-described value Si represents the distance of the line segment OMi, and the value So represents the distance of the line segment OMo. Further, the color of the outermost color reproduction range of each of the input system device and the output system device with respect to the hue angles h and θ not found in the color reproduction range data table is obtained using a known interpolation method based on the color reproduction range data. It is done.

Color Reproduction Range Outermost Outline Conversion Table FIG. 10 is a diagram showing an example of the color reproduction range outermost outline conversion table created in step 302 of FIG. As shown in the figure, the color reproduction range outermost conversion table is for the discrete h (hue angle) and θi (the value of Θ in the hΘS coordinate system), and the color reproduction range of the input system device indicated by h and θi. It is the table which stored the value (theta) o of (theta) of the color of the output type apparatus corresponding to the color on the outermost frame.

  FIG. 12 is a diagram showing an equi-hue (constant h) surface for explaining the relationship between θi and θo. In this figure, 1201 is the outermost color reproduction range of the input system device, 1202 is the outermost color reproduction range of the output system device, point O is a predetermined compression fixed point, and straight line G passes through point O and is the saturation axis (horizontal axis). The point A is a point indicating the color of the outermost color reproduction range of the input system device at θi, the point B is a point indicating the color of the output system device corresponding to the point A, and the above θo is a line G and It is an angle formed with the straight line OB. Θo corresponding to an arbitrary θi can be obtained from the color reproduction range outermost conversion table using a known interpolation method. Similarly, θi corresponding to an arbitrary θo can be obtained from the color reproduction range outermost contour conversion table using a known interpolation method.

  FIG. 13 is a flowchart showing a procedure for creating the above-described color reproduction range outermost conversion table. The color reproduction range outermost conversion table in the present embodiment is created according to the following procedure. First, in step S1301, the color of the point A on the outermost contour of the color reproduction range of the input system device at discrete h and θi is obtained using the color reproduction range data described above.

  Next, in step S1302, the point B corresponding to the point A is a point within the color reproduction range of the output system device, for example, as described above with reference to FIG. It determines so that the color difference with the color to show becomes the minimum. In step S1303, the θ-coordinate value θo of the point B is obtained, and in step S1304, θi and θo are stored in the color reproduction range outermost conversion table shown in FIG.

  Next, in step S1305, it is confirmed whether or not the storage has been completed for all θi. If not, the process returns to step S1301. In step S1306, it is confirmed whether or not the storage has been completed for all h. If not, the process returns to step S1301.

Color reproduction range outside judgment Figure 14 is a flowchart showing the detailed procedure of the color reproduction range outside judgment carried out in step S304 of FIG. The inside / outside determination of the color reproduction range is performed by the following procedure. First, in step S1401, the coordinates of the color signal P (Lp, Cp, hp) input in step S303 in FIG. 3 are converted into hθS coordinates (hp, θp, Sp). Next, in step S1402, using the hθS coordinates (hp, θp, Sp) obtained in step S1401 and the color reproduction range table described above, the points on the outermost color reproduction range of the output system at these hp and θp. The value So of S is obtained.

  Next, in step S1403, the above Sp and So are compared. If Sp is equal to or less than So, the color signal P is within the color reproduction range of the output system device. If Sp is greater than So, the color described above is used. It is determined that the signal P is outside the color reproduction range of the output system device.

  As described above, the color gamut conversion method of the present embodiment, as shown in FIG. 1, the point of compression on the achromatic color axis is the point O, the point of the color reproduction range of the input system device is the point A, When a point indicating the color of the output system device corresponding to the color indicated by the point A is a point B, the point B is determined so as to minimize the color difference between the point B and the point A, for example. A point on the line segment OBA indicating the color of the apparatus is converted into a point on the line segment OB indicating the color of the output system apparatus. By this conversion, it is possible to optimize the ratio of compression in the lightness direction and the saturation direction, and an image reproduced by the input system device can be suitably reproduced by the output device.

<Second Embodiment>
In the color reproduction range outermost conversion table in the first embodiment, the correspondence between the color on the outermost color reproduction range of the input system device and the color on the outermost color reproduction range of the output system device, for example, the color difference It is created so that is minimized. However, according to the conversion relationship, the gradation may be lost particularly in high-saturation colors.

FIG. 15 is a schematic diagram illustrating a correspondence relationship between the input system device and the output system device in the outermost color reproduction range. In the figure, 1501 is the outermost outline of the color reproduction range of the input system apparatus, 1502 is the outermost outline of the color reproduction range of the output system apparatus, point R0 is a point on the outermost outline of the color reproduction range of the output system apparatus, and points R1 and R2 are The points on the outermost color reproduction range of the input device are shown. Here, the point R0 is a color within the color reproduction range of the output system device when the color on the line segment Lr connecting the points R1 and R2 on the outermost color reproduction range of the input system device is Cr. This is a point indicating a color having a minimum color difference from the color Cr. That is, since the CIE / L ^* C ^* h color space shown in FIG. 15 is a uniform color space, the distance in FIG. 15 almost represents a color difference. As can be seen from this, the point on the outermost color reproduction range 1502 of the output device is the shortest distance from the point on the line segment Lr connecting the points R1 and R2 on the outermost color reproduction range of the input device. The point is R0. Therefore, all the colors on the line segment Lr are converted to the color indicated by the point R0 of the input system device, and as a result, the gradation may be lost.

  In view of the above problems, the second embodiment of the present invention is based on the correspondence relationship that minimizes the color difference with respect to the color reproduction range outermost conversion table creation method in the first embodiment, but this is corrected. Related to color gamut conversion. This makes it possible to perform color gamut conversion so as to maintain a moderate gradation. In the color gamut conversion method of the present embodiment, the steps other than the creation of the color reproduction range outermost contour conversion table are the same as those in the first embodiment, and thus the description thereof is omitted.

Color Reproduction Range Outermost Outline Conversion Table FIG. 16 is a flowchart showing a procedure for creating a color reproduction range outermost outline conversion table in the second embodiment of the present invention. The color reproduction range outermost conversion table of this embodiment is created by the following procedure. First, in step S1601, conversion of the maximum saturation color of the input system device is determined at discrete h (hue angle).

FIG. 17 is a schematic diagram for explaining the conversion of the maximum chroma color, where the horizontal axis indicates a saturation hue cross section with chroma C ^* and the vertical axis with lightness L ^* . In this figure, 1701 is the outermost contour of the color reproduction range of the input system device, 1702 is the outermost contour of the color reproduction range of the output system device, point O is a predetermined compression point, and straight line G passes through point O and is the saturation axis (horizontal axis). The point Qi is the maximum saturation color of the input system device at the hue angle shown in the figure, the point Qo is the point indicating the color of the output system device corresponding to the point Qi and the minimum color difference, and Φi is The value of θ in the hθS coordinate system of the point Qi, Φo indicates the value of θ of the point Qo. In step S1601, the value of Φo corresponding to Φi is determined.

  Next, in step S1602, regarding a discrete θi within a range of Φi to 90 °, a point A that is a point on the outermost contour of the color reproduction range of the input system device corresponding to this θi is obtained. Next, in step S1603, the point B corresponding to the point A is a point within the color reproduction range of the output system device having the hue angle h, and the color difference between the color indicated by the point and the color indicated by the point A is minimized. Decide to be. Next, in step S1604, the θ-coordinate value θo of the point B is obtained, and in step S1605, the relationship between θi and θo is stored in the color reproduction range outermost conversion table described with reference to FIG.

Next, in step S1606, it is checked whether or not the storage has been completed for all θi of Φi and 90 degrees or less. If not, θi is incremented and the process returns to step S1602, and if completed, step S1607 is completed. Proceed to
In step S1607, the value of θo relating to θi stored in the outermost color conversion range conversion table is corrected.

  FIG. 18 is a diagram showing the color reproduction range outermost conversion table obtained by the correspondence relationship between θi and θo, that is, the correspondence relationship with the smallest color difference, and the horizontal axis is on the color reproduction range outermost contour of the input system device. Θi related to the color, and the vertical axis indicate θo related to the color on the outermost contour of the color reproduction range of the output system device.

As shown in the figure, Φk and Φk ′ are the θ coordinates of the point Qk and the point Qk ′ in FIG. 17, and in the color gamut conversion based on the minimum color difference correspondence, the range of Φk ′ to Φk of the input system device 17 (points on the line segment connecting point Qk and point Qk ′ on the outermost color reproduction range of the input system device in FIG. 17) all correspond to Φo of the output system device (in FIG. 17). It can be seen that the tone is lost due to the conversion to the point Qo) on the outermost color reproduction range of the output device. Accordingly, the color reproduction range outermost conversion table is corrected so that Θo regarding Θi shown in FIG. 18 becomes a monotonically increasing function. That is, in step S1607, the value of θo related to θi is corrected to a value given by the following expression using a predetermined value Φa that is not less than Φk and not more than 90 degrees.
When Φi ≦ θi ≦ Φa Θo = (F (Φa) −Φo) / (Φa−Φi) × (θi−Φi) + Φo (7)
When Φa ≦ θi ≦ 90 Θo = F (θi) (8)
However, the function F represents the conversion from θi to θo in the color reproduction range outermost conversion table before correction based on the minimum color difference, that is, the conversion function of FIG.

  FIG. 19 shows a state after the correction based on the above equations (7) and (8), including the correction according to the equations (9) and (10) regarding the discrete θi in the range of −90 degrees or more and Φi or less as described below. It is a schematic diagram which shows a color reproduction range outermost contour conversion table. As shown in the figure, according to the method of the present embodiment, by appropriately setting Φa, θi indicating the color on the outermost contour of the color reproduction range of the input system device can be output while maintaining the gradation. It is possible to convert to θo indicating a color on the outermost contour of the color reproduction range of the apparatus. It should be noted that the setting of Φa can be determined in advance while confirming the gradation or the like through experiments or the like.

In step S1608 and subsequent steps, the same processing as steps S1602 to S1606 is performed for discrete θi of −90 degrees or more and Φi or less. That is, first, in step S1608, a point A on the outermost contour of the color reproduction range of the input device corresponding to θi is obtained. In step S1609, the point B corresponding to the point A is determined so that the color difference between the color indicated by the point and the color indicated by the point A is minimized at a point within the color reproduction range of the output system device. . Next, in step S1610, the θ coordinate value θo of the point B is obtained. Next, in step S1611, it is confirmed whether or not the storage has been completed for all θi of −90 degrees or more and Φi or less. If not, θi is incremented and the process returns to step S1608, and if completed, step S1612 is performed. Proceed to In Step A1612, the value of θo related to θi is corrected to a value given by the following equation using a predetermined value Φb that is greater than −90 degrees and less than or equal to Φk ′.
When −90 ≦ θi ≦ Φb Θo = F (θi) (9)
When Φb ≦ θi ≦ Φi Θo = (Φo−F (Φb)) / (Φi−Φb) X (θi−Φb) + F (Φb) · (10)

  According to the method of the present embodiment, it is possible to convert the color on the outermost color reproduction range of the input device to the color on the outermost color reproduction range of the output device while maintaining the gradation.

<Third Embodiment>
Image Processing Apparatus Each of the above-described embodiments has described the method of color gamut conversion without limiting the input system apparatus and the output system apparatus. However, the present invention provides an image of an image output apparatus connected with an image signal of the input system apparatus. Of course, the present invention can also be implemented as an image processing apparatus that converts signals. In this embodiment, an image processing apparatus that controls a color printer will be described. According to the image processing apparatus of the present embodiment, it is possible to reproduce an image having a close impression even on an apparatus and an image recording medium having different color reproduction ranges.

Basic Configuration FIG. 20 is a block diagram showing the basic configuration of an image processing apparatus according to the third embodiment of the present invention.
In FIG. 20, reference numeral 2001 denotes a CPU, which controls the entire apparatus using programs and data stored in a RAM and ROM described later, and also executes image processing described later. Reference numeral 2002 denotes a RAM, which includes a storage area for temporarily storing programs and data loaded from an external storage device or a recording medium drive, which will be described later, and various data being processed. Reference numeral 2003 denotes a ROM which stores a program and control data for controlling the entire apparatus. Reference numeral 2004 denotes an operation unit, which is configured by a pointing device such as a keyboard and a mouse, and can input information on an input system device and information on a print sheet to the apparatus. Reference numeral 2005 denotes a display unit, which is composed of a CRT or a liquid crystal screen, and displays images and characters. An external storage device 2006 stores an operating system (hereinafter referred to as OS), an image processing program 2007 for performing various image processing, and parameters 2008 such as various profiles and color reproduction range data of a connected color printer described later. Reference numeral 2009 denotes a storage medium drive, which reads various data including images from the storage medium and outputs them to the external storage device 2006 and the RAM 2002. Also, the created conversion table is saved. Reference numeral 2010 denotes a bus for connecting the above-described units.

Functional Configuration FIG. 21 is a diagram showing a functional configuration of the image processing apparatus shown in FIG.
As shown in FIG. 21, the image processing apparatus according to the present embodiment includes an input system profile conversion unit 2101, a color conversion unit 2102, a printer profile conversion unit 2103, a color separation unit 2104, an input system device profile storage unit 2105, and a color reproduction. A color data constituting an input image is constituted by a range data storage unit 2106, a color reproduction range outermost part conversion table storage unit 2107, a printer profile storage unit 2108, and a color separation table storage unit 2109. R, G, B are converted into color signals C, M, Y, K of the connected color printer. Hereafter, each said component is demonstrated in detail.

  In the above configuration, the input system profile conversion unit 2101 uses the input system profile stored in the input system profile storage unit 2105 to convert the color signals R, G, and B constituting the input image into three color perception attributes. To color signals L, C, and h in a color space based on the above. The input system device profile is composed of, for example, a look-up table (hereinafter referred to as LUT) that holds color signals L, C, and h related to discrete color signals R, G, and B. Conversion to signals L, C, and h is performed using a known interpolation method in combination.

  The color conversion unit 2102 includes the color reproduction range data stored in the color reproduction range data storage unit 2106 and the color reproduction range outermost contour stored in the color reproduction range outermost conversion table storage unit 2107 described in the above embodiments. Using the conversion table, the input color signals L, C, and h are converted into color signals L ′, C ′, and h that can be reproduced by the connected color printer. That is, the color reproduction range data and the color reproduction range outermost conversion table have the same functions as the components having the same names described in the first embodiment, and convert the color signal LCh to the color signal L′ C′h. The conversion is performed by the color conversion method of the first embodiment or the second embodiment.

  The printer profile conversion unit 2103 uses the printer profile stored in the printer profile storage unit 2108 to input the color signals R ′, G ′, B depending on the color printer to which the input color signals L ′, C ′, h are connected. Convert to '. The printer profile is constituted by, for example, an LUT that holds L ′, C ′, and h color signals related to discrete R ′, G ′, and B ′ color signals, and the color signals are obtained from the color signals L ′, C ′, and h. Conversion to R ′, G ′, and B ′ is performed using a known search and interpolation method. The color separation unit 2104 uses the color separation table stored in the color separation table holding unit 2109 to use the color signals C, M, Y, and K of the color printer to which the input color signals R ′, G ′, and B ′ are connected. Convert to The color separation table is, for example, an LUT that holds CMYK color signals related to discrete R ′, G ′, and B ′ color signals, and conversion from the color signal R′G′B ′ to the color signal CMYK is publicly known. Interpolation is used together.

Image Processing Procedure FIG. 22 is a flowchart showing the processing procedure of the image processing apparatus. The image processing apparatus of this embodiment converts an input image signal into an image signal of a connected color printer in the following procedure. First, in step S2201, the input system device profile and the color reproduction range data of the input system device are acquired. Next, in step S2202, a color reproduction range outermost conversion table is created using the color reproduction range data of the connected color printer held in advance and the color reproduction range data of the input system device.

  In step S2203, the color signal RGB constituting the input image is converted into color signals L, C, and h in the color perception space. In step S2204, the color signals L, C, and h are converted into color signals L ', C', and h that can be reproduced by the connected color printer. In step S2205, the color signals L ', C', and h are converted into color signals R ', G', and B 'depending on the connected color printer. In step S2206, the color signals R ', G', and B 'are converted into color signals C, M, Y, and K of the connected color printer. Next, in step S2207, it is confirmed whether or not conversion has been completed for all color signals constituting the input image. If not completed, the process returns to step S2203.

Other Configuration The image processing apparatus according to the present embodiment converts input color signals R, G, and B into color signals L, C, and h in a color perception space in an input system profile conversion unit that is a functional component. However, in place of L, C, and h, other color spaces that can be converted into a color perception space, such as CIE / L ^* a * b ^* color space, may be used. it can. In this case, a component that converts the color signal in the other color space into the color signals L, C, and h in the color perception space between the input system profile conversion unit 2101 and the color conversion unit 2102 in FIG. Is added.

  Further, the image processing apparatus according to the present embodiment converts the color signals R, G, and B constituting the input image into the color signals L, C, and h, and the color signals L, C, and h to the color signals L ′ and C. Conversion to ', h, conversion from these color signals L', C ', h to color signals R', G ', B', conversion from these color signals R ', G', B 'to color signals C, M, Although the above four conversions to Y and K are performed, these conversions may be combined. For example, the color signals R, G, and B can be converted to the color signals C, M, Y, and K at a time. In this case, the color signals C, M, Y, and K relating to the discrete color signals R, G, and B are obtained, and an LUT is created. By using a known interpolation method using the LUT, the color signals R, G and B can be converted into color signals C, M, Y, and K.

  According to the image processing apparatus of the present embodiment, an image reproduced by the input system apparatus can be suitably reproduced by the output apparatus. For example, the impression is close to an image in which the input image data is displayed on the monitor. It is possible to print with an image, print with an image close to an image printed with another printer, or print with an image close to an image printed with another image recording medium (printing paper) Become.

<Other embodiments>
An object of the present invention is to provide a system or apparatus using a storage medium that records a program code of software that realizes the functions of the embodiments shown in FIGS. 3, 13, 14, 16, and 22. Needless to say, this can also be achieved by reading and executing the program code stored in the storage medium by the computer (or CPU or MPU) of the system or apparatus.

  In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention. As a storage medium for supplying the program code, for example, a floppy (registered trademark) disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CD-R, magnetic tape, nonvolatile memory card, ROM, or the like is used. I can do it. Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) operating on the computer based on the instruction of the program code. It goes without saying that a part of the actual processing is performed and the functions of the above-described embodiments are realized by the processing.

  Further, after the program code read from the storage medium is written to a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. It goes without saying that the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing.

It is a schematic diagram explaining the color gamut conversion method concerning the 1st Embodiment of this invention. It is a figure explaining the method to convert the color corresponding to the point on line segment OBA shown in FIG. 1 into the color corresponding to the point on line segment OB. It is a flowchart which shows the color gamut conversion procedure of 1st Embodiment. It is a figure explaining the method of calculating | requiring the color B on the color reproduction range outermost contour of an output type apparatus based on the color signal P. FIG. It is a figure explaining the method of calculating | requiring the color B outside the color reproduction range of an output type | system | group apparatus based on the color signal P. FIG. It is a figure explaining the coordinate system utilized by this data regarding the color reproduction range data acquired by step S301 of FIG. It is a figure explaining the method which concerns on 1st Embodiment and sets the position of the said point B in consideration of the ratio of the compression of a brightness direction and a saturation direction. FIG. 6 is a diagram for explaining a method for setting the position of the point B in consideration of the compression ratio in the lightness direction and the saturation direction according to the first embodiment. It is a figure which shows an example of the color reproduction range data acquired by step S301 of FIG. It is a figure which shows an example of the color reproduction range outermost contour conversion table produced by step 302 of FIG. It is a figure which shows the equi-hue surface explaining the relationship between (theta) shown in FIG. 9, Si, and So. It is a figure which shows the same hue (h constant) surface explaining the relationship between (theta) i and (theta) o of the color reproduction range outermost contour conversion table shown in FIG. It is a flowchart which shows the preparation procedure of the color reproduction range outermost conversion table shown in FIG. 4 is a flowchart showing details of a color reproduction range inside / outside determination procedure performed in step S304 of FIG. 3. It is a schematic diagram explaining the correspondence in the color reproduction range outermost part of an input system device and an output system device. It is a flowchart which shows the preparation procedure of the color reproduction range outermost contour conversion table in the 2nd Embodiment of this invention. It is a figure explaining the conversion destination of the maximum chroma color in the process shown in FIG. It is a figure which shows the said color reproduction range outermost contour conversion table calculated | required by the correspondence of the minimum color difference. FIG. 17 is a schematic diagram showing a color reproduction range outermost conversion table after correction in the processing shown in FIG. 16. It is a block diagram which shows the basic composition of the image processing apparatus which concerns on the 3rd Embodiment of this invention. It is a figure which shows the function structure of the image processing apparatus shown in FIG. It is a flowchart which shows the process sequence of the image processing apparatus in 3rd Embodiment. It is a figure explaining the conventional color gamut conversion method. It is a figure which shows the clipping input / output conversion function in the conventional color gamut conversion method. It is a figure which shows the linear input / output conversion function in the conventional color conversion method. It is a figure which shows the nonlinear input-output function in the conventional color conversion method.

Explanation of symbols

2001 CPU
2002 RAM
2003 ROM
2004 Operation unit 2005 Display unit 2006 External storage device 2007 Image processing program 2008 Parameter 2009 Storage medium drive 2101 Input system profile conversion unit 2102 Color conversion unit 2103 Printer profile conversion unit 2104 Color separation unit 2105 Input system profile storage unit 2106 Reproduction range data storage Unit 2107 color reproduction range outermost part conversion table storage unit 2108 printer profile storage unit 2109 color separation table storage unit

Claims (8)

A color gamut conversion method for performing color gamut conversion based on a relationship in which a color of a first area is associated with a color of a second area smaller than the area in a color space,
The color on the outermost contour on the second region corresponding to the color on the outermost contour of the first region, the color on the outermost contour on the outermost contour of the first region, and a predetermined fixed point in the color space It is determined on the basis of a predetermined correspondence not associated with a straight line connecting points in the color space indicating the color, and the color on the outermost contour on the determined second region and the first corresponding to the color Preparing the relationship associating the color of the first region with the color of the second region, determined based on the color on the outermost contour of the region;
Perform color gamut conversion according to the relationship.
A color gamut conversion method characterized by comprising steps.   The predetermined correspondence relationship is a relationship in which a color difference between a color on the outermost contour of the first region and a color on the outermost contour on the corresponding second region is minimized. The described color gamut conversion method.   The predetermined correspondence relationship is when a point indicating a color on the outermost contour of the first region moves in a predetermined direction along the outermost contour and moves to a point indicating a color on the outermost contour of the different first region. The point indicating the color on the outermost contour of the second region moves in the same predetermined direction along the outermost contour and moves to the point indicating the color on the outermost contour of the different second region. The color gamut conversion method according to claim 1.   The relationship of associating the color of the first area with the color of the second area is determined by the line segment OB connecting the predetermined fixed point O and the point B indicating the outermost color of the second area, and the second For the color on the line segment BA connecting the point B indicating the color on the outermost contour of the area and the corresponding point A indicating the color on the outermost contour of the first area, the predetermined fixed point in the line segment OB. The color on the line segment from O to the predetermined point C is associated with the color as it is, and the line segment CB from the predetermined point C to the point B in the line segment OB and the color on the line segment BA are 4. The color gamut conversion method according to claim 1, wherein the color gamut conversion method has a linear relationship with the color on the line segment CB.   The color gamut conversion method according to claim 1, wherein the color space is a color space corresponding to color perception of brightness, saturation, and hue.   6. The color gamut conversion method according to claim 1, wherein the color of the predetermined fixed point is an achromatic color. An image processing apparatus that performs color gamut conversion and color signal conversion based on a relationship in which a color of a first area is associated with a color of a second area smaller than the area in a color space,
The color on the outermost contour on the second region corresponding to the color on the outermost contour of the first region, the color on the outermost contour on the outermost contour of the first region, and a predetermined fixed point in the color space It is determined on the basis of a predetermined correspondence not associated with a straight line connecting points in the color space indicating the color, and the color on the outermost contour on the determined second region and the first corresponding to the color The table includes the relationship defined based on the color of the outermost area of the region and associating the color of the first region with the color of the second region,
An image processing apparatus that performs color gamut conversion using the table. In a computer of an image processing apparatus that performs color gamut conversion based on a relationship in which the color of the first area is associated with the color of the second area smaller than the area in the color space,
The color on the outermost contour on the second region corresponding to the color on the outermost contour of the first region, the color on the outermost contour on the outermost contour of the first region, and a predetermined fixed point in the color space It is determined on the basis of a predetermined correspondence not associated with a straight line connecting points in the color space indicating the color, and the color on the outermost contour on the determined second region and the first corresponding to the color Obtaining the relationship associating the color of the first region with the color of the second region, determined based on the color on the outermost contour of the region;
Perform color gamut conversion according to the relationship.
A program for executing a process having steps.

Images