This application claims the benefit of Korean Patent Application No. 2004-43088 filed on Jun. 11, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present general inventive concept relates to a gamut mapping apparatus using a vector stretching, and a method thereof; and more particularly, to a gamut mapping apparatus using a vector stretching that increases brightness based on the shape of a gamut while maintaining chromaticity of a signal from a source device consistently, and a method thereof.
2. Description of the Related Art
Generally, image reproducing apparatuses such as monitors, scanners, printers and so on adopt different color spaces or color models depending on fields of the application. For instance, color printing apparatuses use a color space of CMY, which stands for cyan, magenta and yellow, and color cathode ray tube (CRT) monitors or computer graphic devices use a color space of RGB, which stands for red, green and blue. Those devices that must manipulate hue, saturation and intensity use a color space of HSI, which stands for hue, saturation and intensity. Also, the CIE color space based on human perception developed by the Commission Internationale de I'Eclairage (CIE) committee is used to reproduce images accurately in any device. That is, the CIE color space is employed when it is necessary to define device independent color systems. Also, the CIE color space is representatively classified into the CIE-XYZ color space, the CIE L*a*b color and CIE L*u*v color space.
Besides the color space, the color reproducing apparatuses may have different color gamuts. While the color space refers to a way of representing colors, that is, a relationship of the colors with respect to one another, the gamut is a range of colors that can be reproduced. Therefore, when an input color signal has a different gamut from that of a color reproducing apparatus, a gamut mapping that converts the input color signal into an adequate form that can be matched with the gamut of the color reproducing apparatus is required to improve color reproducibility.
Although the color reproducing apparatuses typically use three primary colors, currently there is an attempt to extend the color gamut using more than four defining colors. For instance, a multi-primary display (MPD) is a display system with extended color reproducibility by using more than four defining colors to expand a color gamut to a greater extend as compared with that of a three channel display system that uses three primary defining colors.
The conventional gamut mapping method using the chroma stretching increases and decreases chroma by maintaining the same lightness (brightness). This gamut mapping method provides images with high definition when the chroma is improved.
Referring to
However, as indicated in the line K in
The present general inventive concept provides a gamut mapping apparatus using a vector stretching to increase and decrease lightness (brightness) calibrated into characteristics of a target device by maintaining colors of color signals of a source device consistent during a gamut mapping between color systems with different color gamuts, and a method thereof.
Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept.
The foregoing and/or other aspects of the present general inventive concept are achieved by providing an apparatus to provide a gamut mapping using a vector stretching, including: a first color space conversion block to convert an input color signal into a first color signal of an LCH (light, chroma and hue) color space; a vector stretching block to output a second color signal obtained as a source point of a source gamut of the first color signal mapped to a transferred target point of a target gamut of a target device to reproduce the input color signal as much as a vector difference between a point at which an extended line of a vector of the source point meets with a boundary line of the source gamut and a point at which the extended line of the vector meets with a boundary line of the target gamut; and a second color space conversion block to convert the second color signal into a color space of the input color signal.
The gamut mapping carried out by the vector stretching block is defined as:
where (cs,ls), (ct,lt), (csg,lsg) and (ctg,lt g) are a source point of the source gamut, a mapped target point, a point at which an extended line of a vector of the source point meets with a boundary line of the source gamut, and a point at which the extended line of the vector meets with a boundary line of the target gamut, respectively.
Also, the gamut mapping apparatus may further include a source gamut calibration unit to calibrate a cusp of a predetermined boundary line of the source gamut to have the same slope of a boundary line of the target gamut adjacent to the source gamut before the vector stretching block performs a gamut mapping.
The source gamut calibration unit calibrates the source gamut on the basis of an equation defined as:
Moreover, the gamut mapping apparatus may further include a target gamut calibration unit to calibrate the boundary line of the target gamut before the source gamut calibration unit calibrates the source gamut. Herein, the target gamut calibration unit calibrates the target gamut when a cusp exists at the boundary line of the target gamut adjacent to the predetermined boundary line of the source gamut calibrated by the source gamut calibration unit.
The gamut mapping apparatus may further include a hue shift unit to perform a shift to reduce the target gamut prior to the gamut mapping executed by the vector stretching block when the source gamut is wider than the target gamut.
Also, the gamut mapping apparatus may further include a chroma stretching unit to perform a chroma stretching to a region that is not mapped after the vector stretching block performs the gamut mapping.
The foregoing and/or other aspects of the present general inventive concept are also achieved by providing a method of performing a gamut mapping using a vector stretching, the method including: converting an input color signal into a first color signal of an LCH color space and outputting the converted signal; outputting a second color signal obtained as a source point of a source gamut of the first color signal mapped to a transferred target point of a target gamut of a target device to reproduce the input color signal as much as a vector difference between a point at which an extended line of a vector of the source point meets with a boundary line of the source gamut and a point at which the extended line of the vector meets with a boundary line of the target gamut; and converting the second color signal into a color space of the input color signal and outputting the converted color signal.
The gamut mapping carried out by using the vector stretching is defined as:
Also, the method may further include calibrating a cusp of a predetermined boundary line of the source gamut to have the same slope of a boundary line of the target gamut adjacent to the source gamut prior to the gamut mapping.
Particularly, the source gamut calibration is carried out on the basis of an equation defined as:
At this time, the predetermined boundary line of the source gamut to which the cusp calibration is applied is a region corresponding to primary colors of the source gamut and has chroma increasing as lightness increases.
The method may further include calibrating the boundary line of the target gamut before the source gamut is calibrated. At this time, this operation of calibrating the boundary line of the target gamut proceeds with the target gamut calibration when a cusp exists at the boundary line of the target gamut adjacent to the predetermined boundary line of the calibrated source gamut.
The method may further include performing a hue shift to reduce the target gamut prior to the gamut mapping when the source gamut is wider than the target gamut.
Additionally, the method may further include performing a chroma stretching to a region that is not mapped after the gamut mapping is carried out by using the vector stretching.
These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept while referring to the figures.
In the following description, same drawing reference numerals are used for the same elements even in different drawings. The matters defined in the description such as a detailed construction and elements are nothing but the ones provided to assist in a comprehensive understanding of the general inventive concept. Thus, it is apparent that the present general inventive concept can be carried out without those defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the general inventive concept in unnecessary detail.
Disclosed is a gamut mapping apparatus and method using a vector stretching in a source device and a target device with different color gamuts. Hereinafter, a three-channel color device and a five-channel color device are exemplified as the source device and the target device, respectively. The disclosed gamut mapping method can be applied to a mapping from a source gamut to a target gamut in color reproducing apparatuses with different color gamuts.
As illustrated in
The first color space conversion block 210 coverts an input color signal into coordinates of LCH, which stands for lightness, chroma and hue since the gamut mapping takes place at a constant hue plane to maintain colors consistently.
Also, the gamut mapping block 220 to which the input color signal converted by the first color space conversion block 210 is input maps a source color gamut of the source apparatus to a target color gamut of the target apparatus at the LCH coordinates.
The hue shift unit 221 shifts the target gamut when the source gamut and the target gamut are highly different from each other. For example, in a case in which the source gamut is wider than the target gamut, a hue shift is performed under the target by decreasing the target gamut to prevent discoloration and desaturation caused by a decrease in chroma and lightness during the gamut mapping.
Prior to the gamut mapping, the source gamut calibration unit 222 calibrates a cusp of the source gamut to be disposed at an extended line having the same slope as a cusp of the target gamut adjacent to the cusp of the source gamut. That is, a region of the source gamut disposed outside the target gamut is downsized prior to the gamut mapping, or a region of the source gamut disposed inside the target gamut is enlarged prior to the gamut mapping.
The target gamut calibration unit 223 calibrates the target gamut when the source gamut calibrated by the source gamut calibration unit 222 is highly different from the target gamut in comparison with the original source gamut that is not calibrated. In other words, the target gamut calibration unit 223 calibrates the target gamut when there is a high discrepancy in chroma and lightness between a cusp of a predetermined boundary line of the source gamut calibrated by the source gamut calibration unit 222 and a cusp of a predetermined boundary line of the target gamut, which is a reference for the source gamut calibration.
The vector stretching unit 224 carries out a gamut mapping by employing a vector stretching method when a predetermined source point of the source gamut is mapped to a target point of the target gamut. That is, a source point at the source gamut is mapped to the target point by being stretched as much as a vector difference between a point at which the extended line of the vector meets with the boundary line of the source gamut and a point at which the extended line of the vector meets with the boundary line of the target gamut.
After the mapping of the source gamut by the vector stretching unit 224, the chroma stretching unit 225 carries out the gamut mapping by employing a chroma stretching for those regions of the target gamut that are not subjected to the vector stretching.
The second color space conversion block 230 converts the input color signal at the LCH color space mapped by the gamut mapping block 220 into a color space of WYV and outputs the converted color signal.
As illustrated in
Coordinates of the LCH color space are converted from a color coordinate system representing brightness and chromaticity. Examples of the color coordinate system are CIE L*a*b, CIE L*u*v, YCbCr and so forth, and these color coordinate systems generally take red-green and yellow-blue as an axis of chromaticity. In this embodiment, WYV coordinates that are linearly converted from XYZ coordinates are described as an example. That is, the color space conversion from the WYV coordinates to the LCH coordinates are defined by the following mathematical equation.
Next, at operation S313, the input color signal converted into the LCH color space is subjected to a hue shift operation by the hue shift unit 221. The hue shift is carried out to prevent discoloration and desaturation caused by a decrease in lightness and chroma which may occur during the gamut mapping when a source gamut is highly different from a target gamut. The discoloration is generally observed when the source gamut is wider than the target gamut. Thus, in a case in which the source gamut is wider than the target gamut, the target gamut is shifted under the target by enlarging the target gamut. However, when the source gamut and the target gamut exhibit a slight difference that does not induce the discoloration, the hue shift of the source gamut or the target gamut is not required. A degree of the hue shift depends on a shifted hue distance, and an amount of the hue shift is also adjusted in order to prevent an incidence of color contour phenomenon caused by the hue shift.
Prior to the gamut mapping, at operation S315, the source gamut calibration unit 222 calibrates the source gamut. The source gamut calibration is carried out such that a cusp of the source gamut is calibrated to be disposed at an extended line having the same slope to a cusp of the target gamut adjacent to the cusp of the source gamut. Depending on a degree of calibrating the cusp of the source gamut, those points in the source gamut are also calibrated. When the source gamut is narrower than the target gamut, the source gamut is calibrated to be enlarged according to the degree of the above cusp calibration. Conversely, when the source gamut is wider than the target gamut, the source gamut is calibrated to be downsized according to the degree of the above cusp calibration.
At operation S317, the target gamut calibration unit 223 calibrates the target gamut to prevent colors from being clustered at a boundary line of the source gamut. The color cluster phenomenon may occur because of the source gamut calibration. More specifically, the color cluster phenomenon arises when the target gamut includes a cusp of another target gamut at a region where the source gamut calibration unit 222 calibrates the cusp of the source gamut to be disposed at the extended line of the cusp of the target gamut. Hence, in a case in which the target gamut does not have another cusp at the region where the aforementioned calibration by the source gamut calibration unit 222 takes place, the target gamut calibration unit 223 does not calibrate the target gamut. That is, when the color cluster of the source gamut does not occur during the gamut mapping by the source gamut calibration, the target gamut calibration is unnecessary.
The target gamut calibration takes place by removing the cusp of the target gamut at the region where the cusp of the source gamut is calibrated to be placed at the extended line of the cusp of the target gamut adjacent to the cusp of the source gamut.
At operation S319, the gamut mapping takes place by performing the vector stretching executed by the vector stretching unit 224. That is, a source point at the source gamut is mapped to a target point at the target gamut as much as a vector difference between a point at which an extended line of a vector of a certain source point at the source gamut meets with a boundary line of the source gamut and a point at which the extended line of the vector meets with a boundary line of the target gamut. At this time, the gamut mapping can be carried out without calibrating the source gamut and the target gamut through employing the vector stretching.
After the gamut mapping by the vector stretching, at operation S321, it is determined whether or not there is a region to which the vector stretching cannot be applied. This region is commonly discovered when the source gamut calibration unit 222 does not calibrate the source gamut, or after the source gamut calibration unit 222 and the target gamut calibration unit 223 make the calibration.
At operation S323, in a case where there is such a region to which the vector stretching cannot be applied, a chroma stretching is applied thereto. Even though the chroma stretching is applied, an incidence of desaturation typically arising after the chroma stretching is not observed during the gamut mapping. The reason for this effect is because the chroma stretching is applied to the region where the vector stretching is not carried out after the source gamut calibration unit 222 and the target gamut calibration unit 223 calibrate the source gamut and the target gamut, respectively, prior to the gamut mapping by the vector stretching.
However, if there is not such a region to which the vector stretching is not applied, the gamut mapping is carried out by the vector stretching without applying the chroma stretching. In addition, the gamut mapping involves only the vector stretching when the source gamut calibration takes place while the target gamut calibration does not take place.
Next, in a case in which the gamut mapping takes place by the gamut mapping block 220 which includes the hue shift unit 221, the source gamut calibration unit 222, the target gamut calibration unit 223, the vector stretching unit 224 and the chroma stretching unit 225, at operation S325, the second color space conversion block 230 converts the LCH coordinates outputted from the gamut mapping block 220 into the WYV coordinates.
Reference denotations S and T in
At this time, the discoloration caused by a discrepancy in the size between the source gamut and the target gamut occurs when the source gamut is wider than the target gamut. As a result, the hue shift takes place when the target gamut is shifted closely toward the source gamut.
Reference denotations S and T refer to a source gamut and a target gamut, respectively. Also, a region A represents a region where the source gamut extends during a vector stretching operation because the source gamut is narrower than the target gamut, and a region B represents a region where the source gamut is downsized during the vector stretching because the source gamut is wider than the target gamut. Further, a region R refers to a region where the vector stretching cannot be applied because the target gamut is wider than the source gamut to which the vector stretching is applied through employing the vector stretching unit 224.
The vector stretching unit 224 performs the vector stretching to each source point on the basis of the following mathematical equation defined as: lsg
Herein, (cs,ls) is a source point of the source gamut and (ct,lt) is a mapped target point. Also, (csg,lsg) is a point at which an extended line of a vector of the source point meets with a boundary line of the source gamut, and (ctg,lt g) is a point at which the extended line of the vector meets with a boundary line of the target gamut.
That is, according to the above mathematical equation 2, the source point is mapped to the target point as much as a vector difference between the point at which the extended line of the vector meets with the boundary line of the source gamut and the point at which the extended line of the vector meets with the boundary line of the target gamut.
With reference to
Referring to
Referring to
As described above with reference to
In the above mathematical equations 3 to 5, (c,l) is a source point of the source gamut, and (c′,l′) is a calibrated source point of the source gamut. Also, (c0,l0) is a cusp of the source gamut prior to the calibration, and (cn,ln) is a cusp of the source gamut after the calibration.
In the case in which o≦l≦lo, that is, in case that the source point of the source gamut has a value less than a lightness (lo) of the cusp of the source gamut, the source point is calibrated in proportion to a calibrated amount of the cusp of the source gamut by the source gamut calibration unit 222. Thus, the lightness of the source point after the calibration is defined as the mathematical equation 3 above. Meanwhile, in the case in which lo≦l≦1, that is, in a case in which the source point of the source gamut has a value greater than the lightness (lo) of the cusp of the source gamut, the source point is calibrated in proportion to a calibrated amount of the cusp of the source gamut by the source gamut calibration unit 222. Thus, the lightness of the source point after the calibration is defined as the mathematical equation 4 above. Since
At this time, a boundary line of the source gamut of which the cusp is calibrated by the source gamut calibration unit 222 corresponds to a region for primary colors of the source gamut. Also, in this region, chroma increases as the lightness of the input color signal increases. In addition, chroma of the cusp of the source gamut is calibrated according to the mathematical equation 5 above.
Similar to
Meanwhile, as described with reference to
The source gamut calibration and the target gamut calibration described with reference to
In comparison with the conventional gamut mapping using the chroma stretching, the disclosed gamut mapping using the vector stretching provides an effect in that the gamut mapping can be carried out under consistently maintained chromaticity. As a result of this effect, it is further possible to reduce a frequency of the discoloration phenomenon. Also, after the source gamut calibration and the target gamut calibration, the gamut mapping using the vector stretching makes it possible to prevent chroma from decreasing.
Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.
Number | Date | Country | Kind |
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2004-43088 | Jun 2004 | KR | national |