Patent Publication Number: US-2011050857-A1

Title: Apparatus and method for displaying 3d image in 3d image system

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority of Korean Patent Application Nos. 10-2009-0083215, 10-2009-0128511, and 10-2010-0080727 filed on Sep. 3, 2009, Dec. 21, 2009, and Aug. 20, 2010, respectively, which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary embodiments of the present invention relate to a 3D image system; and, more particularly, to an apparatus and a method for displaying 3D images, which do not need to be reversed, so that users can watch the 3D images without modification. 
     2. Description of Related Art 
     3D images can be defined from two points of view. Firstly, 3D images are constructed using depth information so that watchers feel as if objects in the images move away from the screen and approach them. As used herein, the depth information refers to information regarding the distance of objects relative to a reference point in 2D images. Therefore, 2D images can be rendered 3D using the depth information. Secondly, 3D images basically provide watchers with various views to provide realistic appearance. 
     Such 3D images are more realistic and closer to our daily experiences than 2D images so that they are increasingly being needed and used in various fields, such as broadcasting, medical care, education, military, gaming, and animation. For these reasons, various methods for reproducing 3D images are being studied. 
     In order to have the perception of depth from 3D images, watchers need at least two images, i.e. left and right images, which are supposed to be incident on their left and right eyes, respectively. 
     However, if the left and right images are erroneously incident on the right and left eyes, respectively, watchers cannot correctly recognize 3D images, not to mention the perception of depth. Such reversing of left and right images may occur during storage, distribution, transmission, or reproduction of 3D images. 
     Therefore, there is a need for a detailed scheme for correcting errors of 3D images, which may occur during storage, distribution, transmission, or reproduction of the 3D images, so that correct 3D images are displayed to watchers. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention is directed to an apparatus and a method for displaying 3D images in a 3D image system. 
     Another embodiment of the present invention is directed to an apparatus and a method for determining whether a 3D image is normal or not, e.g. whether left and right images of the 3D image are reversed or not, so that 3D images are displayed correctly. 
     Another embodiment of the present invention is directed to an apparatus and a method for determining whether 3D images are reversed or not and automatically correcting the 3D images, when reversed, so that watchers have the perception of depth from correctly displayed 3D images. 
     Other objects and advantages of the present invention can be understood by the following description, and become apparent with reference to the embodiments of the present invention. Also, it is obvious to those skilled in the art to which the present invention pertains that the objects and advantages of the present invention can be realized by the means as claimed and combinations thereof. 
     In accordance with an embodiment of the present invention, a 3D image display apparatus in a 3D image system includes: a receiving unit configured to receive left and right images taken by a plurality of imaging devices; a calculation unit configured to select an image among the left and right images, determine a reference element in the selected image, determine a selection element corresponding to the reference element in the non-selected image, and calculate a disparity value between the left and right images by using the reference element and the selection element; a determination unit configured to determine whether the left and right images are normal or not by using the disparity value; and a correction unit configured to correct the left and right images based on the determination result. 
     In accordance with another embodiment of the present invention, a 3D image display method in a 3D image system includes: selecting an image among left and right images taken by the imaging devices; determining a reference element in the selected image, and determining a selection element corresponding to the reference element in the non-selected image; calculating a disparity value between the left and right images by using the reference element and the selection element; determining whether the left and right images are normal or not by using the disparity value; and correcting the left and right images based on the determination result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the schematic structure of an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
         FIG. 2  illustrates rectification of left and right images by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates a process of dividing an image by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
         FIGS. 4A and 4B  schematically illustrate a process of selecting a selection element by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
         FIGS. 5A and 5B  illustrate a process of determining whether a 3D image is reversed or not by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
         FIG. 6  schematically illustrates a process of displaying a 3D image by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present invention. 
     The present invention proposes an apparatus and a method for displaying 3D images in a 3D image system. The apparatus for displaying 3D images refers to a video device for reproducing 3D images, such as a 3D video multiplexer. For convenience of description, the video device will hereinafter be referred to as a 3D image display apparatus. In accordance with an embodiment of the present invention, described later, it is determined whether a 3D image is normal or not, e.g. whether left and right images of the 3D image are reversed or not, in a 3D image system and, based on the determination result, reversing of the 3D image is automatically corrected so that the watcher has the perception of depth from the correctly displayed 3D image. An apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention will now be described in more detail with reference to  FIG. 1 . 
       FIG. 1  illustrates the schematic structure of an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
     Referring to  FIG. 1 , the apparatus for displaying 3D images include a receiving unit  100 , a rectification unit  110 , a processing unit  120 , and a correction unit  130 . The receiving unit  100  includes a storage unit  101  and an acquisition unit  102 . The processing unit  120  includes a calculation unit  121  and a determination unit  122 . 
     The storage unit  101  is configured to receive and store left and right images taken by a plurality of imaging devices e.g. cameras. The left and right images may be taken not only by cameras, but also by other types of various imaging devices. The acquisition unit  102  acquires the characteristic information of the imaging devices, which take the left and right images, from the left and right images. Herein, the acquisition unit  102  can acquire characteristic information by extracting the characteristic information not only using the horizontal disparity of the left and right images, but also using various types of algorithms other than the horizontal disparity-based algorithm. In addition, the acquisition unit  102  can acquire the characteristic information of the imaging devices, which take the left and right images, in additional information, when the receiving unit  100  separately receives the additional information regarding camera characteristics, etc. from an outside to obtain information regarding the cameras. Herein, the characteristic information is information indicating characteristic about a type, etc of the imaging devices e.g. cameras, which take the left and right images. 
     The rectification unit  110  is configured to receive left and right images from the storage unit  101  and receive the characteristic information from the acquisition unit  102 . The rectification unit  110  is configured to rectify the left and right images using the characteristic information acquired by the acquisition unit  102 . More specifically, the rectification unit  110  determines whether to perform rectification or not based on characteristic information regarding the cameras used to take the left and right images. For example, if it is confirmed based on the characteristic information that the left and right images have been taken by parallel-axis cameras or horizontal-axis cameras, the rectification unit  110  is not configured to rectify the left and right images. If it is confirmed based on the characteristic information that the left and right images have been taken by cross-axis cameras, the rectification unit  110  is configured to rectify the left and right images using a rectification algorithm, e.g. a homography algorithm, an affine algorithm, etc. In other words, the rectification unit  110  determines whether the left and right images have been taken by cross-axis cameras or not based on the characteristic information and, based on the determination result, rectifies the left and right images. Rectification of left and right images of a 3D image taken by cross-axis cameras in a 3D image system will now be described in more detail with reference to  FIG. 2 . And, when cameras taken the left and right images are the cross-axis cameras, rectification of the left and right images proceed, therefore the exemplary embodiments of the present invention will be described below when the cross-axis cameras take the left and right images. 
       FIG. 2  illustrates rectification of left and right images by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
     Referring to  FIG. 2 , the cross-axis cameras take images of a triangle  202  and a circle  204  and output left and right images  210  and  220  of the triangle  202  and the circle  204 . In this case, the point of intersection of axes extending from center points of the cross-axis cameras, which take images of the circle  202  and the triangle  204 , is defined as the point of convergence  206 . When an object lying farther than the point of convergence  206 , e.g. the triangle  202 , is displayed through a 3D monitor, the triangle  202  appears as triangles  212 ,  222  positioned in the outside on the screen. On the contrary, an object lying closer than the point of convergence, i.e. the circle  204 , appears as triangles  214 ,  224  positioned in the inside on the screen. 
     When the calculation unit  121  calculates the disparity value based on left and right images, the left and right images  210  and  220  serve as left and right images  250  and  260  for disparity value calculation. The triangles  252  in the left and right images  250  and  260  for disparity value calculation have negative disparity values, while the circles  254  have positive disparity values. This causes errors by disparity values when the determination unit  122  determines whether the 3D image is normal or not, e.g. whether the 3D image is reversed or not. Therefore, in order to correctly determine whether the 3D image is normal or not, the left and right images  210  and  220  need to be rectified by the rectification unit  110  so that the triangles  252  and circles  254  in the left and right images  250  and  260  for disparity value calculation have positive disparity values. 
     Specifically, the rectification unit  110  rectifies the left and right images  210  and  220  using a rectification algorithm, e.g. homography algorithm or affine algorithm, etc, so that the left and right images  210  and  220  become rectified left and right images  230  and  240 . Herein, triangles  232 ,  242  and circles  234 ,  244  in the left and right images  230  and  240  become rectified, so that the triangles  232 ,  242  and circles  234 ,  244  have positive disparity values. The rectified left and right images  230  and  240  then serve as left and right images  270  and  280  for disparity value calculation by the calculation unit  121 . The triangles  272  and  282  and the circles  274  and  284  in the left and right images  270  and  280  for disparity value calculation have positive disparity values, so that the determination unit  122  can accurately determine whether the 3D image is normal or not. 
     As such, in order to guarantee that the determination unit  122  correctly determine whether left and right images of a 3D image are reversed or not using the sign of disparity values, the rectification unit  110  projects left and right images  210  and  220  taken by cross-axis cameras onto two parallel planes so that the images are rectified. That is, by projecting the taken left and right images  210  and  220  onto two parallel planes, the rectification unit  110  rectifies the images so that the resulting rectified left and right images  230  and  240  are the same as images taken by cameras positioned on parallel axes. Therefore, the taken left and right images  210  and  220  are displayed as the rectified left and right images  230  and  240  by rectification of the rectification unit  110 . 
     Based on characteristics between the left and right images, e.g. disparity value, the determination unit  122  determines whether they are reversed or not, and the calculation unit  120  calculates the disparity value between the left and right images. In order to calculate the disparity value, the calculation unit  121  selects an image between the left and right images, i.e. the calculation unit  121  selects the left or right image. And the calculation unit  121  selects a reference element in the selected image, and then designates a selection element, which corresponds to the reference element, in the non-selected image. Herein, the left and right images, which are used to the calculation unit  121  in order to calculate the disparity value, are the rectified left and right images  230  and  240  by rectification of the rectification unit  110 . 
     For example, when the calculation unit  121  selects the right image among the left and right images, the calculation unit  121  determines the reference element in the right image, and determines the selection element in the left image. As used herein, the reference element refers to at least one pixel selected from the group consisting of a pixel corresponding to a partial region of a M×N block or a circle, for example, a pixel chosen by a predetermined criterion, and a pixel chosen randomly, wherein M and N refer to the horizontal and vertical sizes of the pixels, respectively. 
     The calculation unit  121  designates the selection element, which corresponds to the reference element, in the left image. As used herein, the selection element refers to a pixel in the left image, which has not been selected, corresponding to the pixel of the reference element in the selected right image. That is, the pixel of the reference element in the selected right image corresponds to the pixel of the selection element in the non-selected left image. And the calculation unit  121  calculates the disparity value between the right and left images by using the reference element and the selection element. 
     Also the calculation unit  121  can divide the selected image, and determine the reference element in the divided images, determine the selection element in the non-selected image, and then calculate the disparity value between the right and left images by using the reference element and the selection element. That is, in order to calculate the disparity value, the calculation unit  121  selects an image between the left and right images, i.e. the calculation unit  121  selects the left or right image. And then, the calculation unit  121  divides the selected image, e.g. left or right image, with reference to its center axis. The process of dividing an image by the calculation unit  121  will now be described in more detail with reference to  FIG. 3 . 
       FIG. 3  illustrates a process of dividing an image by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
     Referring to  FIG. 3 , the calculation unit  121  selects an image, which is to be divided, from left and right images. Herein, the left and right images, which are used to the calculation unit  121  in order to calculate the disparity value, are the rectified left and right images  230  and  240  by rectification of the rectification unit  110 . It will be assumed for convenience of description that the right image is selected. 
     And, the calculation unit  121  determines a center axis  300 , which vertically extends through the horizontal center of the selected image, e.g. a right image, and divides the right image with reference to the center axis  300 . Besides the above-mentioned manner of dividing the image, the calculation unit  121  may employ an alternative process of, for example, dividing the image with reference to a center axis, at which both images have zero disparity. Herein, the calculation unit  121  divides the selected image in order to easily calculate the disparity values. Specifically, the calculation unit  121  secures a search region for easily selecting elements having large disparity values by dividing the selected image. 
     This is based on the finding that, in the case of 3DTV camera photography, the point of convergence is generally positioned at the center of the screen, in accordance with the point of convergence is positioned at the center of the screen, it is existed objects having large disparity values in the left and right with reference to the point of convergence. Therefore, the calculation unit  121  determines the center axis  300 , which vertically extends through the horizontal center of the selected image, e.g. a right image, and divides the right image with reference to the center axis  300 . 
     And, the calculation unit  121  selects a separate image among the plurality of separate images obtained by dividing the right image, e.g. selects a separate image between first and second separate images. The calculation unit  121  then determines the reference element in the selected separate image. As used herein, the reference element refers to at least one pixel selected from the group consisting of a pixel corresponding to a partial region of a M×N block or a circle, for example, a pixel chosen by a predetermined criterion, and a pixel chosen randomly, wherein M and N refer to the horizontal and vertical sizes of the pixels, respectively. 
     Also, the calculation unit  121  determines the selection element, which corresponds to the reference element, in the non-selected image, i.e. the left image. As used herein, the selection element refers to a pixel in the left image corresponding to the pixel of the reference element in the separate image. That is, the pixel of the reference element in the selected right image corresponds to the pixel of the selection element in the non-selected left image. The calculation unit  121  calculates the disparity value between the left and right images using the reference element and the selection element. The process of determining the selection element, which corresponds to the reference element, will now be described in more detail with reference to  FIGS. 4A and 4B . 
       FIGS. 4A and 4B  schematically illustrate a process of determining a selection element by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. Specifically,  FIG. 4A  shows a process of selecting the left image, and determining the selection element, which corresponds to the reference element, in non-selected right image, and  FIG. 4B  shows a process of selecting the right image, and determining the selection element, which corresponds to the reference element, in non-selected left image. 
     Referring to  FIG. 4A , the calculation unit  121  selects an image (e.g. left image) among the left and right images. The calculation unit  121  determines a reference element  420  in the selected left image, and determines a selection element  430 , which corresponds to the reference element  420 , in the non-selected right image. Herein, the calculation unit  121  determines the selection element  430  based on an algorithm such as Sum of Squared Differences (SSD) for analyzing the brightness difference between pixels, Sum of Absolute Differences (SAD), or Normalized Cross Correlation (NCC) for analyzing cross correlation. And then the calculation unit  121  calculates the disparity value between the reference element  420  and the selection element  430 . 
     Also as above-described, the calculation unit  121  can divide the selected image, determine the reference element  420  in the divided images, and then determine the selection element  430  corresponding to the reference element  420  in the non-selected image. That is, the calculation unit  121  selects an image, e.g. left image, among the left and right images. The calculation unit  121  divides the selected left image with reference to the center axis  410  into first and second separate images. The calculation unit  121  determines the reference element  420  in a separate image, e.g. the first separate image among the separate images, and determines the selection element  430 , which corresponds to the reference element  420 , in the non-selected right image. Herein, the calculation unit  121  determines the selection element  430  based on an algorithm such as Sum of Squared Differences (SSD) for analyzing the brightness difference between pixels, Sum of Absolute Differences (SAD), or Normalized Cross Correlation (NCC) for analyzing cross correlation. And then the calculation unit  121  calculates the disparity value between the reference element  420  and the selection element  430 . 
     The calculation unit  121  does not consider the vertical disparity between the left and right images, but the horizontal disparity only, to calculate the disparity value. Assuming that the disparity value is D, the horizontal coordinate of the reference element  420  is Sx, and the horizontal coordinate of the selection element  430  corresponding to the reference element is Cx, the calculation unit  121  calculates the difference (Sx−Cx) between the horizontal coordinates of the left and right images as the disparity value (D). 
     For example, to be more specific, the calculation unit  121  determines the reference element  420  in the selected left image or the separate images of the selected left image, determines the selection element  430  corresponding to the reference element  420  in the non-selected right image, and then calculates the disparity value between the left and right images by using the reference element  420  and the selection element  430 . Assuming that the horizontal coordinate of the reference element  420  is Lx and the horizontal coordinate of the selection element  430  is Rx, the disparity value is: D=Lx−Rx. 
     Besides the above-described manner, the calculation unit  121  may calculate the disparity value in an alternative manner. For example, the calculation unit  121  calculates the global disparity value between the reference element  420  and the selection element  430  a plurality of times using a binocular disparity model, and adopts the calculated global disparity value as the disparity value. Alternatively, the calculation unit  121  aligns the reference element  420  with the selection element  430 , averages the disparity values of respective pixels inside the aligned elements, and adopts the average as the disparity value. Alternatively, the calculation unit  121  aligns the reference element  420  with the selection element  430 , selects a pixel lying at the center of the aligned elements, calculates the disparity between the selected pixel and other pixels, and adopts the calculated disparity as the disparity value. 
     As such, the calculation unit  121  calculates the disparity value in the above-mentioned manners and, in order to obtain a more precise disparity value, the calculation unit  121  calculates disparity values with regard to a number of reference elements inside an image frame or over a number of frames for a predetermined period of time, and obtains the final disparity value through statistical analysis of the calculated disparity values. The obtained disparity value has a size or sign (±), so that the determination unit  122  identifies the disparity value with regard to a predetermined reference value to determine whether the 3D image is reversed or not. The reference value varies according to whether the reference element exists in the left or right image. 
     Referring to  FIG. 4B , the calculation unit  121  selects an image (e.g. right image), among the left and right images. The calculation unit  121  determines a reference element  450  in the selected right image. Herein, the calculation unit  121  can divide the selected right image with reference to the center axis  440  into first and second separate images, and then determine the reference element  450  in a separate image, e.g. the second separate image among the separate images. 
     This way, the calculation unit  121  determines the reference element  450  in the selected right image or the divided second separate image, and then determines a selection element  460 , which corresponds to the reference element  450 , in the non-selected left image. Herein, the calculation unit  121  determines the selection element  460  based on an algorithm such as Sum of Squared Differences (SSD) for analyzing the brightness difference between pixels, Sum of Absolute Differences (SAD), or Normalized Cross Correlation (NCC) for analyzing cross correlation. And then the calculation unit  121  calculates the disparity value between the reference element  450  and the selection element  460 . 
     The calculation unit  121  does not consider the vertical disparity between the left and right images, but the horizontal disparity only, to calculate the disparity value. Assuming that the disparity value is D, the horizontal coordinate of the reference element  450  is Sx, and the horizontal coordinate of the selection element  460  corresponding to the reference element  450  is Cx, the calculation unit  121  calculates the difference (Sx−Cx) between the horizontal coordinates of the left and right images as the disparity value (D). 
     Besides the above-described manner, the calculation unit  121  may calculate the disparity value in an alternative manner. For example, the calculation unit  121  calculates the global disparity value between the reference element  450  and the selection element  460  a plurality of times using a binocular disparity model, and adopts the calculated global disparity value as the disparity value. Alternatively, the calculation unit  121  aligns the reference element  450  with the selection element  460 , averages the disparity values of respective pixels inside the aligned elements, and adopts the average as the disparity value. Alternatively, the calculation unit  121  aligns the reference element  450  with the selection element  460 , selects a pixel lying at the center of the aligned elements, calculates the disparity between the selected pixel and other pixels, and adopts the calculated disparity as the disparity value. 
     As such, the calculation unit  121  calculates the disparity value in the above-mentioned manners and, in order to obtain a more precise disparity value, applies the above-mentioned manners to the calculated disparity value again. The obtained disparity value has a size or sign (±), so that the determination unit  122  identifies the disparity value with regard to a predetermined reference value to determine whether the 3D image is reversed or not. The reference value  420 ,  450  varies according to whether the reference element exists in the left or right image. The process of determining whether the 3D image is reversed or not based on the disparity value will now be described in more detail with reference to  FIGS. 5A and 5B . 
       FIGS. 5A and 5B  illustrate a process of determining whether a 3D image is reversed or not by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. Specifically,  FIG. 5A  corresponds to a case of determined by the determination unit  122  that a 3D image is not reversed based on a disparity value, and  FIG. 5B  shows a case of determined by the determination unit  122  that a 3D image is reversed based on a disparity value. 
     Referring to  FIG. 5A , as above-described, the calculation unit  121  selects an image (e.g. left image) among the left and right images, determines a reference element  520  in the selected left image, determines a selection element  510  corresponding to the reference element  520  in the non-selected right image. Also, as another embodiment of the present invention, the calculation unit  121  divides the selected left image with reference to the center axis  500  into first and second separate images, determines the reference element  520  in a separate image, e.g. the second separate image among the separate images, and determines the selection element  510  corresponding to the reference element  520  in the non-selected right image. 
     The calculation unit  121  calculates the disparity value between the reference element  520  and the selection element  510 . Assuming that the horizontal coordinate of the reference element  520  is Lx  540 , the horizontal coordinate of the selection element  510  is Rx  550 , and the disparity value is D, the calculation unit  121  defines the difference (Lx−Rx) between the horizontal coordinates of the left and right images as the disparity value (D). 
     The determination unit  122  compares the disparity value with a predetermined reference value to determine whether the 3D image is reversed or not. The disparity value has a sign, therefore the determination unit  122  determines whether the 3D image is reversed or not based on the sign of the disparity value. 
     For example, when the disparity value is larger than the reference value (e.g. when the reference value is zero and thus the disparity value is a positive value), the 3D image does not need to be reversed. On the contrary, when the reference value is smaller than the reference value (e.g. when the disparity value is a negative value), the 3D image needs to be reversed. The disparity value in  FIG. 5A , e.g. D  530 , is larger than the reference value (and thus is a positive value), the 3D image of  FIG. 5A  does not need to be reversed. 
     Referring to  FIG. 5B , as above-described, the calculation unit  121  selects an image (e.g. left image) among the left and right images, determines a reference element  552  in the selected left image, determines a selection element  551  corresponding to the reference element  552  in the non-selected right image. Also, as another embodiment of the present invention, the calculation unit  121  divides the selected left image with reference to the center axis  501  into first and second separate images, determines the reference element  552  in a separate image, e.g. the second separate image among the separate images, and determines the selection element  551  corresponding to the reference element  552  in the non-selected right image. 
     The calculation unit  121  calculates the disparity value between the reference element  552  and the selection element  551 . Assuming that the horizontal coordinate of the reference element  552  is Lx  555 , the horizontal coordinate of the selection element  551  is Rx  554 , and the disparity value is D, the calculation unit  121  defines the difference (Lx−Rx) between the horizontal coordinates of the left and right images as the disparity value (D). 
     The determination unit  122  compares the disparity value (D)  553  with a predetermined reference value to determine whether the 3D image is reversed or not. The disparity value has a sign, therefore the determination unit  122  determines whether the 3D image is reversed or not based on the sign of the disparity value based on the sign of the disparity value. 
     For example, when the disparity value is larger than the reference value (e.g. when the reference value is zero and thus the disparity value is a positive value), the 3D image does not need to be reversed. On the contrary, when the reference value is smaller than the reference value (e.g. when the disparity value is a negative value), the 3D image needs to be reversed. The disparity value in  FIG. 5B , e.g. D  530 , is small than the reference value (and thus is a negative value), the 3D image of  FIG. 5A  needs to be reversed. 
     After the determination unit  122  determines whether the 3D image is reversed or not based on the disparity value, the correction unit  130  receives the determination result from the determination unit  122 , and corrects the left and right images of the 3D image based on the determination result. More specifically, the correction unit  130  stores each of the left and right images in a buffer. When it has been determined that the 3D image needs to be reversed, the correction unit  130  switches positions of the left and right images to correct the reversed 3D image. When it has been determined that the 3D images does not need to be reversed, the correction unit  130  outputs the inputted left and right images without modification. A process of displaying a 3D image in a 3D image system in accordance with an embodiment of the present invention will now be described in more detail with reference to  FIG. 6 . 
       FIG. 6  schematically illustrates a process of displaying a 3D image by an apparatus for displaying 3D images in a 3D image system in accordance with an embodiment of the present invention. 
     Referring to  FIG. 6 , the 3D image display apparatus receives left and right images taken by a plurality of cameras and stores each of the images at step  5600 . Besides the cameras, other various types of imaging devices may be employed to obtain the left and right images. The 3D image display apparatus acquires the characteristic information of the imaging devices, which take the left and right images, by extracting the characteristic information from the left and right images, or acquires the characteristic information of the imaging devices in the additional information, when the 3D image display apparatus separately receives the additional information regarding camera characteristics, etc. from the outside to obtain information regarding the cameras at step  5610 . Herein, the 3D image display apparatus can acquire the characteristic information by extracting not only using the horizontal disparity of the left and right images, but also using various types of algorithms other than the horizontal disparity-based algorithm. 
     The 3D image display apparatus determines whether to rectify the left and right images or not based on the characteristic information regarding the cameras used to take the left and right images at step  5620 . When cross-axis cameras have been used to take the left and right images, the 3D image display apparatus rectifies the left and right images using a rectification algorithm, such as a homography algorithm or an affine algorithm, at step  5630 . The process of rectifying the left and right images has already been described in detail with reference to  FIG. 2 , and repeated description thereof will be omitted herein. 
     The 3D image display apparatus selects an image among the left and right images at step  5640 . It will be assumed for convenience of description that the right image is selected. The 3D image display apparatus determines a reference element in the selected right image, determines a selection element corresponding to the reference element in the non-selected left image, and then calculates the disparity value between the left and right images by using the reference element and the selection element at step S 650 . 
     Also, as another embodiment of the present invention, the 3D image display apparatus selects an image, e.g. the right image, determines a center axis vertically extending through the horizontal center in the selected right image, and divides the selected right image with reference to the center axis at step  5640 . Besides the above-mentioned manner of dividing the image, the 3D image display apparatus can divide the image in various other manners, i.e. divide the image with reference to a center axis, at which the disparity of both images is zero. 
     The 3D image display apparatus determines the reference element in a separate image among the first and second separate images divided the right image, determines the selection element corresponding to the reference element in the non-selected left image, and calculates the disparity value between the left and right images by using the reference element and the selection element at step S 650 . The process of calculating the disparity value has already been described in detail, and repeated description thereof will be omitted herein. 
     The 3D image display apparatus compares the disparity value with a reference value at step  5660 . As based on the result of the comparison at step  5660 , when the disparity value is larger than the reference value, the 3D image display apparatus switches the left and right images of the 3D image so that the reversed 3D image is corrected at step S 670 . More specifically, the 3D image display apparatus stores each of the left and right images in a buffer and, when the determination requires that the 3D image be reversed by the correction based on the result of the comparison, switches positions of the left and right images to correct the reversed 3D image. 
     As based on the result of the comparison at step  5660 , when the disparity value is smaller than the reference value, i.e. when the 3D image does not need to be reversed by the correction, the 3D image display apparatus displays the stored left and right images. The disparity value has a size or sign, so that the 3D image display apparatus can identify the disparity value with regard to a predetermined reference value to determine whether the 3D image is reversed or not. The reference value varies according to which part of the left or right image the reference element belongs to. For example, when the disparity value is smaller than the reference value (e.g. when the reference value is zero and thus the disparity value is a negative value), the 3D image does not need to be reversed. On the contrary, when the reference value is larger than the reference value (e.g. when the disparity value is a positive value), the 3D image needs to be reversed. 
     In accordance with the exemplary embodiments of the present invention, it is determined in a 3D image system whether a 3D image is normal or not, e.g. whether left and right images of the 3D image are reversed or not, so that the 3D image is displayed correctly to the watcher. Furthermore, after determining whether the 3D image is reversed or not, reversing of the 3D image is automatically corrected so that the watcher has the perception of depth from the correctly displayed 3D image. 
     While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.