Abstract:
An image display apparatus calculating correction information to correct measured values of an orientation sensor includes an orientation sensor that senses orientation of a viewpoint and outputs measured values, a correction unit for correcting the measured values by using correction information, and a generation unit for generating a virtual image based on the corrected measured values. A display unit displays the generated virtual image, and a calculation unit calculates the corrected information. The calculation unit obtains the measured values of the sensor at the point of when the virtual image is displayed at an appropriate position, generates the correction information from the measured values and predetermined orientation information, and generates the virtual image based on orientation information relating to the viewpoint obtained using the measured values of the orientation sensor and the correction information.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image display apparatus and method in which an image in a virtual space fused with a real space is displayed based on output values from a posture sensor. More particularly, the invention relates to an image display apparatus and method in which an image in a virtual space drawn in accordance with a posture of a viewpoint of an image pickup apparatus is superposed on an image in a real space photographed by the image pickup apparatus, and the superposed image is displayed on a display picture surface, an image display apparatus and method in which an image in a virtual space drawn in accordance with a posture of a viewpoint of a user is displayed on an optical see-through-type display picture surface through which a real space is transparently observable with the virtual image, and a storage medium. 
   2. Description of the Related Art 
   Recently, mixed reality (hereinafter abbreviated as “MR”) aiming at seamless connection of a real space and a virtual space is intensively being studied. MR is obtained according to a video see-through method in which an image in a virtual image (such as a virtual object drawn according to computer graphics (hereinafter abbreviated as “CG”), character information or the like) is superposed on an image in a real space photographed by an image pickup apparatus, such as a video camera or the like, and the resultant image is displayed, or an optical see-through method in which an image in a virtual space is displayed on an optical see-through-type display picture surface of a display apparatus through which a real space is transparently observable with the virtual image. 
   It is being expected to apply such MR to new fields qualitatively different from conventional virtual reality, such as navigation in which, for example, the name and the guide of a building is superposed and displayed on a building in a real town, scenery simulation in which a CG image of a building to be built on a planned site is displayed in a superposed state, and the like. A common request for such applications is how exactly a real space and a virtual space are to be aligned, and many attempts have been done for this request. 
   The problem of alignment in MR according to the video see-through method relates to the problem of obtaining the three-dimensional position/posture of a viewpoint of an image pickup apparatus in a world coordinate system set in a real space (hereinafter simply termed a “world coordinate system”). The problem of alignment in MR according to the optical see-through method relates to the problem of obtaining the three-dimensional position/posture of a viewpoint of a user in the world coordinate system. 
   In order to solve such problems, acquisition of the three-dimensional position/posture of a viewpoint of a photographing apparatus or a user (hereinafter simply termed a “viewpoint”) in the word coordinate system by utilizing a three-dimensional position/posture sensor, such as a magnetic sensor, an ultrasonic sensor or the like, has been generally practiced. 
   In situations in which fixed values may be used for the position of a viewpoint, for example, in a case in which the distance to an object is sufficiently large outdoors, acquisition of the three-dimensional position/posture of a viewpoint by obtaining the three-dimensional posture of the viewpoint using a three-dimensional posture sensor including a gyro-sensor and an accelerometer has been generally practiced. 
   For example, when using a posture sensor TISS-5-40 made by Tokimec Kabushiki Kaisha as a three-dimensional posture sensor, output values from the sensor represent the sensor&#39;s three-dimensional posture in a sensor coordinate system in which a direction opposite to the direction of gravity is defined as the y axis, and the forward direction of the sensor on the x-z plane defined by the y axis when starting the sensor is defined as the -z axis. Accordingly, output values from a three-dimensional sensor do not generally represent the three-dimensional posture of a viewpoint in the world coordinate system to be measured. That is, output values from the sensor cannot be used for the three-dimensional posture of the viewpoint in the world coordinate system, and it is necessary to perform some coordinate transformation. More specifically, coordinate transformation in which the posture of a sensor is transformed into the posture of a viewpoint, and coordinate transformation in which a posture in the sensor coordinate system is transformed into a posture in the world coordinate system are required. In this specification, data for performing coordinate transformation between output values from a sensor and the three-dimensional posture of a viewpoint in the world coordinate system is termed “correction information”. 
     FIG. 1  illustrates the configuration of an ordinary image display apparatus for providing MR according to the optical see-through method. 
   An optical see-through-type display picture surface  110  is fixed on a head mounting unit  100  together with a posture sensor  120 . When a user (not shown) mounts the head mounting unit  100  so that the display picture surface  110  is positioned in front of the user&#39;s eyes, the user can observe a real space in front of the display picture surface  110  via an optical system (not shown in  FIG. 1 ) of the display picture surface  110 . The posture sensor  120  measures its posture in the sensor coordinate system, and outputs measured values of the posture having three degrees of freedom. The posture sensor  120  incorporates a tiltmeter (not shown) that can measure the direction of gravity of the earth. As described above, one axis (the y axis in this case) of the sensor coordinate system is set to a direction opposite to the direction of gravity. 
   A posture-information output unit  130  calculates the posture of the user&#39; viewpoint in the word coordinate system by transforming measured values input from the posture sensor  120  in accordance with correction information stored in a memory  140 , and outputs the calculated values as posture information. An image generation unit  150  generates and outputs a virtual image corresponding to the user&#39;s posture in accordance with the posture information input from the posture-information output unit  130 . The display picture surface  110  inputs the virtual image from the image generation unit  150  and displays the input virtual image. According to the above-described configuration, the user sees the virtual image displayed on the display picture surface  110  which is fused with a view of the real space seen via the display picture surface  110 . 
   Next, a description will be provided of a method for calculating the posture of the user&#39;s viewpoint in the world coordinate system by the posture-information output unit  130 , with reference to  FIG. 2 . 
   In  FIG. 2 , the posture of a sensor coordinate system  210  in a world coordinate system  200  is represented by R TW , the posture of the posture sensor  120  in the sensor coordinate system  210  is represented by R ST , the relative posture of a viewpoint  220  of the user as seen from the posture sensor  120  is represented by R VS , and the posture of the user&#39;s viewpoint  220  in the world coordinate system  200  is represented by R VW . 
   R is a 4×4 matrix, and R BA  represents the posture of an object B in a coordinate system A. In other words, R is a coordinate transformation matrix from the coordinate system A into a coordinate system B defined by the object B, and defines a transformation formula P B =R BA P A  to transform coordinates P A =(X A ,Y A ,Z A ,1) T  in the coordinate system A into coordinates P B =(X B , Y B Z B , 1) T  in the coordinate system B. That is, the posture R VW  of the user&#39;s viewpoint  220  in the world coordinate system  200  can also be represented by a coordinate transformation matrix (P V =R VW P W ) for transforming coordinates P W =(X W ,Y W ,Z W ,1) T  in the world coordinate system  200  into coordinates P V =(X V ,Y V ,Z V ,1) T  in a user&#39;s viewpoint coordinate system  230 . 
   The matrix R is the product of a rotation matrix Rx defined by an angle of rotation θ around the x axis, a rotation matrix Ry defined by an angle of rotation (azimuth) φ around the y axis, and a rotation matrix Rz defined by an angle of rotation φ around the z axis, and a relationship of R=RzRxRy holds. These matrices are expressed as follows: 
               R   ⁢           ⁢     x   ⁡     (   θ   )         =     [         1       0       0       0           0         cos   ⁢           ⁢   θ           sin   ⁢           ⁢   θ         0           0           -   sin     ⁢           ⁢   θ           cos   ⁢           ⁢   θ         0           0       0       0       1         ]       ,     
     ⁢       R   ⁢           ⁢     y   ⁡     (   ϕ   )         =     [           cos   ⁢           ⁢   ϕ         0           -   sin     ⁢           ⁢   ϕ         0           0       1       0       0             sin   ⁢           ⁢   ϕ         0         cos   ⁢           ⁢   ϕ         0           0       0       0       1         ]       ,     
     ⁢       R   ⁢           ⁢     z   ⁡     (   ψ   )         =     [           cos   ⁢           ⁢   ψ           sin   ⁢           ⁢   ψ         0       0               -   sin     ⁢           ⁢   ψ           cos   ⁢           ⁢   ψ         0       0           0       0       1       0           0       0       0       1         ]             
R VW  can be expressed by R VW =R VS ·R ST ·R TW  - - - (equation A).
 
   Since the y axis of the sensor coordinate system  210  is set to a direction opposite to the direction of gravity, by defining the y axis of the world coordinate system  200  to be perpendicular to the ground level, the direction of the y axis of the world coordinate system  200  can coincide with the y axis of the sensor coordinate system  210 . At that time, each of components Rx TW  and Rz TW  around the x axis and the y axis, respectively, of R TW  is a unit matrix, and R TW  is equivalent to a rotation matrix Ry TW  defined by an angle of rotation φ TW  around the y axis. Accordingly, the above-described (equation A) is transformed into R VW =R VS ·R ST ·Ry TW  (equation B). 
   In the above-described equations, R ST  is an input from the posture sensor  120  to the posture-information output unit  130 , R VW  is an output from the posture-information output unit  130  to the image generation unit  150 , and R VS  and Ry TW  (in other words, angles of rotation θ VS , φ VS  and φ VS  around three axes defining R VS , and an angle of rotation φ TW  defining Ry TW ) constitute correction information necessary for transforming R ST  into R VW . The posture-information output unit  130  calculates R VW  based on (equation B) using R ST  input from the posture sensor  120 , and R VS  and Ry TW  stored in the memory  140 , and outputs the calculated R VW  to the image generation unit  150 . 
   In order to perform exact alignment between the real space and the virtual space, exact correction information must be set in the memory  140  by some means. Only when exact correction information is provided, display of the virtual image exactly aligned with the real space is realized. 
   In one known method for setting correction information, the user or the operator interactively increases or decreases the values of θ VS  φ VS , φ vs and φ TW  stored in the memory  140  via input means (not shown), and adjustment of the respective values is performed in a trial-and-error approach until exact alignment is achieved. 
   In this approach, since the four parameters must be simultaneously adjusted, troublesome operations are required and a large amount of time is required for adjustment. 
   One method for mitigating the above-described trouble has been proposed in Japanese Patent Application Laid-Open (Kokai) No. 2001-050990 (2001) filed by the assignee of the present application. This method for setting correction information will now be described. 
     FIG. 3  is a block diagram illustrating the configuration of an image display apparatus in which this method for setting correction information is incorporated in the image display apparatus shown in  FIG. 1 . As shown in  FIG. 3 , in this configuration, a correction-information calculation unit  310 , an instruction-information input unit  320  and a switching unit  330  are added to the configuration shown in  FIG. 1 . The functions of components of this image display apparatus corresponding to the posture-information output unit  130  and the memory  140  shown in  FIG. 1  differ from the functions of the corresponding components shown in  FIG. 1 . Hence, in the image display apparatus shown in  FIG. 3 , these components are indicated by a posture-information output unit  130 ′ and a memory  140 ′. 
   Correction information is calculated by moving the posture of the user&#39;s viewpoint  220  to a predetermined posture Ry 0   VW  having a visual line horizontal to the ground level in the world coordinate system, and acquiring an output R 0   ST  of the posture sensor  120  at that time. The memory  140 ′ stores the posture Ry 0   VW  of the predetermined viewpoint (or an angle of rotation φ 0   VW  around the y axis defining the posture), in addition to the correction information. 
   The switching unit  330  sets the mode of the posture-information output unit  130 ′ to an ordinary mode or a correction-information calculation mode by receiving an input from the user or the operator (both not shown in  FIG. 3 ). 
   When the mode is the ordinary mode, the posture-information output unit  130 ′ calculates R VW  from R ST  input from the posture sensor  120  using correction information, as the posture-information output unit  130  described with reference to  FIG. 1 , and outputs the calculated R VW  to the image generation unit  150  as posture information. 
   When the mode is the correction-information calculation mode, Ry 0   VW  is input from the memory  140 ′, and is output to the image generation unit  150  as posture information. The instruction-information input unit  320  receives an input from the user or the operator, and transmits an instruction to execute correction-information calculation processing to the correction-information calculation unit  310 . More specifically, in the correction-information calculation mode, the user or the operator adjusts the posture of the viewpoint  220  so that a virtual image displayed on the display picture surface  110  is correctly aligned with the real space seen via the display picture frame  110 . When it is determined that they are sufficiently aligned (i.e., when it is determined that the viewpoint  220  is positioned at the posture Ry 0   VW ), input to the instruction-information input unit  320  is performed, for example, by depressing a specific key. 
   The correction-information calculation unit  310  inputs an instruction to execute correction-information calculation processing from the instruction-information input unit  320 , inputs the output R 0   ST  from the posture sensor  120  at that time (i.e., when the user or the operator determines that the viewpoint  220  is positioned at the posture Ry 0   vw ), and calculates correction information based on the posture Ry 0   vw  and the posture R 0   st . 
   In the above-described method, it is necessary that a rotation component Ry SV  (or an angle of rotation φ SV  around the y axis defining the rotation component) around the y axis of an inverse matrix R SV  (representing a relative posture of the posture sensor as seen from the user&#39;s viewpoint) of the posture R VS , serving as one component of correction information, is known by some means, and is already stored in the memory  140 ′. 
   At that time, a relationship of Ry 0   VW =R 0   VS ·R 0   ST· Ry TW  (equation C) holds between data processed at the correction-information calculation unit  310 , according to (equation B). By modifying (equation C), the following equation is obtained:
 
 Ry   0   VW =( Rz   SV   Rx   SV   Ry   SV ) −1    R   0   st   Ry   TW  (equation D).
 
   The correction-information calculation unit  310  inputs Ry 0   VW  and Ry SV  from the memory  140 ′ and R 0   ST  from the sensor  120 , and calculates unknown components Rz SV , Rx SV  and Ry TW  of correction information based on (equation D), according to the following procedure. 
   By further modifying (equation D), the following equation is obtained:
 
 Rz   SV   Rx   SV   Ry   SV   Ry   0   VW   =Rz   0   ST   Rx   0   ST   Ry   0   ST   Ry   TW   (equation E).
 
Since each of the left and right sides of (equation E) is the product of rotation components around the z, x and y axes, an identity holds for each rotation component around each of the z, x and y axes.
 
   First, an identity of a rotation component around each of the z and x axes is as follows. That is,
 
Rz SV =Rz 0   ST   (equation F)
 
Rx SV =Rx 0   ST   (equation G).
 
Thus, it is possible to obtain Rz SV  and Rx SV .
 
   An identity of a rotation component around the y axis is as follows. That is,
 
Ry SV  Ry 0   VW =Ry 0   ST Ry TW .
 
From this equation, the following equation is obtained:
 
 Ry   TW   =Ry   SV   Ry   0   VW    Ry   0   ST   −1   (equation H).
 
Thus, it is possible to obtain Ry TW .
 
   The correction-information calculation unit  310  calculates Rz SV , Rx SV  and Ry TW  according to the above-described processing, further calculates R VS  (=(Rz SV Rx SV Ry SV ) −1 ) from these values, and outputs R VS  and Ry TW  (or angles of rotation θ SV, φ   SV  and φ TW  defining these values) to the memory  140 ′. 
   Ry SV  may be obtained from measured values by a protractor or the like in a trial-and-error approach, or by using any other appropriate measuring means. 
   As described above, by obtaining only Ry SV  in correction information, it is possible to easily obtain other unknown components of correction information, and realize exact alignment. 
   However, it is generally difficult to exactly measure correction information Ry SV  in advance. Accordingly, it is actually necessary to repeat a series of operations of rough setting of Ry SV , processing to obtain other components of correction information, fine adjustment of Ry SV , processing to obtain other components of correction information, fine adjustment of Ry sv , . . . until exact alignment is realized, and therefore troublesome operations and a large amount of time are required for acquiring correction information. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in consideration of the above-described problems. 
   It is an object of the present invention to more simply acquire correction information for transforming measured values of a sensor into the posture of a viewpoint in the world coordinate system. 
   According to one aspect of the present invention, an image display apparatus for displaying a virtual image in accordance with a posture of a viewpoint of a user observing a display picture surface on the display picture surface includes a posture sensor that outputs measured values of the posture, first display means for generating the virtual image based on predetermined posture information stored in a memory in advance, and displaying the generated virtual image on the display picture surface, calculation means for calculating correction information based on the measured values of the posture from the posture sensor and the predetermined posture information, when it is notified that the posture of the viewpoint and a posture indicated by the predetermined posture information coincide, second display means for generating the virtual image based on posture information relating to the viewpoint obtained using the measured values of the posture obtained from the posture sensor and the correction information, and displaying the generated virtual image on the display picture surface, and modification means for modifying the correction information in response to a predetermined operation, while displaying the virtual image by the second display means. 
   According to another aspect of the present invention, an image display method for displaying an image in a virtual space in accordance with a posture of a viewpoint of a user observing a display picture surface on the display picture surface, while optically transmitting a real space on the display picture surface includes a switching step of performing switching between a first mode and a second mode, a posture-information output step of calculating the posture of the viewpoint based on correction information and measured values of the posture having three degrees of freedom input from a posture sensor and outputting the calculated posture as posture information, in the first mode, and outputting a predetermined posture as posture information relating to the viewpoint, in the second mode, an image generation step of generating the image based on the posture information output in the posture-information output step, an instruction-information input step of inputting instruction information instructing that the posture of the viewpoint coincides with the predetermined posture, a correction-information calculation step of calculating the correction information based on the measured values of the posture when and/or before and after the instruction information has been input, and the predetermined posture, a modification information input step of inputting modification information for modifying the correction information, and a correction-information modification step of modifying the correction information in accordance with the modification information. 
   According to still another aspect of the present invention, an image display method for generating a superposed image obtained by superposing an image in a virtual space in accordance with a posture of a viewpoint of an image pickup apparatus on a real image photographed by the image pickup apparatus, and displaying the generated superposed image on a display picture surface includes a switching step of performing switching between a first mode and a second mode, a posture-information output step of calculating the posture of the viewpoint based on correction information and measured values of the posture having three degrees of freedom input from a posture sensor and outputting the calculated posture as posture information, in the first mode, and outputting a predetermined posture as posture information relating to the viewpoint, in the second mode, an image generation step of generating the superposed image based on the posture information output in the posture-information output step, an instruction-information input step of inputting instruction information instructing that the posture of the viewpoint coincides with the predetermined posture, a correction-information calculation step of calculating the correction information based on the measured values of the posture when and/or before and after the instruction information has been input, and the predetermined posture, a modification information input step of inputting modification information for modifying the correction information, and a correction-information modification step of modifying the correction information in accordance with the modification information. 
   According to yet another aspect of the present invention, a computer readable storage medium stores program codes for causing a computer to execute one of the above-described image display methods. 
   According to a further aspect of the present invention, a control program causes a computer to execute one of the above-described image display methods. 
   The foregoing and other objects, advantages and features of the present invention will become more apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating the configuration of an ordinary image display apparatus that provides MR according to an optical see-through method; 
       FIG. 2  is a diagram illustrating a method for calculating a posture of a user&#39;s viewpoint  220  in a world coordinate system  200 ; 
       FIG. 3  is a block diagram illustrating the configuration of an image display apparatus including a correction-information calculation unit; 
       FIG. 4  is a block diagram illustrating the configuration of an image display apparatus according to an embodiment of the present invention; and 
       FIG. 5  is a flowchart illustrating control in the image display apparatus shown in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of the present invention will now be described in detail with reference to the drawings. 
   In the following embodiment, a description will be provided of a case in which an image display apparatus and method of the present invention is applied to a simulation apparatus and method for performing scenery simulation of a building to be built. 
     FIG. 4  illustrates the configuration of a simulation apparatus according to the embodiment. As shown in  FIG. 4 , in the configuration of the apparatus of the embodiment, a correction-information modification unit  400  and a correction-information input unit  410  are added to the configuration of the image display apparatus shown in  FIG. 3 . It is assumed that an image generation unit  150 ′ has the function of generating a CG image of a building to be built, as a virtual image. A head mounting unit  100 , a display picture surface  110 , a posture sensor  120 , a posture-information output unit  130 ′, a memory  140 ′, a correction-information calculation unit  310 , an instruction-information input unit  320  and a switching unit  330  operate in the same manner as corresponding ones of the image display apparatus shown in  FIG. 3 . Hence, further description thereof will be omitted. 
   In this embodiment, correction information φ SV  stored in the memory  140 ′ in advance need not have an exact value. An outline of processing to acquire correction information in this embodiment is as follows: 
   (1) First, components R VS  and Ry TW  (or angles of rotation θ SV , φ SV  and φ TW  defining these values) of correction information are calculated based on roughly set correction information φ SV , according to the correction-information calculation method described with reference to  FIGS. 2 and 3 . (2) Then, fine adjustment of Ry SV  and Ry TW  (or angles of rotation φ SV  and φ TW  defining these values), that are components of indeterminate correction information, is performed according to the following method. 
   Fine adjustment of the angles of rotation φ SV  and φ TW  will now be described. The modification-information input unit  410  outputs a command to increase the value of the correction information φ SV  by a constant value (an increase command), or a command to decrease the value of the correction information φ SV  by a constant value (a decrease command) to the correction-information modification unit  400 , in response to an input from a user or an operator (both not shown). 
   The correction-information modification unit  400  inputs the values of the correction information φ SV  and φ TW  from the memory  140 ′, performs the following modification to the correction information in accordance with the increase/decrease command transmitted from the modification-information input unit  410 , and outputs the result of modification to the memory  140 ′. 
   That is, when the increase command has been input, the values of φ SV  and φ TW  are increased by a constant value            φ.
 
φ SV ←φ SV +         φ
 
φ TW ←φ TW +         φ  (equation I).

   Similarly, when the decrease command has been input, the values of φ SV  and φ TW  are decreased by the constant value            φ.
 
φ SV ←φ SV −         φ
 
φ TW ←φ TW −         φ  (equation J).

   The reason why the values of the correction information φ SV  and φ TW  are operated by the same amount is that the following relationship holds between these values from (equation H):
 
φ TW =φ SV +φ 0   VW −φ 0   ST   (equation K).
 
     FIG. 5  is a flowchart illustrating the processing procedure of the image display apparatus of the embodiment. Program codes for the flowchart shown in  FIG. 5  are stored within a memory (not shown), such as a RAM (random access memory), a ROM (read-only memory) or the like, within the image display apparatus, and are read and executed by a CPU (central processing unit) (not shown). 
   First, in step S 1010 , it is determined whether or not the current mode is a correction-information calculation mode. Switching of a mode is performed by the switching unit  330 . For example, the switching unit  330  determines which of a correction-information calculation mode and an ordinary mode has been instructed by detecting a predetermined key operation, and notifies the posture-information output unit  130 ′ and the correction-information calculation unit  310  of the result of the determination. 
   When the mode is the correction-information calculation mode, the process proceeds to step S 1140 , where the correction-information calculation unit  310  determines whether or not instruction information has been input from the instruction-information input unit  320 . If the result of the determination in step S 1140  is negative, the correction-information calculation unit  310  does not execute processing indicated by steps S 1150 –S 1180 , and the process skips to step S 1190 , where the posture-information output unit  130 ′ inputs predetermined posture information from the memory  140 ′. Then, in step S 1090 , the input posture information is output to the image generation unit  150 ′ as the posture information relating to the viewpoint. 
   In step S 1100 , the image generation unit  150 ′ inputs the posture information of the viewpoint from the posture-information output unit  130 ′, and sets this information as drawing parameters for a virtual object. Then, in step S 1110 , a virtual image is drawn using the set drawing parameters to generate a CG image. Then, in step S 1120 , the generated CG image is output to the display picture surface  110 . Thus, the virtual image corresponding to a predetermined posture is displayed on the display picture surface  110 . This processing is repeated during the correction-information calculation mode. 
   The user or the operator adjusts the posture of the head so that the virtual image displayed on the display picture surface  110  is correctly aligned with the real space (real world). Upon completion of the adjustment, input of instruction information is performed by the instruction-information input unit  320 . For example, the instruction-information input unit  320  detects a predetermined key operation by the user, and notifies the correction-information calculation unit  310  of the fact. 
   If the result of the determination in step S 1140  is affirmative, the process proceeds to step S 1150 , where the correction-information calculation unit  310  inputs measured values from the posture sensor  120 . Then, in step S 1160 , a predetermined posture and correction information Ry SV  (or an angle of rotation φ SV  defining this value) are input from the memory  140 ′. Then, in step S 1170 , unknown correction information is calculated in accordance with the above-described (equation F), (equation G) and (equation H). Then, in step S 1180 , the calculated correction information is output to the memory  140 ′. 
   Thereafter, the switching unit  330  sets the ordinary mode, and fine adjustment of Ry SV  and Ry TW  (or angles of rotation φ SV  and φ TW  defining these values), that are indeterminate components of correction information, is performed. The ordinary mode in which such fine adjustment can be performed will now be described. 
   When the mode has been set to the ordinary mode by the switching unit  330 , the process proceeds from step S 1010  to step S 1020 . In step S 1020 , it is determined whether or not modification information has been input from the modification-information input unit  410 . 
   If the result of the determination in step S 1020  is negative, the process proceeds to step S 1060 , where the posture-information output unit  130 ′ inputs measured values from the posture sensor  120 . Then, in step S 1070 , correction information is input from the memory  140 ′. Then, in step S 1080 , the posture-information output unit  130 ′ calculates the posture of the viewpoint based on the input measured values and correction information in accordance with the above-described (equation F). Then, in step S 1090 , the calculated posture is output to the image generation unit  150 ′ as the posture information relating to the viewpoint. 
   The image generation unit  150 ′ inputs, in step S 1100 , the posture information of the viewpoint from the posture-information output unit  130 ′, generates, in step S 1110 , a CG image by drawing a virtual object based on the posture information, and outputs, in step S 1120 , the generated CG image to the display picture surface  110 . Thus, the virtual image corresponding to the posture of the viewpoint is displayed on the display picture surface  110 . This processing is repeated during the ordinary mode. 
   The user or the operator confirms whether or not the virtual image displayed on the display picture surface  110  is correctly aligned with the real space, when the posture of the head has been changed. If misalignment is produced due to a change in the posture, fine adjustment of correction information is performed by inputting modification information from the modification-information input unit  410 . 
   When the modification information has been input, the correction-information modification unit  400  executes the following steps S 1030 –S 1050 . 
   First, in step S 1030 , components Ry SV  and Ry TW7590   (or angles of rotation φ SV  and φ TW  defining these values) of correction information are input from the memory  140 ′. Then, in step S 1040 , modification of the correction information is performed in accordance with the above-described (equation I) and (equation J). Then, in step S 1050 , the modified correction information is output to the memory  140 ′. Since the modified correction information is immediately reflected on calculation of the posture by the posture-information calculation unit  130 ′, the user or the operator can interactively input modification information while watching the result of fusion of the image in the real space and the image in the virtual space. Accordingly, fine adjustment of correction information can be easily performed so that correct alignment is realized. Although in this embodiment, modification information is a command to increase or decrease the value by a predetermined amount, the amount of increase or decrease may be instructed. 
   The posture-information output unit  130 ′, the memory  140 ′, the image generation unit  150 ′, the correction-information calculation unit  310 , the instruction-information input unit  320 , the switching unit  330 , the correction-information modification unit  400  and the modification-information input unit  410  may, for example, be configured by a single general-purpose computer (such as a notebook-type personal computer or the like). Although in this embodiment, the posture sensor TISS-5-40 made by Tokimec Kabushiki Kaisha is illustrated as the posture sensor  120 , any other posture sensor may, of course, be used. 
   It is desirable to perform an input operation from the switching unit  330 , the instruction-information input unit  320  or the modification-information input unit  410  via the same user interface using, for example, a keyboard or a control pad. 
   Although in the above-described embodiment, MR according to the optical see-through method is realized, the present invention is not limited to the optical see-through method, but may also be applied to MR according to the video see-through method. In that case, instead of the posture of the user s viewpoint  220  in the above-described embodiment, a viewpoint of an image pickup apparatus (for example, a video camera) is to be measured. In the video see-through method, the image generation unit  150 ′ draws a CG image in a state of being superposed on an image in a real space photographed by the image pickup apparatus, and displays the resultant image on the display image surface  110 . 
   As described above, according to the foregoing embodiment, it is possible to easily acquire correction information for transforming a posture of a sensor in the sensor coordinate system into a posture to be measured in the world coordinate system. 
   MODIFICATION EXAMPLE 1 
   Although in the above-described embodiment, a CG image of a virtual object to be displayed is used in order to perform alignment, any geographic information may be used provided that it serves as a visual cue for confirming that alignment between a virtual space and a real space is performed. For example, alignment may be performed by performing modeling of topography or the shape of a real building, and drawing a wire-frame model of the shape at the same position in the real space in a superposed state. It is, of course, also possible to perform display by combining a wire-frame model of a real object and a virtual object to be displayed. 
   MODIFICATION EXAMPLE 2 
   Although in the above-described embodiment, the image display apparatus and method of the invention is applied to scenery simulation, the present invention may, of course, also be applied to any other appropriate MR system than scenery simulation. The present invention is not limited to a MR system, but may also be applied to various uses in which the position/posture of a viewpoint is measured using a posture sensor. 
   MODIFICATION EXAMPLE 3 
   Although in the above-described embodiment, the posture sensor TISS-5-40 made by Tokimec Kabushiki Kaisha is used as the posture sensor, the posture sensor is not limited to this sensor. The present invention may be applied to any other posture sensor provided that one axis of the sensor coordinate system is set by being correlated with the world coordinate sensor. 
   MODIFICATION EXAMPLE 4 
   In the above-described embodiment, the correction-information calculation unit  310  inputs an output from the posture sensor  120  when instruction information has been input, as R 0   ST , and this is used for calculation of correction information (step S 1170 ). However, an output from the posture sensor  120  when instruction information has been input is not necessarily used as R 0   ST . For example, measured values (for example, the mean value of these values) from the posture sensor  120  before and after instruction information has been input may also be used. Alternatively, the viewpoint  220  may be brought to a standstill at the posture Ry 0   VW  for a constant time period, and R 0   ST  may be obtained based on the mean value of the output values from the posture sensor  120  during that time period. Furthermore, if it is possible to acquire an output from the posture sensor  120  when the viewpoint  220  is positioned at the posture Ry 0   VW , R 0   st  may be acquired according to any appropriate method. 
   OTHER EMBODIMENT 
   The object of the present invention may, of course, also be achieved by supplying a system or an apparatus with a storage medium (or a recording medium) storing program codes of software for realizing the functions of the above-described embodiments, and reading and executing the program codes stored in the storage medium by means of a computer (or a CPU or an MPU (microprocessor unit)) of the system or the apparatus. In such a case, the program codes themselves read from the storage medium realize the functions of the above-described embodiments, so that the storage medium storing the program codes constitute the present invention. The present invention may, of course, be applied not only to a case in which the functions of the above-described embodiments are realized by executing program codes read by a computer, but also to a case in which an OS (operating system) or the like operating in a computer executes a part or the entirety of actual processing, and the functions of the above-described embodiments are realized by the processing. 
   The present invention may, of course, be applied to a case in which, after writing program codes read from a storage medium into a memory provided in a function expanding card inserted into a computer or in a function expanding unit connected to the computer, a CPU or the like provided in the function expanding card or the function expanding unit performs a part or the entirety of actual processing, and the functions of the above-described embodiments are realized by the processing 
   When applying the present invention to the storage medium, program codes corresponding to the above-described flowchart (shown in  FIG. 5 ) are stored in the storage medium. 
   The individual components shown in outline or designated by blocks in the drawings are all well known in the image display apparatus and method arts and their specific construction and operation are not critical to the operation or the best mode for carrying out the invention. 
   While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.