Recently, studies have been conducted actively on mixed reality (hereinafter abbreviated to MR). MR aims at seamless connection between real space and virtual space. It is an important technique in virtual reality (hereinafter abbreviated to VR).
MR aims at coexistence of real space and a VR world which can conventionally be experienced only in situations isolated from real space. It attracts attention as a technique for enhancing VR.
MR includes video see-through mode that involves superimposing a picture of virtual space (for example, virtual objects drawn by computer graphics (hereinafter abbreviated to CG) or text information) over a picture of captured real space captured by an imaging apparatus such as a video camera, and optical see-through mode that involves superimposing a picture of virtual space over a picture of the real space transmitted optically through a display screen.
MR is expected to find applications in medical aids which will show the inside of the patient's body to the surgeon as if it were transparent, applications in work aids which will show assembly procedures of a product superimposed over the actual objects in a factory, and other applications totally different in quality from those of conventional VR.
What is commonly required of these applications is a technique for aligning the position/attitude of real space and virtual space and many attempts have been made so far.
The problem of position/attitude alignment in video see-through mode MR resolves itself down to a matter of finding the 3D position/attitude of a video camera in a global coordinate system set up in real space (hereinafter referred to simply as a global coordinate system). Similarly, the problem of position/attitude alignment in optical see-through mode MR comes down to a matter of finding the 3D position/attitude of the user's viewpoint.
As a method of solving these problems, it is common practice to derive the 3D position/attitude of the video camera's or user+s viewpoint in a global coordinate system by using a 3D position/attitude sensor such as a magnetic sensor or ultrasonic sensor.
The output values of a sensor represent, in the case of a magnetic sensor, for example, the 3D position/attitude of the sensor itself in the sensor coordinate system whose center is defined as the source of an AC magnetic field described later, and not the 3D position/attitude of the video camera's or user's viewpoint in a global coordinate system being measured. In other words, the output values of the sensor cannot be used directly as the 3D position/attitude of the video camera's or user's viewpoint, but some coordinate transformation is necessary. Hereinafter the data needed for coordinate transformation between the output values of the sensor and the 3D position/attitude of the video camera's or user's viewpoint will be referred to as calibration data (calibration information). Also, the process of setting or calculating calibration data beforehand will be referred to as calibration. Only after accurate calibration is performed, the output of a 3D position/attitude sensor can be converted to the 3D position/attitude of the video camera's or user's viewpoint and accurate position/attitude alignment in MR can be performed.
Conventionally, however, it is difficult to:                acquire the position/attitude (the information needed to convert sensor output to the 3D position/attitude of the sensor in a global coordinate system) of the sensor coordinate system in a global coordinate system, and        acquire the position/attitude (the information needed to transform the position/attitude of the sensor into the position/attitude of the measuring object) of the measuring object (camera's or user's viewpoint) as viewed from the sensor.        
The present invention has been made in view of the above problems. Its object is to acquire the information needed to transform a sensor coordinate system into a global coordinate system and the information needed to transform the position/attitude of a sensor into the position/attitude of a measuring object.