Patent Publication Number: US-11660526-B2

Title: Estimation apparatus, estimation method, and program

Description:
TECHNICAL FIELD 
     The present invention relates to an estimation apparatus, an estimation method, and a program. 
     BACKGROUND ART 
     A known body tracking technique uses inverse kinematics (IK) to estimate the positions and orientations of a plurality of parts included in the body of the user on the basis of pieces of data representing the positions and orientations of a plurality of trackers worn by the user. The plurality of parts includes parts with no trackers worn thereon. 
     SUMMARY 
     Technical Problems 
     At present, a user needs to wear a tracker on a chest or a waist to realize accurate body tracking. However, wearing the tracker on the chest or waist is troublesome for the user. 
     Further, in body tracking, even if there is data representing the position and orientation of the tracker worn on a hand, it is not possible, in some cases, to accurately estimate the position of an elbow without data representing the orientation of a wrist. For example, when a motion of only moving a hand, such as a hand wave motion, is performed, the estimation result obtained may be, in some cases, the position of the elbow that has moved significantly, despite the fact that the elbow has not moved significantly. However, wearing the tracker on the wrist is also troublesome for the user. 
     The present invention has been made in view of the above-described problems. One of objects of the present invention is to provide an estimation apparatus, an estimation method, and a program that enable accurate body tracking without having the user wear many trackers. 
     Solution to Problems 
     In order to solve the above-described problems, an estimation apparatus according to the present invention includes: a time-series data acquisition section configured to acquire a plurality of pieces of time-series data each representing positions, postures, or motions of a part of a body; an input section configured to input the plurality of pieces of time-series data into a conversion section; and an output acquisition section configured to acquire a result of estimation of a position, a posture, or a motion of another part of the body that is closer to a center of the body than the part, the result of the estimation being an output obtained when the pieces of time-series data are input into the conversion section. 
     In a mode of the present invention, the plurality of pieces of time-series data includes pieces of time-series data for parts different from each other. 
     In this aspect, the input section may input first time-series data representing orientations of a head included in the body, second time-series data representing angular velocities of a left hand included in the body, and third time-series data representing angular velocities of a right hand included in the body into the conversion section, and the output acquisition section may acquire a result of estimation of an orientation of a chest or a waist included in the body or a result of estimation of an angular velocity of the chest or the waist included in the body. 
     Further, in a mode of the present invention, the plurality of pieces of time-series data includes at least two of time-series data representing orientations of the part, time-series data representing angular velocities of the part, time-series data representing positions of the part, and time-series data representing velocities of the part. 
     In this mode, the input section may input first time-series data representing orientations of a hand included in the body, second time-series data representing angular velocities of the hand, and third time-series data representing positions or velocities of the hand into the conversion section, and the output acquisition section may acquire a result of estimation of an orientation or an angular velocity of a wrist on a side of the hand included in the body. 
     Further, in a mode of the present invention, the conversion section includes a machine learning model that has finished learning. 
     Further, an estimation method according to the present invention includes: a step of acquiring a plurality of pieces of time-series data each representing positions, postures, or motions of a part of a body; a step of inputting the plurality of pieces of time-series data into a conversion section; and a step of acquiring a result of estimation of a position, a posture, or a motion of another part of the body that is closer to a center of the body than the part, the result of the estimation being an output obtained when the pieces of time-series data are input into the conversion section. 
     Further, a program according to the present invention causes a computer to execute: a procedure of acquiring a plurality of pieces of time-series data each representing positions, postures, or motions of a part of a body; a procedure of inputting the plurality of pieces of time-series data into a conversion section; and a procedure of acquiring a result of estimation of a position, a posture, or a motion of another part of the body that is closer to a center of the body than the part, the result of the estimation being an output obtained when the pieces of time-series data are input into the conversion section. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a configuration diagram illustrating an example of an entertainment system according to an embodiment of the present invention. 
         FIG.  2    is a configuration diagram illustrating an example of an entertainment apparatus according to the embodiment of the present invention. 
         FIG.  3    is a diagram illustrating an example of a skeleton model. 
         FIG.  4    is a diagram illustrating an example of a machine learning model used to estimate the orientation of a chest node. 
         FIG.  5    is a diagram illustrating an example of a machine learning model used to estimate the orientation of a left-wrist node or a right-wrist node. 
         FIG.  6    is a functional block diagram illustrating an example of functions implemented in the entertainment apparatus according to the embodiment of the present invention. 
         FIG.  7    is a flow diagram illustrating an example of a flow of processing performed by the entertainment apparatus according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
       FIG.  1    is a diagram illustrating an example of a configuration of an entertainment system  10  according to an embodiment of the present invention.  FIG.  2    is a diagram illustrating an example of a configuration of an entertainment apparatus  14  according to the present embodiment. 
     As illustrated in  FIG.  1   , the entertainment system  10  according to the present embodiment includes a plurality of trackers  12  (trackers  12   a  to  12   e  in the example in  FIG.  1   ), the entertainment apparatus  14 , a relay apparatus  16 , a display  18 , and a camera-microphone unit  20 . 
     Each tracker  12  according to the present embodiment is, for example, a device for tracking the position and orientation thereof. Here, each tracker  12  may include various types of sensors such as a camera, an inertial sensor (IMU), a geomagnetic sensor (azimuth sensor), an acceleration sensor, a motion sensor, and a GPS (Global Positioning System) module, for example. In addition, each tracker  12  may identify the position and orientation thereof on the basis of sensing data, that is, measurement results of sensors included in the tracker  12 . 
     Further, for example, the position and orientation of each tracker  12  may be identified on the basis of an image including an image of each tracker  12  captured by a camera  20   a . The camera  20   a  is included in the camera-microphone unit  20  to be described later. 
     In the present embodiment, the tracker  12   a , the tracker  12   b , the tracker  12   c , the tracker  12   d , and the tracker  12   e  are worn on the user&#39;s head, left hand, right hand, left foot, and right foot, respectively. Here, as illustrated in  FIG.  1   , the tracker  12   b  and the tracker  12   c  may be held by the user&#39;s hands. In the present embodiment, the position and orientation identified by each of the tracker  12   a , the tracker  12   b , the tracker  12   c , the tracker  12   d , and the tracker  12   e  correspond to the position and orientation of each of the user&#39;s head, left hand, right hand, left foot, and right foot, respectively. In the present embodiment, the positions and orientations of the plurality of parts included in the body of the user are identified by the plurality of trackers  12  in this manner. 
     The entertainment apparatus  14  according to the present embodiment is a computer such as a game console, a DVD (Digital Versatile Disc) player, or a Blu-ray (registered trademark) player, for example. The entertainment apparatus  14  according to the present embodiment generates a video and audio by executing a game program, reproducing content, or the like, for example. The game program and the content are stored in the entertainment apparatus  14  or recorded on an optical disc. In addition, the entertainment apparatus  14  according to the present embodiment outputs video signals representing the generated video and audio signals representing the generated audio to the display  18  via the relay apparatus  16 . 
     As illustrated in  FIG.  2   , the entertainment apparatus  14  according to the present embodiment includes a processor  30 , a storage section  32 , a communication section  34 , and an input/output section  36 , for example. 
     The processor  30  is, for example, a program control device, such as a Central Processing Unit (CPU), that operates according to programs installed in the entertainment apparatus  14 . The processor  30  according to the present embodiment also includes a GPU (Graphics Processing Unit) that draws an image in a frame buffer on the basis of a graphics command and data supplied from the CPU. 
     The storage section  32  is, for example, a storage element, such as a Read-Only Memory (ROM) or a Random Access Memory (RAM), a hard disk drive, or the like. The programs and the like to be executed by the processor  30  are stored in the storage section  32 . Further, an area of a frame buffer in which the GPU draws an image is reserved in the storage section  32  according to the present embodiment. 
     The communication section  34  is, for example, a communication interface or the like such as a wireless Local Area Network (LAN) module. 
     The input/output section  36  is an input/output port, such as an HDMI (registered trademark) (High-Definition Multimedia Interface) port or a Universal Serial Bus (USB) port. 
     The relay apparatus  16  according to the present embodiment is a computer that relays video signals and audio signals output from the entertainment apparatus  14  and outputs the video signals and audio signals to the display  18 . 
     The display  18  according to the present embodiment is, for example, a liquid crystal display or the like, and displays a video and the like represented by the video signals output from the entertainment apparatus  14 . 
     The camera-microphone unit  20  according to the present embodiment includes the camera  20   a  and a microphone  20   b , for example. The camera  20   a  outputs an image obtained by capturing a subject to the entertainment apparatus  14 . The microphone  20   b  acquires surrounding sound, converts the sound into audio data, and outputs the audio data to the entertainment apparatus  14 . Further, the camera  20   a  according to the present embodiment is a stereo camera. 
     The trackers  12  and the relay apparatus  16  can, for example, wirelessly transmit and receive data to and from each other. The entertainment apparatus  14  and the relay apparatus  16  are, for example, connected via an HDMI cable, a USB cable, or the like, and can transmit and receive data to and from each other. The relay apparatus  16  and the display  18  are, for example, connected via an HDMI cable or the like. The entertainment apparatus  14  and the camera-microphone unit  20  are, for example, connected via an Auxiliary (AUX) cable or the like. 
     In the present embodiment, for example, while the game program is executed by the entertainment apparatus  14 , various types of processes of game processing or the like corresponding to the positions or orientations of a plurality of parts included in the body of the user in a skeleton model  40  illustrated in  FIG.  3    are performed. Then, a video corresponding to the result of the processes is displayed on the display  18 , for example. 
     As illustrated in  FIG.  3   , the skeleton model  40  according to the present embodiment includes a head node  42   a , a left-hand node  42   b , a right-hand node  42   c , a left-foot node  42   d , and a right-foot node  42   e . The head node  42   a  corresponds to the head of the user wearing the tracker  12   a . The left-hand node  42   b  corresponds to the left hand of the user wearing the tracker  12   b . The right-hand node  42   c  corresponds to the right hand of the user wearing the tracker  12   c . The left-foot node  42   d  corresponds to the left foot of the user wearing the tracker  12   d . The right-foot node  42   e  corresponds to the right foot of the user wearing the tracker  12   e.    
     Further, in addition to the above-described nodes  42 , the skeleton model  40  also includes a chest node  42   f , a waist node  42   g , a left-shoulder node  42   h , a left-elbow node  42   i , and a left-wrist node  42   j . Further, the skeleton model  40  also includes a right-shoulder node  42   k , a right-elbow node  42   l , a right-wrist node  42   m , a left-knee node  42   n , a left-ankle node  42   o , a right-knee node  42   p , and a right-ankle node  42   q.    
     Here, as illustrated in  FIG.  3   , the head node  42   a  and the chest node  42   f  are connected to each other by a link. Further, the chest node  42   f  and the waist node  42   g  are connected to each other by a link. 
     Further, the chest node  42   f  and the left-shoulder node  42   h  are connected to each other by a link. Further, the left-shoulder node  42   h  and the left-elbow node  42   i  are connected to each other by a link. Further, the left-elbow node  42   i  and the left-wrist node  42   j  are connected to each other by a link. Further, the left-wrist node  42   j  and the left-hand node  42   b  are connected to each other by a link. 
     Further, the chest node  42   f  and the right-shoulder node  42   k  are connected to each other by a link. Further, the right-shoulder node  42   k  and the right-elbow node  42   l  are connected to each other by a link. Further, the right-elbow node  42   l  and the right-wrist node  42   m  are connected to each other by a link. Further, the right-wrist node  42   m  and the right-hand node  42   c  are connected to each other by a link. 
     Further, the waist node  42   g  and the left-knee node  42   n  are connected to each other by a link. Further, the left-knee node  42   n  and the left-ankle node  42   o  are connected to each other by a link. Further, the left-ankle node  42   o  and the left-foot node  42   d  are connected to each other by a link. 
     Further, the waist node  42   g  and the right-knee node  42   p  are connected to each other by a link. Further, the right-knee node  42   p  and the right-ankle node  42   q  are connected to each other by a link. Further, the right-ankle node  42   q  and the right-foot node  42   e  are connected to each other by a link. 
     These nodes  42  correspond to respective parts included in the body of the user. In addition, the present embodiment enables body tracking based on the positions and orientations identified for the plurality of trackers  12 , for example. Here, the position of each of the plurality of nodes  42  included in the skeleton model  40  relative to a reference position in an initial state and the orientation thereof relative to a reference orientation in the initial state are estimated, for example. 
     Here, for example, the position and orientation of the head node  42   a  can be determined on the basis of data representing the position and orientation identified for the tracker  12   a . Similarly, the position and orientation of the left-hand node  42   b  can be determined on the basis of data representing the position and orientation identified for the tracker  12   b . Further, the position and orientation of the right-hand node  42   c  can be determined on the basis of data representing the position and orientation identified for the tracker  12   c . Further, the position and orientation of the left-foot node  42   d  can be determined on the basis of data representing the position and orientation identified for the tracker  12   d . Further, the position and orientation of the right-foot node  42   e  can be determined on the basis of data representing the position and orientation identified for the tracker  12   e.    
     One conceivable method here is to use inverse kinematics (IK) to estimate the positions and orientations of the other nodes  42  on the basis of the pieces of data representing the determined positions and orientations of the head node  42   a , the left-hand node  42   b , the right-hand node  42   c , the left-foot node  42   d , and the right-foot node  42   e.    
     In this case, however, unless there is information about the orientation of the chest node  42   f  or the waist node  42   g , the positions and orientations of the other nodes  42  cannot be estimated using inverse kinematics. 
     Further, even if there is data representing the position and orientation of the left-hand node  42   b , it is, in some cases, difficult to estimate the position and orientation of the left-elbow node  42   i  using inverse kinematics unless there is information about the orientation of the left-wrist node  42   j . For example, when a motion of only moving the hand, such as a hand wave motion, is performed, the estimation result obtained may be, in some cases, the position of the elbow that has moved significantly, despite the fact that the elbow has not moved significantly. 
     Similarly, even if there is data representing the position and orientation of the right-hand node  42   c , it is, in some cases, difficult to estimate the position and orientation of the right-elbow node  42   l  using inverse kinematics unless there is information about the orientation of the right-wrist node  42   m.    
     In view of the foregoing, the present embodiment enables accurate estimation of the orientation of the chest node  42   f  or the waist node  42   g  on the basis of the pieces of data representing the positions and orientations of the head node  42   a , the left-hand node  42   b , the right-hand node  42   c , the left-foot node  42   d , and the right-foot node  42   e  as described below. Further, the present embodiment enables accurate estimation of the orientations of the left-wrist node  42   j  and the right-wrist node  42   m  on the basis of the pieces of data representing the positions and orientations of the head node  42   a , the left-hand node  42   b , the right-hand node  42   c , the left-foot node  42   d , and the right-foot node  42   e  as described below. 
     It is noted that in the present embodiment, the position and orientation of each of the trackers  12   a  to  12   e  are identified at a predetermined sampling rate, for example. Then, pieces of data representing the positions and orientations of the trackers  12  are transmitted to the entertainment apparatus  14  in response to the identification of the positions and orientations of the trackers  12 . Then, the entertainment apparatus  14  performs the above-described orientation estimation based on these pieces of data. 
     Hereinafter, the estimation of the orientation of the chest node  42   f  will be described. It is noted that the orientation of the waist node  42   g  can be similarly estimated by the method described below. 
       FIG.  4    is a diagram illustrating an example of a machine learning model used to estimate the orientation of the chest node  42   f . As illustrated in  FIG.  4   , the machine learning model used to estimate the orientation of the chest node  42   f  includes an input layer  44 , an intermediate block  46 , a first intermediate layer  48 , a second intermediate layer  50 , and an output layer  52 . 
     In the present embodiment, for example, time-series data D1, time-series data D2, and time-series data D3 are input into the input layer  44  in response to the identification of the positions and orientations performed by the trackers  12 . The time-series data D1 represents a time series of orientations of the head. The time-series data D2 represents a time series of angular velocities of the left hand. The time-series data D3 represents a time series of angular velocities of the right hand. 
     In the present embodiment, for example, data representing a time series of orientations of the tracker  12   a  for the last  200  identifications is input into the input layer  44  as the time-series data D1 (D1(t−199) to D1(t)). 
     Here, for example, data representing the orientation of the tracker  12   a  at the latest identification timing is represented as the data D1(t). Further, data representing the orientation of the tracker  12   a  at the immediately preceding identification timing is represented as the data D1(t−1). Further, data representing the orientation of the tracker  12   a  at the identification timing 199 times before is represented as the data D1(t−199). 
     Further, in the present embodiment, as illustrated in  FIG.  4   , the time-series data D1(D1(t−199) to D1(t)) each includes three elements of a pitch angle (Pitch), a yaw angle (Yaw), and a roll angle (Roll). For example, the pitch angle, the yaw angle, and the roll angle respectively correspond to the pitch angle, the yaw angle, and the roll angle relative to the above-described reference orientation. 
     Further, in the present embodiment, the difference between a value of data representing the previously identified orientation and a value of data representing the orientation identified this time is used as a value representing an angular velocity. In addition, in the present embodiment, data representing a time series of angular velocities of the tracker  12   b  based on pieces of data representing the orientations of the tracker  12   b  for the last  201  identifications is input into the input layer  44  as the time-series data D2 (D2(t−199) to D2(t)). 
     Here, for example, data representing the angular velocity corresponding to the difference between the orientation of the tracker  12   b  at the latest identification timing and the orientation of the tracker  12   b  at the immediately preceding identification timing is represented as the data D2(t). Further, data representing the angular velocity corresponding to the difference between the orientation of the tracker  12   b  at the immediately preceding identification timing and the orientation of the tracker  12   b  at the identification timing two times before is represented as the data D2(t−1). In addition, data representing the angular velocity corresponding to the difference between the orientation of the tracker  12   b  at the identification timing 199 times before and the orientation of the tracker  12   b  at the identification timing 200 times before is represented as the data D2(t−199). 
     Further, in the present embodiment, as illustrated in  FIG.  4   , the time-series data D2(D2(t−199) to D2(t)) each includes three elements of a pitch angular velocity (ΔPitch), a yaw angular velocity (ΔYaw), and a roll angular velocity (ΔRoll). Here, the pitch angular velocity, the yaw angular velocity, and the roll angular velocity respectively correspond to the difference between pitch angles, the difference between yaw angles, and the difference between roll angles at two consecutive identification timings. Each of these angles is relative to the above-described reference orientation. 
     Further, in the present embodiment, data representing a time series of angular velocities of the tracker  12   c  based on pieces of data representing the orientations of the tracker  12   c  for the last  201  identifications is input into the input layer  44  as the time-series data D3 (D3 (t−199) to D3(t)). 
     Here, for example, data representing the angular velocity corresponding to the difference between the orientation of the tracker  12   c  at the latest identification timing and the orientation of the tracker  12   c  at the immediately preceding identification timing is represented as the data D3(t). Further, data representing the angular velocity corresponding to the difference between the orientation of the tracker  12   c  at the immediately preceding identification timing and the orientation of the tracker  12   c  at the identification timing two times before is represented as the data D3(t−1). In addition, data representing the angular velocity corresponding to the difference between the orientation of the tracker  12   c  at the identification timing 199 times before and the orientation of the tracker  12   c  at the identification timing 200 times before is represented as the data D3(t−199). 
     Further, in the present embodiment, as illustrated in  FIG.  4   , the time-series data D3 (D3(t−199) to D3(t)) each includes three elements of a pitch angular velocity (ΔPitch), a yaw angular velocity (ΔYaw), and a roll angular velocity (ΔRoll). As described above, the pitch angular velocity, the yaw angular velocity, and the roll angular velocity respectively correspond to the difference between pitch angles, the difference between yaw angles, and the difference between roll angles at two consecutive identification timings. Each of these angles is relative to the above-described reference orientation. 
     As described above, in the present embodiment, the pieces of data including 1800 (200×3×3) elements are input into the input layer  44 . 
     In addition, the above-described pieces of data input into the input layer  44  are input into the intermediate block  46 . In the present embodiment, the intermediate block  46  is, for example, an RNN (Recurrent Neural Network) (LSTM block) with LSTM (Long short-term memory) implemented. 
     Then, an output from the intermediate block  46  is input into the first intermediate layer  48 . Then, an output from the first intermediate layer  48  is input into the second intermediate layer  50 . Each of the first intermediate layer  48  and the second intermediate layer  50  is, for example, a fully connected layer that uses a rectified linear function (ReLU) as an activation function. 
     Then, an output from the second intermediate layer  50  is input into the output layer  52 . The output layer  52  is, for example, a layer that uses a linear function as an activation function. Then, the output layer  52  outputs data D4(t) as an output  54 . The data D4(t) corresponds to the orientation of the chest at the latest identification timing. As illustrated in  FIG.  4   , the time-series data D4(t) includes three elements of a pitch angle (Pitch), a yaw angle (Yaw), and a roll angle (Roll). The pitch angle, the yaw angle, and the roll angle respectively correspond to the pitch angle, the yaw angle, and the roll angle relative to the above-described reference orientation, for example. 
     In the present embodiment, the machine learning model illustrated in  FIG.  4    is assumed to have learned from data representing the orientation of the chest associated with a combination of the orientation of the head, the angular velocity of the left hand, and the angular velocity of the right hand in advance. Therefore, in the present embodiment, the data D4(t), which is the output obtained when the time-series data D1, the time-series data D2, and the time-series data D3 are input into the machine learning model in  FIG.  4    that has finished learning, corresponds to the result of the estimation of the orientation of the chest node  42   f.    
     Here, the machine learning model illustrated in  FIG.  4    may have learned from data representing the orientation of the waist associated with a combination of the orientation of the head, the angular velocity of the left hand, and the angular velocity of the right hand in advance. In this case, the data D4(t), which is the output obtained when the time-series data D1, the time-series data D2, and the time-series data D3 are input into the machine learning model in  FIG.  4    that has finished learning, corresponds to the result of the estimation of the orientation of the waist node  42   g.    
     Further, the machine learning model illustrated in  FIG.  4    may have learned from data representing the angular velocity of the chest associated with a combination of the orientation of the head, the angular velocity of the left hand, and the angular velocity of the right hand in advance. In this case, the data D4(t), which is the output obtained when the time-series data D1, the time-series data D2, and the time-series data D3 are input into the machine learning model in  FIG.  4    that has finished learning, corresponds to the result of the estimation of the angular velocity of the chest node  42   f.    
     Further, the machine learning model illustrated in  FIG.  4    may have learned from data representing the angular velocity of the waist associated with a combination of the orientation of the head, the angular velocity of the left hand, and the angular velocity of the right hand in advance. In this case, the data D4(t), which is the output obtained when the time-series data D1, the time-series data D2, and the time-series data D3 are input into the machine learning model in  FIG.  4    that has finished learning, corresponds to the result of the estimation of the angular velocity of the waist node  42   g.    
     In this manner, in the present embodiment, the orientation of the chest node  42   f  or the waist node  42   g  or the angular velocity of the chest node  42   f  or the waist node  42   g  based on the pieces of time-series data D1 to D3 is estimated. 
     Hereinafter, the estimation of the orientation of the left-wrist node  42   j  will be described. It is noted that the orientation of the right-wrist node  42   m  can be similarly estimated by the method described below.  FIG.  5    is a diagram illustrating an example of a machine learning model used to estimate the orientation of the left-wrist node  42   j . As illustrated in  FIG.  5   , the machine learning model used to estimate the orientation of the left-wrist node  42   j  includes the input layer  44 , the intermediate block  46 , the first intermediate layer  48 , the second intermediate layer  50 , and the output layer  52 , as in the case of the machine learning model illustrated in  FIG.  4   . 
     In the present embodiment, for example, data D5, data D6, and data D7 are input into the input layer  44  in response to the identification of the positions and orientations performed by the trackers  12 . The data D5 represents a time series of orientations of the left hand. The data D6 represents a time series of angular velocities of the left hand. The data D7 represents a time series of velocities of the left hand. 
     In the present embodiment, for example, data representing a time series of orientations of the tracker  12   b  for the last  200  identifications is input into the input layer  44  as the time-series data D5(D5(t−199) to D5(t)). 
     Here, for example, data representing the orientation of the tracker  12   b  at the latest identification timing is represented as the data D5(t). Further, data representing the orientation of the tracker  12   b  at the immediately preceding identification timing is represented as the data D5(t−1). In addition, data representing the orientation of the tracker  12   b  at the identification timing 199 times before is represented as the data D5(t−199). 
     Further, in the present embodiment, as illustrated in  FIG.  5   , the time-series data D5(D5(t−199) to D5(t)) each includes three elements of a pitch angle (Pitch), a yaw angle (Yaw), and a roll angle (Roll). For example, the pitch angle, the yaw angle, and the roll angle respectively correspond to the pitch angle, the yaw angle, and the roll angle relative to the above-described reference orientation. 
     Further, in the present embodiment, the difference between a value of data representing the previously identified orientation and a value of data representing the orientation identified this time is used as a value representing an angular velocity. In addition, in the present embodiment, data representing a time series of angular velocities of the tracker  12   b  based on pieces of data representing the orientations of the tracker  12   b  for the last  201  identifications is input into the input layer  44  as the time-series data D6(D6(t−199) to D6(t)). 
     Here, for example, data representing the angular velocity corresponding to the difference between the orientation of the tracker  12   b  at the latest identification timing and the orientation of the tracker  12   b  at the immediately preceding identification timing is represented as the data D6(t). Further, data representing the angular velocity corresponding to the difference between the orientation of the tracker  12   b  at the immediately preceding identification timing and the orientation of the tracker  12   b  at the identification timing two times before is represented as the data D6(t−1). In addition, data representing the angular velocity corresponding to the difference between the orientation of the tracker  12   b  at the identification timing 199 times before and the orientation of the tracker  12   b  at the identification timing 200 times before is represented as the data D6(t−199). 
     Further, in the present embodiment, as illustrated in  FIG.  5   , the time-series data D6(D6(t−199) to D6(t)) each includes three elements of a pitch angular velocity (ΔPitch), a yaw angular velocity (ΔYaw), and a roll angular velocity (ΔRoll). As described above, the pitch angular velocity, the yaw angular velocity, and the roll angular velocity respectively correspond to the difference between pitch angles, the difference between yaw angles, and the difference between roll angles at two consecutive identification timings. Each of these angles is relative to the above-described reference orientation. 
     Further, in the present embodiment, data representing the time series of velocities of the tracker  12   b  based on pieces of data representing the positions of the tracker  12   b  for the last  201  identifications is input into the input layer  44  as the time-series data D7 (D7(t−199) to D7(t)). 
     Here, for example, data representing the velocity corresponding to the difference between the position of the tracker  12   b  at the latest identification timing and the position of the tracker  12   b  at the immediately preceding identification timing is represented as the data D7(t). Further, data representing the velocity corresponding to the difference between the position of the tracker  12   b  at the immediately preceding identification timing and the position of the tracker  12   b  at the identification timing two times before is represented as the data D7(t−1). In addition, data representing the velocity corresponding to the difference between the position of the tracker  12   b  at the identification timing 199 times before and the position of the tracker  12   c  at the identification timing 200 times before is represented as the data D7(t−199). 
     Further, in the present embodiment, as illustrated in  FIG.  5   , the time-series data D7(D7(t−199) to D7(t)) each includes three elements of Δx, Δy, and Δz. Each of Δx, Δy, and Δz corresponds to the velocity in a corresponding one of the three-axis directions. The velocities in the three-axis directions respectively correspond to the difference between x coordinate values, the difference between y coordinate values, and the difference between z coordinate values at two consecutive identification timings. Each coordinate value is relative to the above-described reference position. 
     As described above, in the present embodiment, the pieces of data including 1800 (200×3×3) elements are input into the input layer  44 . 
     Then, eventually, the output layer  52  outputs data D8(t) as the output  54  on the basis of the above-described pieces of data input into the input layer  44  in a similar manner described with reference to  FIG.  4   . The data D8(t) corresponds to the orientation of the left wrist at the latest identification timing. As illustrated in  FIG.  5   , the time-series data D8(t) includes three elements of a pitch angle (Pitch), a yaw angle (Yaw), and a roll angle (Roll). The pitch angle, the yaw angle, and the roll angle respectively correspond to the pitch angle, the yaw angle, and the roll angle relative to the above-described reference orientation, for example. 
     In addition, in the present embodiment, the data D8(t) is used as the result of the estimation of the orientation of the left-wrist node  42   j.    
     In the present embodiment, the machine learning model illustrated in  FIG.  5    is assumed to have learned from data representing the orientation of the left wrist associated with a combination of the orientation of the left hand, the angular velocity of the left hand, and the velocity of the left hand in advance. Therefore, in the present embodiment, the data D8(t), which is the output obtained when the time-series data D5, the time-series data D6, and the time-series data D7 are input into the machine learning model in  FIG.  5    that has finished learning, corresponds to the result of the estimation of the orientation of the left-wrist node  42   j . It is noted that instead of the data representing a time series of velocities of the left hand, data representing a time series of positions of the left hand may be used as the time-series data D7. 
     Here, the machine learning model illustrated in  FIG.  5    may have learned from data representing the angular velocity of the left wrist associated with a combination of the orientation of the left hand, the angular velocity of the left hand, and the position or the velocity of the left hand in advance. In this case, the data D8(t), which is the output obtained when the time-series data D5, the time-series data D6, and the time-series data D7 are input into the machine learning model in  FIG.  5    that has finished learning, corresponds to the result of the estimation of the angular velocity of the left-wrist node  42   j.    
     Further, in the present embodiment, a machine learning model similar to the machine learning model illustrated in  FIG.  5    learns from data representing the orientation of the right wrist associated with a combination of the orientation of the right hand, the angular velocity of the right hand, and the velocity of the right hand. Then, when data representing a time series of orientations of the right hand, data representing a time series of angular velocities of the right hand, and data representing a time series of velocities of the right hand are input into the machine learning model that has finished learning, the output corresponds to the result of the estimation of the orientation of the right-wrist node  42   m . It is noted that instead of the data representing a time series of velocities of the right hand, data representing a time series of positions of the right hand may be used. 
     Here, for example, the machine learning model similar to the machine learning model illustrated in  FIG.  5    may have learned from data representing the angular velocity of the right wrist associated with a combination of the orientation of the right hand, the angular acceleration of the right hand, and the position or the velocity of the right hand. Then, when the data representing a time series of orientations of the right hand, the data representing a time series of angular velocities of the right hand, and the data representing a time series of velocities of the right hand are input into the machine learning model that has finished learning, the output corresponds to the result of the estimation of the angular velocity of the right-wrist node  42   m.    
     In this manner, in the present embodiment, the orientation or the angular velocity of each of the left-wrist node  42   j  and the right-wrist node  42   m  is estimated. 
     In addition, in the present embodiment, for example, inverse kinematics is used for the estimation of the positions and orientations of the other nodes  42  based on the above-described estimation results. 
     In the present embodiment, the position and orientation of each node  42  included in the skeleton model  40  are estimated without having the user wear the tracker  12  on the chest or waist. Further, in the present embodiment, the orientation of the left-wrist node  42   j  and the orientation of the right-wrist node  42   m  are estimated from the pieces of data representing the positions and orientations of the trackers  12  limited to five. In addition, the positions of the left-elbow node  42   i  and the right-elbow node  42   l  can be accurately estimated using inverse kinematics on the basis of the above estimation results. According to the present embodiment, accurate body tracking can be performed in this manner without having the user wear many trackers  12 . Specifically, accurate body tracking can be performed without having the user wear the trackers  12  on the chest, waist, and wrists, for example. 
     Hereinafter, functions of the entertainment apparatus  14  according to the present embodiment and processing performed by the entertainment apparatus  14  will be further described. 
       FIG.  6    is a functional block diagram illustrating an example of the functions implemented in the entertainment apparatus  14  according to the present embodiment. It is noted that all of the functions illustrated in  FIG.  6    do not need to be implemented in the entertainment apparatus  14  according to the present embodiment. Further, functions other than the functions illustrated in  FIG.  6    may be implemented therein. 
     As illustrated in  FIG.  6   , the entertainment apparatus  14  functionally includes a conversion section  60 , a posture data acquisition section  62 , a posture data storage section  64 , a time-series data generation section  66 , a time-series data input section  68 , an output acquisition section  70 , and a body tracking execution section  72 , for example. Further, the conversion section  60  includes a first conversion section  60   a , a second conversion section  60   b , and a third conversion section  60   c . Further, the time-series data generation section  66  includes a first time-series data generation section  66   a , a second time-series data generation section  66   b , and a third time-series data generation section  66   c . Further, the time-series data input section  68  includes a first time-series data input section  68   a , a second time-series data input section  68   b , and a third time-series data input section  68   c . Further, the output acquisition section  70  includes a first output acquisition section  70   a , a second output acquisition section  70   b , and a third output acquisition section  70   c.    
     The conversion section  60  is mainly implemented by the processor  30  and the storage section  32 . The posture data acquisition section  62  is mainly implemented by the processor  30  and the input/output section  36 . The posture data storage section  64  is mainly implemented by the storage section  32 . The time-series data generation section  66 , the time-series data input section  68 , the output acquisition section  70 , and the body tracking execution section  72  are mainly implemented by the processor  30 . The entertainment apparatus  14  according to the present embodiment plays a role as a body part orientation estimation apparatus that estimates the orientations of the parts included in the body of the user. 
     The above functions may be implemented by the processor  30  executing a program that includes instructions corresponding to the above functions and that is installed in the entertainment apparatus  14 , which is a computer. This program may be supplied to the entertainment apparatus  14  via a computer-readable information storage medium, such as an optical disc, a magnetic disc, a magnetic tape, a magneto-optical disk, or a flash memory, or via the Internet or the like, for example. 
     In the present embodiment, for example, the first conversion section  60   a  corresponds to the machine learning model that has finished learning and that is used to estimate the orientation of the chest node  42   f . This machine learning model is exemplified in  FIG.  4   . It is noted that the first conversion section  60   a  does not need to be implemented by the machine learning model and may be implemented by general software or hardware such as a circuit, for example. 
     In the present embodiment, for example, the second conversion section  60   b  corresponds to the machine learning model that has finished learning and that is used to estimate the orientation of the left-wrist node  42   j . This machine learning model is exemplified in  FIG.  5   . It is noted that the second conversion section  60   b  does not need to be implemented by the machine learning model and may be implemented by general software or hardware such as a circuit, for example. 
     In the present embodiment, for example, the third conversion section  60   c  corresponds to the machine learning model that has finished learning and that is used to estimate the orientation of the right-wrist node  42   m . This machine learning model is similar to the machine learning model exemplified in  FIG.  5   . It is noted that the third conversion section  60   c  does not need to be implemented by the machine learning model and may be implemented by general software or hardware such as a circuit, for example. 
     In the present embodiment, for example, the posture data acquisition section  62  acquires pieces of posture data representing the positions and orientations of the trackers  12   a  to  12   e  identified at the predetermined sampling rate. Here, for example, each tracker  12  may generate posture data representing the position and orientation thereof at the predetermined sampling rate and then, in response to the generation, transmit the posture data generated thereby to the entertainment apparatus  14  via the relay apparatus  16 . Further, for example, the camera-microphone unit  20  may generate posture data representing the positions and orientations of the trackers  12   a  to  12   e  at the predetermined sampling rate and then, in response to the generation, transmit the posture data generated by the camera-microphone unit  20  to the entertainment apparatus  14 . 
     In the present embodiment, for example, the posture data storage section  64  stores the pieces of posture data acquired by the posture data acquisition section  62 . 
     In the present embodiment, for example, the first time-series data generation section  66   a  generates pieces of time-series data to be input into the first conversion section  60   a  on the basis of the pieces of posture data stored in the posture data storage section  64 . Here, for example, the three pieces of time-series data including the time-series data D1 representing a time series of orientations of the head, the time-series data D2 representing a time series of angular velocities of the left hand, and the time-series data D3 representing a time series of angular velocities of the right hand are generated. In this manner, the plurality of pieces of time-series data generated by the first time-series data generation section  66   a  may be pieces of time-series data for parts different from each other (here, for example, the head, the left hand, and the right hand) that are included in the body. 
     It is noted that the first time-series data generation section  66   a  may retain the generated time-series data. Then, for example, the time-series data D1(t) may be generated on the basis of the posture data at the latest identification timing. Then, the first time-series data generation section  66   a  may generate the time-series data D1 that includes the retained time-series data D1(t−198) to D1(t) as the time-series data D1 (t−199) to D1(t−1) and that further includes the generated time-series data D1(t). It is noted that each of the time-series data D2 and D3 may also be similarly generated on the basis of the retained time-series data and the posture data at the latest identification timing. 
     In the present embodiment, for example, the second time-series data generation section  66   b  generates pieces of time-series data to be input into the second conversion section  60   b  on the basis of the piece of posture data stored in the posture data storage section  64 . Here, for example, the above-described three pieces of time-series data including the time-series data D5 representing a time series of orientations of the left hand, the time-series data D6 representing a time series of angular velocities of the left hand, and the time-series data D7 representing a time series of velocities of the left hand are generated. In this manner, the plurality of pieces of time-series data generated by the second time-series data generation section  66   b  may include time-series data representing the orientations of a part (here, for example, the left hand) included in the body and time-series data representing the angular velocities of the part (here, for example, the left hand). It is noted that each of the time-series data D5 to D7 may also be generated on the basis of the retained time-series data and the posture data at the latest identification timing, as described above. 
     In the present embodiment, for example, the third time-series data generation section  66   c  generates pieces of time-series data to be input into the third conversion section  60   c  on the basis of the pieces of posture data stored in the posture data storage section  64 . Time series data for orientations of the right hand, time-series data for angular velocities of the right hand, and time-series data for velocities of the right hand are generated. In this manner, the plurality of pieces of time-series data generated by the third time-series data generation section  66   c  may include time-series data representing the orientations of a part (here, for example, the right hand) included in the body and time-series data representing the angular velocities of the part (here, for example, the right hand). It is noted that each of these pieces of time-series data may also be generated on the basis of the retained time-series data and the posture data at the latest identification timing, as described above. 
     In the present embodiment, for example, the time-series data input section  68  acquires the plurality of pieces of time-series data each representing the positions, postures, or motions of a part of the body. Then, in the present embodiment, the time-series data input section  68  inputs the plurality of pieces of time-series data to the conversion section  60  that has finished learning, for example. It is noted that the pieces of time-series data are not limited to the above-described pieces of data representing orientations, angular velocities (the differences between orientations), and velocities (the difference between positions). For example, the pieces of time-series data may be data representing positions, data representing accelerations (the difference between velocities), and data representing angular accelerations (the differences between angular velocities). Further, the time-series data may be, for example, data representing relative positions, postures, or motions of a certain part relative to another part. More specifically, for example, the time-series data may be data representing the differences between the orientations of a hand and the orientations of an arm (the relative orientations of the arm with respect to the orientations of the hand). 
     Here, in the present embodiment, the first time-series data input section  68   a  acquires the pieces of time-series data generated by the first time-series data generation section  66   a , for example. Then, in the present embodiment, the first time-series data input section  68   a  inputs the pieces of time-series data into the first conversion section  60   a , for example. Further, in the present embodiment, the second time-series data input section  68   b  acquires the pieces of time-series data generated by the second time-series data generation section  66   b , for example. Then, in the present embodiment, the second time-series data input section  68   b  inputs the pieces of time-series data into the second conversion section  60   b , for example. Further, in the present embodiment, the third time-series data input section  68   c  acquires the pieces of time-series data generated by the third time-series data generation section  66   c , for example. Then, in the present embodiment, the third time-series data input section  68   c  inputs the pieces of time-series data into the third conversion section  60   c , for example. 
     In the present embodiment, for example, the output acquisition section  70  acquires the result of the estimation of the position, posture, or motion of another part of the body that is closer to the center of the body than the part(s) of the body whose positions, postures, or motions are represented by the pieces of time-series data. The result of the estimation is an output obtained when the pieces of time-series data are input into the conversion section  60 . 
     Here, in the present embodiment, the first output acquisition section  70   a  acquires an output obtained when the pieces of time-series data generated by the first time-series data generation section  66   a  are input into the first conversion section  60   a , for example. 
     For example, as described above, the pieces of time-series data generated by the first time-series data generation section  66   a  are the pieces of data representing the orientations of the head, the angular velocities of the left hand, and the angular velocities of the right hand. In addition, in the skeleton model  40 , the head node  42   a  and the chest node  42   f  are connected to each other via one link. Further, in the skeleton model  40 , the left-hand node  42   b  and the chest node  42   f  are connected to each other via four links. Further, in the skeleton model  40 , the right-hand node  42   c  and the chest node  42   f  are connected to each other via four links. In addition, the chest node  42   f  is the node  42  that is closer to the center of the body than any of the head node  42   a , the left-hand node  42   b , and the right-hand node  42   c . In addition, in the present embodiment, for example, the result of estimation of the position, posture, or motion of the chest node  42   f  is acquired. The chest node  42   f  corresponds to the chest that is the part closer to the center of the body than the parts of the body whose positions, postures, or motions are represented by the pieces of time-series data generated by the first time-series data generation section  66   a . It is noted that here, an estimated value of the position, posture, or motion, such as an estimated value of the orientation (a combination of the pitch angle, the yaw angle, and the roll angle), an estimated value of the angular velocity, or the like, may be acquired. Further, a score value serving as a basis for calculating the estimated value of the position, posture, or motion may be acquired. 
     Further, in the present embodiment, the second output acquisition section  70   b  acquires an output obtained when the pieces of time-series data generated by the second time-series data generation section  66   b  are input into the second conversion section  60   b , for example. 
     For example, as described above, the pieces of time-series data generated by the second time-series data generation section  66   b  include at least two of the data representing the orientations of the left hand, the data representing the angular velocities of the left hand, the data representing the positions of the left hand, and the data representing the velocities of the left hand. In addition, in the skeleton model  40 , the left-hand node  42   b  and the left-wrist node  42   j  are connected to each other via one link. In addition, the left-wrist node  42   j  is the node  42  that is closer to the center of the body than the left-hand node  42   b . In addition, in the present embodiment, for example, the result of estimation of the position, posture, or motion of the left-wrist node  42   j  is acquired. The left-wrist node  42   j  corresponds to the wrist on a side of the left hand. The wrist is the part closer to the center of the body than the part of the body whose positions, postures, or motions are represented by the pieces of time-series data generated by the second time-series data generation section  66   b . It is noted that here, an estimated value of the position, posture, or motion, such as an estimated value of the orientation (a combination of the pitch angle, the yaw angle, and the roll angle), an estimated value of the angular velocity, or the like, may be acquired. Further, a score value serving as a basis for calculating the estimated value of the position, posture, or motion may be acquired. 
     Further, in the present embodiment, the third output acquisition section  70   c  acquires an output obtained when the pieces of time-series data generated by the third time-series data generation section  66   c  are input into the third conversion section  60   c , for example. In addition, in this case, the result of estimation of the position, posture, or motion of the right-wrist node  42   m  is acquired. The right-wrist node  42   m  corresponds to the wrist on a side of the right hand. The wrist is the part closer to the center of the body than the right hand that is the part of the body whose positions, postures, or motions are represented by the pieces of time-series data generated by the third time-series data generation section  66   c . It is noted that here, an estimated value of the position, posture, or motion, such as an estimated value of the orientation (a combination of the pitch angle, the yaw angle, and the roll angle), an estimated value of the angular velocity, or the like, may be acquired. Further, a score value serving as a basis for calculating the estimated value of the position, posture, or motion may be acquired. 
     It is noted that the data output by the conversion section  60  and acquired by the output acquisition section  70  is not limited to the above-described data representing the orientation or the angular velocity (the difference between orientations). For example, the conversion section  60  may output data representing the velocity (the difference between positions), data representing the position, data representing the acceleration (the difference between velocities), or data representing the angular acceleration (the difference between angular velocities). Further, the conversion section  60  may output data representing the relative position, posture, or motion of a certain part relative to another part. More specifically, for example, the conversion section  60  may output data representing the difference between the orientation of a hand and the orientation of an arm (the relative orientation of the arm with respect to the orientation of the hand). 
     In the present embodiment, for example, the body tracking execution section  72  performs body tracking processing on the basis of the results of the estimation of the orientations acquired by the output acquisition section  70  and the pieces of posture data acquired by the posture data acquisition section  62 . Here, for example, the position and the orientation of each of the plurality of nodes  42  included in the skeleton model  40  are estimated. It is noted that the body tracking execution section  72  may perform the body tracking processing on the basis of the pieces of posture data stored in the posture data storage section  64 . 
     Here, for example, the position and orientation of the head node  42   a  may be determined on the basis of the posture data representing the position and orientation of the tracker  12   a . Further, for example, the position and orientation of the left-hand node  42   b  may be determined on the basis of the posture data representing the position and orientation of the tracker  12   b . Further, for example, the position and orientation of the right-hand node  42   c  may be determined on the basis of the posture data representing the position and orientation of the tracker  12   c . Further, for example, the position and orientation of the left-foot node  42   d  may be determined on the basis of the posture data representing the position and orientation of the tracker  12   d . Further, for example, the position and orientation of the right-foot node  42   e  may be determined on the basis of the posture data representing the position and orientation of the tracker  12   e.    
     Further, as described above, the body tracking execution section  72  may use inverse kinematics to estimate, on the basis of the determined or estimated positions or orientations of the nodes  42 , the positions of the nodes  42  that have not been estimated and the orientations of the nodes  42  that have not been estimated. 
     Conceivably, it is possible to accurately estimate the orientations of the nodes  42  corresponding to the parts included in the body by using temporal motion patterns of rotation of the other nodes  42  that are away from the center of the body than the parts and that are connected to the nodes  42  via one or more links. Thus, the present embodiment focuses on this point. As described above, on the basis of an output obtained when a plurality of pieces of time-series data each representing the orientations or angular velocities of a part of the body is input into the conversion section  60 , the orientation of another part of the body that is closer to the center of the body than the part is estimated. 
     Here, an example of a flow of the processing performed by the entertainment apparatus  14  according to the present embodiment will be described with reference to a flow diagram illustrated in  FIG.  7   . In this processing example, the following processes, which are illustrated in S 101  to S 112 , are repeatedly performed at the predetermined sampling rate. 
     First, the posture data acquisition section  62  acquires pieces of posture data generated by the trackers  12  and causes the posture data storage section  64  to store these pieces of posture data (S 101 ). These pieces of posture data represent the positions and orientations of the respective trackers  12  at the latest identification timing. Here, for example, the posture data acquisition section  62  acquires the posture data of each of the trackers  12   a  to  12   e . Each posture data represents the position and orientation of the corresponding tracker  12 . 
     Then, the first time-series data generation section  66   a  generates pieces of time-series data to be input into the first conversion section  60   a  on the basis of the pieces of posture data stored in the posture data storage section  64  (S 102 ). 
     Then, the second time-series data generation section  66   b  generates pieces of time-series data to be input into the second conversion section  60   b  on the basis of the pieces of posture data stored in the posture data storage section  64  (S 103 ). 
     Then, the third time-series data generation section  66   c  generates pieces of time-series data to be input into the third conversion section  60   c  on the basis of the pieces of posture data stored in the posture data storage section  64  (S 104 ). 
     Then, the first time-series data input section  68   a  acquires the pieces of time-series data generated in the process illustrated in S 102  and inputs the pieces of time-series data into the first conversion section  60   a  (S 105 ). 
     Then, the second time-series data input section  68   b  acquires the pieces of time-series data generated in the process illustrated in S 103  and inputs the pieces of time-series data into the second conversion section  60   b  (S 106 ). 
     Then, the third time-series data input section  68   c  acquires the pieces of time-series data generated in the process illustrated in S 104  and inputs the pieces of time-series data into the third conversion section  60   c  (S 107 ). 
     Then, the first output acquisition section  70   a  acquires the result of estimation of the orientation of the chest node  42   f  that is an output from the first conversion section  60   a  corresponding to the input in the process illustrated in S 105  (S 108 ). 
     Then, the second output acquisition section  70   b  acquires the result of estimation of the orientation of the left-wrist node  42   j  that is an output from the second conversion section  60   b  corresponding to the input in the process illustrated in S 106  (S 109 ). 
     Then, the third output acquisition section  70   c  acquires the result of estimation of the orientation of the right-wrist node  42   m  that is an output from the third conversion section  60   c  corresponding to the input in the process illustrated in S 107  (S 110 ). 
     Then, the body tracking execution section  72  determines the position and orientation of each of the head node  42   a , the left-hand node  42   b , the right-hand node  42   c , the left-foot node  42   d , and the right-foot node  42   e  in this cycle (S 111 ). Here, for example, the position and orientation of each of the head node  42   a , the left-hand node  42   b , the right-hand node  42   c , the left-foot node  42   d , and the right-foot node  42   e  may be determined on the basis of the corresponding posture data acquired in the process illustrated in S 101 . 
     Then, the body tracking execution section  72  uses inverse kinematics to estimate the remaining positions and orientations that have not been determined or acquired as the estimation results in the processes illustrated in S 108  to S 111  among the positions and orientations of the nodes  42  included in the skeleton model  40  (S 112 ). 
     In this manner, the positions and orientations of all the nodes  42  included in the skeleton model  40  in one cycle are estimated. Then, the processing returns to the process illustrated in S 101 . 
     As for the processes illustrated in the above-described processing example, the execution order of each step is not limited to the one described above. For example, the execution order of the processes illustrated in S 102  to S 104 , the execution order of the processes illustrated in S 105  to S 107 , and the execution order of the processes illustrated in S 108  to S 110  are not limited to those described above. 
     Further, the processes illustrated in S 105  and S 108  may be sequentially performed following the process illustrated in S 102 . Similarly, the processes illustrated in S 106  and S 109  may be sequentially performed following the process illustrated in S 103 . Similarly, the processes illustrated in S 107  and S 110  may be sequentially performed following the process illustrated in S 104 . 
     Further, each of the above-described steps may be performed in parallel. For example, the processes illustrated in S 102 , S 105 , and S 108 , the processes illustrated in S 103 , S 106 , and S 109 , and the processes illustrated in S 104 , S 107 , and S 110  may be performed in parallel. 
     It is noted that as described above, the first conversion section  60   a  may be the machine learning model used to estimate the orientation of the waist node  42   g , not the chest node  42   f . In this case, the first output acquisition section  70   a  may acquire the result of the estimation of the orientation of the waist node  42   g.    
     It is noted that the present invention is not limited to the above-described embodiment. 
     For example, the tracker  12   a  may be a head-mounted display (HMD). In this case, a video corresponding to the result of various types of processes of the game processing or the like corresponding to the positions or orientations of the plurality of parts included in the user may be displayed on a display section of the HMD, for example. 
     Further, for example, a part or all of the functions illustrated in  FIG.  6    may be implemented by the trackers  12 . 
     Further, the above-described specific character strings and numerical values and the specific character strings and numerical values in the drawings are examples, and character strings and numerical values are not limited to these character strings and numerical values.