Patent Publication Number: US-2017364144-A1

Title: Systems and Methods for Displaying Virtual-Reality Images on a Convertible Wired/Mobile Head-Mounted Display

Description:
TECHNICAL FIELD 
     This application relates generally to wearable technology and virtual-reality technology, including but not limited to a head-mounted display capable of displaying images generated on a stationary computer (e.g., a desktop computer) and images generated on a mobile computer (e.g., a smartphone) on the same headset. 
     BACKGROUND 
     Virtual-reality (VR) head-mounted displays (HMDs) have wide applications in various fields, including engineering design, medical surgery practice, military simulated practice, and video gaming. Some head-mounted displays include a mobile device for rendering and displaying images and a headset configured to dock with the mobile device. 
     The aforementioned conventional head-mounted displays lack the ability for a user to enjoy the convenience of viewing high-quality VR video as well as enjoy the convenience of being immersed in the VR experience without any constraints on the user movement. Furthermore, the aforementioned HMD having the mobile device docked in the front of the HMD creates additional weight on the front of the user&#39;s head, thereby causing user fatigue due to the imbalance of the HMD on the user&#39;s head. 
     SUMMARY 
     Accordingly, there is a need for virtual-reality systems capable of producing and rendering VR images from high-quality videos provided by stationary computers and VR images from video provided by mobile computers on the same headset so as to allow the user to view high-quality images during intensive use (e.g., “high-CPU” gameplay) and allow movement of the user to not be constrained by long cables to a standalone computer while using the HMD. There is a further need for virtual-reality systems capable of reducing user fatigue which is due to an imbalance of a weight of the HMD exerted on the front of the headset. 
     In accordance with some embodiments, a VR system includes a HMD to receive video input from a stationary computer in a first mode of operation and from a mobile computer in a second mode of operation, and to display images corresponding to the video input. The VR system further includes a headband to secure the HMD on a user&#39;s head, and a holder mounted on the headband to detachably support the mobile computer. 
     In some embodiments, the virtual-reality system further includes a first cable to communicatively couple the stationary computer to the HMD in the first mode of operation, and a second cable to communicatively couple the mobile computer to the HMD in the second mode of operation. The second cable is shorter than the first cable. 
     In some embodiments, the holder is mounted to a back section of the headband to at least partially balance a weight of the HMD on the user&#39;s head when the mobile computer is supported in the holder. 
     In some embodiments, the holder is configured to support the mobile computer and a power source on the headband. 
     In some embodiments, the power source is a battery selected from the group consisting of a lithium-polymer battery and a lithium-ion battery. 
     In some embodiments, the holder is rotatably coupled to the back section for adjustment of an angle of orientation of the mobile computer. 
     In some embodiments, the holder is detachably coupled to the back section. 
     In some embodiments, the holder is integrally formed with the back section. 
     In some embodiments, the holder is adjustable to accommodate a plurality of sizes of the mobile computer. 
     In some embodiments, the HMD includes an interface positioned on an outer surface of the HMD to allow for access during switching between the first and second modes, the first cable communicatively couples the stationary computer to the HMD through the interface in the first mode of operation, and the second cable communicatively couples the mobile computer to the HMD through the interface in the second mode of operation. 
     In some embodiments, the mobile computer includes at least one position sensor to track a motion of the user&#39;s head in the second mode. 
     In some embodiments, the mobile computer includes at least one camera to track a position of the user in the second mode. 
     In some embodiments, a display of the mobile computer is configured to deactivate in the second mode to conserve power of the mobile computer. 
     In some embodiments, the virtual-reality system further includes a plurality of light-emitting diodes (LEDs) coupled to at least one of the HMD and the headband. The plurality of LEDs is configured to be activated in the first mode and deactivated in the second mode. 
     In accordance with some embodiments, a method for displaying VR images produced by a VR system includes in a first mode of operation, receiving, by a HMD of the VR system, a first video input from a stationary computer, and displaying, by the HMD, images corresponding to the first video input. The method further includes in a second mode of operation, receiving, by the HMD, a second video input from a mobile computer mounted in a holder on a headband of the VR system. The holder is configured to detachably support the mobile computer and the headband is configured to secure the HMD on a user&#39;s head. The method further includes displaying, by the HMD, images corresponding to the second video input. 
     In some embodiments, the method further includes, in the second mode, deactivating a display of the mobile computer to conserve power of the mobile computer. 
     In some embodiments, in the first mode, the HMD receives the first video input through a first cable communicatively coupling the stationary computer to a VR interface of the HMD, and in the second mode, the HMD receives the second video input through a second cable communicatively coupling the mobile computer to the VR interface. The second cable is shorter than the first cable. 
     In some embodiments, the holder is mounted to a back section of the headband to at least partially balance a weight of the HMD on the user&#39;s head when the user wears the HMD. 
     In some embodiments, the method further includes, in the second mode, tracking a motion of the user&#39;s head, by at least one position sensor of the mobile computer, and translating the motion to produce a corresponding motion of the images displayed on the HMD. 
     In some embodiments, the method further includes, in the second mode, tracking a position of the user, by at least one camera of the mobile computer, to produce a corresponding motion of the images displayed on the HMD. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the various described embodiments, reference should be made to the Detailed Description below, in conjunction with the following drawings. Like reference numerals refer to corresponding parts throughout the figures and description. 
         FIG. 1  is a perspective view of a head-mounted display of a VR system in accordance with some embodiments. 
         FIG. 2  is a back view of a VR system including a head-mounted display communicatively coupled to a stationary computer in a first mode of operation in accordance with some embodiments. 
         FIG. 3A  is a back view of a VR system including a head-mounted display communicatively coupled to a mobile computer in a second mode of operation, in accordance with some embodiments. 
         FIG. 3B  and  FIG. 3C  are side perspective views of a VR system including the head-mounted display in accordance with some embodiments. 
         FIG. 4  is a block diagram illustrating an electrical configuration of the VR system in accordance with some embodiments. 
         FIG. 5  is a flow diagram illustrating a method of displaying VR images produced by a VR system in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to embodiments, examples of which are illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide an understanding of the various described embodiments. However, it will be apparent to one of ordinary skill in the art that the various described embodiments may be practiced without these specific details. In other instances, well-known systems, methods, procedures, components, circuits, and networks have not been described in detail so as not to unnecessarily obscure aspects of the embodiments. 
     It will also be understood that, although the terms first, second, etc. are, in some instances, used herein to describe various elements, these elements should not be limited by these terms. These terms are used only to distinguish one element from another. For example, a first mode could be termed a second mode, and, similarly, a second mode could be termed a first mode, without departing from the scope of the various described embodiments. The first mode and the second mode are both modes, but they are not the same mode. 
     The terminology used in the description of the various embodiments described herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various described embodiments and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “includes,” “including,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
       FIG. 1  is a side perspective view of a head-mounted display  110  coupled to a headband  120  of a virtual-reality (VR) system  100  in accordance with some embodiments. In some embodiments, the VR system  100  includes a head-mounted display (HMD)  110 , a headband  120  to secure the HMD  110  on a user&#39;s head, and a holder  115  ( FIGS. 3A-3C ) mounted on the headband  120 , to detachably support a mobile computer  135  ( FIGS. 3A-3C ) in a second mode of operation. In some embodiments, the holder  115  is detachably coupled to the headband  120  so as to be removable in the first mode of operation. In some embodiments, the VR system further includes an audio subsystem  180 , which may be detachable. 
     As shown in  FIG. 1 , the head-mounted display  110  includes an opaque front cover  145  to cover the front of the head-mounted display  110 , flexible circuits distributed inside the head-mounted display  110  (not shown), an opaque housing  155  to house components of the head-mounted display  110 , a foam  165  coupled to the opaque housing  155  to rest against a user&#39;s face when the user wears the head-mounted display  110 , and electrical connectors (e.g., cables, circuits, wires). The front cover  145  may be coupled to the housing  155  using one or more connectors, such as screws, by inserting the connectors through screw holes on the front cover  145 . The front cover  145  and the opaque housing  155 , when connected, may be considered a single opaque housing of the head-mounted display  110 . In some embodiments, the housing  155  is opaque at visible wavelengths but not at infrared wavelengths. 
     Various embodiments of the head-mounted display  110  are described in U.S. patent application Ser. No. 14/861,910, filed on Sep. 22, 2015, the disclosure of which is incorporated herein by reference in its entirety. 
     In some embodiments, the head-mounted display  110  further includes a VR interface  175  positioned on an outer surface of the head-mounted display  110 , or alternatively on the headband  120 . The VR interface  175  may be a port through which the head-mounted display  110  is communicatively coupled to the stationary computer  125  in the first mode or the mobile computer  135  in the second mode. For example, the VR interface  175  may be, but is not limited to, a HDMI port through which video input is received from the stationary computer in the first mode and from the mobile computer in the second mode. Additionally the head-mounted display  110  may include a one or more interfaces through which additional data and power are received from at least one cable coupled to an auxiliary device (e.g. a hand-held controller or headphones) and/or a power source. 
     The headband  120  is used for mounting the head-mounted display  110  on a user&#39;s head. In the example of  FIG. 1 , the headband  120  comprises a rigid segment  130 , a semi-rigid segment  140 , and a rigid segment  150  that are coupled to each other to adjustably wrap around side and back portions of the user&#39;s head. 
     In some embodiments, the headband  120  comprises a single and continuous semi-rigid segment  140  including two arc portions, and each arc portion is to extend from above a user&#39;s ears to below the user&#39;s occipital lobe to conform to a portion of the user&#39;s head. Alternatively, the headband  120  may comprise two separate and symmetric semi-rigid segments each including an arc portion. 
     In some embodiments, the rigid segments  130  and  150  are respectively connected to the semi-rigid segment  140 . The rigid segments  130  and  150  are also respectively coupled to the head-mounted display  110  and positioned on respective sides of the user&#39;s head to extend along the lateral dimension (e.g., the Z dimension in  FIG. 1 ). The headband  120  may further include flexible segments (not shown) that are stretchable within the rigid segments  130  and  150  respectively to adjust the headband  120  in accordance with the user&#39;s head. 
     In some embodiments, the headband  120  comprises a back section  160  coupled with the semi-rigid segment  140  to rest against the back of the user&#39;s head (e.g., around the user&#39;s occipital lobe). The semi-rigid segment  140  extends to wrap around a portion at the back of the user&#39; head (e.g., around the user&#39;s occipital lobe). The semi-rigid segment  140  includes a portion that conforms to the shape of the back section  160  and the back section  160  is coupled to the portion of the semi-rigid segment  140  at the back of the user&#39;s head. Alternatively, the headband  120  may have a back section that is part of a segment (e.g., the semi-rigid segment  140 ). The back section may be any part of the headband  120  against the back of the user&#39;s head. 
     In some embodiments, the headband  120  comprises a top strap  170  coupled to the back section  160  (or the semi-rigid segment  140 ) and the head-mounted display  110  to adjustably conform to the top of the user&#39;s head when the user is wearing the head-mounted display  110 . In some embodiments, the VR interface  175  may be positioned on the top strap  170  or the rigid segment  150 . 
     In some embodiments, various electrical connection mechanisms (e.g., flat flexible circuits and/or electric cables) are used in the VR system  100  to provide power management, signal transmission, and/or other functionalities to the head-mounted display  110  and the detachable audio subsystem  180 . For example, the head-mounted display  110  is integrated with the detachable audio subsystem  180  using suitable electrical connection mechanisms to provide both visual and audio virtual-reality experiences to the user. 
     Various embodiments of the headband  120  and the head-mounted display  110  are described in U.S. patent application Ser. No. 14/603,335, filed on Jan. 22, 2015, and U.S. patent application Ser. No. 14/681,001, filed on Apr. 7, 2015, the disclosures of which are incorporated herein by reference in their entireties. 
     As discussed above, the headband  120  of the present disclosure is used for mounting the head-mounted display on a user&#39;s head. The headband  120  (e.g., the rigid guide segments and the back rigid piece) offers rigidity to balance the head-mounted display  110  on the user&#39;s head and provide accurate head-motion tracking. The headband  120  (e.g., the stretchable bands of the flexible segments and the semi-rigid segment) also provides adjustability to accommodate different users with different head sizes and shapes. In some embodiments, the rigid guide segments of headband  120  are rotatable relative to the head-mounted display  110  to allow a better compatibility with users&#39; head and face geometries and wearing preferences. In some embodiments, the headband  120  comprises plastic materials, and thus is light-weight and comfortable to wear. When the user looks up and down or makes other head motions, the head-mounted display will not fall off the user&#39;s head. 
     In some embodiments, as illustrated in  FIGS. 3A-3C , the holder  115  is configured to support at least one of the mobile computer  135  and an additional power source  185  on the headband  120 . (In the example of  FIG. 2 , the holder  115  has been detached from the headband  120  and is not shown.) The additional power source  185  is to provide additional power to the mobile device  135  during intensive VR use (e.g. high-CPU gameplay). In some embodiments, the power source  185  may be a lithium-polymer battery or a lithium-ion battery. 
     In some embodiments, the holder  115  is rotatably coupled to the back section  160  for adjustment of an angle of orientation of the mobile computer  135 . The holder  115  may be rotatably coupled to the headband  120  through a bolt, a screw, or any other similar fastener. The holder  115  is mounted to the back section  160  of the headband  120  with at least one of the mobile computer  135  and the additional power source  185  supported (e.g., cradled) in the holder  115 . The holder, with the mobile computer  135  and/or the additional power source  185  supported therein, is mounted on the back section  160  (e.g., at a center of the back section  160 ) to at least partially balance the weight of the HMD  110  on the user&#39;s head. This configuration provides the advantage of reducing fatigue of the user due to wearing the HMD. Conventional VR systems providing a mobile device mounted to the front of an HMD fail to provide this advantage as the additional weight of the mobile device in the conventional VR systems is exerted in the front of the HMD where the mobile device is mounted, thereby actually further contributing to the user fatigue. 
     In some embodiments, the holder  115  is integrally formed with the headband  120 . In these embodiments, the holder  115  and the headband  120  form one continuous part. In other embodiments, the holder  115  is fixedly coupled to the headband  120 . In these embodiments, the holder  115  may be sewn, stapled, mechanically fused (e.g. ultrasonically welded or melted) or otherwise connected to the headband  120 . In other embodiments, the holder  115  is detachably coupled to the headband  120  and comprises at least one connector to couple to the headband  120 . In some embodiments the at least one connector comprises first and second connectors provided at corresponding positions on each of the headband  120  and the holder  115  to couple the holder  115  to the headband  120 . 
     In some embodiments, the at least one connector is permanently coupled to at least one of the holder  115  and the headband  120 . For example, a first connector may be glued to the holder  115  and a second connector may be glued to the headband  120  at a position corresponding to the first connector so as to couple the holder  115  and the headband  120  to each other. Alternatively, each connector may be sewn, stapled, mechanically fused (e.g. ultrasonically welded or melted) or otherwise connected to the holder  115  and/or the headband  120 . In other embodiments, each connector is detachably coupled to the holder  115  and/or the headband  120 . 
     In some embodiments, the at least one connector comprises a magnet to magnetically couple the holder  115  and the headband  120  to each other. 
     In other embodiments the connector comprises a first connector which is a hook surface of a hook-and-loop fastener disposed on one of the holder  115  and the headband  120 , and a second connector which is a loop surface of the hook-and-loop fastener disposed on the other of the holder  115  and the headband  120 . The hooks are configured to hook and engage the loops thereby coupling the holder  115  and the headband  120 . 
     In some embodiments the connector comprises a first disc on one of the headband  120  and the headset  110  and the second connector  170  comprises a second disc on the other of the headband  120  and the headset  110 . The first disc has a protrusion protruding from one of the headband  120  and the headset  110 . The second disc has a groove at a position on the other of the headband  120  and the headset  110  corresponding to a position of the protrusion on the first disc. The holder  115  and the headband  120  are detachably mated by insertion of the protrusion into the groove. In these embodiments, the connector comprises a snap fastener. 
     In some embodiments, at least one of a length and a width of the holder  115  is adjustable to accommodate a plurality of sizes of the mobile computer  135 . In some embodiments, the holder  115  is formed of a rigid plastic material including, but not limited to high density polyethylene providing rigidity in structure. Alternatively, any other suitable materials may be used. 
     As described above, the VR system  100  of the present invention provides two modes of operation, using the same HMD, which allows for a user to view VR images based on high definition video processed by a stationary (e.g., desktop) computer  125  in the first mode and allows for the user to view VR images based on video processed by a mobile device  135  attached to the HMD  110 , thereby removing physical constraints on the user&#39;s movements normally associated with connecting the HMD  110  to the stationary computer  125 , in the second mode. This provides the user with the advantage of being able to view high-quality high-definition video during intensive VR use (e.g., gameplay) requiring high video processing power, in a first mode, and in a second mode utilizing a mobile computer  135  coupled to the headband  120  thereby offering greater freedom of movement not otherwise possible with the first mode where movement is restricted due to the cable coupling the HMD to the stationary computer  125 . 
     In some embodiments, the VR system  100  further includes a first cable  187  ( FIG. 2 ) to communicatively couple the stationary computer  125  to the HMD  110  in the first mode of operation and a second cable  189  ( FIGS. 3A-3C ) to communicatively couple the mobile computer  135  to the HMD  110  in the second mode of operation. The first cable  187  is longer than the second cable  189 . In some embodiments, the first cable  187  and/or second cable  189  are HDMI cables or cables specific to particular computer-HMD combinations. 
     In some embodiments, the VR system  100  further comprises a plurality of light-emitting diodes (LEDs)  190  positioned on the HMD  110 , on the headband  120  or on both the HMD  110  and the headband  120 . In some embodiments, the plurality of light-emitting diodes (LEDs)  190  is distributed on outer surfaces (e.g., front, top, bottom, left-side, and/or right-side surfaces) of the head-mounted display  110  and/or the headband  120 . For example, the LEDs  190  may be mounted on or embedded within the outer surface of the HMD  110  and/or the headband  120 . The LEDs  190  may be infrared (IR) LEDs. In some embodiments, the IR LEDs are molded into the back section  160  of the headband  120  so that the IR LEDs are flush with the back section  160  yet still exposed. Alternatively, an IR transmissive material may be used on the back section  160  (e.g., on an outer surface of the back section  160 ). The IR LEDs are positioned under the outer surface of the back section  160  and covered by the IR transmissive material, so that IR light can still be transmitted through the outer surface of the back section  160 . In some examples, the LEDs  190  are distributed along the edges of the back section  160 . The LEDs  190  can also be arranged in any other suitable patterns. The rigidity of the back section  160  allows accurate positioning of the LEDs  190  for head-motion tracking. The LEDs  190 , when working with a motion tracking video camera and the related software, provide head-motion tracking (e.g., over 360°) using the head-mounted display system in the first mode. In some embodiments, the LEDs  190  are activated in the first mode and disabled in the second mode, to save power. For example, the HMD  110  or mobile computer  135  includes one or more processors and memory (e.g., a non-transitory computer-readable medium) storing instructions that, when executed by the one or more processors, activate the LEDs  190  in the first mode and deactivate the LEDs  190  in the second mode. 
     The LEDs  190  are electrically connected to a power source which may or may not be the same power source providing power to the HMD  110  (e.g., additional power source  185  in the second mode). The HMD  110  may be wireless; therefore, the power source may be one or more batteries. The LEDs  190  may be housed in diffused cases including a current limiting resistor to keep the current from the power source to the LED below the LED&#39;s maximum current rating so as to ensure maximum life of the LEDs  190 . The LEDs  190  may be activated in the first mode when a suitable voltage is applied. By virtue of the LEDs  190  being positioned in an area on the HMD  110  and/or headband  120  detectable by an external camera, motion of the light produced by the LEDs that is detected by the external camera is used as an indication of the positions and motion of the user&#39;s head, as described above. In this way, motion of the user&#39;s head is tracked by the camera, allowing for corresponding virtual-reality motions to be shown. For example, when the user shakes his/her head vigorously while playing a guitar during a VR activity, movement of the LEDs in a manner corresponding to the head shaking may be detected and used to model the user&#39;s motion. 
     To track motion of the user&#39;s head, the external camera captures a sequence of images of the HMD  110  and/or headband  120 . Variation in positions of the LEDs  190  over time is used to determine movement, based on which motion of an image subject is modeled in virtual reality in accordance with actual physical head motions made by the user. Virtual-reality images are generated and presented to the user accordingly. The head-mounted display  110  is thus configured to display a view which shifts as a user shifts their head in some direction or tilts their head at an angle. 
       FIG. 2  is a back view of the virtual-reality (VR) system including the head-mounted display  110  communicatively coupled to the stationary computer  125  in the first mode of operation. In some embodiments, as illustrated in  FIG. 2 , in the first mode of operation, the HMD is communicatively coupled to the stationary computer  125  through the first cable  187 . In the illustration of  FIG. 2 , the stationary computer  125  is a desktop computer and the first cable  187  is shown as being connected to the monitor of the desktop computer, however, the first cable may also be connected to an enclosure (e.g., the processing tower (CPU tower)) of the desktop computer. Since stationary computers (e.g. the desktop computer) generally have significantly greater processing power than mobile computers, the aforementioned configuration provides the high processing power for generating high-definition video to produce high-quality VR images which may not be possible using only a mobile computer to generate the video. 
       FIG. 3A  is a back view and  FIGS. 3B-3C  are side perspective views of the virtual-reality (VR) system  100  including the head-mounted display  110  communicatively coupled to the mobile computer  135  in the second mode of operation. In some embodiments, as illustrated in  FIGS. 3A-3C , in the second mode of operation, the HMD  110  is communicatively coupled to the mobile computer  125  through the second cable  189 . The aforementioned configuration provides the advantage over the HMD  110  connected to the stationary computer  125  in that the user&#39;s movements are not physically constrained based on a length of the second cable  189 . Because the mobile computer  135  is coupled to the headband  120  of the HMD  110 , the user may freely move without interrupting the cable connection between the HMD  110  and the mobile computer  135 . 
     In some embodiments, the first cable  187  is longer than the second cable  189 . The first cable may for example be of a length ranging from about 0-20 feet, or about 5-15 feet, or about 8-12 feet, or in some cases approximately 10 feet. Such length provides consistently good video signal to the HMD  110  without significant degradation, while remaining reasonably light in weight so as to not overly constrain user movements. The second cable may for example be of a length ranging from 0-2 feet, or about 0.5 to 1.5 feet, or about 0.8 to 1.2 feet, or in some cases approximately 1 foot. 
     In some embodiments, the mobile computer  135  comprises at least one camera  137  to track a position of the user in the second mode. The camera  137  may be a rear-facing or a front-facing camera. As the user moves their head, variation in positions of the user&#39;s head over time are tracked by the camera  137  (e.g., by identifying and tracking the relative locations of external edges and/or objects) and used to determine movement of the HMD  110 , based on which motion of an image subject is modeled in virtual reality in accordance with actual physical head motions made by the user. Virtual-reality images are generated and presented to the user accordingly. The HMD  110  is thus configured to display a view which shifts as a user shifts their head in some direction or tilts their head at an angle. 
     In some embodiments, a display  139  of the mobile computer  135  is configured to deactivate in the second mode to conserve power of the mobile computer  135 . For example, a VR application on the mobile computer  135  includes instructions, stored in memory (e.g., in a non-transitory computer-readable medium), that when executed by a processor of the mobile computer  135  disable the display  139  in the second mode. In the second mode, the mobile computer  135  thus acts as a processing device processing video to be transmitted to the HMD  110  through the second cable  189 , but does not display the video. Instead, the video is displayed on the HMD  110 , thus conserving life of the battery of the mobile computer  135  or the additional power source  185 . 
     In some embodiments, the mobile computer  135  includes at least one position sensor to track a motion of the user&#39;s head in the second mode. For example, the mobile computer  135  includes one or more sensors  320  (e.g. a gyroscope, or accelerometer) which provide sensor readings to the HMD  110  for use in image or video rendering. The sensor readings may include, but are not limited to, coordinates relating to various positions of the user&#39;s head during VR activities. The sensor readings are used by a VR application running on the mobile computer  135  and/or transmitted to the HMD  110  through the second cable  189  to the VR interface  175  of the HMD  110 . In some embodiments, the HMD  110  includes one or more sensors (e.g., gyroscopes, accelerometers, magnetometers) which obtain sensor readings  104  for use in image or video rendering. 
     The VR system  100  thus provides an advantage in that the HMD is capable of displaying both high-quality VR images from a stationary computer and VR images from a mobile computer for which the user&#39;s freedom of movement is increased. Additionally, as described above, the aforementioned configuration of the mobile computer  135  supported in the holder  115  on the back portion  160  of the headband  120  provides the further advantage of balancing the HMD  110  on the user&#39;s head, thereby reducing fatigue of the user due to most of the weight otherwise being on the front of the user&#39;s head. Furthermore, mounting the mobile computer  135  on the back of the headband  120  reduces user fatigue, whereas systems that mount a mobile computer to the front of the HMD  110  actually increase user fatigue. 
       FIG. 4  is a block diagram illustrating an electrical configuration of an exemplary VR system (e.g., VR system  100 ) in accordance with some embodiments. In some embodiments, the VR system includes a mainboard  403 . The mainboard  403  includes a controller  410 , power path  450 , motion/position tracking sensors  430 , a VR interface  175 , and a light-emitting diode (LED) driver  440 . The mainboard  403  may be coupled to a power source (e.g., additional power source  185 ) to provide power to the HMD  110 . The power may be supplied to the mainboard  403  through the power path  450 . 
     In some embodiments, the VR interface  175  includes a converter  460  to convert video from a format provided by the mobile computer  135  and/or stationary computer  125  to a format processed by the HMD  110 . 
     The LED driver  440  drives LEDs  190  under the control of the controller  410 , and thus turns the LEDs  190  on in the first mode of operation and off in the second mode of operation. The mobile computer  135  comprises a processor  314 , a camera  316 , a communication interface  318 , and motion tracking sensors  320 . In the second mode, as described above, the controller  314  processes the VR video and transmits the processed video to the VR interface  175  of the HMD  110  through the communication interface  318  and the second cable  189 . The stationary computer  125  comprises one or more processors  315 , memory  317 , a communication interface  319 , and a camera  321 . In the first mode, as described above, the processor(s)  315  processes the VR video and transmits the high-quality high-definition processed video to the VR interface  175  of the HMD  110  through the communication interface  319  and the first cable  187 . 
     The motion tracking sensors  320 ,  430  include a plurality of motion sensors (e.g. accelerometers and/or gyroscopes) which tracks motion of the HMD  110  based on motions made by the user. 
       FIG. 5  is a flow diagram illustrating a method  500  of displaying virtual-reality (VR) images produced by a VR system (e.g., VR system  100 ) in accordance with some embodiments. In some embodiments, the method  500  for displaying VR images produced by the VR system  100  includes receiving ( 502 ), in a first mode of operation, by a HMD  110  of the VR system, a first video input from the stationary computer  125 , and displaying ( 506 ), by the HMD  110 , images corresponding to the first video input. In some embodiments ( 504 ), the stationary computer is a desktop computer. Due to the high processing power of the stationary computer as compared to a mobile computer, high-quality, high definition video is transmitted from the stationary computer to the HMD, thereby enhancing the feeling of “reality” a user experiences during VR activities. The method  500  further includes in a second mode of operation, receiving ( 510 ), by the HMD, a second video input from the mobile computer  135  mounted in the holder  115  on a headband  120  of the VR system. In some embodiments ( 512 ), the mobile computer  135  is a smartphone. In some embodiments ( 514 ), the holder  115  detachably supports the mobile computer  135  and the headband  120  secures the HMD on a user&#39;s head. The method further includes displaying ( 516 ), by the HMD  110 , images corresponding to the second video input. In some embodiments, the method  400  further comprises, in the second mode, deactivating a display of the mobile computer ( 518 ) to conserve power of the mobile computer. In the second mode, the mobile computer  135  thus processes video to be transmitted to the HMD  110  through the second cable  189 , but does not display the video, in accordance with some embodiments. Instead, the video is displayed on the HMD  110 , thus conserving life of the battery of the mobile computer  135  or additional power supply  185 . 
     As described above, in the first mode, the HMD  110  receives the first video input through the first cable  187  communicatively coupling the stationary computer  125  to the VR interface  175  of the HMD  110 , and in the second mode, the HMD  110  receives the second video input through the second cable  197  communicatively coupling the mobile computer  135  to the VR interface  175 , the second cable being shorter than the first cable. In some embodiments, the method  500  further comprises, in the second mode, tracking a motion of the user&#39;s head ( 520 ) by at least one motion-tracking sensor  320  of the mobile computer  135 , and translating ( 522 ) the motion to produce a corresponding motion of the images displayed on the HMD  110 . In other embodiments, the method further includes, in the second mode, tracking a position of the user, by at least one camera  137  of the mobile computer  135 , to produce a corresponding motion of the images displayed on the HMD  110 . 
     The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the scope of the claims to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen in order to best explain the principles underlying the claims and their practical applications, to thereby enable others skilled in the art to best use the embodiments with various modifications as are suited to the particular uses contemplated.