Patent Publication Number: US-11025880-B2

Title: ROI-based VR content streaming server and method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of International Patent Application No. PCT/KR2018/014572, filed on Nov. 23, 2018, which claims priority to Korean patent application No. 10-2018-0134700 filed on Nov. 5, 2018, contents of both of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a virtual reality (VR) content streaming technology and, more particularly, to an ROI-based VR content streaming server and method with a reduced streaming delay time. 
     Description of Related Technology 
     With the recent growths of IT technologies, high-quality content transmission has been enabled and the demand for high-quality media content video streaming service is increasing. Thus, researches on various techniques for more efficiently streaming and rendering of high-quality media content more than ultra-high definition (UHD) are being actively conducted. 
     According to a recently proposed spatial relationship description (SRD), a video is divided into spatial units and encoded by HEVC, and the encoded video is divided again into spatial and time units to generate tiles. Then, bitstream stitching is performed for a tile corresponding to a region of interest (ROI) of a user as a high-quality tile and for the other tiles as low-quality tiles. This can selectively improve the network efficiency and the quality of streaming video. 
     SUMMARY 
     When a viewpoint change occurs before final streaming and rendering to a head mounted display (HMD) device, a switching delay may be generated. In order to solve this problem, not only tracking and rendering processing is required at the user device, but also a fast response to the dashing segment is required at the streaming server. 
     An aspect of the present invention is to provide an ROI-based VR content streaming server and method with a reduced streaming delay time through tile caching of a tiled video using an ROT. 
     Embodiments of the present invention provide an ROI-based VR content streaming server that includes a communication unit that receives a request signal for a media presentation description (MPD) file regarding tiles of a tiled video, region of interest (ROI) information, and a request signal for a segment file from an electronic device, and transmits the MPD file and the segment file corresponding to the request signals to the electronic device. The server further includes a controller configured to, when the request signal for the MPD file is received, control the MPD file corresponding to the request signal to be transmitted to the electronic device, when the ROI information is received, to generate and cache the segment file in which a first resolution and second to nth resolutions (‘n’ is a natural number of two or more), which are lower than the first resolution, are assigned to the tiles of the tiled video, respectively, based on the ROI information, and when the request signal for the segment file is received, to control the cached segment file to be transmitted to the electronic device. 
     In addition, the controller is further configured to predict next ROI information based on the ROI information and thereby to generate the segment file in which the first to nth resolutions are assigned. 
     In addition, the controller is further configured to, based on a base tile corresponding to the ROI information among the tiles, predict a moving direction of an object contained in the base tile and thereby to predict the next ROI information. 
     In addition, the controller is further configured to assign the first resolution to a tile corresponding to the ROI among the tiles and to assign the second to nth resolutions to the remaining tiles. 
     In addition, the controller is further configured to control the cached segment file to be transmitted in real time. 
     Embodiments of the present invention provide an ROI-based VR content streaming method that includes, at a streaming server, when a request signal for a media presentation description (MPD) file regarding tiles of a tiled video is received from an electronic device, transmitting the MPD file corresponding to the request signal to the electronic device; at the streaming server, when region of interest (ROI) information is received from the electronic device, generating and caching a segment file in which a first resolution and second to nth resolutions, which are lower than the first resolution, are assigned to the tiles of the tiled video, respectively, based on the ROI information; and at the streaming server, when a request signal for the segment file is received from the electronic device, transmitting the cached segment file to the electronic device. 
     In addition, the caching includes predicting next ROI information based on the ROI information and thereby generating the segment file in which the first to nth resolutions are assigned. 
     In addition, the caching includes, based on a base tile corresponding to the ROI information among the tiles, predicting a moving direction of an object contained in the base tile and thereby predicting the next ROT information. 
     In addition, the caching includes assigning the first resolution to a tile corresponding to the ROI among the tiles and assigning the second to nth resolutions to the remaining tiles. 
     In addition, the cached segment file is transmitted in real time. 
     The ROI-based VR content streaming server and method according to embodiments of the present invention can cache in advance high-quality and low-quality tiles to be transmitted, by using ROT information of a user, and transmit the cached tiles in response to a transmission request. Therefore, it is possible to reduce a load on transmission request, minimize a response delay time, and thereby reduce a streaming delay time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a VR content streaming system according to an embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating a streaming server according to an embodiment of the present invention. 
         FIG. 3  is a diagram illustrating a dash connector shown in  FIG. 2 . 
         FIG. 4  is a diagram illustrating a segment cache shown in  FIG. 2 . 
         FIG. 5  is a block diagram illustrating an electronic device according to an embodiment of the present invention. 
         FIG. 6  is a diagram illustrating an ROI detection according to an embodiment of the present invention. 
         FIG. 7  is a diagram illustrating a texture of a tiled video according to an embodiment of the present invention. 
         FIG. 8  is a diagram illustrating a 360-degree VR video modeling according to an embodiment of the present invention. 
         FIG. 9  is a diagram illustrating a tiled video rendering according to an embodiment of the present invention. 
         FIG. 10  is a flow diagram illustrating a VR content streaming method of a streaming server according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings and descriptions, the same or corresponding elements are denoted by the same reference numerals. Also, in describing features of the present invention, techniques that are well known in the art will not be described. This is to avoid obscuring a subject matter of embodiments of the present invention by omitting an unnecessary explanation. 
       FIG. 1  is a schematic diagram illustrating a VR content streaming system according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the VR content streaming system  300  reduces a streaming delay time through tile caching of a tiled video using a region of interest (ROI). The VR content streaming system  300  includes a streaming server  100  and an electronic device  200 . 
     The streaming server  100  generates a media presentation description (MPD) file for tiles of the tiled video and transmits the generated MPD file to the electronic device  200 . The streaming server  100  generates and caches a segment file in which different resolutions are assigned to the respective tiles of the tiled video, based on ROI information. That is, the streaming server  100  may store and manage segmented video tiles having different resolutions. The streaming server  100  transmits, to the electronic device  200 , the cached segment file that contains the tiles of the tiled video having different resolutions. The MPD file may correspond to a manifest file that contains information about contents available for the electronic device  200 , provided alternative bit rates, and URL address values. The ROI may be generated based on field of view (FOV) information of a user. The streaming server  100  may be implemented as a desktop, a laptop, a server computer, a cluster computer, or the like. 
     By parsing the MPD file, the electronic device  200  may acquire information about program timing, media-content availability, media type, image quality, minimum and maximum bandwidths, usable encoded-alternatives, DRM, and the like. The electronic device  200  may select an encoding scheme corresponding to a network state or the status of the electronic device and then perform a streaming request through an HTTP GET Request. For example, the electronic device  200  may establish a communication channel with the streaming server  100  in response to a user input. Through this, the electronic device  200  transmits the ROI information to the streaming server  200  and receives the segment file generated by using the ROI information. The electronic device  200  generates a single video by combining segmented video tiles of different resolutions contained in the received segment file, and outputs the generated video. Because high-resolution video tiles are located in ROIs and low-resolution video tiles are located in non-ROIs, the electronic device  200  can provide a high-quality VR content to the user in real time while reducing the load of data processing. The electronic device  200  may be implemented as a desktop, a laptop, a smart phone, a tablet PC, a handheld PC, an HMD, or the like, and preferably, the HMD. 
     The VR content streaming system  300  may be implemented based on a streaming protocol for transmission/reception of information between the streaming server  100  and the electronic device  200 . The streaming protocol may use, but is not limited to, the HTTP 1.1 GET protocol. 
       FIG. 2  is a block diagram illustrating a streaming server according to an embodiment of the present invention.  FIG. 3  is a diagram illustrating a Dash connector shown in  FIG. 2 .  FIG. 4  is a diagram illustrating a segment cache shown in  FIG. 2 . 
     Referring to  FIGS. 1 to 4 , the streaming server  100  may include a communication unit  10 , a controller  30 , and a storage  50 . 
     The communication unit  10  communicates with the electronic device  200 . The communication unit  10  may perform wired or wireless communication with the electronic device  200 . The communication unit  10  receives a request signal for the MPD file regarding tiles of a tiled video, the ROI information, and a request signal for the segment file from the electronic device  200 . The communication unit  10  transmits the MPD file and the segment file corresponding to the request signals to the electronic device  200 . 
     When the request signal for the MPD file is received, the controller  30  controls the MPD file corresponding to the request signal to be transmitted to the electronic device  200 . When the ROI information is received, the controller  30  generates and caches the segment file in which a first resolution and second to nth resolutions (‘n’ is a natural number of two or more), which are lower than the first resolution, are assigned to tiles of a tiled video, respectively, based on the ROI information. Through this, the controller  30  may assign different resolutions to the tiled video. When the request signal for the segment file is received, the controller  30  controls the cached segment file to be transmitted to the electronic device  200 . 
     Specifically, the controller  30  may predict the next ROI information based on the ROI information and thereby generate the segment file in which the first to nth resolutions are assigned. That is, based on a base tile corresponding to the ROI information among a plurality of tiles, the controller  30  predicts a moving direction of an object contained in the base tile and thereby predicts the next ROI information. In addition, the controller  30  assigns the first resolution to a tile corresponding to the ROI among the plurality of tiles and also assigns the second to nth resolutions to the remaining tiles. Through this, the controller  30  may lower the system load. Also, the controller  30  generates and caches in advance the segment file to be transmitted to the electronic device  200 , thereby minimizing a response delay time and enabling a real-time transmission of the segment file to the electronic device  200 . 
     Particularly, in order to generate the segment file, the controller  30  may include a dynamic adaptive streaming over HTTP (Dash) connector  35 . The Dash connector  35  includes a Dash live connection module  41  and a Dash live connection thread  43 . 
     The Dash live connection module  41  performs a function of receiving an HTTP Connection from the electronic device  200 . In addition, the Dash live connection module  41  performs functions of generating a DashConnectionThread when the HTTP Connection is established, and delivering and executing the HTTP Connection. The Dash live connection module  41  may receive the Connection in the same structure as that of the MultiThread Socket in the limit allowed by the system. 
     The Dash live connection thread  43 , which is a thread related to HttpServletRequest and Response, parses a HTTP URI with a SubThread generated at the Dash live connection module  41  and performs a corresponding work. That is, in accordance with a URI message parsed by a thread&#39;s core method, the Dash live connection thread  43  determines whether to generate and transmit a ClientID, transmit an IndexList, transmit an MPD, and transmit a segment. The Dash live connection thread  43  generates a thread by checking the presence of a base tile, configures an adaptation set to be predicted by a user using the electronic device  200  based on the requested base tile, and requests caching. 
     The storage  50  stores various programs or various instructions necessary for operating the streaming server  100 . For example, the storage  50  may store instructions related to the MPD file, the ROI information, and the segment file in connection with the tiles of the tiled video. The storage  50  stores the MPD file, the ROI information, and the segment file. In particular, the storage  50  may include a segment cache  55  for caching the segment file. The segment cache  55  is composed of three level caches  61 ,  63 , and  65 , and each cache manages a file loaded on memory in a first-in first-out (FIFO) scheme. The Lv1 cache  61  is a segment cache for predicted adaptation tile, the Lv2 cache  63  is a segment cache for a recently Dashed tile, and the Lv3 cache  65  is a segment cache for the least recently used (LRU) tile. The Lv1 cache  61 , the Lv2 cache  63 , and the Lv3 cache  65  do not have overlapped tiles. When there is a request for the segment file, the segment cache  55  performs a search in the order of the Lv1 cache  61 , the Lv2 cache  63 , and the Lv3 cache  65 , and finally searches a file system if there is no cache. The segment cache  55  has a key/value map architecture and searches for a file name as a key. The segment cache  55  increases a requestCount corresponding to a retrieved and returned tile by one, and records a time stamp of the moment of return to use it as a factor for determining whether to move to the Lv3 cache  65  in case of deletion from the cache. 
     Whenever the Dash live connection thread  43  receives a request for a new center tile segment, the controller  30  caches surrounding tiles of the center tile into the Lv1 cache  61 . This process is as shown in Equation 1. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           M 
                           
                             m 
                             × 
                             n 
                           
                         
                         ⁡ 
                         
                           ( 
                           GoT 
                           ) 
                         
                       
                       ij 
                     
                     ∋ 
                     
                       
                         
                           [ 
                           
                             a 
                             
                               k 
                               , 
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                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 1, ‘M m×n ’ denotes indexes of tiles in the entire tile, ‘GoT’ (Grouping of Tiles) denotes a set of adjacent tiles, and ‘k,l’ denotes an index of an i, j array in adjacent tiles. 
     If a set of elements adjacent to {a 2,2 } is calculated from a M 4×4  tiled video, {a 1,1 , a 1,2 , a 1,3 , a 2,1 , a 2,3 , a 3,2 , a 3,3 } are defined as adjacent tiles as shown in Equation 2, and such tiles are cached into the Lv1 cache  61 . 
     
       
         
           
             
               
                 
                   
                     
                       
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                     ⁢ 
                     
                         
                     
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                     2 
                   
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                       } 
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     The storage  50  may include at least one of a flash memory type storage, a hard disk type storage, a multimedia card micro type storage, a card type memory (e.g., SD or XD memory), a random access memory (RAM), a static random access memory (SRAM), a read-only memory (ROM), an electrically erasable programmable read-only memory (EEPROM), a programmable read-only memory (PROM), a magnetic memory, a magnetic disk, or an optical disk. 
       FIG. 5  is a block diagram illustrating an electronic device according to an embodiment of the present invention. 
     Referring to  FIGS. 1 and 5 , the electronic device  200  includes a device communication unit  210 , a sensor unit  220 , a device controller  230 , a display  240 , an input/output unit  250 , and a device storage  260 . 
     The device communication unit  210  communicates with the streaming server  100 . The device communication unit  210  may perform wired or wireless communication with the streaming server  100 . The device communication unit  210  transmits a request signal for the MPD file, the ROI information, and a request signal for the segment file to the streaming server  100 . The device communication unit  210  receives the MPD file and the segment file corresponding to the request signals from the streaming server  100 . 
     The sensor unit  220  tracks user&#39;s eyes and collects tracking information. The sensor unit  220  transmits information about a direction indicated by a user&#39;s pupil to the device controller  230  as viewpoint information. When the electronic device  200  is the HMD, the sensor unit  220  is installed in the HMD and may be activated, when the user wears the HMD, to detect a change in the position of the user&#39;s pupil. When the electronic device  200  is not the HMD, the sensor unit  220  is implemented as a separate sensor module and may be activated, when the user enters a user input, to detect a change in the position of the user&#39;s pupil. 
     The device controller  230  controls the request signal for requesting the MPD file to be transmitted to the streaming server  100 . When the MPD file corresponding to the request signal is received, the device controller  230  controls the MPD file to be outputted through the display  240 . At this time, the sensor unit  220  may collect viewpoint information of the user who watches a displayed image. The device controller  230  generates the ROI information by using the viewpoint information collected by the sensor unit  220  and controls the generated ROI information to be transmitted to the streaming server  100 . 
     When receiving the segment file for the tiles of the tiled video having different resolutions from the streaming server  100 , the device controller  230  performs the multi-resolution shading (MRS) rendering of the tiles of the tiled video to generate VR content, and controls the display  240  to output the generated VR content. In particular, the VR content has high-resolution tiles in the ROIs and low-resolution tiles in the non-ROIs, thereby reducing the overall data load and enabling a low-latency live streaming service. Additionally, when further receiving audio information, the device controller  230  may render the received audio information together with the tiles of the tiled video. 
     The display  240  outputs a screen related to the operation of the electronic device  200 . For example, the display  240  may output a screen associated with access to the streaming server  100 , a screen associated with the MPD file provided by the streaming server  100 , and the VR content generated by the segment file provided by the streaming server  100 . When the electronic device  200  is the HMD, the display  240  may include separated display areas included in the HMD to output the VR video. The separated display areas may output a left-eye image and a right-eye image, respectively. The display  240  may include at least one of a liquid crystal display (LCD), a thin film transistor-liquid crystal display (TFT-LCD), an organic light emitting diode (OLED), a flexible display, or a three-dimensional (3D) display. 
     The input/output unit  250  may include at least one input device for entering a user input related to a manipulation of the electronic device  200 , such as one or more buttons, a touch pad, a touch screen, and a voice input microphone. The input/output unit  250  may include an audio device capable of outputting an audio signal related to the operation of the electronic device  200 , a lamp capable of outputting light, or a vibrating element. For example, the input/output unit  250  may output audio signals received from the streaming server  100  to be synchronized with the tiles of the tiled video. 
     The device storage  260  stores programs and instructions related to the operation of the electronic device  200 . That is, the device storage  260  may store an application program associated with the operation of the electronic device  200 . The device storage  260  may store the MPD file and the segment file received from the streaming server  100 , and store the VR content generated based on the ROI. The device storage  260  may include at least one of a flash memory type storage, a hard disk type storage, a multimedia card micro type storage, a card type memory (e.g., SD or XD memory), a RAM, an SRAM, a ROM, an EEPROM, a PROM, a magnetic memory, a magnetic disk, or an optical disk. 
       FIG. 6  is a diagram illustrating an ROI detection according to an embodiment of the present invention.  FIG. 7  is a diagram illustrating a texture of a tiled video according to an embodiment of the present invention.  FIG. 8  is a diagram illustrating a 360-degree VR video modeling according to an embodiment of the present invention.  FIG. 9  is a diagram illustrating a tiled video rendering according to an embodiment of the present invention. 
     Referring to  FIGS. 6 to 9 , the electronic device  200  generates an ROI-based VR content and outputs the generated VR content. 
     To this end, the electronic device  200  defines an ROI in the entire texture. The electronic device  200  defines a field of view (FOV) to use high-quality tiles by using position tracking (see  FIG. 6 ). The electronic device  200  may receive position tracking values as x, y, and z vector values, and the received vector values are normalized as shown in Equation 3.
 
res=√{square root over ( x   2   +y   2   +z   2 )}  [Equation 3]
 
     if(res≠0|res≠1)x=y=z=1.0−res 
     Here, ‘res’ means a vector length between x, y, and z. 
     The electronic device  200  calculates FOV coordinates, based on the normalized position tracking coordinates. Coordinates x1 and x2 are calculated as shown in Equation 4, and coordinates y1 and y2 are calculated as shown in Equation 5. 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
                     
                       atan 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         
                           z 
                           , 
                           x 
                         
                         ) 
                       
                     
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                       π 
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
             
               
                 
                   
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                     · 
                     fov 
                   
                   = 
                   
                     
                       
                         
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                           · 
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                         × 
                         
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                           · 
                           width 
                         
                       
                       
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                         · 
                         height 
                       
                     
                     ÷ 
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                     1 
                   
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                   [ 
                   
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                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
             
               
                 
                   
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     Here, ‘h.fov’ means the FOV with respect to height, and ‘w.fov’ means the FOV with respect to width. 
     The electronic device  200  defines the ROI in the FOV by using viewpoint tracking coordinates (x, y, z) in the FOV, based on the position tracking. Using Equation 6, the electronic device  200  calculates position coordinates in the FOV from the viewpoint tracking coordinates (x,y,z=1). 
     
       
         
           
             
               
                 
                   
                     Eye 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     X 
                   
                   = 
                   
                     
                       
                         x 
                         × 
                         width 
                       
                       2 
                     
                     + 
                     
                       width 
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     Eye 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Y 
                   
                   = 
                   
                     
                       
                         y 
                         × 
                         height 
                       
                       2 
                     
                     + 
                     
                       height 
                       2 
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     Using the defined ROI, the electronic device  200  may divide a viewport area of the rendered texture  71  or  72  into a high resolution (1.0), a medium resolution (0.7), and a low resolution (0.2). That is, the electronic device  200  may generate a multi-resolution texture by assigning the high resolution (1.0) to a tile located in the ROI, the medium resolution (0.7) to surrounding tiles, and the low resolution (0.2) to the other tiles, and also quantify  73  such resolutions for the respective tiles. 
     Specifically, the electronic device  200  basically supports the MPEG-DASH standard and detects the ROI by using eye-view tracking coordinates. For example, tiles represented by x and y coordinate values of a video tiled into 6×6 with a resolution of 7680×3840 may be calculated as shown in Equation 7. 
     
       
         
           
             
               
                 
                   row 
                   = 
                   
                     { 
                     
                       
                         
                           6 
                         
                         
                           
                             if 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ( 
                               
                                 y 
                                 = 
                                 3840 
                               
                               ) 
                             
                           
                         
                       
                       
                         
                           
                             [ 
                             
                               1 
                               ≤ 
                               
                                 
                                   y 
                                   640 
                                 
                                 + 
                               
                               &lt; 
                               7 
                             
                             ] 
                           
                         
                         
                           
                             if 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ( 
                               
                                 0 
                                 ≤ 
                                 y 
                                 &lt; 
                                 3840 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
             
             
               
                 
                   col 
                   = 
                   
                     { 
                     
                       
                         
                           6 
                         
                         
                           
                             if 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ( 
                               
                                 x 
                                 = 
                                 7680 
                               
                               ) 
                             
                           
                         
                       
                       
                         
                           
                             [ 
                             
                               1 
                               ≤ 
                               
                                 
                                   x 
                                   1280 
                                 
                                 + 
                                 1 
                               
                               &lt; 
                               7 
                             
                             ] 
                           
                         
                         
                           
                             if 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ( 
                               
                                 0 
                                 ≤ 
                                 x 
                                 &lt; 
                                 7680 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     If the x coordinate is 4100 and the y coordinate is 2000, a tile including this coordinate value is (3+1, 3+1), which is 
               (       [       4100   1280     +   1     ]     ,     [       2000   640     +   1     ]       )     ,         
and finally represented as (4, 4) when expressed as an integer except a decimal point.
 
     The electronic device  200  may configure a tile corresponding to the ROI with a high resolution  81  and also configure a tile corresponding to the non-ROI with a medium or low resolution  82 . At this time, the electronic device  200  requests and receives the segment file for tiles having different resolutions based on the ROI from the streaming server  100 . Meanwhile, if there is no pre-stored MPD file, the electronic device  200  first receives the MPD file from the streaming server  100  before ROI detection. 
     After receiving the segment file, the electronic device  200  performs bitstream stitching and decoding  83 . Thereafter, the electronic device  200  maps the decoded video image to a 3D texture  84  for rendering, and selects a projection view point  85 . 
     The electronic device  200  creates and displays a multi-view port projection texture from a single projection texture  91  of the selected projection view point video image by using multi-resolution rendering. The multi-resolution rendering may include a graphics pipeline  92  and three multi-view port outputs  93 . 
       FIG. 10  is a flow diagram illustrating a VR content streaming method of a streaming server according to an embodiment of the present invention. 
     Referring to  FIGS. 1 and 10 , the VR content streaming method caches in advance high-quality and low-quality tiles to be transmitted, by using ROI information of a user, and transmits the cached tiles in response to a transmission request. 
     At step S 110 , the streaming server  100  determines whether a request signal for an MPD file is received. When the MPD file request signal is received from the electronic device  200 , the streaming server  100  performs step S 120 . If the MPD file request signal is not received, the streaming server  100  terminates a process. 
     At step S 120 , the streaming server  100  transmits the MPD file to the electronic device  200 . That is, the streaming server  100  transmits the MPD file corresponding to the request signal to the electronic device  200 . 
     At step S 130 , the streaming server  100  determines whether ROI information is received. When the ROI information is received from the electronic device  200 , the streaming server  100  performs step S 140 . If the ROI information is not received, the streaming server  100  performs step S 130  again. 
     At step S 140 , the streaming server  100  generates and caches a segment file. Specifically, the streaming server  100  generates the segment file in which a first resolution and second to nth resolutions, which are lower than the first resolution, are assigned to tiles of a tiled video, respectively, based on the ROI information. That is, the streaming server  100  assigns the first resolution to a tile corresponding to the ROI among the tiles and assigns the second to nth resolutions to the remaining tiles. The streaming server  100  caches the generated segment file. 
     At step S 150 , the streaming server  100  determines whether a request signal for the segment file is received. When the segment file request signal is received from the electronic device  200 , the streaming server  100  performs step S 160 . If the segment file request signal is not received, the streaming server  100 ) performs step S 150  again. 
     At step S 160 , the streaming server  100  transmits the cached segment file to the electronic device  200 . That is, the streaming server  100  transmits the segment file corresponding to the request signal to the electronic device  200 . 
     At step S 170 , the streaming server  100  determines whether to terminate the process. Unless terminating the process, the streaming server  100  returns to step S 130 . 
     As described hereinbefore, the streaming server  100  generates in advance the segment file to be transmitted by predicting the next ROT information even if a request for the segment file is not received from the electronic device  200 . Therefore, the streaming server  100  can provide live streaming to the user with a reduced system load and a minimized response delay time. 
     While the present invention has been particularly shown and described with reference to some embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the appended claims.