Patent Publication Number: US-7221366-B2

Title: Real-time rendering system and process for interactive viewpoint video

Description:
BACKGROUND 
   1. Technical Field 
   The invention is related to the rendering of video, and more particularly to a system and process for rendering an interactive viewpoint video in which a user can watch a dynamic scene while changing the viewpoint at will. 
   2. Background Art 
   For several years now, viewers of TV commercials and feature films have been seeing the “freeze frame” effect used to create the illusion of stopping time and changing the camera viewpoint. The earliest commercials were produced by using a film-based system, which rapidly jumped between different still cameras arrayed along a rail to give the illusion of moving through a frozen slice of time. 
   When it first appeared, the effect was fresh and looked spectacular, and soon it was being emulated in many productions, the most famous of which is probably the “bullet time” effects seen in the movie entitled “The Matrix”. Unfortunately, this effect is a one-time, pre-planned affair. The viewpoint trajectory is planned ahead of time, and many man hours are expended to produce the desired interpolated views. Newer systems are based on video camera arrays, but still rely on having many cameras to avoid software view interpolation. 
   Thus, existing systems would not allow a user to interactively change to any desired viewpoint while watching a dynamic image-based scene. Most of the work on image-based rendering (IBR) in the past involves rendering static scenes, with two of the best-known techniques being Light Field Rendering [10] and the Lumigraph [6]. Their success in high quality rendering stems from the use of a large number of sampled images and has inspired a large body of work in the field. One exciting potential extension of this groundbreaking work involves interactively controlling viewpoint while watching a video. The ability of a user to interactively control the viewpoint of a video enhances the viewing experience considerably, enabling such diverse applications as new viewpoint instant replays, changing the point of view in dramas, and creating “freeze frame” visual effects at will. 
   However, extending IBR to dynamic scenes is not trivial because of the difficulty (and cost) of synchronizing so many cameras as well as acquiring and storing the images. Not only are there significant hurdles to overcome in capturing, representing, and rendering dynamic scenes from multiple points of view, but being able to do this interactively provides a significant further complication. To date attempts to realize this goal have not been very satisfactory. 
   In regard to the video-based rendering aspects of an interactive viewpoint video system, one of the earliest attempts at capturing and rendering dynamic scenes was Kanade et al.&#39;s Virtualized Reality system [9], which involved 51 cameras arranged around a 5-meter geodesic dome. The resolution of each camera is 512×512 and the capture rate is 30 fps. They extract a global surface representation at each time frame, using a form of voxel coloring [13] based on the scene flow equation [16]. Unfortunately, the results look unrealistic because of low resolution, matching errors, and improper handling of object boundaries. 
   Carranza et al. [3] used seven synchronized cameras distributed around a room looking towards its center to capture 3D human motion. Each camera is at CIF resolution (320×240) and captures at 15 fps. They use a 3D human model as a prior to compute 3D shape at each time frame. 
   Yang et al. [18] designed an 8×8 grid of cameras (each 320×240) for capturing a dynamic scene. Instead of storing and rendering the data, they transmit only the rays necessary to compose the desired virtual view. In their system, the cameras are not genlocked; instead, they rely on internal clocks across six PCs. The camera capture rate is 15 fps, and the interactive viewing rate is 18 fps. 
   As a proof of concept for storing dynamic light fields, Wilburn et al. [17] demonstrated that it is possible to synchronize six cameras (640×480 at 30 fps), and compress and store all the image data in real time. They have since hooked up 128 cameras. 
   A lot of images are required for realistic rendering if the scene geometry is either unknown or known to only a rough approximation. If geometry is known accurately, it is possible to reduce the requirement for images substantially [6]. One practical way of extracting the scene geometry is through stereo, and a lot of stereo algorithms have been proposed for static scenes [12]. However, there have been a few attempts at employing stereo techniques with dynamic scenes. As part of the Virtualized Reality work [9], Vedula et al. [16] proposed an algorithm for extracting 3D motion (i.e., correspondence between scene shape across time) using 2D optical flow and 3D scene shape. In their approach, they use a voting scheme similar to voxel coloring [13], where the measure used is how well a hypothesized voxel location fits the 3D flow equation. 
   Zhang and Kambhamettu [19] also integrated 3D scene flow and structure in their framework. Their 3D affine motion model is used locally, with spatial regularization, and discontinuities are preserved using color segmentation. Tao et al. [15] assume the scene is piecewise planar. They also assume constant velocity for each planar patch in order to constrain the dynamic depth map estimation. 
   In a more ambitious effort, Carceroni and Kutulakos [2] recover piecewise continuous geometry and reflectance (Phong model) under non-rigid motion with known lighting positions. They discretize the space into surface elements (“surfels”), and perform a search over location, orientation, and reflectance parameter to maximize agreement with the observed images. 
   In an interesting twist to the conventional local window matching, Zhang et al. [20] use matching windows that straddle space and time. The advantage of this method is that there is less dependence on brightness constancy over time. 
   Active rangefinding techniques have also been applied to moving scenes. Hall-Holt and Rusinkiewicz [7] use projected boundary-coded stripe patterns that vary over time. There is also a commercial system on the market called ZCam™ manufactured by 3DV Systems of Israel, which is a range sensing video camera add-on used in conjunction with a broadcast video camera. However, it is an expensive system, and provides single viewpoint depth only, which makes it less suitable for multiple view-point video. 
   However, despite all the advances in stereo and image-based rendering, it is still very difficult to interactively render high-quality, high resolution views of dynamic scenes. The present invention tackles this problem in a cost efficient manner. 
   It is noted that in the preceding paragraphs, as well as in the remainder of this specification, the description refers to various individual publications identified by a numeric designator contained within a pair of brackets. For example, such a reference may be identified by reciting, “reference [1]” or simply “[1]”. Multiple references will be identified by a pair of brackets containing more than one designator, for example, [2, 3]. A listing of references including the publications corresponding to each designator can be found at the end of the Detailed Description section. 
   SUMMARY 
   The present invention is directed toward a system and process for rendering and displaying an interactive viewpoint video in which a user can watch a dynamic scene while manipulating (freezing, slowing down, or reversing) time and changing the viewpoint at will. The ability to interactively control viewpoint while watching a video is an exciting new application for image-based rendering. Because any intermediate view can be synthesized at any time, with the potential for space-time manipulation, this type of video has been dubbed interactive viewpoint video. 
   The rendering and displaying system that is used to play the interactive viewpoint video has a user interface sub-system for inputting user viewpoint selections and displaying rendered interactive viewpoint video frames to the user. To this end, the system includes an input device of some type that is employed by the user to input viewpoint selections on an ongoing basis, and a display device used to display the rendered interactive viewpoint video frames to the user. In addition, the user interface sub-system can include a graphic user interface that allows the user to graphically indicate the viewpoint via the input device. 
   The rendering and displaying system further includes a computing device on which a rendering program is run. This rendering program is used to render each frame of the interactive viewpoint video and generally includes a selective decoding module and a rendering module. The selective decoding module decodes data in the form of layered video frame, as well as calibration data. The calibration data provides location and viewpoint information for each video camera used to capture the data associated with the aforementioned layered representations of video frames. The layered representation data and calibration data may be input by the decoding module in the form of an interactive viewpoint video file, and may also be compressed. If so, the decoding module is tasked with decompressing the data. In order to speed up processing and provide real time rendering, just the data needed to render the desired view are decoded by the decoding module. The particular data needed are identified by the aforementioned rendering module. 
   In general, for each interactive viewpoint video frame rendered, the rendering module first identifies a current user-specified viewpoint. Next, the frame or frames that are needed to render the current frame of the interactive viewpoint video from the identified viewpoint are identified from the group of contemporaneous frames associated with the current temporal portion of the video being played. This generally entails using the calibration data to determine the viewpoints associated with each frame in the current group, and then determining if the current user-selected viewpoint coincides with one of the frame viewpoints or falls between two of the frames. When the identified viewpoint coincides with a viewpoint of one of the video frames, that frame is identified as the frame needed to render the scene. However, when the identified viewpoint falls between the viewpoints of two of the video frames, both of these frames are identified as the frames needed to render the scene. 
   Once the required frame or frames are identified, the layered representations corresponding to those frames are obtained. This, at a minimum, entails extracting just the needed frame data from the interactive viewpoint video data. If the video data is contained in a file, it will typically need to be decoded. A selective decoding module of the rendering program can be employed for this purpose. Further, if the layered frame data have been compressed, the decoder module is responsible for decompressing the portion of the video data needed to recover the particular frames required to render the scene from the desired viewpoint. The layered representations of each frame include a main layer made up of background color and background disparity values of pixels found within pre-determined sized areas in the frame surrounding pixels exhibiting depth discontinuities exceeding a prescribed threshold, as well as color and disparity values of pixels found outside said areas. In addition, there is a boundary layer having foreground color, foreground disparity and alpha values of pixels found within the aforementioned depth discontinuity areas. 
   The decoded frame data are used to render the next frame of the interactive viewpoint video from the viewpoint currently specified by the user. This is a straightforward process if the specified viewpoint coincides with the viewpoint associated with a decoded frame. More particularly, it involves compositing the boundary layer over the main layer using the alpha values from the boundary layer. However, if the desired viewpoint falls between two frames, the rendering process is more involved. In one embodiment of the rendering process, this entails, for each of the two input frames in turn, projecting the main and boundary layers of the frame to a virtual view corresponding to the current user-specified viewpoint. For each pixel in the first rendered frame an alpha-blended color C 0  is computed. The color C 0  is computed for the corresponding pixel of the main and boundary layers associated with a first of the frames using the equation C 0 =C boundary0 α color0 +C main0 (1−α color0 ), where α color0 =1+α main0  (α boundary0 −1) if the corresponding depth of the boundary layer at that pixel is less than or equal to the depth of the main layer and α color0 =0 otherwise. The value C boundary     0    is the color of the boundary layer pixel of the first frame, α boundary0  is the alpha value of the boundary layer pixel of the first frame and C main     0    is the color of the main layer pixel of the first frame. The main and boundary layers are then combined in the same manner of the second frame, creating a color C 1  at each pixel using C 1 =C boundary1 α color1 +C main1 (1−α color1 ), where α color1 =1+α main1  (α boundary1 −1) if the corresponding depth of the boundary layer at that pixel is less than or equal to the depth of the main layer and α color1 =0 otherwise. It is noted that in both cases, the alpha value of the boundary layer pixels falling outside the depth discontinuity areas is assumed to be zero, the alpha value of the main layer pixels outside the depth discontinuity areas is assumed to be one, and the alpha value of the main layer pixels inside the depth discontinuity areas is assumed to be zero. Once the two layer pairs have been composited into the new virtual viewpoint they are combined. A weight w 0  and (1−w 0 ) are assigned to the first and second composited layer pairs respectfully. This weight is in direct proportion to how close the viewpoint associated with the layers used to create the composited layer pair is to the current user-specified viewpoint. Once the weights have been established, the composited layer pairs are blended based on their assigned weights using the equation: 
             C   f     =       (         w   0     ⁢     α   0     ⁢     C   0       +       (     1   -     w   0       )     ⁢     α   1     ⁢     C   1         )       (         w   0     ⁢     α   0       +       (     1   -     w   0       )     ⁢     α   1         )             
where C f  is the final color. The alpha values α 0  and α 1  are computed as α 0 =α boundary0 +α main0 (1−α boundary0 ) and α 1 =α boundary1 +α main1 (1−α boundary1 ). In order to maintain color resolution, on the fixed point arithmetic of GPUs, when computing C 0  and C 1 , their values are not multiplied by α 0  and α 1  until computing the final color C f .
 
   In a variation of the blending procedure, correspondingly located pixels of the composited layer pairs are blended based on not only the assigned weights, but also based on their relative disparity values. More particularly, the color value of the pixel having the smaller disparity value is assigned to the corresponding pixel location in the frame of the interactive viewpoint video being rendered, whenever one of the pixels in the pair exhibits a disparity value that is less than the other by more than a prescribed tolerance (e.g., 4 levels). However, if the pair of pixels exhibit disparity values that differ by an amount that is equal to or less than the prescribed tolerance, then the color values of the pixels are blended in accordance with the weights assigned to each composited layer pair and assigned to the corresponding pixel location in the frame of the interactive viewpoint video. 
   In addition to the just described benefits, other advantages of the present invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it. 

   
     DESCRIPTION OF THE DRAWINGS 
     The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawing(s) will be provided by the U.S. Patent and Trademark Office upon request and payment of the necessary fee. 
     The specific features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
       FIG. 1  is a diagram depicting a general purpose computing device constituting an exemplary system for implementing the present invention. 
       FIG. 2  is a block diagram showing the general computer program module architecture for one embodiment of the interactive viewpoint video rendering program according to the present invention. 
       FIGS. 3(   a )–( c ) are a series of images showing an example of the results of the interactive viewpoint video rendering process according to the present invention.  FIGS. 3(   a ) and ( c ) represent contemporaneously captured frames from two adjacent video cameras located at different viewpoints.  FIG. 3(   b ) is an example of the frame that is rendered when the user-specified viewpoint is between the viewpoints associated with the images of  FIGS. 3(   a ) and ( c ). 
       FIG. 4  is an image showing a frame from an interactive viewpoint video created in accordance with the present invention, into which an extra copy of a breakdancer has been inserted. 
       FIG. 5  is a flow chart diagramming the overall interactive viewpoint video rendering process according to the present invention. 
       FIGS. 6A–B  are a flow chart diagramming one embodiment of the rendering portion of the interactive viewpoint video rendering process of  FIG. 5  where data from two camera viewpoints is required to render a view of the scene from the user specified viewpoint, and where the main and boundary layers from the same camera view are projected and composited, and then the composited layers associated with each of the camera views are blended. 
       FIG. 7  is a flow chart diagramming a variation of the process of  FIGS. 6A–B  where a soft Z compare is employed in the blending portion thereof. 
       FIGS. 8(   a )–( d ) are a series of images showing an example of the intermediate and final stages of the foregoing rendering process.  FIG. 8(   a ) represents the rendered main layer of one camera view with the areas associated with depth discontinuities erased.  FIG. 8(   b ) represents the composited main and boundary layers of this first view, whereas  FIG. 8(   c ) represents the composited layers of the second view. Finally,  FIG. 8(   d ) represents the final blended frame of the interactive viewpoint video. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following description of the preferred embodiments of the present invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
   1.0 Interactive Viewpoint Video 
   The present rendering system and process renders and displays dynamic scenes with interactive viewpoint control, in real time, from interactive viewpoint video data. In general, interactive viewpoint video is video in which a user can watch a dynamic scene while manipulating (freezing, slowing down, or reversing) time and changing the viewpoint at will. This video is generated using a relatively small number of cameras to simultaneously capture multiple views of a scene from different viewpoints to produce a set of contemporaneous frames of the video. This is done on a continuing basis to produce a sequence of these frame sets, which are then characterized using unique two-layer frame representations, to form the video. While the generation of the interactive viewpoint video is beyond the scope of the present rendering system and process, it is the subject of a co-pending application entitled “Interactive Viewpoint Video System And Process”, which was filed on Jun. 28, 2004 and assigned Ser. No. 10/880,774. 
   The aforementioned two layer representations of each frame include a main layer and a boundary layer. The main layer has pixels exhibiting background colors and background disparities associated with correspondingly located pixels of depth discontinuity areas of the frame being represented, as well as pixels exhibiting colors and disparities associated with correspondingly located pixels of the frame not found in these depth discontinuity areas. The boundary layer is made up of pixels exhibiting foreground colors, foreground disparities and alpha values associated with the correspondingly located pixels of the depth discontinuity areas in the frame. The depth discontinuity areas correspond to prescribed sized areas surrounding depth discontinuities found in the frame. 
   2.0 Interactive Viewpoint Video Rendering 
   A key advantage of the present interactive viewpoint video rendering system and process is that view-interpolated frames of the highest possible quality are provided to enhance the viewing experience, even though relatively few camera views are employed in generating the video data. This is not easily achieved. One approach, as suggested in the Light Field Rendering paper [10], is to simply resample rays based only on the relative positions of the input and virtual cameras. As demonstrated in the Lumigraph [6] and subsequent work, however, using a 3D impostor or proxy for the scene geometry can greatly improve the quality of the interpolated views. Another approach is to create a single texture-mapped 3D model [9], but this generally produces inferior results to using multiple reference views. 
   The present system and process employs the geometry-assisted image-based rendering approach, which requires a 3D proxy. One possibility is to use a single global polyhedral model, as in the Lumigraph and Unstructured Lumigraph reference [1]. Another possibility is to use per-pixel depth, as in Layered Depth Images [14], offset depth maps in Facade [4], or sprites with depth [14]. In general, however, using different local geometric proxies for each reference view [11, 5, 8] produces higher quality results, so that approach is adopted. 
   The following sections will present details of the present rendering system and process. A computing environment suitable for implementing the present invention is presented first. This is followed by a more detailed description of the rendering system and process. 
   2.1 The Computing Environment 
   Before providing a description of the preferred embodiments of the present invention, a brief, general description of a suitable computing environment in which portions of the invention may be implemented will be described.  FIG. 1  illustrates an example of a suitable computing system environment  100 . The computing system environment  100  is only one example of a suitable computing environment and is not intended to suggest any limitation as to the scope of use or functionality of the invention. Neither should the computing environment  100  be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary operating environment  100 . 
   The invention is operational with numerous other general purpose or special purpose computing system environments or configurations. Examples of well known computing systems, environments, and/or configurations that may be suitable for use with the invention include, but are not limited to, personal computers, server computers, hand-held or laptop devices, multiprocessor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments that include any of the above systems or devices, and the like. 
   The invention may be described in the general context of computer-executable instructions, such as program modules, being executed by a computer. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote computer storage media including memory storage devices. 
   With reference to  FIG. 1 , an exemplary system for implementing the invention includes a general purpose computing device in the form of a computer  110 . Components of computer  110  may include, but are not limited to, a processing unit  120 , a system memory  130 , and a system bus  121  that couples various system components including the system memory to the processing unit  120 . The system bus  121  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus also known as Mezzanine bus. 
   Computer  110  typically includes a variety of computer readable media. Computer readable media can be any available media that can be accessed by computer  110  and includes both volatile and nonvolatile media, removable and non-removable media. By way of example, and not limitation, computer readable media may comprise computer storage media and communication media. Computer storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by computer  110 . Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of the any of the above should also be included within the scope of computer readable media. 
   The system memory  130  includes computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM)  131  and random access memory (RAM)  132 . A basic input/output system  133  (BIOS), containing the basic routines that help to transfer information between elements within computer  110 , such as during start-up, is typically stored in ROM  131 . RAM  132  typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit  120 . By way of example, and not limitation,  FIG. 1  illustrates operating system  134 , application programs  135 , other program modules  136 , and program data  137 . 
   The computer  110  may also include other removable/non-removable, volatile/nonvolatile computer storage media. By way of example only,  FIG. 1  illustrates a hard disk drive  141  that reads from or writes to non-removable, nonvolatile magnetic media, a magnetic disk drive  151  that reads from or writes to a removable, nonvolatile magnetic disk  152 , and an optical disk drive  155  that reads from or writes to a removable, nonvolatile optical disk  156  such as a CD ROM or other optical media. Other removable/non-removable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include, but are not limited to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. The hard disk drive  141  is typically connected to the system bus  121  through a non-removable memory interface such as interface  140 , and magnetic disk drive  151  and optical disk drive  155  are typically connected to the system bus  121  by a removable memory interface, such as interface  150 . 
   The drives and their associated computer storage media discussed above and illustrated in  FIG. 1 , provide storage of computer readable instructions, data structures, program modules and other data for the computer  110 . In  FIG. 1 , for example, hard disk drive  141  is illustrated as storing operating system  144 , application programs  145 , other program modules  146 , and program data  147 . Note that these components can either be the same as or different from operating system  134 , application programs  135 , other program modules  136 , and program data  137 . Operating system  144 , application programs  145 , other program modules  146 , and program data  147  are given different numbers here to illustrate that, at a minimum, they are different copies. A user may enter commands and information into the computer  110  through input devices such as a keyboard  162  and pointing device  161 , commonly referred to as a mouse, trackball or touch pad. Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  120  through a user input interface  160  that is coupled to the system bus  121 , but may be connected by other interface and bus structures, such as a parallel port, game port or a universal serial bus (USB). A monitor  191  or other type of display device is also connected to the system bus  121  via an interface, such as a video interface  190 . In addition to the monitor, computers may also include other peripheral output devices such as speakers  197  and printer  196 , which may be connected through an output peripheral interface  195 . A camera  192  (such as a digital/electronic still or video camera, or film/photographic scanner) capable of capturing a sequence of images  193  can also be included as an input device to the personal computer  110 . Further, while just one camera is depicted, multiple cameras could be included as input devices to the personal computer  110 . The images  193  from the one or more cameras are input into the computer  110  via an appropriate camera interface  194 . This interface  194  is connected to the system bus  121 , thereby allowing the images to be routed to and stored in the RAM  132 , or one of the other data storage devices associated with the computer  110 . However, it is noted that image data can be input into the computer  110  from any of the aforementioned computer-readable media as well, without requiring the use of the camera  192 . 
   The computer  110  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  180 . The remote computer  180  may be a personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer  110 , although only a memory storage device  181  has been illustrated in  FIG. 1 . The logical connections depicted in  FIG. 1  include a local area network (LAN)  171  and a wide area network (WAN)  173 , but may also include other networks. Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet. 
   When used in a LAN networking environment, the computer  110  is connected to the LAN  171  through a network interface or adapter  170 . When used in a WAN networking environment, the computer  110  typically includes a modem  172  or other means for establishing communications over the WAN  173 , such as the Internet. The modem  172 , which may be internal or external, may be connected to the system bus  121  via the user input interface  160 , or other appropriate mechanism. In a networked environment, program modules depicted relative to the computer  110 , or portions thereof, may be stored in the remote memory storage device. By way of example, and not limitation,  FIG. 1  illustrates remote application programs  185  as residing on memory device  181 . It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used. 
   The exemplary operating environment having now been discussed, the remaining parts of this description section will be devoted to a description of the interactive viewpoint video rendering system program architecture and process. 
   2.2 Interactive Viewpoint Video Rendering Program Architecture 
   Referring to  FIG. 2 , an interactive viewpoint video file  204  is first fed into a selective decoding module  200 . The video file contains encoded versions of the aforementioned two-layer video frame representations as well as calibration data. The calibration data includes the location and viewpoint information for each of the video cameras used to capture the scene associated with the video being rendered. Given this information the locations of the camera viewpoints can be computed. Generally, the selective decoding module  200  decodes just those portions of the incoming file that are needed to render a current frame of the video from a vantage point selected by a user viewing the video. More particularly, the module  200  decodes the portions of the file identified by the rendering module  202  (which will be described shortly) to recover the layered video frame data associated with the particular video frames needed to render the scene from the desired viewpoint. In this way, the minimum amount of data possible must be decoded, thereby speeding up the process and providing real-time rendering capability. 
   The decoded layered frame data is provided to the rendering module  202 . Generally, this module  202  takes the frame data and renders a view of the scene for the current portion of the video being viewed by the user from a vantage point specified by the user. This involves first obtaining the current user input  206  and then generating the desired view. 
   2.2.1 Selective Decoding Module 
   The purpose of the selective decoding module is to decode only the information needed to render the scene captured in the video from the current user-selected vantage point. In essence this entails decoding the frame or frames from the group of contemporaneously capture frames associated with the current part of the video being played that must be decoded in order to obtain the layered video frame data needed to render a view of the scene depicted in the current portion of the video from a particular user selected vantage point. Should this vantage point coincide with the view of the scene captured by one of the cameras, then only the data associated with that frame need be decoded. However, if the desired viewpoint falls somewhere between two of the camera views, then the frame data associated with both of these adjacent camera views must be decoded in order to render the scene from the desired viewpoint. 
   The particular frame or frames needed to render the scene from the desired vantage point is identified by the rendering module (which will be described next). Once identified, the layered video frame data associated with the identified frame or frames is decoded using the appropriate decoding technique applicable to the type of compression (if any) and encoding schemes employed in creating the interactive viewpoint video file. 
   In addition to the decoding of frame data from the interactive viewpoint video file, the decoding module also decodes the aforementioned camera calibration data. This data could be found in the file header or as metadata. 
   2.2.2 Rendering Module 
   It is the job of the rendering module to first process user input concerning the viewpoint that is desired for the scene to be rendered, and to identify the frame or frames from the group of contemporaneously capture frames associated with the current temporal portion of the video that are needed to render the desired view. In order to accomplish this task, the rendering module is initialized with the aforementioned camera calibration data contained in the interactive viewpoint video file. This calibration data includes the location and viewpoint information for each of the video cameras used to capture the scene associated with the video being viewed. Given this information the rendering module computes the locations of the camera viewpoints. The user is able to specify any viewpoint along the path connecting the camera viewpoints, with the two outside cameras representing the endpoints of the possible viewpoint selections. As discussed above, the selected viewpoint can coincide with the view of the scene captured by one of the cameras. In such a case only the “current” frame associated with that camera is identified as being needed to render the desired view. However, the usual case will be that the viewpoint falls between the viewpoints of two adjacent cameras. In this latter case, the rendering module identifies current frames associated with both of these adjacent cameras. 
   As for the user input, this information can be obtained in any appropriate conventional manner, such as via a user interface of some type used to input and process user viewpoint selections. For example, this interface can include a graphics user interface (GUI) that is presented to the user on a display device (e.g., computer monitor, display screen, 3D goggles, among others). This GUI would include some graphic arrangement that allows the user to indicate the viewpoint, among the possible viewpoints, from which he or she wants to view the scene captured in the video for the current portion of the video being rendered. The user can change the desired viewpoint as the video plays as well. These selections would be made by the user interfacing with the GUI using any standard input device (e.g., mouse, joystick, eye tracking device, among others). 
   Once the frame or frames needed to render the desired view have been identified, the rendering module directs the selective decoding module to decode the needed frame data. The frame data output of the selective decoding module consists of two layers with a total of five data planes for each frame provided: the main layer color, main layer disparity, boundary layer alpha matte, boundary layer color, and boundary layer disparity. In the case where the desired viewpoint coincides with one of the camera viewpoints, only the main layer and boundary layer data planes of that camera are used to reconstruct the scene. However, in the case where the desired viewpoint falls between two of the camera viewpoints, the main layer and boundary layer data planes from both views are needed to reconstruct the scene. The details on how the desired view can be rendered will be described shortly. Once the desired view is rendered it is displayed to the user in the manner appropriate to the display device involved. 
     FIG. 3(   a )–( c ) shows an example of the results of the rendering operation.  FIGS. 3(   a ) and ( c ) represent contemporaneously captured frames from two adjacent video cameras located at different viewpoints.  FIG. 3(   b ) is an example of the frame that is rendered when the user-specified viewpoint is between the viewpoints associated with the images of  FIGS. 3(   a ) and ( c ). 
   It is also possible for the rendering module to introduce synthesized elements into the rendered scenes. Thus, in one embodiment of the rendering module (as shown in  FIG. 2 ), 3D object data  208  is input into the rendering module for incorporation into the frame being currently rendered. In one embodiment, this input would include the data necessary to render an animated object or objects from a viewpoint corresponding to the selected viewpoint and locating information for incorporating the object(s) in a pre-established position within the frame being rendered. The object(s) could change shape over time (i.e., so as to have a different appearance in different ones of the rendered frames) or have a static appearance. Further, the position in the rendered frame where the object(s) are incorporated can change with time (i.e., so as to have a different location in different ones of the rendered frames) or can be located in the same place in each successive rendered frame. Image-based objects can also be inserted into the scene during the rendering process. For example,  FIG. 4  shows a frame from an interactive viewpoint video created in accordance with the present invention, into which an extra copy of a break-dancer has been inserted. This effect was achieved by first “pulling” a matte of the dancer using a depth threshold and then inserting the pulled sprite into the original video using a standard z-buffering technique. 
   It is noted that the foregoing rendering architecture can be implemented using graphics processing units (GPU), software rendering techniques, or both. However, if processing solely using the central processing unit (CPU) imposes limitations that would preclude real-time interactive rendering, then the use of a GPU, or a combination of GPU and CPU, is desirable in order to achieve this goal. This is because recent advances in the programmability of GPUs has made it possible to render directly from the output of a decoder without going back to a CPU for any additional processing, thereby increasing the processing speed significantly and making real-time rendering more feasible. 
   2.2.2 Interactive Viewpoint Video Rendering Process 
   In general, the foregoing rendering program architecture can be employed to perform the following process to render the interactive viewpoint video as shown in  FIG. 5 . Generally, for each frame of the video rendered, the current user-specified viewpoint is first input (process action  500 ). However, it is noted that instead of inputting the viewpoint each time a new frame of the video is rendered, only changes in the specified viewpoint could be input. In this case, unless a change in viewpoint has been received, it would be assumed the last-specified viewpoint is still valid and would be used in rendering the current frame of the video. 
   Once the user-specified viewpoint is established, the next process action  502  is to identify the frame or frames from the group of contemporaneously captured input frames associated with the current frame of the video being rendered, which are needed to generate the desired view. The identified frame or frames are then decoded (process action  504 ). 
   Next, the current frame of the interactive viewpoint video is produced using the decoded video data (process action  506 ). This frame will depict the scene associated with the current temporal portion of the video as seen from the viewpoint currently specified by the user. This may require synthesizing the frame if the desired viewpoint falls between the viewpoints of two of the adjacent cameras used to capture the scene. It is noted that the foregoing process can optionally be modified to also insert computer-generated or image-based objects into the scene during the rendering process as described previously, although this action is not shown in  FIG. 5 . 
   In cases where the user-specified viewpoint coincides with one of the camera views, the process used to produce the current frame of the interactive viewpoint video is straightforward and conventional. Essentially, the layers of the coinciding camera view are converted to textured meshes as will be described shortly in connection with rendering views that are between the viewpoints of two of the cameras. The boundary layer mesh is then composited over the main layer mesh based on the alpha values of the boundary layer. The result is the finalized frame of the video, which is ready for display to the user. 
   In cases where the user-specified viewpoint falls between two camera views, the rendering process is more complex. In generally, it involves first projecting the various layers of the frames needed to render the user-specified view to the associated virtual viewpoint. This can be accomplished using conventional methods and the camera calibration data provided in the interactive viewpoint video file. Once projected, the layers are combined to produce the final frame. There are various ways that this can be accomplished as will now be described. 
   In one embodiment of the present rendering process where data from two camera viewpoints is required to render a view of the scene from the user specified viewpoint, the main and boundary layers from the same camera view are projected and composited, and then the composited layers associated with each of the camera views are blended. More particularly, referring to the process flow diagram of  FIGS. 6A–B , one of the camera views involved is selected (process action  600 ). The selected view&#39;s main layer disparity values are then converted to depth values using conventional methods to produce a depth map of the main layer of the frame under consideration (process action  602 ). It is noted that if depth values are already provided in the layer data rather than disparity values, this first step is skipped. The depth map is then used to generate a 3D mesh (process action  604 ). To accomplish this task, a vertex shader can be employed. A vertex shader is a programmable unit on current generation GPUs and a simple vertex shader program can be used to generate a 3D mesh from a depth map. However, the conversion from depth map to 3D mesh could also be performed by a more general purpose processor. In essence, the shader takes two input streams, i.e., the X-Y positions and the depth values from the depth map, and produces the mesh. The color of the pixels from the main layer is next applied as a texture map to this mesh (process action  606 ), again using standard methods. This layer mesh is projected to the virtual view which generates projected color and depth maps (process action  608 ). To reduce the amount of memory required, the X-Y positions can optionally be stored for only a 256×192 block. The shader is then repeatedly called with different offsets to generate the required 3D mesh and texture coordinates. 
   Once the mesh is rendered, those triangles crossing depth discontinuities in the frame are erased, Thus, in process action  610 , the depth discontinuities are identified by finding regions in the depth map that have a large depth change (e.g., 4 disparity levels). The erasing procedure is then accomplished by projecting transparent (alpha=0) triangles that cross the identified discontinuities and compositing this with the main layer result (process action  612 ). This effectively erases those regions. The erase action is required because current GPUs are not capable of terminating triangles already in the pipeline. It would be advantageous to design future generations of GPUs to (in addition to processing triangle vertices, which they currently do) also process the triangles themselves. This additional functionality would allow for a more efficient implementation of this system. 
   Next, the boundary layer is rendered. The boundary layer data is fairly sparse, typically containing about 1/64th the amount of data as the background. Accordingly, the boundary layer only needs to be rendered where it is non-zero, resulting in a mesh that only has vertices at pixel locations with non-zero alpha values. Thus, the first process action  614  in rendering the boundary layer is to convert the selected view&#39;s boundary layer disparity values to depth values to produce a depth map of the boundary layer. Again, it is noted that if depth values are already provided in the layer data rather than disparity values, this step is skipped. The depth map is next converted to a 3D mesh (process action  616 ). The color of the pixels from the boundary layer is also stored in each vertex, resulting in the 3D position and color of each pixel being stored with the associated mesh vertex. The resulting boundary layer mesh is then projected to the user-specified viewpoint and the resulting color and depth maps are stored (process action  618 ). The depth values will be needed in the final blending stage as described shortly. The process of generating a 3D mesh from the boundary depths is performed on the CPU. If future generations of GPUs allowed for the creation of triangles, this process could be offloaded to the GPU, resulting in a performance gain. 
   It is noted that the boundary layer and main layer meshes share vertices in the region near the depth discontinuities. This avoids cracks and aliasing artifacts. In addition, to reduce processing costs, the same CPU pass used to generate the erase mesh of the main layer could be used to generate a boundary layer mesh. 
   Once both the main layer and boundary layers associated with a camera view under consideration have been rendered as described above and projected to the user-specified viewpoint, the boundary layer mesh is composited over the main layer mesh (process action  620 ). More particularly, for each pixel in the first rendered frame an alpha-blended color C 0  is computed. The color C 0  is computed for the corresponding pixel of the main and boundary layers associated with the frame using the equation C 0 =C boundary0 α color0 +C main0 (1−α color0 ), where α color0 =1+α main0 (α boundary0 −1) if the corresponding depth of the boundary layer at that pixel is less than or equal to the depth of the main layer and α color0 =0 otherwise. The value C boundary     0    is the color of the boundary layer pixel of the first frame, α boundary0  is the alpha value of the boundary layer pixel of the first frame and C main     0    is the color of the main layer pixel of the first frame. It is noted that in these calculations the alpha value of the projected boundary layer pixels falling outside the depth discontinuity areas is assumed to be zero, the alpha value of the projected main layer pixels outside the depth discontinuity areas is assumed to be one, and the alpha value of the projected main layer pixels inside the depth discontinuity areas is assumed to be zero. It is then determined if both the camera views involved have been processed (process action  622 ). If not, process actions  600  through  622  are repeated as indicated in  FIG. 6 , except that in this case the main and boundary layers of the second frame are composited to create an alpha-blended color C 1  at each pixel using the equation C 1 =C boundary1 α color1 +C main1 (1−α color1 ), where α color1 =1+α main1 (α boundary1 −1) if the corresponding depth of the boundary layer at that pixel is less than or equal to the depth of the main layer and α color1 =0 otherwise. The value C boundary     1    is the color of the boundary layer pixel of the second frame, α boundary1  is the alpha value of the boundary layer pixel of the second frame and C main     1    is the color of the main layer pixel of the second frame. If, however, both views have been selected and processed, the rendering process continues as described below. 
   At this point, there is a composited rendering associated with each of the camera viewpoints under consideration. These rendered images are now blended to produce an interactive viewpoint video frame. More particularly, weights (w 0  and w 1 ) are first computed for each camera view (process action  624 ). The weights are in proportion to how close the associated camera&#39;s viewpoint is from the user-specified viewpoint, i.e., view-dependent weighting [4]. In other words, if the desired viewpoint is closer to a first one of the camera&#39;s viewpoints than the other, the composited image associated with the first of these cameras is weighted more heavily than the other, i.e., w 1 =1−w 0 . The computed weights are next provided to a blending shader, or the equivalent. The blending shader blends each composited layer pair using a weighted averaging of the view-dependent weights (process action  626 ). More particularly, the blending is based on their assigned weights using the equation: 
             C   f     =       (         w   0     ⁢     α   0     ⁢     C   0       +       (     1   -     w   0       )     ⁢     α   1     ⁢     C   1         )       (         w   0     ⁢     α   0       +       (     1   -     w   0       )     ⁢     α   1         )             
where C f  is the final color. The alpha values α 0  and α 1  are computed as α 0 =α boundary0 +α main0 (1−α boundary0 ) and α 1 =α boundary1 +α main1 (1−α boundary1 ). In order to maintain color resolution when computing C 0  and C 1 , their values are not multiplied by α 0  and α 1  until computing the final color C f . The result is the final frame of the interactive viewpoint video rendered from the desired viewpoint and ready for display to the user.
 
   In a variation of the foregoing embodiment of the rendering process, a customized blending procedure is used. In this blending procedure, for each pair of pixels to be blended, the shader performs a soft Z compare in order to compensate for noise in the depth estimates and reprojection errors. In essence, pixels that are sufficiently close together are blended using the view-dependent weights; for pixels that differ too much in depth, the front-most pixel is used. More particularly, referring to the process flow diagram of  FIG. 7 , this alternate blending procedure is outlined as it would be performed for each pair of correspondingly located pixels in the two composited images being blended. First, the pair of pixels under consideration are compared to determine their relative depths (process action  700 ). It is next determined if one of the pixels in the pair exhibits a disparity value that is less than the other by more than a prescribed tolerance (process action  702 ). In tested embodiments of the present invention, the prescribed tolerance was set to 4 disparity levels. If one of the pixels does have a disparity value that is less than the others by more than the prescribed tolerance, then that pixel&#39;s color value is assigned to the pixel location under consideration in the frame of the video currently being rendered (process action  704 ). Whereas, if the two pixels exhibit disparity values that differ by an amount that is equal to or less than the prescribed tolerance, then the color value assigned to the pixel location under consideration in the frame of the video currently being rendered is blended from the two pixels&#39; color based on their view-dependent weights as described previously (process action  706 ). More particularly, the two pixels are blended using a weighted average C f =(w 0 α 0 C 0 +(1−w 0 )α 1 C 1 )/(w 0 α 0 +(1−w 0 )α 1 ). The normalization performed here is important since some pixels might only be visible or partially visible in one camera&#39;s view. 
   It is noted that in the foregoing soft Z compare blending procedure the pixel disparity values are still needed in order to perform the blending operation. However, a conventional shader program may not have access to a z-buffer of a GPU. Thus, if a GPU is employed in the rendering process, rather than storing the depth map in the GPU&#39;s z-buffer, the disparity values associated with the composited version of each view can be stored in a separate buffer to facilitate access by the shader program. It would be advantageous if future generations of GPUs allowed shader access to the hardware depth values. If this was possible, the current invention could be implemented more efficiently. 
     FIGS. 8(   a )–( d ) shows examples of the intermediate and final stages of the foregoing rendering process.  FIG. 8(   a ) represents the rendered main layer of one camera view with the areas associated with depth discontinuities erased.  FIG. 8(   b ) represents the composited main and boundary layers of this first view, whereas  FIG. 8(   c ) represents the composited layers of the second view. Finally,  FIG. 8(   d ) represents the final blended frame of the interactive viewpoint video. It can be seen from these images how the depth discontinuities are correctly erased, how the soft alpha-matted foreground elements are rendered, and how the final view-dependent blend produces high-quality results. 
   3.0 Potential Applications 
   The present interactive viewpoint video rendering system and process can be employed in a variety of interesting applications. At its basic level, a user is able to play a video and continuously change their viewpoint as they are watching. Thus, interactive viewpoint video allows users to experience video as an interactive 3D medium. This has a high potential for changing the way dynamic events are watched and enhancing realism of games. Examples of dynamic events of interest are sports events (baseball, basketball, skateboarding, tennis, etc.), instructional videos (how-to videos for golf, martial arts, etc.), and performances (Cirque de Soleil, ballet, modern dance, etc.). Further, if sufficient bandwidth is available, the video could be broadcast or multicast, thus providing a viewing experience that could be described as 3D television. 
   However, the present invention is not just limited to changing viewpoints while watching the video. It can also be used to produce a variety of special effects such as space-time manipulation. For example a user can freeze the video and view the depicted scene from a variety of viewpoints. A user can also play the video while viewing the depicted scene from one or more viewpoints, and then reverse the video and watch the scene from different viewpoints. Still further the video can be played forward or backward at any speed, while changing viewpoints as desired. 
   The foregoing features of interactive viewpoint are not only interesting to a casual viewer, but would be particularly useful to the television and film industry. Instead of the painstaking process of determining what part of a scene is to be captured and from what viewpoint ahead of time, with the possibility that most desirable shot would be missed, the system and process of the present invention can be used. For example, a scene would first be captured as an interactive viewpoint video. Then, the filmmaker can view the video and for each shot (even down to a frame-by-frame basis) chose the viewpoint that is desired for the final film. Further, the previously described object insertion feature is also a tool that could be advantageous to the filmmaker. Thus, the techniques presented bring us one step closer to making image-based (and video-based) rendering an integral component of future media authoring and delivery. 
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