MODEL-BASED VIEW EXTRAPOLATION FOR INTERACTIVE VIRTUAL REALITY SYSTEMS

Abstract: A method by which a client and a server cooperate to render views of a virtual reality world within which a user of the client navigates. The server sends the client a reference view and a model to use to extrapolate the reference view in response to the user's virtual movement. The client sends the server a record of the user's virtual movement. Both the client and the server perform the model-based extrapolation of the reference view. The server also performs an exact update of the reference view. When the extrapolated reference view deviates too much from the updated reference view, the server sends the client the difference between the extrapolated and updated reference views, so that the client can update its reference view.

Description: MODEL-BASED VIEW EXTRAPOLATION FOR INTERACTIVE VIRTUAL REALITY SYSTEMS 5 FTF.T.O AND BACKGROUND OF THE INVENTION The present invention relates to interactive networks and, more particularly, to a network in which a server interactively provides views of a virtual reality world to a client. Unlike text-based media, video must be transmitted in a predictable, I o synchronized manner, and requires a guaranteed quality of service, with guaranteed bandwidth and guaranteed bounds on other properties such as latency and jitter. Protocols that support guaranteed quality-of-service media connections soon will be provided by ATM-based networks, or by other technologies such as FDDI and Fast Ethernet. Such protocols establish a virtual connection between a sender (a 15 multimedia server) and a receiver (a client) provided that sufficient resources can be reserved along the path to support the minimum level of quality of service required by the connection. Photo-realistic virtual reality applications are similar to video-based real-time applications, but provide full interaction. In many virtual reality systems, the user 0 must have a real perception of the environment that is being explored or discovered, and a smooth interaction with the environment. In an interactive web-system scenario, the client carries the virtual camera and navigates through the virtual environment. The server constantly receives details regarding the client camera position and orientation, as well as its activities which may modify the virtual 25 environment. All the information concerning the entire setting is held at the server. According to the client movement, the server updates the client with essential data which enables the generation of new views. Time lag and low quality images are the main reasons for a decrease in the sense of reality. High fidelity and photo-realism are achieved by using a fully textured (photo-mapped) environment. Today we are witnessing a rapidly increasing presence of 3D virtual worlds on the world wide web, described using a virtual reality modeling language (VRML). However, the interaction with remote virtual environments on the web is still extremely limited. The common approach is to first download the entire VRML 3D world to the client. Then the client renders the scene locally. This approach is successful as long as the environment is not too complex; otherwise it causes a critical penalty in the downloading time. This prevents the use of photo-textures, which are necessary for a photo-realistic impression. It should be emphasized that the downloading time is required for every change of session, for example, if the user moves to an upper floor in a shopping application or to another planet in a video game. To avoid the above drawbacks, an alternative approach has been suggested in which the server computes the new views and sends them compressed to the client. Although each image is compressed (e.g., JPEG), the volume of transmission is still quite large and would either require an expensive bandwidth or lower the quality of the images. Video compression techniques such as MPEG, which exploit temporal data redundancy, are based on inter-frame dependencies and may be compressed on¬ line, but with a time lag which prohibits real-time feedback. There is thus a widely recognized need for, and it would be highly advantageous to have, a method for providing views of a remote complex virtual reality world, at the client of an interactive server-client system, fast enough to preserve the illusion of virtual reality. RELATED WORK In visual navigation applications there is always a need to balance the imaging quality and the frame rate. In interactive real-time systems, one is required to maintain a user-specified minimal frame rate. T. A. Funkhouser and C. H. Sequin (Adaptive display algorithm for interactive frame rates during visualization of complex virtual environments, Computer Graphics (SIGGRAPH '93 Proceedings), pp. 247-254, August 1993) proposed an algorithm that adjusts the image quality adaptively by choosing the level-of-detail and rendering algorithm according to its estimated rendering cost. P. W. C. Maciel and P. Shirley (Visual navigation of large environments using textured clusters, 1995 Symposium on Interactive 3D Graphics, pp. 95-102, April 1995) suggested the use of an imposture to trade speed for quality. An imposture must be faster to draw than the true model while visually resembling the real image. Textures mapped on simplified models are a common form of imposture. J. Shade, D. Lischinski, D. H. Salesin. J. Snyder and T. Derose (Hierarchical image caching for accelerated walkthroughs of complex environments, Computer Graphics (SIGGRAPH '96 Proceedings)), G. Schauffler and W. Sturzlinger (A three dimensional image cache for virtual reality, Eurographics ' 96, Computer Graphics- Forum Vol. 15 No. 3 pp. 227-235, 1996) and D. G. Aliaga (Visualization of complex models using dynamic texture-based simplification. Proceedings of Visualization 96) all used a single texture polygon. These image-based primitives are view-dependent and form a compact representation: thus they have the potential to be more appropriate in applications which also need to sustain a user-specified communication bandwidth. S. Eric Chen and L. Williams (View interpolation for image synthesis, Computer Graphics (SIGGRAPH '93 Proceedings), pp. 279-288, August 1993) and T. Kaneko and S. Okamoto (View interpolation with range data for navigation applications, Computer Graphics International, pp. 90-95, June 1996) generated novel images from a number of precalculated reference images by "view interpolation " '. Along with the images, corresponding maps are necessary so that one image can be morphed into another. The user can stroll through restricted paths connecting successive locations at which the precomputed views are stored, providing the sensation of continuous in-between views. The advantage of view interpolation and any other image-based rendering technique is that the generation of a new image is independent of the scene complexity. The technique gives more freedom than strolling back and forth within a video sequence. However, it works well only if adjacent images depict the same object from different viewpoints. The interpolated views may introduce some distortions because linear interpolation does not ensure natural or physically valid in- between images. Recently, S. M. Seitz and C. R. Dyer (View morphing, Computer Graphics (SIGGRAPH '96 Proceedings)) proposed a new method, called "view morphing", which better preserves the in-between shape appearance. Image-based methods usually do not consider the underlying 3D model, and some inherent problems, known as holes and overlaps, need to be alleviated. In the paper by Kaneko and Okamoto cited above, a full range of data, acquired from a range scanner, is associated with each reference image. The exact range simplifies the generation of the in-between images. No correspondence is required, and overlaps are easily resolved by a Z-buffer approach. P. E. Debevec, C. J. Taylor and J. Malik (Modeling and rendering architecture from photographs: a hybrid geometry- and image-based approach, Computer Graphics (SIGGRAPH '96 Proceedings)) use a set of viewpoints to approximate the 3D model, and new views are then rendered from arbitrary viewpoints by a view-dependent texture-mapping technique. SI IMMARY OF THE INVENTION According to the present invention there is provided, in a system in which a server and a client cooperate to render a plurality of views of a virtual world, each view corresponding to a viewpoint, a method for generating the views in real time, comprising the steps of: (a) transmitting a first reference view to the client; (b) transmitting at least a portion of a model to the client; (c) extrapolating said first reference view, based on said at least portion of said model, thereby providing an extrapolated view; (d) transmitting at least one correction dataset to the client; and (e) correcting said extrapolated view based on said at least one correction dataset. thereby providing at least one second reference view. According to the present invention there is provided, In a system in which a server and a client cooperate to render a plurality of views of a virtual world, a method for updating the views in real time, comprising the steps of: (a) transmitting a first reference view to the client; (b) extrapolating said first reference view, thereby providing an extrapolated view; (c) transmitting at least one correction dataset to the client: and (d) correcting said extrapolated view based on said at least one correction dataset. thereby providing at least one second reference view; wherein said extrapolating is effected at least twice before said correcting. The present invention is based on a new paradigm for interaction with complex virtual worlds, on a network such as the world wide web, which uses both the client and the server simultaneously. The client generates (extrapolates) the new views based on the locally available data, while the server transmits only the data necessary to prevent an accumulation of errors. The "polygon-assisted" compression introduced by Marc Levoy (Polygon-assisted JPEG and MPEG compression of synthetic images. Computer Graphics (SIGGRAPH '95 Proceedings), pp. 21 -28, August 1995) was previously suggested to partition the rendering task between the server and the client. The client renders a low-quality image and receives from the server the compressed difference between the high-quality image and the low-quality image. This requires the transmission of the difference image for every frame, in contrast to the present invention, wherein the client can generate several frames autonomously. The present invention is based on the principle of view interpolation discussed above. It enables the user to have a smooth "exploration" of the virtual environment in which (s)he is traveling. However, the present invention does not interpolate between precomputed views, but "extrapolates" the last reconstructed reference view towards the new view. RRIF.F DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 is a flow diagram of the model-based extrapolation scheme: FIG. 2 is a flow diagram of novel view generation. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a method for rendering views of a virtual reality world, at the client of any interactive client-server network, from the full world wide web to even a simple communication line, fast enough to preserve the illusion of virtual reality. Consider an interactive web-system in which the user roams in a remote virtual environment. According to the present invention, the client extrapolates the new views based on the locally available data, which include previous images, camera position and range data. Because the client cannot extrapolate the exact new view, the server needs to transmit to the client a correction dataset, for example a difference image, representing the difference between the client approximated view and the exact new view. The correction dataset is better compressed and reduces the network transmission volume. Moreover, the server does not need to correct the client- extrapolated view in every frame, but at lower frequencies than the client frame rate, to further reduce the network requirements. A novel view R + i is an extrapolation of a reference view R The server needs to transmit correction datasets to guarantee that the reference view is sufficiently close to the current frame to improve the quality of the extrapolated views. Because the transmitted data do not necessarily reconstruct the current view, no latency occurs. The extrapolation of the novel view is based on a model-based backprojection technique. Maneesh Agrawala. Andrew Beers and Navin Chaddha (Model-based motion estimation for synthetic animations, Proc. ACM Multimedia '95) and D. S. Wallach, S. Kunapalli and M. F. Cohen (Accelerated MPEG compression of dynamic polygonal scenes, Computer Graphics (SIGGRAPH '94 Proceedings), pp. 193-197. July 1994) used model-based techniques for synthetic animations to guide the motion estimation for block-based video compression algoritlims. These techniques show that a model-based approach significantly improves the exploitation of inter-frame coherence. In the view extrapolation scheme, the motion compensation is computed by the client and need not be transmitted. Only the difference values need to be transmitted. In terms of network requirements, this guarantees a higher bitrate compression (lower bandwidth requirement) or higher image quality. The virtual environment consists of textured models stored at the server. The model of the relevant parts is transmitted to the client according to its relative position within the viewer. The transmitted model includes only its geometry and no textures. (It should be emphasized that the texture-space can be significantly larger than the geometry-space.) The transmitted model may include all or only part of the geometry' of the true model, or may be an approximation of the geometry of all or part of the true model. The 3D model does not need to be constantly transmitted over the network, but can be transmitted incrementally, and is dynamically transformed by the client. The server only transmits the model data when new models enter the viewing frustrum or when a new level of detail of an existing model is required. Referring now to the drawings, Figure 1 is a block diagram of the model-based extrapolation scheme of the present invention. The scheme is initialized by the transmission from the server to the client of both the portion M of the model that is needed to reconstruct the view of the virtual reality world from the client's initial viewpoint, and an exact reference view R from that viewpoint. The user navigates through the virtual reality world by entering coordinates of new viewpoints. Both the client and the server transform model M to the new viewpoints. The transformed model M is combined with reference view R, as described below, to provide an extrapolated view W. This also is done by both the client and the server. Meanwhile, the server computes exact views V that correspond to the new viewpoints, using the full model as well as texture T. As needed, the server computes correction datasets and transmits them to the client, which used the correction datasets to correct reference view R. According to the embodiment of the present invention depicted in Figure 1 , the correction dataset is the difference D between an exact view V and a corresponding extrapolated view W: D = V - W. D is transmitted to the client and composed with W (in this embodiment by adding D to W) to give an updated reference view R. Again, this composition is done by both the client and the server, so that the server always has available to it the state of the client. Optionally, as shown in Figure 1 , the server compresses D to a compressed difference image D' before transmission to the client. If a lossy compression such as JPEG is used, then the new reference view R is only an approximation of V. If a lossless compression is used, then the new R is identical to V. Also as needed, additional portions of the model are transmitted from the server to the client, so that the client always has as much of the model as it needs to extrapolate to new viewpoints. The extrapolation scheme of the present invention is in a sense similar to MPEG compression technology. An MPEG video stream consists of intra frames (I), predictive frames (P) and interpolated frames (B). The I frames are coded independently of any other frames in the sequence, while the P and B are coded using motion estimation and interpolations. The P and B frames are substantially smaller than the I frames. The motion estimation of a P frame is based on preceding frames as well as successive frames. According to the present invention, successive frames are not available, and instead of the P and B frames, only extrapolated frames W are used. Figure 2 is a flow diagram of the generation of a novel view according to the present invention. This is done in three steps. The first step renders model M to create a Z-map. The second step generates extrapolated view W by backprojecting to reference view R. The third step corrects warped view W using the transmitted data. including the correction dataset. As noted above, the third step is performed only as needed, and not necessarily in every cycle. The embodiment of the invention described herein uses only one reference view for view extrapolation, and uses a difference image as an correction dataset. It will be appreciated that these are not inherent limitations of the invention. The scope of the invention includes other kinds of correction datasets, and extrapolation based on several reference views, as will be obvious to one ordinarily skilled in the art. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims: WHAT IS CLAIMED IS: 1. In a system in which a server and a client cooperate to render a plurality of views of a virtual world, each view corresponding to a viewpoint, a method for generating the views in real time, comprising the steps of: (a) transmitting a first reference view to the client; (b) transmitting at least a portion of a model to the client; (c) extrapolating said first reference view, based on said at least portion of said model, thereby providing an extrapolated view; (d) transmitting at least one correction dataset to the client; and (e) correcting said extrapolated view based on said at least one correction dataset, thereby providing at least one second reference view. 2. The method of claim 1, wherein said extrapolating is effected by the steps of: (i) transforming said model to the viewpoint of said extrapolated view; (ii) rendering said model; and (iii) backprojecting said model to said first reference view. 3. The method of claim 1 , further comprising the steps of: (t) rendering an exact view, by the server; and (g) subtracting said extrapolated view from said correct view, thereby providing a difference image to use as one of said at least one correction dataset. 4. The method of claim 3, further comprising the step of: (h) compressing said difference image. 5. The method of claim 1 , further comprising the step of: (f) replacing said first reference view with one of said at least one second reference view. 6. The method of claim 1 , wherein said correction dataset includes a difference image. 7. In a system in which a server and a client cooperate to render a plurality of views of a virtual world, a method for updating the views in real time, comprising the steps of: (a) transmitting a first reference view to the client: (b) extrapolating said first reference view, thereby providing an extrapolated view; (c) transmitting at least one correction dataset to the client; and (d) correcting said extrapolated view based on said at least one correction dataset, thereby providing at least one second reference view; wherein said extrapolating is effected at least twice before said correcting. 8. The method of claim 7, further comprising the steps of: (e) rendering an exact view, by the server; and (f) subtracting said extrapolated view from said correct view, thereby providing a difference image to use as one of said at least one correction dataset. 9. The method of claim 8, further comprising the step of: (g) compressing said difference image. 10. The method of claim 7, further comprising the step of: (e) replacing said first reference view with one of said at least one second reference view. 1 1. The method of claim 1 , wherein said correction dataset includes a difference image.

Inventors: COHEN-OR DANIEL, COHEN-OR, DANIEL
Applicants: COHEN OR DANIEL, UNIV RAMOT, COHEN-OR, DANIEL, RAMOT UNIVERSITY AUTHORITY FOR APPLIED RESEARCH &
Application year: 1997
Main classification: H04N
IPC codes: G06T15, G06T17, H04N7