Abstract:
A system for providing interactive views of 3-dimensional models with surface properties is disclosed. The system provides a compact representation of a 3D model and its surface features and provides for efficiently viewing and interacting with the model using dynamically switched texture maps. The compact representation is beneficial for transmission of the model across a network as well as for local storage of the model in the computer memory. The dynamically switched texture maps allow for more accurate surface details on the 3D model, as well as speedy interaction between a user and the 3D model.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to computer-generated graphics and interactive display systems in general, and, in particular, to the generation of three-dimensional imagery using an efficient and compact form. 
     2. Description of the Related Art 
     The use of 3D imagery, whether displayed on a screen or in virtual reality goggles, is becoming increasingly important to the next generation of technology. Besides CAD/CAM (Computer Aided Design/Computer Aided Modelling), 3D imagery is used in videogames, simulations, and architectural walkthroughs. In the future, it may be used to represent filing systems, computer flowcharts, and other complex material. In many of these uses, it is not only the image of a 3D model that is required, but also the ability to interact with the image of the 3D model. 
     The process of creating and displaying three-dimensional (3D) objects in an interactive computer environment is a complicated matter. In a non-interactive environment, 3D objects require memory storage for the shape, or geometry, of the object depicted, as well as details concerning the surface, such as color and texture. However, in an interactive environment, a user has the ability to view the 3D object from many angles. This means that the system has to be able to generate the images of geometry and surface details of the 3D object from any possible viewpoint. Moreover, the system needs to generate images from these new viewpoints quickly, in order to maintain the sensation that the user is actually interacting with the 3D object. 
     As an example, consider viewing a 3D model of a house on a computer system with a monitor and a joystick. Although you may be viewing it on the flat computer screen, you will see the shape of the house, the black tiles on the roof, the glass windows, the brick porch, etc. But if you indicate “walk to the side” by manipulating the joystick, the viewpoint on the screen shifts, and you begin to see details around the side corner of the house, such as the aluminum siding. The system needs to keep up with your manipulations of the joystick, your “virtual movement,” in order to provide a seamless, but continuously changing image of the 3D object, the house. So the system needs to redraw the same object from a different perspective, which requires a large amount of memory and a great deal of processing. 
     The problem of memory usage becomes even more acute when in a networked environment, where the image is being shown on a client system, while the original 3D object information is stored on a server. The client display system will have to receive the 3D object information over a communication link, thereby slowing down the process. In most cases, a compromise will have to be made between the interactive speed, how quickly the display will redraw based on user input, and the accuracy of the 3D object, how many details can be recalculated with each change in perspective. Although the following discussion will assume a networked environment, the problems addressed are also applicable to stand-alone computer systems, since an analogous situation exists between the hard drive (server) and the local memory (client) in a computer. 
     Before addressing the memory and processing problems, a brief discussion of 3D imaging is in order. As indicated above, the features of 3D objects can be separated into geometry, the shape and volume of the object, and surface details, such as texture, color, and shading. The first issue is how these two attributes, geometry and surface details, are stored and transmitted. The storage of geometry involves the description of the various edges and vertices of the object in 3D space. One way to speed the network time of transmitting and displaying a 3D object is to simplify the geometry, but this can make it hard to represent the original surface information on the reduced geometry, as the vast majority of simplification algorithms use only a geometric approximation and ignore the importance of surface details. A technique such as “texture mapping” [Blinn] maps an image of the rendered object onto the simplified geometry, but it is difficult to create and map images onto a surface of arbitrary topology. Furthermore, the corresponding texture coordinates would have to be sent along with the model, which would increase the transmission load. 
     Texture mapping is one of a class of techniques called “image-based rendering,” where an image of the surface details are “rendered” for each new perspective on the 3D object. Every time the user moves around the 3D object, a new surface image must be “rendered” from the new perspective. The image of surface details is taken and stored in various ways. One approach, which is used in texture mapping, is to break the surface of the 3D object into polygons, render every surface polygon as a small image, and then assemble all the images into a large montage that acts as a single texture image for the entire surface [Cignoni98]. Each polygon would then appear fully rendered and without any occlusion somewhere in the montage image, and the corresponding texture coordinates of the polygon corners would be well-defined. Occlusion occurs when some surface details are blocked from the user&#39;s perspective. Also in this approach is a method that packs the images into the montage efficiently to minimize wasted space in the texture image. 
     There are other approaches to taking a surface detail image. A single texture image of an object can be created by doing a “flyover” of the 3D object. This is equivalent to passing a camera over every surface of the object and taking pictures either continuously, creating one large seamless image, or discretely, creating multiple images corresponding to coverage of the entire 3D object. 
     Some image-based rendering techniques use these surface detail images directly as hardware texture maps [Cignoni98, Rademacher, Foran98, Erdahl97, Jackson96]; in other words, they use one or more images to completely describe the texture of the 3D object from any perspective the user may take. Others rely on data structures and algorithms that are implemented in software [Harashima98, Oliveira98, Oliveira99]; meaning that new perspectives are created or interpolated from image data by computer processing. Furthermore, some approaches are optimized for creating as realistic view as possible from a finite set of images, while others seek a compromise between accuracy and interaction speed. 
     There are many problems of memory usage, data transmission and excessive processing when employing the above techniques. For instance, the image-based rendering techniques that rely extensively on software algorithms to perform elaborate operations during every change of viewpoint are inapplicable to CAD/CAM and related applications that demand both high performance and high accuracy. This is even more of a problem in client-server applications in which the client may be much less powerful than the server machine. For the rest of this application, we will focus on texture mapping as the best technique for a client-server application. 
     When using texture mapping over a client-server/networked system, there is the need to send additional information such as texture coordinates, details, and color along with the geometry. To reduce bandwidth requirements it is important to send as little data as possible while still allowing the client to reconstruct the scene. However, this is a problem with complex rendering processes that don&#39;t reduce to simple 2D renderings of a 3D object with a given viewpoint. Complex rendering requirements will more tightly couple the process that creates the viewable geometry with the process that renders the images, which is inappropriate, for example, when the creation process is performed on the server and the rendering process is performed on the client. Thus, in a client-server system, multiple 2D renderings or multiple texture maps of the 3D object are preferable. 
     When using the polygon method of texture mapping, multiple images are used to represent the surface of an object at the same time. In this situation, any difference in rendering parameters between images may result in a discontinuity between regions that can stand out as a disturbing artifact. Surfaces that do not appear seamless may be hard to interpret because the eye is drawn to the abrupt changes in surface characteristics rather than the actual features on the surface. 
     If, in a client-server system, the server performs the original rendering, and transmits it to the client, the client is stuck with the conditions of the original rendering. For instance, if a server renders the 3D object with a light source, resulting in shading, the client is unable to change the shading on the renderings it receives from the server. The inability of the client to view the object under different lighting and shading conditions may limit the applicability of the technique, particularly when multiple objects rendered under different conditions are collected in a single scene that has a different type of lighting. 
     When it is important to view the facets of a model, and yet have the client display simplified geometry, there is a fundamental problem in showing individual surface details of the original geometry using only simplified geometry. This is particularly a problem when showing shading on the facets, since if the lighting is done on the client side there is no way to subdivide a polygon into the polygons in the original model. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide an improved system and method for representing a 3D model with surface features in a compact form. 
     Another object of this invention is to provide a system and a method for representing a 3D model with surface features in a compact form for rapid transmission over a network and/or for storage in computer memory. 
     Another object of this invention is to provide a system and a method for representing a 3D model with one surface texture at a time, thus eliminating discontinuities in surface features between regions of the 3D model. 
     Another object of the invention is to provide a system for interactively viewing a 3D model in an efficient manner using simplified geometry while displaying the appearance of the original model using the original surface features, including original connections and shading. 
     Another object of the invention is to enable a client to recreate a 3D model with a minimum of transmitted information across a network, particularly without the need to receive texture coordinates to map the images onto the geometry of the model. 
     To accomplish the above and other objects, a server renders a 3D model, described using geometry and surface features, from a number of viewpoints. These image renderings, or “camera views,” contain the original detailed facets of the original model on the server. The data concerning the “camera position” of each viewpoint is also saved with each “camera view.” The geometry of the original model is simplified to reduce the amount of information that needs to be sent across the network. 
     The simplified model, the rendered images, and the camera orientations are sent to a remote client that dynamically texture maps the best image based upon the current view direction. A pre-processing stage determines which portions of the model&#39;s geometry are occluded from a given view direction and colored accordingly, to indicate that the texture information for these portions are invalid. Because there are multiple texture maps for the same model, a given spot on the model is typically depicted in more than one texture map. As the object is rotated to different view directions, the texture map changes, giving the impression that the entire object has been texture mapped. 
     By separating the pure geometrical component of the model from all the data and surface features that contribute to its rendered appearance, the client need only be able to drape a texture onto a geometric mesh to recreate the rendered appearance on the client. The server need only create a few images of the model from a view camera direction, and the client need only receive the (possibly simplified) geometry of the model without any additional surface data or annotations that are represented in the texture images. The client is both thin and generic, because all the specifics of rendering a model remain the job of the server application. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of preferred embodiments of the invention with reference to the following drawings: 
     FIG. 1 a  is a block diagram of a preferred system embodying the present invention; 
     FIG. 1 b  is a block diagram of a client-server architecture; 
     FIG. 2 is a view of a 3D model with surface features, shown together with camera view directions, according to an embodiment of the present invention; 
     FIG. 3 is a rendered image of the 3D model of FIG. 2 corresponding to one of the view directions (orientations), according to an embodiment of the present invention; 
     FIG. 4 is a rendered image of the 3D model of FIG. 2 using the view orientation of FIG. 3, and showing occluded regions from this orientation in black, according to an embodiment of the present invention; 
     FIGS. 5 a - 5   c  are an illustration of how a client maps the 3D model&#39;s geometry (FIG. 5 a ) onto the 2D rendered image (FIG. 5 b ) to create a rotatable 3D textured object (FIG. 5 c ) that shows partially occluded triangles, according to an embodiment of the present invention; 
     FIG. 6 is a rotated view of the 3D textured object of FIG. 5 c  viewed from the first camera and revealing backfacing and occluded regions, according to an embodiment of the present invention; 
     FIG. 7 is a further rotated view of the 3D textured object of FIG. 5 c  viewed from the second camera and showing the second camera&#39;s texture swapped in, wherein the view orientation is closer to the second camera&#39;s orientation, according to an embodiment of the present invention; 
     FIG. 8 is an illustration of the data structures that comprise the original 3D model and its renderable features, according to an embodiment of the present invention; 
     FIG. 9 is an illustration of the parameters that comprise the data structure for a view camera, whereby the parameters provide enough information to specify how to render a 3D scene into a 2D image in terms of the geometric mapping of the 3D geometry to the 2D location in the image, according to an embodiment of the present invention; 
     FIG. 10 is an illustration of the data structures that the server sends to the client that allows the client to reconstruct a rotatable model using geometry and a number of camera/image pairs, according to an embodiment of the present invention; 
     FIG. 11 is an illustration of the data structures created by the client based on the structures shown in FIG. 10, in which the geometric data are decompressed and the texture coordinates are calculated with consideration for occlusion, backfacing and the presence of an unused pixel available in each texture image, according to an embodiment of the present invention; 
     FIG. 12 is a flowchart of the server-side process that simplifies the 3D model&#39;s geometry, selects the locations of the cameras for viewing the 3D model, creates the 2D rendered image for each of the cameras, and transmits the information over a network, according to an embodiment of the present invention; 
     FIG. 13 is a flowchart of the pre-processing performed by the client-side display system that creates the 3D textured object from the 3D model&#39;s (simplified) geometry and the 2D rendered images, according to an embodiment of the present invention; 
     FIG. 14 is a flowchart of the display process performed by the client-side display system that selects the texture corresponding to the camera with the view orientation closest to a current viewing orientation, according to an embodiment of the present invention; and 
     FIG. 15 corresponds to a close-up of the occluded polygon in FIG. 5, and depicts the algorithm for subdividing partially occluded polygons into polygons that are either fully occluded or fully visible, according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, we assume a client-server system, although, as stated above, the ideas are applicable to a stand-alone computer system. 
     In a preferred embodiment, a server stores the original 3D object data, including geometry and surface features. The server also has a rendering function, which can be applied to the stored 3D object data to create texture maps from particular viewpoints. A network connects the server to the client. The client has a display and the ability to take input from a user in order to alter the display. The server sends data, including texture maps and geometry, about the 3D object over the network to the client, and the client stores and processes that data in order to provide interactivity with the user. 
     In the preferred embodiment, the steps involved are as follows: 
     First, on the server side, a number, N, of cameras based on the 3D object&#39;s shape and relevant features is generated. For example, on a cube, six views, one from each of the directions around a cube, may be generated, as shown in FIG.  2 . Images of the model using each of the cameras are created. For the rest of this application, the term “camera” or “view camera” in relation to data will signify the information identifying each of these N cameras: location, direction of viewpoint, view angle, up direction, etc. If desired, the geometry of the model is simplified. The simplified geometry, the view cameras for the series of images, and the images themselves are sent to the client. Note that the view cameras contain a small amount of data and do not contribute significantly to the data that are sent. 
     Second, the client receives the simplified geometry, cameras, and images and performs a pre-processing step, once per camera. Based on the geometry and a given camera, a visibility algorithm decides which portions of the model&#39;s geometry are fully visible in the image and which are either backfacing or partially occluded by other geometry. The corners of the fully visible portions of geometry are mapped into texture coordinates corresponding to the current image and camera, while the corners of the remaining geometry are assigned a unique value corresponding to an empty region of the image. One or more of the pixels in the empty region of the image are assigned to a color to convey the lack of texture image data for this particular view direction. The pre-processing step runs once per camera and results in N sets of texture coordinates for the simplified geometry. 
     Third, in order to show the model in an interactive manner, the client display system chooses an initial view direction and determines the camera whose orientation most closely matches it. The system displays the simplified geometry using the texture image and coordinates fitting that camera. As the user rotates and interacts with the object, the system calculates which camera is the best match to the current view orientation. When the best matching camera is different from that of the current texture map, the new texture image and coordinates are swapped in. 
     From this overview, additional advantages of the techniques of the present invention are clear. When creating the original images of the 3D model on the server, rendering the surface details of the model&#39;s original geometry allows the client to view the original surface details on the surface of simplified geometry. This results in the client having a more accurate view of the model, even though only simplified geometry is being used on the client&#39;s computer. 
     In the above description, it is assumed the original rendering by the server was done with no shading, i.e. all surfaces were lit up equally. However, to provide the client with additional visual cues, the server can render the 3D model with shading turned on. As the client is interactively viewing the textured model, direct illumination can be turned off, thereby allowing the client to see the shading of the original model on the simplified geometry. This additional information may have a slight drawback in that the light on the client will move with the object as the object rotates, and will jump to a new position when the texture map changes. However, the result is a rotatable model with highly accurate shading that rotates quickly on the client side because it has simplified geometry. Such quick rotation was not possible with the prior art techniques that map the entire object to a single, complicated texture image, rather than the various camera views of the present invention. 
     FIG. 1 is a block diagram showing an overview of a typical networking environment using the present invention. The figure describes a system  110  comprising a server computer  122 , a computer network  109 , and a client computer  121 . The server and client each further comprise a memory  104  where one or a plurality of geometric models are stored. A CPU  102  accesses the model via the internal bus  105  and assists a graphics processor  103  in rendering the image and conveying it via the I/O subsystem  106  and the graphics accelerator  107  to the display  108 . 
     FIG. 2 shows a 3D object  200  with surface features surrounded by symbols representing six cameras C 1 - 6  used to view the surface of the object. Since this model has an approximately cubic shape with many square features, the cameras were located slightly offset from the x and y directions in order to avoid view directions containing surface planes that are viewed edge-on. The six cameras include four that go around the cube from slightly above the horizontal plane, plus two more that look on to the top and bottom of the model. The exact number and location of cameras is not critical, although important regions on the model surface should have a camera that renders them well in an image. 
     FIG. 3 shows the 3D model  200  again, along with the six cameras C 1 - 6 . This view is directly along the direction of camera one C 1  and shows the corresponding image obtained by rendering the 3D model with camera one C 1 . The image conveys that the camera is looking slightly down on the cube and slightly off the right side. This is one of the images that the client will use when displaying the 3D geometry using a texture map. 
     FIG. 4 is the same as FIG. 3 except that the polygons occluded by part of the model are colored black. The 3D model  200  is shown in the context of the six cameras C 1 -C 6  while region  408 , which is partially occluded by the forward projecting right side of the cube, is colored black. 
     When a 3D geometric model is texture mapped with a 2D image, the corners of each polygon in the model are assigned a 2D coordinate, often denoted (u,v), where 0≦u≦1, 0v≦1. The (u,v) coordinate represents the (x,y) location of the polygon corner relative to the lower left corner of the texture image, where u is the x location normalized to the image width and v is the y location normalized to the image height. The process of texture mapping an object involves creating images of the object and then calculating (u,v) for each of the polygon corners. 
     FIG. 5 details how the geometric coordinates of the model are texture mapped using the image with consideration for occluded regions of the surface.  501  and  502  show the separate features that make up the 3D object: geometry  501  and a rendered image  502  of the surface details. The rendered image of the original model  200  is taken from the viewpoint of camera one C 1 , without any occlusion, as in FIG.  3 . The unused areas that are outside the bounds of the original model  200  are black in  502 . In this case and in many applications, the rendered image will have an empty border of unused pixels around it, guaranteeing that the image corners will be unused; i.e., they will not correspond to the surface of the model. In  501  the polygon connections are shown schematically as struts connecting the vertices to show the exact location of the polygons of the model, although they do not appear in the rendering  502 . 
     In order to texture map the image  502  onto the geometry  501  and accommodate occlusion, an unused region of  502  must be identified and a pixel assigned to represent the color of occluded polygons. In the rendered image  502 , the lower left corner pixel  510  is designated as the pixel to be used for coloring occluded regions and will be used as the texture coordinate of polygons with occluded corners. The lower left corner pixel is black so the occluded regions will also be black. Alternatively, if no unused pixel is available or if the identification of an unused pixel is problematic, an additional row of pixels may be added to the image to create an unused region of the image for texture mapping the occluded and backfacing parts of the model. 
     With continued reference to FIG. 5, two polygons  507  and  508  (in this case, triangles) are shown texture mapped into the image  502 . Polygon  507  is fully visible in this view so its corners are projected into the image as shown by the curved arrows  511 . In the image  502  is a polygonal patch  509  with a section of a ‘B’ corresponding to the surface detail that will cover the visible polygon  507 . In contrast, the partially occluded polygon  508  has a corner  512  that is occluded by part of the model in this view. Any polygon with at least one corner occluded will be incorrectly texture-mapped with surface information due to the occluding polygons, which means that occluded polygons such as  508  must be treated specially to indicate the lack of good texture information across the entire polygon. 
     The preferred embodiment texture maps the entire polygon to a special pixel in the image that has been colored to indicate “no texture information available in this view,” which produces a natural appearance of the model  505  similar to having a shadow cast on the object where no features are visible  504 . The fact that one texture map is coloring the entire surface including the occluded and backfacing regions, and that the geometry on which the texture map is draped has been simplified, guarantees a highly interactive view of the model while maintaining surface detail comparable to the original model. 
     To reduce the size of the shadows caused by occlusion the client can subdivide the partially occluded polygon into fully visible and fully occluded polygons, as depicted in FIG. 15, which corresponds to a close-up view of the partially occluded polygon  508  in FIG.  5 . In FIG. 15, a polygon  508  is partially occluded by an edge of the geometry, which leaves one corner  1503  hidden. The subdivision process  1507  subdivides the occluded polygon into three—two fully visible polygons  1504  and  1505  and a fully occluded polygon  1506 . This has the benefits of reducing the size of the shadowed regions, forcing them to match the appearance of the true shadow cast by occluding edge (which may appear more natural), and of not throwing away any parts of the image that have usable texture information. The drawbacks are additional pre-processing for the client to do to subdivide the partially occluded polygons, along with the rendering expense of having more polygons in the displayed geometry. 
     FIG. 6 is a view of the model  200  after the client rotates the object to the right in the horizontal plane. After a small rotation the texture map remains the same and backfacing regions  608  are now exposed to view, while the occluded region  607  simply rotates with the object. 
     If the rotation is continued further from that of FIG. 6, until the view direction is now closer to camera two C 2  than camera one C 1 , the client will automatically switch the texture image from that of camera one C 1  to that of camera two C 2 , as shown in FIG.  7 . Now regions that were previously occluded, such as  607  in FIG. 6, or backfacing, such as  608  in FIG. 6, become fully visible and fully textured, and from a different direction, as shown in FIG.  7 . This shows that if a region of the model is not textured from one direction, it may become textured by rotating the object until those regions are more squarely visible in the new view direction. 
     FIG. 8 depicts the components  808  of the original model on the server, including its geometry, surface features and data. The components that are required in the preferred embodiment are the original positions  801  and the original connections  802 —which are enough for the client to reconstruct the basic appearance of the object based on its shape. Positions are also referred to as “vertices” or “points” and refer to the actual geometry of the 3D model. Connections refer to the edges that exist between positions. Surface normals  803  may be deduced from the positions and connections themselves or be specified separately in the model in case they differ from what is dictated by the geometry, as is the case when a particular shading effect is desired in the model. The remaining items: original colors  804 , which are assigned to the original positions  801 , original annotations  805 , higher order data  806 , and other data  807  are used if available during the rendering process to refine the resulting appearance of the model. Annotations refer to additional notations, such as special symbols to highlight certain positions of the model, thus bringing the viewer&#39;s attention to those positions. Higher order data is used when representing high resolution information on a grid with much lower resolution, which sometimes occurs in CAD/CAM applications. Other data is any type of additional information, such as material type, density, stock number, thermal coefficient, etc., which may be embedded in parts of the model. Any information that determines the appearance of the model but does not affect its geometry need not be sent to the client since it will be represented in the texture image that results from rendering. 
     FIG. 9 shows a typical embodiment  907  of the data structures representing a view camera used to render the model in a particular orientation. The eye point  901  represents a particular point in space from which the model is being viewed, and the view point or direction  902  is either another point that is in the center of the view or the direction vector in which the eye is looking, respectively. The two most common types of views are perspective and orthonormal, and a perspective/orthographic flag  903  indicates which type of view the camera is using. Depending on the value of the perspective/orthographic flag  903 , there will be an additional parameter  904  corresponding to the angle of view or the width of the view, if the perspective/orthographic flag is true or false, respectively. The pixel dimensions of the image, X and Y Pixel Count  905 , must also be specified in the camera, along with the up direction  906 , which specifies a vector used to determine the up direction of the rendered image. 
     FIG. 10 shows the data structures sent from the server to the client, which typically is a subset of the original model plus a number of texture images and cameras. In the preferred embodiment the original geometry is simplified to reduce the number of polygons to be sent and rendered while retaining the original geometry within some allowed tolerance. The original positions and connections are simplified to simplified positions  1001  and simplified connections  1002 . If the surface normals are specified in the model, they are simplified ( 1003 ) also and sent. These three components: positions  1001 , connections  1002 , and normals  1003  comprise the simplified model geometry  1004 . A number, N, of view cameras  907  are created based on the model shape and the location of its most important features, and for each camera an image  1005  is rendered of the original model. The set of images and corresponding view cameras comprise the texture images with corresponding cameras  1006  sent with the simplified geometry  1004 . 
     FIG. 11 shows the data structures for the two main components reconstructed by the client: the model geometry with texture coordinates  1105  and the texture images with associated cameras and unused pixels  1107 . The client receives the compressed simplified geometry and decompresses it to create simplified positions  1101 , simplified connections  1102 , and simplified normals  1103 , if provided. The client then takes the first texture image and its associated camera, and identifies a pixel  1106  in the image that does not correspond to a part of the model using projection. The client then uses an occlusion algorithm with the simplified geometry and camera to calculate the texture coordinates  1104  using the scheme depicted in FIG.  5 . Once the first texture coordinates have been calculated the model can be displayed using the first camera and texture image, while the others continue to arrive. For each view camera that arrives, the client repeats the occlusion calculation and derives corresponding unused pixels  1106  and texture coordinates  1104 . As the new images are processed, the client is able to provide the best available texture if the model rotates to a view direction closer to one of the already processed view cameras. 
     In the embodiment based on subdividing the regions occluded by edges of the model, the geometry must be processed for each view camera to create a new geometry containing all the subdivided polygons resulting from all the view occlusions. Then the new geometry is texture mapped as before, but this time no polygons are partially occluded; the polygons are either fully visible, fully occluded, or backfaced. 
     FIG. 12 is a flowchart of the server process that sends the geometry and texture information to the client. The process starts at step  1201  when a 3D model is loaded at step  1203  from storage  1202  on disk or in memory. The geometry is then simplified and compressed at step  1204  in preparation for transmission to the client. A number, N, of cameras is selected to cover the model adequately for display at step  1205 . If N is large, many images will have to be rendered and transmitted over the network; if N is small, there may be inadequate texture information, leading to gaps and abrupt discontinuities in the appearance of the model as it is rotated. 
     A rendering system then creates an image of the model for each of the N cameras at step  1206 . Note that the rendering system need not be tightly integrated into the server process; it can be a standalone legacy application with the one requirement that it be able to render the model with a specified camera. The rendering system has its own customized or proprietary visualization techniques based on the data in the model, and the other components of the embodiment, in particular the client that displays the results, are able to present those visualizations without any knowledge of them. In other words, for purposes of this invention, the rendering system can be viewed as a “black box,” which receives a 3D model and a camera view as input and outputs a 2D image. However, the rendering process occurs completely on the server side; and is only done for the selected camera views. The decoupling of the server rendering application from the client display process allows the client to be “thinner” and contain no code customized specifically for a particular server rendering application. The server rendering system is preferably a large monolithic legacy system, and, as long as it can create images with a specified camera, the client can drape the results onto the mesh without knowing exactly how the server created the images. 
     The simplified and compressed geometry, along with the texture images and cameras, are delivered to the client, when they become available, at step  1207 . In the preferred embodiment the images are compressed at step  1208  individually and as a collection, to take advantage of the similarity between adjacent views that results in good interframe compression. Sending geometry concurrently with the images allows the client to display something useful to the viewer as soon as it is available. If the geometry arrives first, before any texture images, the geometry can be displayed as a shaded shape with a default color. As the textures arrive, they are draped on the surface based on the current view camera. On the other hand, if the texture image arrives before the geometry, the 2D texture image can be shown to give an immediate sense of the model&#39;s appearance prior to the availability of the shape itself. 
     A flowchart showing the client pre-processing algorithm of the preferred embodiment is depicted in FIG.  13 . The process starts at step  1301 , when results from the server process arrive at step  1302  and the streaming information is decompressed in order to load the geometry, N cameras and N images at step  1303 . As described above, for each image, an unused pixel or region must be identified or created for coloring the occluded and backfacing polygons at step  1305 . Also, for each camera the polygons that are occluded or backfacing in that view must be identified at step  1304 . Optionally, the polygons can be subdivided further based on the occluding edge of the model at step  1306 . At step  1307 , the results from the last two steps  1304  and  1305  are used to create texture coordinates for each view camera, giving special treatment for polygons that are found occluded or backfacing at step  1304 . The occluded and backfacing polygons should texture map to the region in the image assigned in step  1305  and colored to distinguish the polygons that lack texture information from those that are fully textured. Now that the texture coordinates are created, they are delivered to the display process at step  1308 . 
     FIG. 14 is a flowchart of the display process performed by the client-side display system according to a preferred embodiment of the present invention. The display process receives the geometry, texture coordinates, texture cameras, and textures from the pre-processing algorithm at step  1401 . It is checked whether this is the first time that the model is displayed at step  1402 . If the model has not been displayed yet, an initial view camera is chosen at step  1403 . The first display will depend upon what information has been received and processed by the client. If the geometry is available for display but there are no texture images, the geometry with shading and default color is displayed. If a texture image is available but not enough geometry has arrived, the texture image can be shown. If this is not the first display of the model at step  1402 , the next viewpoint of the user is calculated, at step  1404 , based on the previous view camera and user interaction since the last view. Next, at step  1405 , it is determined which of the N view cameras is closest to the viewpoint found in step  1404 . If the closest view camera to the user&#39;s current viewpoint is different than the view camera currently in use, the closer view camera, with its texture coordinates, and texture mapping, is swapped in at step  1406 , and the previous view camera is removed. At step  1407 , the model is displayed using the current view camera, whether it is a new view camera swapped in at step  1406  or the previous view camera. At step  1408 , user interaction is processed and the procedure goes to step  1402 . 
     When the client displays the model locally there is an option of using the normals for smooth shading, faceted shading, or no shading at all. The best choice may depend on the model and the application area, but if the goal is to display the model with as few polygons as possible, yet convey the faceted shading of the original model, there is a fundamental problem in that the simplified model by definition does not contain the polygons corresponding to all the “facets” of the original model. If the model is intended to be displayed with smooth shading the problem is reduced, because the graduation of shading resulting from interpolating adjacent normals tends to smooth over the rougher edges created by simplification. But if the model pertains to CAD/CAM, for example, and the shading of the original facets is desired, this invention provides a unique solution to the problem as described below. 
     When a model is rendered by the server and displayed on the client, shading is done only once on the combined system. In most cases the server renders the model without shading and the client provides appropriate shading and lighting based on the normals on the model and user preference. This has the advantage that the shading will smoothly change as the 3D model is viewed from different angles. However, the shading may appear coarse, because the client is providing the shading based on simplified geometry. On the other hand, if the original, faceted shading of the original 3D model is desired, the server renders each image with shading enabled. When the client displays the 3D model, the client disables shading and illumination, thus conveying the original, faceted shading in the texture image itself. As the object rotates, the shading remains constant until a new texture camera is closer, at which point a different image is used for the texture map and there is an abrupt change in the shading. Although the abrupt changes in shading may appear jarring at first, the fact that they are linked to changes in view orientation makes it an acceptable sacrifice of quality in order to retain a view of the original model structure. 
     In many situation the model will have a well-defined shape and the number and location of cameras that provide optimal coverage will be obvious. Furthermore, if the model is being modified in small steps as its design evolves, the ideal set of cameras may remain fixed while the design changes. Note that the cameras need not be evenly spaced; they can be a mixture of uniformly distributed and special cameras located to provide optimal views of known regions of particular interest. In most cases, a non-model-specific algorithm for locating cameras will provide a good and interactive view of the geometry. It is also possible to allow the client to request additional renderings on demand from the server. 
     One way to guarantee that important views of a model have a good texture image associated with them is to allow the client to request additional texture images on demand. In the embodiment described above, most of the server process can run once per model, pre-rendering the images and compressing the geometry for later streaming to the client. But in addition the client could request new renderings on demand if a particular orientation is important and lacks good texture information on important parts of the model. 
     Finally, the client and server pieces can work to advantage on a single computer without a network, since the server process creates components that allow the client to view a 3D model with much higher performance while maintaining the appearance of the original model. The reverse is also true: the functions of server and client could be broken up onto different computers in a network. For instance, parallel processing could be used to make renderings of the original 3D model, which would mean different computers or processors would simultaneously perform the same server function. Or the 3D models could be stored on a distributed network and various servers and clients could be working to create images on various user displays. 
     While the present invention has been described with respect to certain preferred embodiments, it should be understood that the invention is not limited to these particular embodiments, but, on the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.