Abstract:
A technique and system for the realistic simulation of visual scenes reduces the three-dimensional computation to two additions and further reduces the need for three-dimensional computations by displaying several screen pixels per three-dimensional computation. The approach when implemented in hardware or software significantly speeds up scene generation time while improving the resolution and realism of the rendered scene.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention generally relates to a technique and system for the generation of images on a display device, and more particularly to real-time computer simulation of visual images of perspective scenes such as landscapes and seascapes.  
         BACKGROUND OF THE INVENTION  
         [0002]    The principal application area for computer image generation has been that of visual training and/or entertainment simulators which present scenes to an operator, usually a trainee or game player, that allows the operator to practice some task such as flying an airplane, helicopter or space vehicle. In a flight simulator, a three-dimensional model of a desired “gaming area”, called a visual data base, is prepared and stored in memory of a computer. Typically, the visual data base is an array of points, generally in the form of Cartesian coordinates. In visual data bases of this type, the z component associated with each x, y position may be an elevation value, or some other attribute required to produce a visually pleasing scene, such as color, hazing, viscosity, or the like.  
           [0003]    In its usual configuration , the simulator combines an image generator, typically a computer, with a display device such as a cathode ray tube (CRT) or liquid crystal display (LCD). The image generator converts the three-dimensional visual data base into a two-dimensional scene for display on the display device. The generated imagery is meant to be representative of the true scenes that the operator would see if the operator were actually performing the task being simulated. The generation of display images is said to be in “real time” if the images are updated fast enough to provide the operator with a sense of motion.  
           [0004]    Real-time generation of visual scenes is usually a compromise between realism and the cost of the image generator and the display device. Generation of visually pleasing and realistic appearing images requires the manipulation of substantial amounts of data by the image generator. This manipulation typically requires a high speed processor and a large amount of associated memory for storing both the raw data and the rendered image. In the past, various techniques have been employed to accelerate processing of the visual data during scene rendering, usually at the cost of reduced resolution and realism. Many prior techniques rendered the terrain of the gaming area using so-called polygonal projections. This type of visual scene rendering, while adequate for simple games, or for games where the focus of attention is not on the overall visual scene, lacks the realism important to providing the user with a true simulation of the operation of the aircraft or vehicle simulated. Such realism is particularly important where the operator of the game is engaging in simulated combat, such as in a dog fight or tank battle. The simulation has two parts: 1) the operation model that simulates the actual operation of the plane or vehicle in response to operator inputs. For example, in a flight simulation, the operator must be presented with a flight model that mimics, as closely as possible, the actual flight characteristics of the plane being simulated. The flight model must present the operator with a realistic flying experience, mimicking the responses of the aircraft in a wide variety of situations.  
           [0005]    Typically, a flight model must provide a realistic simulation of the aircraft&#39;s response to changes such as operator inputs controlling climb, descent, banking for maneuvering, as well as determining whether the aircraft is in level flight or landing or taking off. While the processor is controlling the flight model, it must also associate the movements of the aircraft with motion over the ground. Thus, the processor must also render and update a visual scene representing a simulated “world” that is experienced by he operator as the operator “flies” the aircraft through the simulation.  
           [0006]    Rendering a realistic visual scene at a rate that provides a visually pleasing and realistic experience places a heavy load on the processor. This, it is important to optimize the rendering of the scent to reduce processor load and data transmittal. In these simulations much of the action involves use of the features of the terrain, either for purpose of evasion or to lend difficulty to enemy detection and attack.  
           [0007]    One prior approach to presenting more realistic terrain to a virual observer moving through a gaming area is described in U.S. Pat. No. 5,550,959 issued to Freeman. This approach forgoes the simplicity of polygonal feature and uses a technique based on volume elements. In this approach, the visual data base contains elevational and color data associated with each point in an array of points defining a gaming area. The location of the operator in relation to the coordinates of the visual data base is determined from the operator&#39;s starting position in the gaming area and the operator inputs. The terrain generator uses this information to divide the area of the visual scene visible to the operator into a number of cross-sections oriented perpendicular to the line of sight of the operator. The horizontal scale of the cross-sections changes proportionately to the distance from the operator, as does the vertical scale of the terrain features being displayed. The horizontally and vertically scaled visual scene is generated one cross-section at a time until the entire scene is rendered, and then displayed. Alternatively, each point on a series of individual cross-sections can be generated, stepping backwards or forwards through the cross-sections. One disadvantage of this approach is that the distance to the visible horizon is limited, resulting in terrain features suddenly appearing on the horizon as the operator moves toward the horizon. Another problem arises as an operator moves toward an object or terrain feature. In this case, the resolution of the feature is limited to the size of the smallest volume area in the data base. This results in terrain features rendered with a visually unrealistic and unpleasing blocky appearance. Another shortcoming of previous methods of terrain generation using volume elements has been difficulty in providing motion in all axes, known as pitch, yaw and roll. Because horizontal and vertical scaling were necessarily fixed to enhance processing speed, prior techniques could provide virtually no adjustment for pitch or roll. The lack of such features is detrimental to the level of realism achieved by the simulation, and thus is disadvantageous in providing the kind of exciting and visually pleasing simulation sought by a game player.  
           [0008]    Additionally, game developers were constrained by the relatively slow processing speed of available processors incorporated in computers and gaming devices, slow data transfer rates across the internal bus of the computer or gaming device, and the amount of rapidly addressable random access memory available for storing the data comprising the visual image to be displayed. With the advent of 4×86 and other advanced processors, VESA and PCI bus architecture and the availability of inexpensive memory, developers are now able to utilize more sophisticated data retrieval and display techniques without sacrificing execution speed of the simulation.  
           [0009]    What has been needed is a technique for real-time generation of visual displays without sacrificing realism or requiring expensive hardware.  
         SUMMARY OF THE INVENTION  
         [0010]    The invention provides a unique real-time display of visual scenes rendered by a computer in accordance with programming commands to provide a realistic and visual pleasing simulation. The invention provides this real-time display of visual scenes by rendering the scenes by altering the scaling of the visual scenes in accordance with the distance of a virtual observer from features in the visual scene to be displayed. The present invention accomplishes the more realistic rendering of visual scenes by using elevational databases whose resolution is varied dependant upon the calculated distance of the observer to features that are to be displayed.  
           [0011]    More particularly, the invention may draw upon a series of databases, each database containing more information, and thus higher resolution, than the preceding database. Thus, the same feature to be rendered is contained, at varying resolution, in more than one database.  
           [0012]    Moreover, the invention may enhance the resolution of features that are relatively close to the virtual observer by expanding the data used to render the feature using expansion tokens that may point to stored images or portions of images that may be rendered or mapped into the visual scene. The invention determines when to expand the data depending on the distance between the feature to be rendered and the virtual observer. In this manner, the invention may render a single visual scene using more than one database containing elevational and/or color data for individual features in the scene, depending on the apparent distance between individual features and the virtual observer.  
           [0013]    Because the enhanced rendering of visual scenes requires substantial processing time by the computer, the invention provides for identifying quadrants of the visual scene to be displayed with elevational databases associated with the quadrants wherein a single elevation, representative of the height of the highest feature in the quadrant, is stored. In this manner, the computer, in determining whether individual elements and features of the visual scene are visible to the virtual observer, or whether the elements or features are obscured by other elements or features, need only determine whether the quadrant would be visible, rather than processing each pixel element of the quadrant individually. Using this feature, the invention substantially reduces the amount of processing required to render complex and fast moving visual scenes to provide a visually pleasing and realistic simulation experience to an operator.  
           [0014]    Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features of the invention. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a block diagram of a general purpose computer system that is programmed by the techniques of the present invention to render the terrain of a simulation;  
         [0016]    [0016]FIG. 2 is a representation in plan view of a gaming area of the simulation, representing a virtual observer&#39;s location and heading therein;  
         [0017]    [0017]FIG. 3 is a representation of the gaming area of FIG. 2 as seen by a game player showing the terrain of the gaming area rendered in perspective;  
         [0018]    [0018]FIG. 4 is a cross-sectional view of the gaming area of FIG. 3 taken along the line  4 - 4 .  
         [0019]    [0019]FIGS. 5A and 5B are block diagrams of the image generator of the present invention; 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    The real-time generation of perspective scenes reflective of a virtual observer&#39;s movement through given terrain is a critical need for aviation and ground simulation trainers and many types of computer games. In order to minimize the number and complexity of the required calculations, the present invention provides a method for generating perspective scenes using data bases such as the elevation grid data bases produced by the Defense Mapping Agency. Such data bases are described more fully in U.S. Pat. No. 4,583,185 to Robert A. Heartz.  
         [0021]    The technique according to the present invention may be illustrated with reference to FIGS.  2 - 5 . Observer (virtual) location and heading with respect to the elevation data base are obtained. In its simplest embodiment, the scene is generated by sequentially generating a series of “slices” or cross-sections at ninety degrees to the observer&#39;s line of sight. Adjustment is made for the observer&#39;s altitude. Each cross-section is horizontally scaled to render the observer&#39;s field of view, and vertically scaled to render perspective. For each point in a given cross-section, a vertical- line is drawn to the respective scaled height. After a pre-selected number of cross-sections have been generated, the scene is displayed.  
         [0022]    An alternative approach, while consuming more computer resources, but more visually pleasing, is to use a technique similar to the “ray casting” approach commonly used for generating polygon graphic images. The method of the present invention, however, differs from the ray casting technique in that images are generated using volume elements rather than polygon constructs to generate the features of the terrain comprising the gaming area. In general, a video display is comprised of a number of pixels arranged in an array of columns and rows. The number of columns and rows is dependent on the resolution of the video device, which is dependent on the amount of video memory, commonly called a video buffer, available in the computer. A typical arrangement, for example, of a video display  18  operating using VGA Mode  13   h  is 320×200, or 64,000 pixels is illustrated by FIG. 3. In this resolution, the display  18  consists of an array of pixels 320 columns wide and 200 rows high. In this arrangement, each pixel can be represented by a unique combination of values for X and Y. For example, the pixel at the top left corner of the display of FIG. 3 is represented by the coordinates ( 0 ,  0 ), while the pixel located at the bottom right corner of the display is represented by the coordinates ( 320 ,  320 ). Other resolutions, such as 1048×768, with an increased number of pixels comprising the display are also possible.  
         [0023]    It will be obvious to one skilled in the art that the area displayed on the display  18  is only a small portion of the gaming area that is available for use in the simulation. The size of the gaming area is limited only by considerations of data base size, memory storage, and processor speed. All of these variables can be manipulated by a simulation designer to enhance the realism of the simulation.  
         [0024]    Referring now to FIG. 1, an exemplary computer system on which the software techniques of the present invention runs is described. Generally, the terrain rendering techniques of the present invention may be implemented by a general purpose computer programmed to carry out the specific methods set forth in reference to FIGS. 5A and 5B. Typically, such a general purpose computer comprises a processor  1  which is connected to either or both a read-only-memory  2  and/or a random-access-memory  3 . The read-only-memory  2  stores program instructions and data of a permanent nature. The random-access-memory  3  is used to store program instructions and data temporarily in response to commands from the processor  1 . For example, the random-access memory  3  may store particular programs or segments of programs used to carry out specific functions, the processor  1  being response to the stored programs to carry out the functions of the programs as if the processor were a hardwired, dedicated device. Additionally, the random-access memory  3  may be used to store current data or intermediate results in a manner well known in the art. In a preferred embodiment, the processor is an Intel x386, x486, pentium or similar processor and the random-access-memory is typically 16 megabytes of dynamic random-access memory or the equivalent.  
         [0025]    The computer system also has one or more storage devices  4 , such as a hard disk drive, CD-ROM, floppy drive or other storage media operably connected to the processor. The storage device  4  may contain programs that are accessed by the processor to program the process to perform specific functions as well as data to be used by programs, such as the present invention.  
         [0026]    The processor  1  receives operator input from one or more input devices  5 , such as a keyboard, joystick or mouse. Acting on the operator input, the processor  1  performs various operations and writes the output of such operations to the display buffer  6 , also called video memory. The contents of the display buffer  6  or video memory are written to the display  18 , forming an image on the display  18 .  
         [0027]    Referring now to FIG. 2, a gaming area  10  represented by a visual data base is shown. The gaming area  10  comprises the background, or terrain, in which the simulation occurs. In a typical simulation, the portion of the gaming area  10  visible on the screen at any particular time is only a small portion of the total gaming area  10  that is stored as an array of discrete points in the memory of the computer or electronic gaming device on which the simulation is operated. As shown in FIG. 2, any location in the entire gaming area may be represented by the Cartesian coordinates X and Y. The gaming area is stored in the storage device of the computer or gaming device as strings of data, each data string comprising values of X and Y coordinates, and having associated with each X and Y coordinate a number of other values, commonly called Z coordinates. In the present invention, the Z coordinate may comprise not only elevational data for the particular location represented by an individual X, Y coordinate, but may also include additional data, such as data representative of a textural value for the location, or color of the location, or some other attribute such as translucence, viscosity or the like. This information may be stored in an individual data string defining each X, Y location of the gaming area  10 . Alternatively, a number of separate databases may be used, with each data base linked by common X, Y coordinates identifying the location within the gaming area. Whether to store the data as one string, or as a plurality of databases linked by common X, Y coordinates is a decision for the game designer and is dependent on the storage capacity of the storage media used by the computer or gaming device, and is also limited by the amount of random access memory available in the computer. Processing speed of the computer or gaming device is also critical in that the realism of the game is determined by the rate at which visual scenes from the gaming area  10  can be displayed.  
         [0028]    It will be apparent that the resolution of terrain details in the gaming area  10  is dependent upon the number of locations within the gaming area  10  represented by X, Y coordinates. A gaming area has higher resolution, that is, contains more detail, when the database defining the gaming area  10  contains more data points. Thus, infinite detail would require an infinite number of data points, each data point representing a particular location within the gaming area and associated with an X, Y coordinate. At present, the degree of detail that can be made available is limited by the amount of storage capacity and processor speed available.  
         [0029]    To overcome this problem, and still provide sufficient resolution to provide a detailed and visually pleasing simulation, prior approaches to the rendering such visual scenes reached a compromise by rendering images having a resolution representative of the resolution of terrain features located at an intermediated distances as seen by a human eye. Because of this compromise, as an observer moves through the simulation, objects that are rendered to appear as if they were located either nearby the location of the virtual observer or at a relatively large distance from the location of the virtual observer appear unnatural.  
         [0030]    A further description of the terrain visible to a virtual observer at viewpoint  12  is shown in FIG. 3. As can be seen in FIG. 3, not all of the gaming area  10  is visible to a virtual observer whose point of view is located at viewpoint  12 , which has a particular reference point described by coordinates X, Y in the gaming area  10 . As will be apparent to one skilled in the art, an observer situated at viewpoint  12  whose line of sight is indicated by line  14  is only able to view terrain features that are presented to the right of the area bounded by lines  16  and  18 . The virtual observer&#39;s field of view, indicated by Φ in FIG. 2, may be any value desired by the game developer. In most simulations, however, a field of view comprising, for example, 90° is preferred because this provides a realistic field of view while not requiring inordinate amounts of processing speed or memory to render the terrain within that field of view. It will also be apparent that besides having a location indicated by coordinates X, Y, a virtual observer located at viewpoint  12  will also have an elevation relative to the terrain around him. In the simplest case, the virtual observer is located in the plane of the ground as rendered in the simulation, and thus his elevation is essentially equivalent to the elevation data associated with that particular X, Y location in the gaming area  10 . In the case of an aircraft simulator, the elevation of the virtual observer can be substantially greater than the elevation of the terrain associated with location X, Y in the gaming area  10 . As will be discussed further below, the elevation of the virtual observer must be taken into account when rendering the scene to provide realism.  
         [0031]    Also shown in FIG. 3 are a series of randomly drawn lines  20 - 28  which serve as reference points to aid in describing the several embodiments of the present invention. These reference lines  20 - 28  represent locations of features which are located in the gaming area  10  at varying distances from the location of the virtual observer. While in FIG. 3, lines  20 - 28  appear equidistant from each adjacent line, it will be understood that the distance between he lines may vary depending on the requirements of the game. For example, and not by way of limitation, the distance between each line moving towards the right of FIG. 2 could be twice the distance of the preceding line. Thus, terrain features located along line  22  may be twice as far from viewpoint  12  as features located along line  20 , and features located along line  24  may be four times as far from viewpoint  12  as features located along line  22 .  
         [0032]    As in real life, terrain features in a game simulation provides perspective to the game player. Thus, terrain features located in the plane of line  20 , will appear larger than objects located along line  22 , since they are closer to the viewpoint  12 . Because of their apparent larger size, fewer features will be able to be rendered within the field of view bounded by lines  16  and  18  along line  20  than may be rendered along line  22 . Accordingly, less stored information is required to be processed to render those features located along line  20  than is required to render features located along line  22 . As the virtual observer visualizes objects further and further from his location, his field of view becomes progressively larger and more objects need to be rendered to fill the scene. At the same time, to provide the horizontal scaling necessary to provide for proper visual perspective dependent upon the distance from the observer, the relative size of each object is decreasing. However, the data required to render the scene, for example, at line  26 , is substantially greater. At some point, the amount of data necessary to render objects along a line at a given distance from viewpoint  12  will exceed the processing speed of the processor and transfer rate of the bus so that further attempts to render terrain will adversely affect the rate at which the terrain can be displayed, thus slowing down the simulation and negatively affecting the performance. Accordingly, simulation developers using prior approaches to terrain rendering set an arbitrary distance for the horizon of the simulation. Because the resolution of each object was the same, objects would suddenly appear on the horizon in a so-called “jack-in-the-box affect”. The appearance of large terrain features could thus prove startling and distracting to a game player.  
         [0033]    In a preferred embodiment of the invention, the gaming area  10  may comprise a series of data bases, with each data base optimized to provide terrain as seen by a virtual observer at a predetermined distance. Since each of these data bases are optimized to provide terrain scaled to appear as if viewed at a predetermined distance, the optimization of each data base could take advantage of the reduced need for data to provide realistic resolution of the terrain features. For example, and not by way of limitation, separate data bases could be used to render terrain features visible along each line  20 , through  28 . For example, while terrain features located along line  20  are closest to viewpoint  12  and thus require the highest resolution to provide a realistic visual scene, the number of features visible within the field of view bounded by lines  16  and  18  is substantially less than the number of features similarly located within the field of view along lines  22 - 28 . However, because features located along line  28  are scaled to appear further from the virtual observer located at viewpoint  12  than features located along line  20 , less resolution is required to render the features located along line  28 . Accordingly, by way of example and not by way of limitation, a data base representing objects located along line  20  might require one data string for each pixel. Because of the horizontal and vertical scaling factors, one data string representing a particular location along line  28  may represent, because of the reduced need for resolution, the same information that required ten data strings for locations along line  20 . Thus, it should be apparent that the data bases may be optimized to provide realistic perspective at varying distances without requiring the large amounts of data storage processing and transfer that would be required if a single data base having features at a single resolution were used to render the entire scene.  
         [0034]    The number of data bases having differing resolutions is limited only by the number of data bases that can be stored in the computer or gaming device storage device  4  or random-access-memory  3  (FIG. 1). Preferably, the number of data bases is approximately ten, with each having approximately 75% less data than the data base representing the preceding highest resolution available.  
         [0035]    For most purposes, while a virtual observer in a simulation needs to be able to look out towards the horizon to detect oncoming terrain features and adversaries, most events requiring operator response and input occur in relatively close proximity to the virtual observer. Thus; in a preferred embodiment, the data base representing that portion of the gaming area  10  within a pre-determined distance of the viewpoint  12  would necessarily be optimized around an intermediate resolution.  
         [0036]    Since it is not unusual during a simulation to pass close by a feature of a terrain, such as a mountainside, use of a single database with one level of resolution may lead to visually unpleasing results. For example, as the virtual observer draws near to a particular feature of the terrain, the simulator may render the feature such that the feature takes on a blocky and unrealistic appearance. Accordingly, in another embodiment of the invention, still another database having terrain features stored at higher resolutions for viewing at close distances may be used to render the scene. As will be apparent to one skilled in the art, the additional detail required to render the close-up scene requires additional information to be stored to describe the appearance of the features located at each X and Y coordinate. A preferred approach to reducing the amount of data necessary to render such a close up visual scene is to incorporated expansion tokens into the data string. Alternatively, associated with each X and Y location can be expansion tokens. As will be discussed more fully below, during the evaluation of the terrain visible to the virtual observer given the apparent heading elevation and viewpoint of the observer, the expansion tokens provide information to the processor or the computer of the gaming device instructing the processor to access and retrieve additional data to enhance the display of the features stored at that particular location in the gaming area  10  to selectively increase the resolution of a particular feature. The features could be stored in a library of such features, for example, where the detail of a mountainside is required to provide a realistic visual scene, such detail could be provided from a library of features representative of a typical mountainside, the particular feature being selected from the library as indicated by the expansion token associated with the particular location in the gaming area  10 . This library of features may be accessed by the processor  1  from the storage device  4 , as required by the processor  1  to render visual scenes in response to operator input from the input device  5 . Alternatively, the library of features may be loaded into the random-access-memory  3  either at the beginning of the simulation, or at some time during the simulation prior to the library being accessed by the processor  1 . Pre-loading the library of features may substantially increase processing speed and thus the rendering of visual scenes in response to operator inputs from the input device  5 , providing a more realistic and visually pleasing experience. By properly selecting the distance from the virtual observer that triggers the processor to select a different database to render the visual scene, the game developer may provide the game player with an apparently seamless rendering of visual scenes from very close to very distant from the virtual observer.  
         [0037]    In an alternative embodiment of the present invention, the gaming area may also be represented by another set of data bases wherein the gaming area is divided into a number of regions called, for convenience only, quadrants. In one embodiment, a single quadrant data base is used to represent the entire gaming area. In this embodiment, the gaming area is divided into areas requiring a 10×10 block of pixels to render on a display screen. Unlike the databases described above, where a height value is associated with each pixel, in the quadrant data base, a single height value may be associated with the entire quadrant. The value of this height value may be, for example, the height of the highest feature located anywhere within the quadrant. Thus, the height of an entire 10×10 block of pixels is represented by a single value.  
         [0038]    As will be discussed more fully below, use of quadrants speeds up the processing and rendering of the terrain data by providing a small, quickly accessible data base containing the maximum heights of all locations within a certain area. Thus, the processor need only check the height of a given quadrant to determine if any of the features in a given quadrant are visible, rather than checking the height of each individual pixel within the quadrant.  
         [0039]    The operation of the present invention will now be described with reference to FIGS.  2 - 5 A and  5 B. The position and heading of the virtual observer  12  within the gaming area  10  are determined based on inputs from an operator. In normal game play, such inputs may be made through the pressing of keys on a keyboard or through signals received from a mouse or joystick or other input device These signals are interpreted by the processor in accordance with the programming instructions of the software of the simulation program and result in the apparent motion of the virtual observer through the visual scene that is displayed on the display  18 . Such signals may, for example, change the direction of movement of the observer, or may, in a flight simulation, change the apparent elevation of the virtual observer above the gaming area.  
         [0040]    Once the position, heading and elevation of the virtual observer have been determined, that portion of the gaming area determined to be visible to the vital observer is rendered in the display buffer  6 , as will be discussed in more detail below. Once the entire scene is rendered and stored in the display buffer  6 , the contents of the display buffer  6  are written to the display  18 . This process is repeated as often as necessary to provide a flow of images on the display  18  that provide for a pleasing and realistic simulation.  
         [0041]    [0041]FIG. 3 depicts an exemplary image rendered on the display screen  18  by the present invention as viewed by an operator at a given instant of simulation or game play. Three horizon lines identified as c 1 , c 2 , and c n  representing a sequence of terrain features visible to a virtual observer are illustrated. It should be appreciated that the screen dimensions given are selected merely for purposes of illustration of the invention, and are nowise limiting. The horizon lines c 1 , c 2 , and c n  represent various topographical features, as they might appear on the display screen  18 , according to the data stored in the visual data base  10 .  
         [0042]    In a preferred embodiment, the visual scene is rendered by the processor  1  by iteratively generating each pixel of the visual scene by beginning with the pixel located at the bottom left of the display screen  18 , identified in FIG. 3 by the array coordinates ( 0 , 200 ). A cross-section of the display  18  screen in FIG. 3 is represented in FIG. 4 by the line  54 . This line  54  is representative of the first column of pixels on the left side of the display screen as viewed by a game player.  
         [0043]    Once the position, heading and elevation  48  of the observer have been determined by the processor  1 , the visual scene is rendered by first calculating a three dimensional step vector that describes the view of the observer passing through a single pixel, in this example, the pixel of the display identified as pixel ( 0 , 200 ). As will be obvious to one skilled in the art, this step vector  44  can be calculated to describe the line originating at the viewpoint  12  and passing through pixel ( 0 ,  200 ). Once this vector is determined, the process can then calculate the extension of the vector to determine if the vector intersects any feature of the terrain of the gaming area  10  that may be visible to the virtual observer located at viewpoint  12 . In one embodiment of the invention, when vector  44  encounters a terrain feature that would be visible, the height of the feature  40  is stored in a memory buffer for later reference. For example, once a feature is determined to be visible, a line is drawn, the length of the line being a scaled height determined from the Z component of the vector  44  scaled to account for the elevation  48  of the virtual observer and the distance from the viewpoint  12  to the feature  40 . Once this line is drawn, the next y coordinate of the step vector is incremented and vector  45  is calculated. In a preferred embodiment, the height of feature  40  has been stored in a buffer, typically a portion of the random-access-memory  3 , although the buffer may also be located in cache memory associated with the processor  1 , or in a file on the storage device  4 , and is thus available to the processor to compare to the scaled height of the feature as determined from vector  45 . As can be seen from FIG. 2, the calculation of vector  45  is not necessary, because its calculation will not render any further features of the gaming area  10  than were rendered with the calculation of vector  44 . Thus, a substantial improvement in overall processing speed is obtained using the present invention because the processor  1 , taking into account the stored value of the height of feature  40 , calculates the next incremental step vector to define vector  46 , rather than vector  45  or any other intermediate vectors, since the vectors passing through those pixels on the display screen  18  will add no further detail. This process is continued for each pixel on the display screen until the entire first column of the display has been rendered. The process is then repeated for the next column of the display screen.  
         [0044]    Alternatively, the X coordinate may be iterated, rendering an entire row of the display screen. In that embodiment, the display would be rendered by generating the display beginning with the bottom row of the display screen  18 , which represents the gaming area closest to the virtual observer, and then generating each subsequent row until the entire visual scene is rendered in the display buffer  6 , the last row representing the area of the gaming area visible to the virtual observer that is farthest away from the virtual observer.  
         [0045]    With this background established, the technique of the present invention may be described alternatively as follows. FIGS. 5A and 5B are flow charts representative of the technique of the present invention. Reference may also be taken to the following table (Table I) which represents summarily the process which is explained below.  
                         TABLE I                       PROGRAM OPERATIONS                                A   Allocate memory for data.       B   Load data into memory.       C   Clear the screen buffer.       D   Get observer position and heading.       E   Add step vector to Observer position       F   Get quadrant data       G   Determine if the altitude of the observer is less than the quadrant           height       H   If altitude is less, compute distance of sample point from the           observer; else add quadrant step vector to position and repeat           from step F       I   Select data set for sampling       J   If the sample point is close to the virtual observer, expand the data       K   Sample the height data of the selected data set       L   If the height is greater than the height of the last feature drawn, draw           a line       M   Repeat from step E until all the visual scene is rendered in the buffer       N   Write the display buffer to the display                  
 
         [0046]    As before, the processor renders the visual scene by computing the position, heading and elevation of the virtual observer relative to the gaming area  10 . The first step vector is computed, and added to the observers position, as indicated in box  72 . Retrieving the height data in box  74  from the quadrant within which the pixel to be generated as determined in box  72  lies, the processor  1  next determines whether there is any feature within the quadrant that would be visible as viewed along the step vector determined in step  72 . If the processor  1  determines that no feature is visible within the quadrant, the program branches to box  78 . In box  78 , a quadrant step vector is calculated. This vector is different from the step vector in that the quadrant vector moves incrementally across quadrants, while the step vector iterates across pixels. Thus, in box  78 , the calculated quadrant step vector is added to the position of the virtual observer. Because the quadrant step vector translates the vector over a much larger distance than the step vector, the same amount of terrain can be generated using substantially fewer steps, thus resulting in a substantial improvement in processing speed.  
         [0047]    If there is a terrain feature visible as determined in box  76 , the distance of the sample point, or the point where the vector  44  intersects the gaming area, from the virtual observer is calculated in box  80 . This distance is used by the processor to select an appropriate data base wherein the elevation data contained in the data base representing the height of features of the gaming area  10  has been scaled to be representative of the scaling caused by the distance of the feature from an observer. Since this data is already scaled for distance, additional improvements in processing speed are obtained because overdrawing of lines previously rendered is minimized. In a preferred embodiment, the number of data bases, each having sequentially less resolution, is limited only by the available capacity of the storage device  4  or the random-access-memory  3  of the system. For example, a series of data bases beginning with a database representing a large area of the gaming area and ending with a data base representing only a small area of the gaming area wherein the height of any feature of the gaming area  10  can be represented by a single pixel could be used. For example, a series of ten databases, each database representing a smaller gaming area at higher resolution than the previous databases could be used.  
         [0048]    Alternatively, the processor  1  may determine that the distance of the sample point from the virtual observer is sufficiently small such that even using the data base having the highest resolution, a pleasing visual display cannot be obtained. If this is the case, the processor may expand the data available in the data base using expansion tokens that are stored within the data base and are associated with each location in the data base. As stated previously, these expansion tokens can be used to provide additional resolution in the visual display for the locations close to the virtual observer. In a preferred embodiment, the expansion token is used as a pointer to a table of features or portions of features stored in the storage device  4  or the random-access-memory  3  having a higher resolution than exists in the currently selected data base. Thus, the table of features could contain a close up of the side of a mountain, or a bush, or a landing strip. The processor  1  may then render this feature pixel by pixel, as for all the other terrain features. In the preferred embodiment, the feature may be mapped by the processor  1  into the display buffer  6  so that the feature is rendered at the location of the gaming area determined from the virtual observer&#39;s position, heading and elevation. Such mapping provides additional resolution without requiring a large increase in processing steps to render that portion of the visual scene close to the virtual observer. The expanded data may also be stored in a buffer located in the random-access memory  3  or the storage device  4  for future reference by the processor in further scene rendering, thus saving additional processing steps that would be needed to re-render the expanded data.  
         [0049]    Once expansion of the data base is completed, or if expansion is not necessary, as determined in box  84 , the height of the feature as a function of (X, Y) steps is sampled in box  88 . If the sampled height is greater than the height of the feature that is stored in the buffer described above, a line  92  is drawn in the display buffer representing the scaled height of the feature at that particular location as viewed by the virtual observer. If the sampled height is not greater than the stored height, then the line is not drawn, because it would simply overdraw a line previously drawn. By preventing the overdrawing of lines, a substantial gain in processing speed is obtained. If the line is not drawn, the processes returns to box  70  and is repeated until the entire visual scene is rendered in the display buffer, and the contents of the display buffer are written to the display.  
         [0050]    It should be appreciated that the invention may be easily implemented in either software or hardware. Other modifications to the hardware or software can be made to the present invention without departing from the spirit and scope thereof. Accordingly, it is not intended that the invention be limited except by the appended claims.