Patent Publication Number: US-2016240107-A1

Title: 3d map display system

Description:
CLAIM OF PRIORITY 
     This application is a Continuation of International Patent Application No. PCT/JP2015/052845, filed on Feb. 2, 2015, which claims priority to Japanese Patent Application No. 2014-055712, filed on Mar. 19, 2014, each of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a three-dimensional (3D) map display system for displaying a 3D map representing a feature that cannot be visually recognized such as an underground structure without a sense of discomfort. 
     2. Description of the Related Art 
     In electronic maps used in a navigation apparatus, a computer screen or the like, a 3D map representing features such as a building three-dimensionally is used in some cases. The 3D map is drawn by projecting a feature arranged in a virtual three-dimensional space by perspective projection or the like. Since the 3D map is to reproduce a state seen from a position of the point of sight, it is inevitable that a dead angle that cannot be visually recognized from the position of the point of sight is generated such as an underground structure including a tunnel or a road behind a high building. 
     In order to alleviate an influence by such dead angles, Japanese Patent Laid-Open No. 11-24556 discloses a technology of drawing a tunnel with a broken line or the like. 
     A merit of the 3D map is that a user can intuitively grasp the geography since a state seen from the position of the point of sight is reproduced realistically. If the tunnel or the like is drawn with the broken line in the map as in Japanese Patent Laid-Open No. 11-24556, reality of the 3D map is lost, and representation gives a sense of extreme discomfort. That is, if an underground structure such as a tunnel is to be displayed, representation without a sense of discomfort which does not damage the merit of the 3D map is desirable. This is a problem common to cases of display of various portions that cannot be seen from the position of the point of sight including not only display of the underground structure but also display of the dead angle behind the building. The various portions that cannot be seen from the position of the point of sight cannot be displayed by a mere actually shot image, and capability of such display is one of the greatest merits of the 3D map. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention was made in view of the aforementioned problems and has an object to enable display of a feature that cannot be visually recognized without a sense of discomfort in a 3D map. 
     One embodiment of the present invention provides a 3D map display system for displaying a 3D map representing a feature three-dimensionally. The 3D map display system may includes (a) a map database for storing a 3D model of the feature, (b) a projecting condition setting unit for setting a projecting condition for projecting the 3D map, (c) a transmissive (transparent) object extracting unit for extracting at least a part of the feature shielded (i.e., its view is blocked) by a ground surface or other features as a transmissive object in accordance with its attribute, (d) a projection processing unit for generating a transmissive object projection view in which the transmissive object is projected and a non-transmissive (non-transparent) object projection view in which a feature other than the transmissive object is projected, and (e) a superposing processing unit for superposing the transmissive object projection view on the non-transmissive object projection view at a predetermined transmittance. 
     In accordance with one embodiment of the present invention, a 3D map is displayed by dividing a feature into a transmissive object and a non-transmissive object, by projecting them individually so as to generate a transmissive object projection view and a non-transmissive object projection view, and by superposing the both. At this time, the transmissive object projection view is made transmissive at a predetermined transmittance. As a result, the transmissive object projection view is superposed on the non-transmissive object projection view in a slightly visible state and thus, the user feels as if the feature on a side of the non-transmissive object projection view is transmissive. Therefore, the transmissive object can be displayed, capable of being visually recognized without a sense of discomfort. 
       FIG. 1  is an explanatory view illustrating a display method of the 3D map display system. The display method of the 3D map in the present invention will be described on the basis of this display example. 
     As illustrated on an upper stage in the figure, a transmissive object projection view is generated by projecting a transmissive object. In this example, a tunnel under the ground and a part of a building are transmissive objects. A non-transmissive object projection view is generated by projecting a feature other than the transmissive object as illustrated in a middle stage in the figure. In the example in the figure, a road and a river are drawn in the non-transmissive object projection view. As indicated by broken-line arrows, by superposing the transmissive object projection view on the non-transmissive object projection view, a superposed view illustrated on a lower stage of the figure is generated. At this time, by adjusting transmittance on a side of the transmissive object projection view, the transmissive object is slightly displayed. By configuring as above, the tunnel can be displayed in a state as if the ground surface can be seen through. 
     The transmissive object can be set arbitrarily and can be a feature shielded by the ground surface or other features, that is, a feature that should not be visually recognized or can be a structure under the ground or a feature behind other buildings or the like, for example, that is, any feature shielded by the ground surface or the like. In the example in  FIG. 1 , the building is drawn without any features shielding in order to avoid overcrowding, but a building shielded by other features is preferably a transmissive object in order to utilize the merit of the present invention. 
     In accordance with the present invention, by displaying the transmissive object so that it can be visually recognized as described above, a dead angle in a 3D map can be reduced, and more information can be given as a map to the user. 
     In the prior art, there are methods in which a building or the like shielding a road behind it is made transmissive. The present invention has a characteristic that a shielding object is not made transmissive but an object to be shielded is set to be a transmissive object and made transmissive. That is, the feature which should be behind is made transmissive and displayed on a front surface of the shielding feature. As a result, the user&#39;s illusion is induced such that the user can recognize as if the shielding feature is transmissive, and since the transmissive object is superposed on the front surface, such a merit can be obtained that the feature can be visually recognized relatively clearly. 
     Moreover, the present invention does not apply transmission processing when the feature is projected but applies the transmission processing to the transmissive object projection view obtained by projection. Therefore, even if a large number of transmissive objects are present, the transmission processing unified as a whole can be applied with a light load, which is a merit. 
     A projecting condition for creating a transmissive object projection view and a non-transmissive object projection view can have various settings. As a projecting method, either one of perspective projection and parallel projection may be used. In the case of perspective projection, the projecting condition, that is, a position of the point of sight and a direction of the line of sight can be arbitrarily set. In the case of parallel projection, the projecting condition, that is, the projecting direction can be arbitrarily set. Though the projecting condition can be arbitrarily set, a transmissive object projection view and a non-transmissive object projection view need to be projected under the same projecting condition so that they are superposed without positional discrepancy. 
     In the 3D map display system in accordance with one embodiment of the present invention, the transmissive object may be an underground structure. The underground structures include for example a tunnel under the ground, a basement part of a building, an underground shopping center and the like. By making them transmissive objects, display as if the ground surface can be seen through can be realized. This aspect is particularly useful for route guidance processing when an underground tunnel is passed through, a guidance of an underground shopping center or guidance to an underground destination such as a subway or a shop present under the ground. 
     If an underground structure is made a transmissive object, the map database may store a 3D model of a tunnel in a form of a polygon representing a line or a road surface. The transmissive object may be the tunnel, and a tunnel model generating unit may further be provided so as to generate a tunnel model by giving a wall to both sides of the road surface in the 3D model of the tunnel stored in the map database. The projection processing unit may generate the transmissive object projection view based on the tunnel model. In the case of a tunnel, its 3D model is prepared in the form of a line or a polygon as a part of a road in many cases. The aforementioned aspect has a merit that the tunnel can be represented more realistically since the tunnel model with the wall is generated and projected even to such a simplified model. 
     However, the aforementioned aspect does not mean that the wall should be given when the tunnel is made to be a transmissive object. Instead of the aforementioned aspect, a 3D model of a tunnel may be prepared with the wall being given in advance. 
     A shape of the wall or the like when the tunnel model is generated can be arbitrarily set. For example, the wall may be generated so as to have a semicircular sectional shape which is a general shape of a tunnel. If the wall is generated as above, a gap having a predetermined width may be provided instead of fully covering an upper part of the tunnel. As a result, a road surface of the tunnel can be made visually recognizable, and more useful representation as a map can be realized. 
     If a tunnel is made to be a transmissive object, the system may further include network data representing a road by a node and a link, and a current position detecting unit for detecting a current position of the user of the 3D map display system. The transmissive object extracting unit may extract from among the tunnels, as the transmissive object, tunnels on the route or tunnels connected to the node ahead of the link where the current position is present, on the basis of route data representing a route to be guided by the 3D map display system by connection of the links and the network data. 
     As a result, a part of the tunnel can be a target to be displayed. If all the tunnels in the map are displayed, the user can visually recognize many tunnels that could not have been visually recognized in addition to the features such as roads which are usually visually recognizable, and there is a concern that an information amount is too large and incurs confusion. On the other hand, in the aforementioned mode, the tunnels are displayed after being narrowed to those considered to have higher importance for the user and thus, appropriate information can be given to the user. 
     In the aforementioned aspect, a tunnel on the route and a tunnel connected to the node ahead of the link where the current position is present are made to be display targets. The front means a side of an advancing direction of the user. Since such node represents a branch present in the advancing direction, unless the tunnel connected to the node is provisionally displayed, a 3D map looking as if there is no branch is displayed, which confuses the user. In order to avoid such a situation, the tunnel connected to such node is made to be a display target in the aforementioned aspect. Any condition for determining the tunnel to be displayed can be set other than the aforementioned two conditions. 
     If an underground structure is made to be a transmissive object, the projection processing unit may perform the projection by perspective projection with a point of sight for the projection set to a position at a predetermined height of the point of sight relatively to the underground structure, and a 3D model of the underground structure may be prepared with a depth under the ground surface being kept smaller than the height of the point of sight. 
     When the position of the point of sight of the perspective projection is set in a relative positional relation with the underground structure as in the aforementioned aspect, if the underground structure is present deep under the ground, there is a concern that the position of the point of sight is set in the ground. For example, in the case of a tunnel provided in a mountain, it is highly concerned that the position of the point of sight is set in the ground as it gets closer to a center of the mountain. 
     On the other hand, since the 3D model of the underground structure is generated with the depth under the ground surface being modified in the aforementioned aspect, the position of the point of sight under the ground can be avoided, and a map without a sense of discomfort can be displayed. In the 3D model in the aforementioned aspect, a shape of the underground structure is not accurately represented in a point that the depth in the ground is regulated, but if the point of sight is set as described above, display without a sense of discomfort can be realized rather better by using the 3D model in the form away from the reality in some cases. 
     If the position of the point of sight in the ground is to be merely avoided, a method that the position of the point of sight is controlled so as to be at a certain height from the ground surface can be considered. However, with this method, a distance from the position of the point of sight to the tunnel is too long in the vicinity of the center of the mountain, and there is a concern that the tunnel can be displayed only in a reduced manner. On the other hand, since a relative positional relation between the underground structure and the position of the point of sight can be maintained in the aforementioned aspect, this aspect has a merit that a negative effect that the underground structure becomes extremely small can be also avoided. 
     In the 3D map display system in accordance with one embodiment of the present invention, regardless of whether the underground structure is made to be a transmissive object or not, the superposing processing unit may perform the superposition with transmittance of an upper part of the transmissive object projection view higher than that of a lower part. The higher transmittance means that the view is brought close to transparent. According to the aforementioned aspect, the transmissive object can be visually recognized relatively clearly in the lower part, that is, in a part close to the position of the point of sight, while in the upper part, that is, in a distant part, the object can be displayed slightly to such a degree that is hardly visually recognizable. As a result, the transmissive object can be displayed so as to be faded out as it goes farther, and the sense of discomfort can be further alleviated. 
     The transmittance can be arbitrarily set. It may be changed linearly as it goes from the lower part to the upper part or may be changed in steps or in a curved manner. Moreover, in a certain region in the upper part, the transmissive object may be made fully transmissive, that is, invisible. 
     In the present invention, since the transmittance is changed with respect to the transmissive object projection view, the transmittance unified as a whole can be given with a light processing load even if a large number of the transmissive objects are present. If the method of individual control of transmittance of the transmissive object is employed for projection unlike the aspect of the present invention, such needs arise that a distance from the position of the point of sight is acquired for each transmissive object and the transmittance according to that is set individually, which requires extremely complicated processing. In the present invention, such a load can be avoided. 
     In the present invention, it is not necessary to comprise all of the aforementioned various characteristics but some of them may be omitted or combined as appropriate in configuration. The present invention may also be configured as a map data generating apparatus for generating data of an underground structure for the 3D map display system. 
     That is, in accordance with one embodiment of the present invention, the map data generating apparatus comprise (a) a map database storing a 3D model of the underground structure, (b) a modifying unit for modifying height data so that, in a portion where a depth under the ground surface is larger than the height of the point of sight, the depth is the height of the point of sight or less in the 3D model of the underground structure, and (c) a map database management unit for storing the modified 3D model in the map database. 
     By generating feature data as above, the depth under the ground surface can be suppressed and thus, nonconformity that the position of the point of sight is set in the ground can be avoided even if the position of the point of sight is determined in a relative relation with the feature. 
     As still another aspect, the present invention may be configured as a 3D map display method for displaying a 3D map by a computer or may be configured as a computer program for performing such display by the computer. Moreover, the present invention may be configured as a computer-readable recording medium such as a CD-R, a DVD or the like in which such computer program is recorded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view illustrating a display method of a 3D map display system. 
         FIG. 2  is an explanatory view illustrating configuration of the 3D map display system. 
         FIG. 3  is an explanatory view illustrating a structure of a 3D map database. 
         FIG. 4  is a flowchart of route guidance processing. 
         FIG. 5  is a flowchart of map display processing. 
         FIG. 6  is a flowchart of tunnel model generation processing. 
         FIG. 7  is an explanatory view illustrating a display example (1) of a 3D map. 
         FIG. 8  is an explanatory view illustrating a display example (2) of the 3D map. 
         FIG. 9  is an explanatory view illustrating a shape example of tunnel data in a second embodiment. 
         FIG. 10  is a flowchart of tunnel data modification processing. 
         FIG. 11  is a flowchart of map display processing in the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Embodiment 1 
     A. System Configuration 
       FIG. 2  is an explanatory view illustrating configuration of a 3D map display system. The 3D map display system of this embodiment is a system for giving route guidance by performing route search while a 3D map is displayed. The 3D map display system may be also configured as a system for displaying a 3D map merely in accordance with an instruction from a user or the like without having route search and route guidance functions. 
     The 3D map display system of the embodiment is configured by connecting a server  200  and a terminal  300  by a network NE 2 . A smart phone is assumed to be used as the terminal  300 , but various apparatuses that can display a map such as a mobile phone, a mobile-side information terminal, a personal computer, a car navigation apparatus and the like can be used. Alternatively, the 3D map display system may be configured as a system in which the server  200  and the terminal  300  are integrated. 
     For the server  200  and the terminal  300 , various illustrated functional blocks are prepared. These functional blocks are configured in a software manner by installing a computer program realizing the respective functions in the server  200  and the terminal  300  in this embodiment, but a part of or the whole of them may be configured in a hardware manner. 
     In this embodiment, configuration comprised of the server  200  and the terminal  300  is employed, but the 3D map display system may be configured as a stand-alone apparatus or may be configured as a discrete system comprised of many more servers and the like. 
     (1) Server  200   
     A map database  210  stores a 3D map database  211  and network data  213 . The 3D map database  211  stores polygon data representing a three-dimensional shape of a feature, line data, and character data. The network data  213  is data for route search representing a road by a link and a node. 
     Contents of the 3D map database  211  will be described. In this embodiment, the features are handled by being divided into linear objects and other general features. The linear object is a collective name of linear features such as a road and refers to an object capable of representing a shape by line data, that is, polyline data. The linear objects include a road, a tunnel, a railway, route guidance display, a river and the like, for example. The general features other than the linear object include a building and the like. In the 3D map database  211 , polygon data representing a three-dimensional shape is prepared for the general feature such as a building. The line data is prepared for the linear object. However, as will be described later, the polygon data may be also prepared for the linear object. 
     A database management unit  202  manages input/output of data of the map database  210 . In this embodiment, the objects stored in the 3D map database  211  are classified into a transmissive object and a non-transmissive object, and drawing is performed. Reading-out of data from the 3D map database  211  and this classification are both functions of the database management unit  202 . 
     In the database management unit  202 , a tunnel model generating unit  204  is prepared. The tunnel model generating unit  204  exerts a function of generating a three-dimensional polygon model by providing a road surface and a wall on the basis of the line data representing a tunnel. 
     A route search unit  203  searches a route from a starting point to a destination specified by a user of the terminal  300  by using the network data  213 . Route search can be made by a known method such as a Dijkstra method. 
     A transmission/reception unit  201  performs transmission/reception of various types of data and commands to/from the terminal  300  through the network NE 2 . 
     (2) Terminal  300   
     A main control unit  304  integrally controls an operation of each functional block provided in the terminal  300 . A transmission/reception unit  301  performs transmission/reception of data and commands with the server  200  to/from the network NE 2 . A command input unit  302  inputs of instructions relating to route guidance and the like from the user. The instructions include specification of a starting point and a destination of route guidance, specification of display scale during map display and the like. A position/traffic information acquiring unit  303  acquires a current position and the like of the terminal  300  from a sensor such as a GPS (Global Positioning System). 
     A map information storage unit  305  temporarily stores the 3D map database  211  acquired from the server  200  when a map is displayed. In this embodiment, the terminal  300  does not store all the map data in advance but acquires the map data required in accordance with a display range of the map as appropriate from the server  200 . The map information storage unit  305  stores the map data acquired as above. At the same time it stores a result of route search. 
     A display control unit  306  displays a map on a display  300   d  of the terminal  300  by using the map data stored in the map information storage unit  305 . In the display control unit  306 , a projection processing unit  307  and a superposing processing unit  308  are provided. The projection processing unit  307  exerts a function of classifying the polygon data and the line data stored in the map information storage unit  305  into the transmissive object and the non-transmissive object and by arranging and projecting them in a virtual 3D space so as to generate a transmissive object projection view and a non-transmissive object projection view. The superposing processing unit  308  generates a superposed view (see a lower stage in  FIG. 1 ) by superposing the generated transmissive object projection view on the non-transmissive object projection view with adjusted transmittance. 
     B. Map Database 
       FIG. 3  is an explanatory view illustrating structure of the 3D map database. In the figure, structures of the line data and the polygon data stored in the 3D map database  211  are illustrated. The line data is data representing a linear feature such as a road and a tunnel and as illustrated, it stores data such as an ID, an attribute, and a configuration point. The ID is identification information of each line data. The attribute is information indicating a type of each line data, that is, whether it is a “road” or a “tunnel”. The attribute information may include a type of the roads such as a national road, a prefectural road and the like, a width of the road, a number of lanes, regulations such as one-way and the like other than the above. The configuration point is a three-dimensional coordinate of a point defining a shape of the road. 
     In the example in the figure, the line data given ID=L 1 D 1  (a portion corresponding to a road indicated by a solid line in the figure) is a “road” by the attribute and indicates that its shape is defined by the configuration points PL 1  and PL 2 . The line data given LD=L 1 D 2  (a portion corresponding to a road indicated by a broken line in the figure) is a “tunnel” by the attribute and indicates that its shape is defined by the configuration points PL 2  to PL 5 . 
     The polygon data is data representing a feature such as a building and has a data structure similar to that of the line data. However, it is the data having the configuration point which gives a three-dimensional coordinate of a top point of the polygon representing a three-dimensional shape. 
     In the example in the figure, the polygon data given ID=P 1 D 1  (a portion corresponding to the road indicated by the solid line in the figure) is a “building on the ground” based on the attribute thereof, and indicates that its shape is defined by the configuration points PP 1  to PP 4  and the like of a surface illustrated in the figure. Since the building on the ground has other surfaces, the configuration point further stores a coordinate indicating a top point of each of the surfaces. The polygon data given LD=P 1 D 2  (a portion corresponding to the road indicated by the broken line in the figure) is an “underground building” based on the attribute thereof, and indicates that its shape is defined by the configuration points PP 3  to PP 6  and the like. Since the underground building also has other surfaces, the configuration point further stores a coordinate indicating a top point of each of the surfaces. 
     In this embodiment, the tunnel and the road are handled as separate features, and the building on the ground and the underground building are also handled as separate features. Instead of this, such a method may be employed that the illustrated building on the ground and the underground building as a whole may be handled as one feature, and attributes such as a part on the ground, an underground part and the like are given to each configuration point or a polygon. 
     C. Route Guidance Processing 
     (1) Route Guidance Processing: 
       FIG. 4  is a flowchart of route guidance processing. The route guidance processing is processing of searching a route from a starting point to a destination specified by the user and of giving guidance for that. This is processing executed mainly by the route search unit  203  of the server  200  and the display control unit  306  of the terminal  300  and the like in collaboration and is processing executed by CPUs of the server  200  and the terminal  300  in terms of hardware. When the processing is started, the terminal  300  receives an input of specification of a starting point and a destination from the user (Step S 10 ). The current position may be used as the starting point. 
     The server  200  receives information on the starting point and the destination from the terminal  300  and performs route search by referring to the network data  213  (Step S 11 ). For the route search, a known method such as the Dijkstra method can be employed. Then, on the basis of the route search result, route guidance data is generated (Step S 12 ). The route guidance data is data representing the result of the route search by a link line of the network data  213 . The route guidance data is transmitted as the route search result to the terminal  300 . 
     The terminal  300  executes processing of guiding a route while displaying the 3D map in accordance with the current position of the user. First, the terminal  300  detects the current position of the user (Step S 13 ). The current position can be detected by using a sensor such as a GPS. Then, the terminal  300  displays the 3D map by the map display processing (Step S 14 ). Contents of the processing will be described later in detail. The terminal  300  repeatedly executes the aforementioned processing until the destination is reached (Step S 15 ). 
     (2) Map Display Processing: 
       FIG. 5  is a flowchart of the map display processing. This is processing corresponding to Step S 14  in the route guidance processing ( FIG. 4 ) and is processing executed mainly by the display control unit  306  of the terminal  300 . 
     When the processing is started, the terminal  300  receives inputs of the point of sight, the direction of the line of sight, and the display scale (Step S 20 ). The point of sight may be determined on the basis of the current position. The direction of the line of sight may be determined on the basis of the current position and the route to be travelled. 
     Then, the map data in a range to be displayed as the 3D map and the route guidance data are read (Step S 21 ). In order to display the 3D map, in this embodiment, the terminal  300  first reads the data stored in the map information storage unit  305  and then, if the map data is insufficient for display of the map, the terminal  300  acquires a shortage from the server  200 . Next, the server  200  executes the tunnel model generation processing (Step S 22 ). This processing is processing for generating a 3D model of a tunnel by generating a road surface and a tunnel wall on the basis of the line data of the tunnel. Details of the processing will be described later. This processing is also executed only for the tunnel with a shortage similarly to reading of the map data and the like (Step S 21 ) since the tunnel model that has been already generated is stored in the map information storage unit  305 . 
     The terminal  300  extracts a transmissive object from a feature displayed in the map (Step S 23 ). In this embodiment, a tunnel is made to be a transmissive object. Then, the terminal  300  arranges the transmissive object in a virtual three-dimensional space and generates a transmissive object projection view by performing perspective projection (Step S 24 ). Moreover, a feature other than the transmissive object, that is, the non-transmissive object is arranged separately in the virtual three-dimensional space and perspective projection is performed so as to generate a non-transmissive object projection view (Step S 25 ). Projecting conditions when the transmissive object projection view and the non-transmissive object projection view are generated, that is, the position of the point of sight, the direction of the line of sight and the like are set the same. 
     Finally, the terminal  300  superposes the transmissive object projection view on the obtained non-transmissive object projection view (Step S 26 ). As a result, a superposed view illustrated on the lower stage in  FIG. 1  can be obtained. In superposition, the transmittance of the transmissive-object projection view is adjusted. In this embodiment, the transmittance is set to such a degree that the user has an illusion that the transmissive object projection view is visually recognizable by making each feature in the non-transmissive object projection view such as the ground surface transmissive. The transmittance may be constant over the whole transmissive-object projection view or may be changed depending on the region. 
     (3) Tunnel Model Generation Processing: 
       FIG. 6  is a flowchart of the tunnel model generation processing. This processing is processing corresponding to Step S 22  in the map display processing ( FIG. 5 ) and processing executed by the server  200 . It may be executed by the terminal  300  if processing capacity of the terminal  300  is sufficient. 
     When the processing is started, the server  200  reads the road data and extracts a tunnel section (Step S 30 ). An example of the processing is illustrated in the figure. In this example, the road data is given in a format of the line data defined by configuration points P 1  to P 6 . Among them, if an attribute of a “tunnel” is given to the section from the configuration points P 2  to P 4  indicated by the broken line, the server  200  extracts this section of the configuration points P 2  to P 4  as a tunnel section. If the road and the tunnel are stored in the 3D map database as separate features, it is only necessary that a feature given the attribute of the “tunnel” is extracted without requiring application of the aforementioned complicated processing. 
     Then, the server  200  expands a width of the tunnel section and generates a road surface polygon (Step S 31 ). A state of the processing is exemplified in the figure. A line segment at a center indicated as “line” is a shape of a line given by the line data. The server  200  expands the width by parallelly moving the line in the right-and-left direction orthogonal to this line. By executing this to all the configuration points of the tunnel section, the road surface polygon can be generated. The width of the road-width expansion may be a constant value or may match the width of the road connected to the tunnel. Alternatively, it may be so configured that attribute information such as the road width or the number of lanes may be prepared in advance for the tunnel section, and the width of road-width expansion is determined on the basis of such information. 
     The server  200  generates a wall polygon on both sides of the road surface polygon (Step S 32 ). An example of the processing is illustrated in the figure. In this example, a wall having a ¼ arc shaped section is installed on the both sides of the road surface polygon. A radius R of the wall can be arbitrarily set but it may match a height regulated by laws and regulations relating to roads. In this embodiment, a gap WS is provided between the wall polygons on the both sides. That is because presence of the gap WS allows visual recognition of the road surface even when the tunnel is displayed three-dimensionally. A value of the gap WS can be also determined arbitrarily, but a value calculated by a calculation formula WS=road width Wr−2×R is employed in this embodiment. That is, a gap inevitably obtained when the wall polygons each having the ¼ arc with the radius R (center angle of 90 degrees) are generated from both ends of the road is WS. To the contrary, the radius R may be adjusted after the road width Wr is determined or the center angle of the wall polygon may be set to a value smaller than 90 degrees by considering visibility. 
     Here, the wall polygon having the arc shape is exemplified, but the shape of the wall polygon is arbitrary and may be a flat plate shape. 
     D. Display Example 
       FIG. 7  is an explanatory view illustrating a display example (1) of the 3D map. An example in which the tunnel is displayed as a transmissive object is illustrated. A curve drawn in a vertical direction in the vicinity of a center is the tunnel. Around it, roads, buildings and the like are drawn as non-transmissive objects. Since the tunnel should be located under the ground surface, the tunnel is not displayed in the 3D map, but it is known that the tunnel is displayed in the 3D map of this embodiment. Therefore, in the route guidance, even if a route passing through the tunnel is selected, a current location mark is displayed on the tunnel, and display without a sense of discomfort can be realized for the user. 
     In the example in  FIG. 7 , when the transmissive object projection view is superposed, transmittance is changed in accordance with the region. The transmittance is lowered for a region on a lower part of the transmissive object projection view, that is, a portion closer to the position of the point of sight, while the transmittance is increased for a region on an upper part, that is, a portion far from the point of sight. By configuring as above, fade-out display can be realized such that the tunnel is displayed clearly in a lower region TA, while it is displayed slightly in a far region TB. As a result, a depth of the tunnel can be represented, and giving of excessive information to the user which causes confusion in the user can be avoided. To change the transmittance of the tunnel which is one of the features in accordance with the distance from the point of sight usually needs processing such as setting of the transmittance for each section of a feature before projection and increases the processing load, but since the transmittance is set for the transmissive object projection view as a two-dimensional image after the projection in this embodiment, the fadeout display can be realized with a light processing load. 
       FIG. 8  is an explanatory view illustrating a display example (2) of the 3D map. An example in which a part of the building is also handled as a transmissive object together with the tunnel is illustrated. Transmittance is set so that it is low in the portion closer to the point of sight and high in the far portion. By handling the building as the transmissive object, a merit that the tunnel and the road can be recognized easily can be obtained. 
     In the illustrated example, all the buildings are transmissive objects, but only the buildings shielded (i.e., which views are blocked or hidden) by other buildings and the like may be the transmissive objects. As a result, similarly to the display that the ground surface becomes transmissive and the tunnel looks as if it is visually recognizable, display that the building on the front becomes transmissive and the building behind it looks as if it is visually recognizable can be realized. 
     According to the 3D map display system of the first embodiment described above, a shielded (blocked, or hidden) feature such as a tunnel can be displayed as if a feature on a shielding side is transmissive, and highly useful display as a map can be realized without a sense of discomfort. 
     Embodiment 2 
     Next, a 3D map display system of a second embodiment will be described. In the second embodiment, a data structure of a tunnel handled as a transmissive object is different from that of the first embodiment. 
     (1) Modification of Tunnel Data: 
       FIG. 9  is an explanatory view illustrating a shape example of the tunnel data in the second embodiment. A tunnel penetrating in a mountain substantially horizontally is shown from a side. A landform is schematically represented on an upper stage of the figure, and a graph indicating a depth D from a ground surface is illustrated on a lower stage. The depth D indicates a distance from the ground surface to the tunnel as illustrated on the upper stage and is positive if the tunnel is in the ground. 
     The ground surface drawn having an upward projecting curved line in the figure on the upper stage is a mountain. The tunnel goes substantially horizontally as indicated by a solid line on the lower part. Since the ground surface rises like a mountain, the depth D from the ground surface becomes the maximum in the vicinity of a center as illustrated in the figure on the lower stage. 
     Assume a case in which the route guidance is displayed during traveling through the tunnel in such a condition. The position of the point of sight (hereinafter also referred to as a camera position) and the direction of the line of sight when the route guidance is displayed is preferably set from rear above of the current position to the current position. For example, for the current position P 1 , perspective projection is performed by using a point at a height h in the rear thereof as a camera position C 1 . As a result, a 3D map including the current position and a route in an advancing direction can be displayed. 
     However, in the case of current positions P 2  and P 3 , camera positions C 2  and C 3  are set in this method. Since these camera positions are in the ground, a 3D map enters a state in which features other than the tunnel are not drawn. In order to avoid this, if the camera positions C 2  and C 3  are set at high positions of the ground surface, for example, then, a distance to the tunnel is too long, and another problem occurs that the tunnel is drawn only in a small form this time. 
     In order to avoid such a negative effect, modification is made in this embodiment such that the depth D from the ground surface of the tunnel becomes a maximum value Dmax or less. Tunnel modification data obtained by applying this modification represents a shape curved upward along the shape of the mountain as indicated by a broken line on the upper stage of the figure. Only height data of the tunnel is modified, and two-dimensional position data is not modified. Assume a case in which the route guidance is given by using the tunnel modification data. 
     At a current position P 4 , the position of the point of sight is set to a camera position C 4  at a height h in the rear along the tunnel modification data. Since this is located above the ground surface, a 3D map without a sense of discomfort can be displayed. At the current position P 3 , the position of the point of sight is set to a camera position C 5  at the height h in the rear along the tunnel modification data. Therefore, in this case, too, it is set above the ground surface, and a 3D map without a sense of discomfort can be displayed. 
     The tunnel modification data does not represent a real tunnel shape. However, by preparing data for convenience of map display with an underground depth of the tunnel modified as above, display of a map without a sense of discomfort can be realized during the route guidance even without using complicated algorithm. 
     Since the maximum value Dmax of the underground depth is a regulating value in order to avoid going of the camera position under the ground as described above, it can be set arbitrarily within a range larger than a value of the height h determining the camera position. 
     (2) Tunnel Data Modification Processing: 
       FIG. 10  is a flowchart of tunnel data modification processing. This processing may be executed by the server  200  (see  FIG. 1 ) or may be executed by another map data generating apparatus connected to the server  200 . In either case, it can be configured in a software manner by installing a computer program for realizing a function illustrated in  FIG. 10 . Here, description will be made assuming that the processing is executed by the server  200 . 
     When the tunnel data modification processing is started, the server  200  first reads the tunnel data (Step S 40 ) and modifies the height data of each configuration point of the tunnel data so as to obtain depth D from the ground surface ≦maximum value Dmax (Step S 41 ). 
     An example of a modification method is illustrated in the figure. A solid line indicates the tunnel data before modification and it is configured by configuration points RP[ 1 ] to RP [ 7 ]. The depth D from the ground surface exceeds the maximum value Dmax at the configuration points RP[ 3 ] to RP[ 5 ] in the vicinity of a center. 
     In a first modification method of the height data, each of the configuration points RP[ 3 ] to RP[ 5 ] is modified so that the ground surface depth becomes the maximum value Dmax. The modification data in this method is the configuration points RPA[ 3 ] to RPA[ 5 ]. In the first modification method, a tunnel shape after the modification is slightly distorted in the vicinity of the center, but the number of configuration points to be modified can be small, which is a characteristic. 
     In a second modification method, first, a height of the configuration point RP[ 4 ] where the ground surface depth is the largest is modified to the maximum value Dmax. This configuration point is RPB[ 4 ]. Then a smooth curved line passing the configuration points RP[ 1 ] and RP[ 7 ] on both ends of the tunnel section and the modified configuration point RPB[ 4 ] after the modification, that is, a spline curve, for example, is acquired, and a height of each configuration point is modified so as to ride on this curved line. As a result, the configuration points RP[ 2 ], RP[ 3 ], RP[ 5 ], and RP[ 6 ] are modified to RPB[ 2 ], RPB[ 3 ], RPB[ 5 ], and RPB[ 6 ], respectively. In the second modification method, there are many configuration points to be modified, but it has a merit that a smooth tunnel shape as a whole can be realized. 
     Either one of the first and second modification methods may be selected. 
     When the tunnel data has been modified by the aforementioned processing, the server  200  stores the data after this modification (Step S 42 ) and finishes the tunnel data modification processing. In the route guidance, the tunnel is displayed by using the data after the modification. 
     (3) Map Display Processing: 
       FIG. 11  is a flowchart of the map display processing in the second embodiment. In the map display processing in the first embodiment ( FIG. 5 ), processing of selecting a tunnel to be displayed is added (Step S 21 A in  FIG. 11 ) before the tunnel model generation processing (Step S 22  in  FIG. 5 ). Since the tunnel is a feature which should not be visually recognized, if all the tunnels are made to be display targets, the map becomes extremely overcrowding, which might confuse the user and thus, in the second embodiment, only the tunnels with higher importance are made to be display targets. 
     The tunnels with higher importance are determined on the basis of the following two conditions: Condition 1: tunnel on the route; and Condition 2: tunnel connected to tip end of current link. In accordance with the second embodiment, the tunnel satisfying at least either one of the condition 1 and the condition 2 is made to be a display target, while the others are excluded from the display targets. 
     In the figure, a determination example of the aforementioned conditions is illustrated. Assume that a route indicated by a solid line in the figure has been obtained as the result of the route search. Respective arrows indicate an advancing direction of the route and also indicate links constituting the route. Black circles at connection portions of the links indicate nodes. 
     A tunnel  1  drawn on the uppermost stage is a tunnel on the route. Therefore, the tunnel  1  is a display target under the aforementioned condition 1. A tunnel  2  on the middle stage is a tunnel connected to the node ahead of the link where the current position is present. Therefore, the tunnel  2  is the display target under the aforementioned condition 2. The reason why the tunnel  2  is made to be a display target though it is not a tunnel on the route is as follows. Since the tunnel  2  is connected to the node on the route, it constitutes a branch point through which the user passes without any fail when traveling on the route. If the tunnel  2  is excluded from the display targets, the map used for the route guidance makes display which looks as if the aforementioned branch point does not exist, which might confuse the user. In this embodiment, in order to avoid such confusion, those constituting a branch ahead of the current position such as the tunnel  2  are made to be display targets. 
     Since a tunnel  3  on the lower stage does not satisfy either of the conditions  1  and  2 , it is excluded from the display targets. The tunnel  3  is also connected to the route and constitutes a branch, but the current position has already passed through the branch, and even if the tunnel  3  is not displayed, there is no concern that the user is confused by that. The tunnel  2  which was previously made to be a display target is also switched to a non-display state similarly to the tunnel  3  when the branch has been passed. 
     Processing after the tunnel is selected is similar to the first embodiment ( FIG. 5 ). A tunnel model is generated for the selected tunnel (Step S 22  in  FIG. 5 ), the transmissive object is extracted (Step S 23 ), the transmissive object projection view is generated (Step S 24 ), the non-transmissive object projection view is generated (Step S 25 ), and the both are superposed (Step S 26 ) so as to display a 3D map. 
     According to the second embodiment, in addition to the effect of the first embodiment, even if the camera position is set in the relative relation with the current position, trouble that the camera goes under the ground can be avoided. Moreover, by selecting a tunnel to be displayed, information offered to the user can be appropriately suppressed. 
     The embodiments of the present invention have been described. It is not necessary to comprise all the various features described in the aforementioned first and second embodiments, but a part of them may be omitted or combined in application as appropriate. For example, the modification of the tunnel data or the tunnel selection processing illustrated in the second embodiment can be also applied to the first embodiment. Moreover, in the second embodiment, modification of the depth or selection of a display target may be performed for the underground buildings as targets similarly to the tunnel. 
     The present invention is capable of employing various variations in addition to the aforementioned embodiments. For example, (1) A feature that can be made to be a transmissive object is not necessarily limited to a tunnel or an underground building. A feature present on the ground may be made to be a transmissive object, and (2) A part processed by software in the embodiments can be replaced by hardware or vice versa. 
     The present invention relates to a 3D map display system for displaying a 3D map representing a feature that cannot be visually recognized such as an underground structure without a sense of discomfort.