Patent Publication Number: US-2016239995-A1

Title: 3d map display system

Description:
CLAIM OF PRIORITY 
     This application is a Continuation of International Patent Application No. PCT/JP2015/052844, filed on Feb. 2, 2015, which claims priority to Japanese Patent Application No. 2014-055711, 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 which displays a 3D map three-dimensionally representing not only features but also characters. 
     2. Description of the Related Art 
     In an electronic map used for a navigation apparatus, a computer screen and the like, a 3D map three-dimensionally representing features such as a building is used in some cases. The 3D map is usually displayed by three-dimensionally drawing a 3D model by perspective projection or the like. 
     In the 3D map, characters are also displayed in various aspects. Japanese Patent No. 3402011 illustrates an output example which draws a road and the like by a bird&#39;s eye view and two-dimensionally displays characters along the road. An output example in which a name of the road and the like are three-dimensionally displayed as a road sign perpendicularly standing on the road is also disclosed. The three-dimensional display can be also realized by preparing a plate polygon on which characters are pasted, and this is arranged in a virtual 3D space and subjected to perspective projection. 
     In order to utilize the three-dimensional display as a 3D map, the characters are also preferably displayed three-dimensionally. However, with a method of perspective projection of the plate polygon on which the characters are pasted, the characters subjected to the perspective projection can be hardly recognized visually if the plate polygon is arranged in parallel with a direction of the line of sight or if it is arranged far from the point of sight in some cases. 
     As a method for avoiding such a negative effect, there can be a method that a 2D image three-dimensionally representing characters is prepared in advance, and this is pasted on the perspective projection. However, if a 3D map is to be displayed from various point of sights and direction of the line of sights by such method, various and diversified character images according to these point of sights and direction of the line of sights need to be prepared, and its data amount becomes huge. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention was made in view of such problems and has an object to realize three-dimensional display while visibility of the characters is ensured in the 3D map. 
     One embodiment of the present invention provides a 3D map display system for displaying a 3D map including a feature and a character string. The 3D map display system may includes: (a) a map database storing feature data representing a 3D shape of the feature and character data for displaying the character string, (b) a feature image generating unit for generating a feature image by perspective projection of the feature arranged in a virtual 3D space by using the feature data, (c) a character image generating unit for generating a character image by using the character data by arranging a polygon representing the character string in the virtual 3D space and applying parallel projection to the polygon, and (d) a superposing unit for superposing and displaying the character image on the feature image. 
     According to one embodiment of the present invention, though the feature is drawn by perspective projection, an image representing a character is generated by parallel projection. Since the parallel projection is a projecting method having no relation with the position of the point of sight, it is capable of projection of the characters which are located far from the point of sight in a visually recognizable state. Moreover, since the character image is generated by parallel projection of the polygon representing the character, diversified images according to an arrangement state of the polygon and a projecting direction of the parallel projection can be prepared without requiring preparation of huge image data in advance. Then, the characters can be three-dimensionally represented in diversified aspects depending on the arrangement state of the polygon representing the character. As described above, according to the present invention, the diversified three-dimensional representation can be realized while visibility of the characters is ensured. 
     The present invention can be applied to various characters displayed in the map, and all the characters or only a part of them may be targets. In embodiments of the present invention, those having various shapes such as a flat plate, a cylindrical shape and the like can be used for the polygon representing the character. 
     A direction of the parallel projection when the character image is to be generated can be set arbitrarily as long as it is a direction diagonally tilted from a perpendicular direction. However, though the projection direction can be represented by tilting from the perpendicular and a projection azimuth, the projection azimuth is preferably kept within a range in which a difference from the direction of the line of sight of the perspective projection is smaller than 90 degrees or more preferably, within an approximate range in which it is smaller than 45 degrees. 
     Superposing of the character image on the feature image can be also performed by various methods. For example, it may be so configured that the polygon is arranged in a virtual space in accordance with a three-dimensional positional coordinate where the character should be displayed and parallel projection is applied, and the obtained character image is superposed on the feature image as it is. Alternatively, such a method may be employed that individual character images are generated for each character by the parallel projection and moreover, a display position on the feature image is acquired by applying coordinate conversion to the display position of the character in accordance with the perspective projection, and the character image is individually arranged at an obtained display position. 
     In embodiments of the present invention, the character image generating unit may perform parallel projection from the same direction as the direction of the line of sight in the perspective projection. As described above, the direction of the parallel projection is represented by the tilting from the perpendicular and the projection azimuth, but this aspect means a state in which the both are substantially matched with the direction of the line of sight. However, the same direction does not mean a strictly identical angle but it has a meaning including an error range to such a degree that a significant difference is not generated in the projected image. As described above, by matching the direction of the line of sight with the direction of the parallel projection, a sense of discomfort caused by the feature image by the perspective projection and the character image can be further alleviated. 
     Moreover, in accordance with one embodiment of the present invention, the system may be configured such that the character image generating unit generates a character plate polygon on which the character string is pasted, arranges the character plate polygon in a virtual 3D space so that an angle between the character plate polygon and a ground surface is a predetermined value set on the basis of an arrangement direction and an arrangement position of the character in the projection image, and performs the parallel projection. 
     By such a configuration, the character can be displayed in a state in which the character plate polygon is diagonally stood on the ground surface like a signboard. Moreover, this angle can be changed in accordance with the arrangement direction and the arrangement position of the character. As a result, not only that the character can be represented three-dimensionally but a depth can be also felt by the tilting of the character. 
     Moreover, in accordance with one embodiment of the present invention, the system it may also configured such that the character image generating unit generates the character individually image for each character string stored in the character data, and the superposing unit sets arrangement of the character image in the projection image individually and performs superposition. 
     In embodiments of the present invention, since the feature is displayed by the perspective projection and the character by the parallel projection, even if the both are arranged in the virtual 3D space, deviation is generated in the coordinate values of a projection result in some cases. However, according to the aforementioned aspect, since the arrangement of the character is set individually, such deviation is not generated but the character can be displayed at an appropriate position. 
     Moreover, in accordance with one embodiment of the present invention, the system may be configured such that the feature includes a road, and the character string includes a name of the road, and the character image generating unit generates the character image by arranging the character string representing the name of the road in a direction along the road. 
     The present invention can be applied to various characters as targets but it is particularly useful when being applied to the name of the road as in the aforementioned aspect. It is useful to display the characters representing the name of the road in the direction along the road so that a correspondence with the road can be grasped easily. Moreover, a width of the road becomes narrow by the perspective projection in a region far from the point of sight. If the two-dimensional characters are displayed in the same size in that situation, such trouble might be caused that the road is hidden by the characters. According to the present invention, the characters can be displayed three-dimensionally by three-dimensionally tilting the characters from a road surface while the characters are displayed in the direction along the road and thus, diversified display according to the road width can be realized. Moreover, since the characters are subjected to parallel projection, its visibility does not deteriorate even in the region far from the point of sight. Therefore, in the present invention, the characters along the road can be displayed in an aspect which avoids troubles that visibility is damaged or the road is hidden by the characters. 
     The present invention does not necessarily have to include all of the aforementioned various characteristics but some of them may be omitted or combined in configuration as appropriate. In addition, 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, it may be configured as a computer-readable recording medium such as a CD-R, a DVD and the like recording such computer program. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view illustrating configuration of a 3D map display system. 
         FIG. 2  is an explanatory view illustrating structures of feature data and character data. 
         FIG. 3  is an explanatory view illustrating tilt display of a character with respect to a road in a right-and-left direction. 
         FIG. 4  is an explanatory view illustrating the tilt display of the character with respect to a road in a perpendicular direction. 
         FIG. 5  is an explanatory view illustrating the tilt display of the character with respect to a road in a lower right direction. 
         FIG. 6  is an explanatory view illustrating the tilt display of the character with respect to a road in an upper right direction. 
         FIG. 7  is an explanatory view illustrating setting of a tilt angle. 
         FIG. 8  is a flowchart of route guidance processing. 
         FIG. 9  is a flowchart ( 1 ) of map display processing. 
         FIG. 10  is a flowchart ( 2 ) of map display processing. 
         FIG. 11  is an explanatory view illustrating a display example of a 3D map. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Embodiment 1 
     A. System configuration: 
       FIG. 1  is an explanatory view illustrating configuration of a 3D map display system  10 . A configuration example as a navigation apparatus performing route guidance from a starting point to a destination specified by a user while displaying a 3D map is illustrated. In this embodiment, the present invention is not limited to the configuration as the navigation apparatus but also can be configured as a system only for displaying a map. Moreover, in this embodiment, a system operated in a stand-alone manner is exemplified but a part of functions illustrated in the figure may be configured by a plurality of servers and the like connected via a network. 
     The 3D map display system  10  is configured by a computer including a CPU, a RAM, and a ROM. Each illustrated functional block is configured by installing software for realizing these functions. The functions of each functional block are as follows. 
     A map database  20  stores map data used for 3D map display. The map data includes network data  21 , feature data  22 , and character data  23 . The network data  21  is data representing a road by a link and a node and is data used for route search. The feature data  22  is data storing 3D models representing three-dimensional shapes of the features such as a road and a building. The character data  23  is data storing character strings displayed in the map. 
     A main control unit  14  exerts a function for controlling an operation of each functional block. A command input unit  11  receives an input of a command by the user. The input commands include specification of a starting point and a destination of route guidance, specification of a display range of the map and the like. A route search unit  12  performs route search from the specified starting point to destination by using the network data  21 . For the route search, a known method such as the Dijkstra method can be applied. A GPS  13  acquires position information from a GPS (Global Positioning System) or the like and specifies a current position. A display control unit  15  performs map display. In this embodiment, a 3D map is displayed but a 2D map may be additionally displayed. 
     The display control unit  15  has a feature image generating unit  16 , a character image generating unit  17 , and a superposing unit  18 . The feature image generating unit  16  arranges the 3D model stored in the feature data  22  in a virtual 3D space and performs perspective projection from a specified position of the point of sight and direction of the line of sight. This projection view is called a feature image in this embodiment. The character image generating unit  17  arranges a polygon on which a character string stored in the character data  23  is pasted (hereinafter also referred to as a “character polygon” in some cases) in the virtual 3D space and performs parallel projection. This projection view is called a character image in this embodiment. The superposing unit  18  displays the 3D map by superposing this character image on the feature image. 
       FIG. 2  is an explanatory view illustrating structures of the feature data  22  and the character data  23 . On an upper side of the figure, the structure of the feature data  22  is illustrated. An ID is identification information given to each feature. A type is information indicating a type of the feature such as a building and a road. A shape is data representing a 3D shape of each feature. Regarding the building, 3D coordinates of apexes PP 1  and PP 2  of a polygon representing a 3D shape are stored. The road is represented not by a polygon but by 3D line data in this embodiment, and 3D coordinates of configuration points LP 1  and LP 2  of the line data are stored. 
     On a lower side of the figure, the structure of the character data  23  is illustrated. An ID is identification information for each character data. The feature is information for specifying a feature with which the character data is associated. In this embodiment, the ID of the feature data  22  is stored. For example, since ID 1  is stored as the feature for the character data with CID 1  in the figure, this character data means that it represents a name of the building or the like indicated by ID 1  of the feature data  22 . Regarding the character data of CID 2 , since ID 2  is stored as the feature, this character data means that it represents a name of the road or the like indicated by ID 2  of the feature data  22 . The character string is characters which should be displayed. A position is a three-dimensional positional coordinate where the character is displayed. An attribute is information indicating a type of the characters, and two types of the attribute, that is, “general” and “street name” are prepared in this embodiment. The “street name” indicates the name of a road or the like, while the “general” indicates the other character strings. The attribute is information used in display control of the characters which will be described later. The character data  23  may also store various types of information such as character sizes and fonts other than the above. 
     B. Setting of tilt angle: 
     In this embodiment, display of the character string stored in the character data  23  is controlled in the following modes. First, the characters are sorted to the street names and the other generals and only the street names are made to be control targets. The general character strings may be displayed two-dimensionally on a map image or may be displayed with the feature by the perspective projection. 
     Regarding the street name, the character string is displayed in a direction along the corresponding road. The character string is displayed as a signboard leaning diagonally on the ground surface by rotating it, that is, by tilting it around lower ends of the characters with respect to the ground surface. This angle of tilting shall be called a tilt angle below. The tilt angle is set in accordance with a direction of the road in the perspective projection view and a display position of the character in the perspective projection view, that is, a distance from the point of sight. A specific setting method of the tilt angle of the character will be described below. 
       FIG. 3  is an explanatory view illustrating tilt display of the character with respect to a road in the right-and-left direction. 
     On an uppermost stage, a state of the roads in a 2D map is illustrated. On a middle stage, a state of the perspective projection of the roads is illustrated. By means of the perspective projection, as it goes farther from the point of sight, an interval between the roads becomes gradually narrower, but the direction of the road is the right-and-left direction also in the perspective projection. The directions of the roads are indicated by angles AV 1 , AV 2 , AV 3  and the like based on a vertical direction (one-dot chain line in the figure) of the perspective projection view (hereinafter this angle shall be called a display direction in some cases). In the case of the road in the right-and-left direction, the display directions AV 1 , AV 2 , and AV 3  are approximately 90 degrees. 
     On a lower stage, a display example of a 3D map displaying the road with the characters superposed thereon is illustrated. The road is displayed in a perspective projection view. The positional coordinates in the perspective projection view are indicated by coordinates of a lateral direction u and a vertical direction v. The tilt angles of character strings CH 1 , CH 2 , and CH 3  of the street names are set such that as they move from the front of the point of sight to the depth as illustrated on the right side in the figure, that is, as the positions V 1 , V 2 , and V 3  in the v-axis direction in the perspective projection view become larger, the angles become gradually larger from  0  degrees as the angles TA 1  to TA 3 . Regarding a range to which such control is applied, the display directions AV 1  to AV 3  do not have to be strictly 90 degrees but may have a certain margin. 
       FIG. 4  is an explanatory view illustrating the tilt display of the character with respect to the road in the perpendicular direction. 
     On the uppermost stage, a state of the road in a 2D map is illustrated. The perpendicular direction means that the two-dimensional direction of the road is substantially a vertical direction. 
     On the middle stage, a state of perspective projection of the roads is illustrated. Even after the perspective projection, the direction of the road stays in the perpendicular direction and does not change. The display direction AV is approximately 0 degrees. 
     On the lower stage, a display example of a 3D map displaying the characters superposed on this road is illustrated. The road is displayed in a perspective projection view. Character strings CH 4  and CH 5  of the street names become unnatural however they are tilted if they are displayed along the road. An example in which diagonal roads are illustrated on both sides of a center road and the characters are tilted and displayed is illustrated. If the characters are displayed by being tilted as above on the perpendicular road, a sense of discomfort remains in the display however they are tilted. Therefore, for the perpendicular road, the tilt angle shall be 0 degrees regardless of the values of the positions V 4  and V 5  in the v-axis direction in the perspective projection view from the front of the point of sight to the depth as illustrated on the right side of the figure. 
     However, in order to give a feeling of the depth by the display state of the characters, regarding the road displayed perpendicularly, as the positions of the characters go from the point of sight to the depth, that is, as the positions V 4  and V 5  in the depth direction become large, a width of the characters in the vertical direction or in a direction along the road is reduced. This display can be realized by tilting with respect to the ground surface by using an end on a side closer to the point of sight in the character string or a side of the character “T” at the end of “SFLOWERST” as an axis in the character string in the figure, for example. This is called character-width reduction in the embodiment. As seen on the right side in the figure, the tilt angle is 0 degrees to the perpendicular road, but reduction of the character width is applied in accordance with the position in the depth direction. A range in which such control is applied does not necessarily have to be limited to the case that the display direction AV is strictly 0 degrees but can have a certain margin. 
       FIG. 5  is an explanatory view illustrating the tilt display of the characters with respect to a road in a lower right direction. 
     On the uppermost stage, a state of the roads in a 2D map is illustrated. Roads in the lower right substantially in parallel are illustrated. 
     On the middle stage, the state of the perspective projection of these roads is illustrated. By means of the perspective projection, as it goes farther from the point of sight, the interval between the roads gradually becomes narrow, and display directions AV 10  and AV 11  of the roads change. Even for the roads in parallel on the 2D, the display directions AV 10  and AV 11  get closer to 90 degrees as they go deeper from the point of sight. In the case of the road in the lower right, the display direction is within a range larger than 0 degrees and smaller than 90 degrees. 
     On the lower stage, a display example of the 3D map displaying the characters superposed on these roads is illustrated. The tilt angles of character strings CH 10  to CH  13  of the street names are set in accordance with the position in the v-axis direction in the perspective projection view and the display direction. 
     First, regarding the relation between the display direction and the tilt angle, as illustrated on the lower side in the figure, the smaller the display direction is, that is, the closer the road is to the perpendicular, the smaller the tilt angle becomes, while the larger the display direction is, that is, the closer to the horizontal the road is, the larger the tilt angle becomes. For example, since the display direction Av 11  of the character string CH  11  is larger than the display direction Av 10  of the character string CH 10 , a tilt angle TA 11  of the character string CH 11  becomes larger than the character string CH 10 . Similarly, a tilt angle TA 13  of a character string CH 13  becomes much larger. 
     The relation between the display position and the tilt angle is set so that the deeper the display position goes, the larger the tilt angle becomes as illustrated on the right side in the figure. For example, when the character strings CH 12  and CH 13  are compared with each other, their display directions are the same but it is the display position V 12 &gt;display position V 13 , which results in the tilt angle TA 12 &gt;tilt angle TA 13 . Since the display positions V 10  and V 11  of the character strings CH 10  and CH 11  are smaller than the display position V 12  of the character string CH 12 , and since the display directions Av 10  and Av 11  are also smaller than the character string CH 12 , the tilt angles of the character strings CH 10  and CH 11  are smaller than the character string CH 12 . 
       FIG. 6  is an explanatory view illustrating the tilt display of the characters with respect to a road in an upper right direction. 
     On the uppermost stage, a state of the roads in a 2D map is illustrated. Roads in the upper right substantially in parallel are illustrated. 
     On the middle stage, the state of the perspective projection of these roads is illustrated. By means of the perspective projection, as it goes farther from the point of sight, the interval between the roads gradually becomes narrow, and display directions AV 20  and AV 21  of the roads change. Even for the roads in parallel on the 2D, the display directions AV 20  and AV 21  get closer to 90 degrees as they go deeper from the point of sight. In the case of the road in the upper right, the display direction is within a range larger than 90 degrees and smaller than 180 degrees. 
     On the lower stage, a display example of the 3D map displaying the characters superposed on these roads is illustrated. The roads are displayed in the perspective projection view. The tilt angles of character strings CH 20  to CH  23  of the street names are set in accordance with the position in the v-axis direction in the perspective projection view and the display direction. 
     Regarding the relation between the display direction and the tilt angle, as illustrated on the lower side in the figure, the larger the display direction is, that is, the closer the road is to the perpendicular, the smaller the tilt angle becomes, while the smaller the display direction is, that is, the closer to the horizontal the road is, the larger the tilt angle becomes. For example, since the display direction Av 21  of the character string CH 21  is larger than the display direction Av 20  of the character string CH 20 , a tilt angle of the character string CH 21  becomes smaller than the character string CH 20 . Similarly, a tilt angle TA 23  of a character string CH  23  becomes much smaller. 
     The relation between the display position and the tilt angle is set so that the deeper the display position goes, the larger the tilt angle becomes as illustrated on the right side in the figure. For example, when the character strings CH 22  and CH 23  are compared with each other, their display directions are the same but it is the display position V 22 &lt;display position V 23 , which results in the tilt angle TA 22 &lt;tilt angle TA 23 . 
       FIG. 7  is an explanatory view illustrating setting of the tilt angle. This summarizes the setting methods of the tilt angle in  FIGS. 3 to 6 . 
     On the upper stage in the figure, a definition of the tilt angle TA is illustrated. The tilt angle refers to an angle formed by the character plate polygon and the ground surface. Assuming that a direction in which the character string is described as illustrated on the upper left of the figure is the x-axis, a rotating angle from the y-axis in the z-axis direction around the x-axis is the tilt angle. 
     As illustrated in  FIGS. 3 to 6 , the tilt angle is set by the display direction AV of the toad and a display position Vc of the character. When it is the display direction AV=0 degrees, that is, when the road in in the perpendicular direction, the tilt angle is 0 degrees as described in  FIG. 4  regardless of the display position Vc. In this embodiment, regardless of the display direction AV, the tilt angle is set to 0 degrees at a position the closest to the point of sight. When the display direction AV=90 degrees, that is, when the road is in the right-and-left direction, the deeper the display position Vc goes to, the larger the tilt angle becomes as described in  FIG. 3 . A range from 0 to 90 degrees of the display direction AV indicates the state of the road in the lower right direction, and the range from 90 to 180 degrees of the display direction AV indicates the state of the road in the upper right direction. In these ranges, the tilt angle is determined in accordance with the display direction AV and the display position Vc. 
     The setting example illustrated in  FIG. 7  is only an example, and the tilt angle can be set arbitrarily in accordance with the display direction AV and the display position Vc. In  FIG. 7 , the example in which the tilt angle is linearly changed with respect to the display direction AV and the display position Vc is illustrated, but setting may be made so that the it is changed in a curved manner. Moreover, instead of monotonic increase or monotonic decrease, the setting may have an extreme value. 
     C. Route guidance processing: 
     The processing example displaying a map by using setting of the tilt angle of the character string described above will be described using a case in which a navigation apparatus (see  FIG. 1 ) performs route guidance as an example in this embodiment. 
       FIG. 8  is a flowchart of the route guidance processing. When the processing is started, the navigation apparatus receives an input of a starting point and a destination (Step S 10 ) and performs the route search (Step S 11 ). The route search can be made by the Dijkstra method or the like using network data. Then, route guidance data representing an obtained route or information specifying a link string which should be a route, for example is generated (Step S 12 ), and the route guidance is performed on the basis of this. In the route guidance, the navigation apparatus detects a current position of a vehicle (Step S 13 ), performs the map display processing and displays the 3D map for guiding the route (Step S 14 ). This is repeatedly performed until the vehicle reaches the destination (Step S 15 ). 
       FIGS. 9 and 10  are flowcharts of the map display processing. It is processing corresponding to Step S 14  of the route guidance processing. In this processing, the navigation apparatus sets the position of the point of sight, the direction of the line of sight, and the display scale for drawing the 3D map by perspective projection (Step S 20 ). For the position of the point of sight, a method of setting it at a predetermined relative position in the rear of the current position can be employed, for example. The direction of the line of sight can be set to a direction of seeing the current position or the like from the position of the point of sight. Other than the above, the position of the point of sight, the direction of the line of sight and the like may follow specification by the user. The navigation apparatus reads the map data required for display of the 3D map (Step S 21 ). Then, first, the feature is subjected to the perspective projection so as to generate a feature image (Step S 22 ). 
     Next, the display position and the display direction in the feature image are calculated for the street name (Step S 23 ). The display direction Av can be represented by an angle from the perpendicular direction as described above in  FIGS. 3 to 6 . First, the street name is extracted on the basis of the attribute of the character data  23 , and the roads corresponding to the respective street names are specified. Then, the display direction of the road in the feature image can be acquired. The display position can be acquired by coordinate conversion of the 3D display position set by the character data in accordance with the perspective projection. 
     Next, the navigation apparatus sets the tilt angle on the basis of the display position and the display direction (Step S 24 ). The tilt angle can be acquired in accordance with the setting illustrated in  FIG. 7 . Then, the navigation apparatus arranges the character plate polygon in the virtual space by the set tilt angle (Step S 25 ). An arrangement example is illustrated in the figure. The plate polygon is arranged in accordance with set values of the tilt angles TA 0 , TA 1  and the like, and the character string is pasted to this surface. Regarding the street name, since the characters are displayed along the road, an arrangement direction of the plate polygon of the character is set such that the lower ends thereof are along the road. The display position is set such that it is arranged in accordance with the 3D coordinate set as the display position of the character. 
     The navigation apparatus generates a character image by applying parallel projection to the character plate polygon (Step S 26 ). The feature image is generated by perspective projection, but the character image is generated not by the perspective projection but by the parallel projection. 
     A projecting direction of the parallel projection can be set arbitrarily. The projecting direction can be represented by a tilt angle from below the perpendicular and a projecting azimuth but the projecting azimuth is preferably matched with the direction of the line of sight. As a result, a sense of discomfort can be alleviated when the character image is superposed on the feature image. The tilt angle from below the perpendicular does not necessarily have to be matched, but if this is also matched with the direction of the line of sight, the sense of discomfort can be further alleviated. 
     The parallel projection is a projecting method with no relation with the position of the point of sight. In the perspective projection, the character string located far from the point of sight is crushed and hard to be seen, but the character string included in the character image obtained by the parallel projection can ensure sufficient visibility regardless of the display position. 
     When the character image is generated as above, the navigation apparatus superposes the character image on the feature image (Step S 27 ). The superposing method may be any one of the following: 
     A first method is a method of superposing the character image and the feature image as they are. Since the feature image is generated by the perspective projection and the character image is generated by the parallel projection, the coordinate systems in the image of the both do not necessarily match each other. Therefore, the character string in the character image can be displayed with deviation from the position which should be. However, depending on the position of the point of sight and the direction of the line of sight of the perspective projection and the projecting direction of the parallel projection, the deviation between the feature image and the character image cannot be visually recognized in actuality in some cases. In such case, even if the character image and the feature image are superposed as they are, a map without the sense of discomfort can be displayed. 
     A second method is a method of arranging the character image on a map image for each character string. In this method, the character image is configured for each character string, and the 3D coordinate representing the display position is subjected to coordinate conversion in accordance with the perspective projection, and the display position on the feature image is acquired. Then, this character image is superposed on the feature image. By executing this for each character string, the respective character strings can be displayed at appropriate positions on the feature image. 
     When superposition of the feature image and the character image is completed, the navigation apparatus displays other characters (Step S 28 ) and finishes the map display processing. 
     D. Output example, effect and variation: 
       FIG. 11  is an explanatory view illustrating a display example of the 3D map. Here, a state in which only the character strings of the street names are displayed is exemplified. As illustrated, the road is drawn by the perspective projection by the position of the point of sight and the direction of the line of sight from up in the sky. The street names are displayed so as to follow the respective roads. As a result, the correspondence between the roads and the street names can be easily grasped visually. The street names are three-dimensionally displayed by various tilt angles. As a result, a three-dimensional feeling of the entire map is given, and the characters can be also represented three-dimensionally in diversified modes. 
     In this embodiment, the tilt angle of the street name can be set on the basis of the display direction and the display position of the road. Therefore, the user can intuitively feel the depth feeling of the character string by the tilt angle of the character and intuitively recognize the positions of the respective streets. 
     Moreover, in this embodiment, the tilt angle is set larger as it goes to the depth from the point of sight. In the perspective projection, the farther it goes from the point of sight, the smaller the road width becomes, and an interval between the roads also becomes smaller and thus, a region in which the character string of the road can be displayed also becomes narrower, but by making the tilt angle of the character string larger as above, the character string far from the point of sight can be represented in a mode according to the smallness of the region. 
     According to the 3D map display system of this embodiment described above, by generating the feature image and the character image separately and by applying the parallel projection to the character image, visibility of the characters can be ensured in a region far from the point of sight while the characters are displayed in diversified three-dimensional modes. Moreover, by setting the tilt angle when the character is displayed in accordance with the display direction of the road and the display position of the character string, a depth of the road can be represented by the display state of the characters and a map whose positional relation can be intuitively grasped easily can be provided. 
     The embodiment of the present invention has been described. The 3D map display system of the present invention does not necessarily have to include all the functions of the aforementioned embodiment but only a part of them may be realized. Moreover, additional functions may be provided to the aforementioned contents. It is needless to say that the present invention is not limited to the aforementioned embodiment and can employ various configurations within a range not departing from its gist. For example, a portion configured in a hardware manner in the embodiment can be also configured in a software manner and vice versa. 
     The present invention can be used for displaying a 3D map three-dimensionally representing not only the features but also the characters.