Realization Of Map Display Module Of Self-Contained Automatic Navigation System

Vehicle Navigation System is the outcome of integration of the technology of GPS, GIS and computer. It is an important part of the domain of automobile electronic, and it is also the trend of the development of vehicle navigation technology. Knowing from the traditional vehicle navigation technology, electronic map data is installed in the vehicle of the customer. As long as the user input the start and the destination, the system will tell the user the best path from the start to the destination, and the user also can adjust the destination at any moment. Through the position signal from the GPS, the system can display the Real-time position of the car in the electronic map. Combining with the voice prompt and menu prompt, the system can direct the driver where to drive. The most advanced navigation system at the present time can communicate with the traffic information center through wireless communication .If there is traffic jam in the front, the system can automatically compute a path which can avoid the traffic jam.The module of map display is the most important module in Vehicle Navigation System excluding the module of position. In the article, the author describes how to display map with GDI. GDI is the abbreviation of Graphics Device Interface. It means the interface of the graphics device. The most important task of GDI is the information exchange between system and the plot program. It deals with all the graphics output of all windows program. In the condition of windows operation system, the most of program with graphics interfaces can not run without GDI. With the numerous functions that the GDI supply, we can display graphics and text in screen, printer and other output device conveniently. With GDI, the programmer can transform the output of the application program to the output in the hardware device without regarding from the hardware device and the device driver. It isolates the programmer and hardware device and supply convenience to our work.The function of the map display module is to display map to the display device according to display condition that set beforehand. The map menu is also called canvas. The canvas include map information (route, background, text note) ,appended information of map (position cursor, the shortest route, track, information of attention point that is labeled, circumjacent information) and traffic information.The geography scope that the map covering with and the data quantity of the map information relates to the map scale. The bigger if the map scale became, the scope that it covering with became much smaller. To balance the vision load, we can make map information with different map scale have same data on the whole with the technology of LOD.The dimension of the canvas relates to the resolution of the display. To get better smoothness effect when browser the map, we usually make the dimension of the canvas be bigger than the dimension of the resolution of the display. For example, the width and the length of the canvas is 1.5 times that of the resolution of the display. The module of operating interface will distribute a view region in the display for the canvas. The canvas will cut out a part of map region that is the same size of the view region and then copy it to the view region, and displays it. When the view region is roaming in the canvas and the size of the view region is changed, the map that cut out from the canvas is changed. The size of the view region is limited by the display, and also is limited by the display strategy. For example, the display strategy of double screen and projecting the crossing by multimedia technology all have effect on the adjusting of the size of the view region.In substance, the process of displaying a map is to get a local part from the vector map data that covering the whole country and then project it in the display. The main process of the projection is: first, cutting out local region map data from vector map data from data of covering the whole country; second, transforming the data to raster display; third, cutting out what we want from the raster display .The cutting object of the first time is vector data, the cutting object of the second time is raster display. The size of the canvas is smaller than the size of the map of the country, and the size of the view region is smaller than the size of the canvas. The whole operation of the module of displaying map is to realize this projection operation with high quality and high speed, so the user can get a beautiful and dynamic map screen from the display.A beautiful map screen rests with the condition of plot. The plot condition includes the colors of map, visual angle of map, graphic parameter, annotation which can have effect on the graphic appearance. The plot condition also includes the strategy of adopting and abandoning automatically, the strategy of steer clear of the conflicts, the strategy of adjusting the scale of the map automatically which have effect on the map display effect. The dynamic map display effect relates to the forecast of the moving direction and the technology of high speed display when the map scale is changed. To improve the map screen effect, we adopt the next three technology. Next we will introduce them to you.The double buffer area technology is a technology to display a map screen completely and quickly. The process of display map screen is: first, drawing the vector map information and the appended map information and saving it in vector buffer area; second, cutting out a part of map region and displaying it in the screen.The process of reading the canvas beforehand is running under the control of the process of map display. When the map display process thinks that it is time to start this process to reading data, the process will read data form the driver layer and copy it to the No.1 buffer area or the No.2 buffer area. After this , the map display process will take over the buffer area. When the map display process think that it is time to start the reading process again, it will be start again, but the data will be saved in a buffer area that is different from the last time. So ,the two process take over the buffer areas by turns.The process of displaying map screen quickly also is running under the control of the process of map display. The difference is that the reading process is start when the user browser the map and the process of displaying map screen quickly is start when the user zooms in or zooms out the map.After running our software in the swatch machine, we find that we have basically finished the task that we have destined. When we browsers the map, the system can display the map smoothly. When we zoom in or zoom out the map, the system can display the map quickly. In the same way, the effect of the annotation is perfect.