Car navigation systems are mounted on, for example, automobiles, and automatically display maps around present positions detected by using the GPS system and the like, and/or guide routes up to preset destination locations. In such car navigation systems, the areas around the present positions and the routes up to the destination locations are required to be displayed as maps. To this end, various sorts of map display apparatuses are proposed. Generally, these map display apparatuses have scroll functions of map images. Here, “scroll”, means that a screen display is moved along upper/lower/right/left directions so as to display a location which is to be viewed. As to this “scroll” operation, two scroll operations are proposed, that is, a two-dimensional map scroll operation capable of scrolling the normal two-dimensional map image, and also, a birds-eye view scroll operation capable of scrolling a birds-eye view of a map.
Generally, a drawing processing operation of a map is very complicated. Also, when a map is drawn, an access operation to a memory is carried out at random. As a result, a high speed access method (burst mode) to a DRAM cannot be applied. For this reason, a display image can be hardly drawn in real time (for instance, 30 to 60 fps). This is explained in more detail with reference to FIGS. 11A, 11B, 12, 13A and 13B.
First, a processing flow operation executed in the normal display mode will be explained with reference to FIG. 11A. A map image is drawn by a drawing processing block. The drawn image data is stored in a memory by a memory control block. When an image is displayed, the image data is read out from the predetermined storage location in the memory by the memory control block. Then, such a processing operation as an RGB conversion and a simple image synthesizing operation is carried out with respect to the read image data by a display processing block. Thereafter, the processed image data is outputted to a display device at necessary timing.
Next, a high speed producing method of a display image while a two-dimensional map is scrolled is described with reference to FIG. 11B. When the two-dimensional map is scrolled, the drawing processing block draws in advance a map image having a larger area than the display area, and then stores this drawn map image in the memory. When the map image is displayed, the memory control block reads out a display area of an image in the memory. At this time, an image having a larger area than a display area has been stored in the memory. As a result, a scroll image is formed in a high speed by merely changing a read out position of the display area from the memory. In this method, the display image can be formed in real time (30 to 60 fps), and can be scrolled in a smooth manner.
In contrast to the above two-dimensional map scroll operation, in the birds-eye view scroll operation shown in FIG. 12, since display images are independently produced for every frame, such a method of the two-dimensional map scroll cannot be applied. The display images must be drawn every frame.
Here, it is assumed that a road having a constant width is drawn as a birds-eye view. In the case that a view point is moved from a point “A” to another point “B” as shown in FIG. 13A, as to both a birds-eye view formed at the point “A” and another birds-eye view formed at the point “B”, roads having the same size must be drawn thereon. However, in a case that the two-dimensional map scrolling method is employed, as shown in FIG. 13B, a birds-eye view larger than a display area is drawn by the drawing processing block, and a read out position of this drawn birds-eye view image is changed. The road size at the point “B” becomes smaller than the road size at the point “A” on the birds-eye view at the point “A” and the birds-eye view at the point “B”. This is not proper. As a result, the birds-eye views should be drawn every frame.
In practice, the time duration of 200 to 300 milliseconds (msec) is required in order to form one frame of a map image. Under such a display speed of 3 to 5 fps (200 msec to 300 msec/frame), movement appears unnatural, and the birds-eye views cannot be scrolled in a smooth display manner.
Thus, since the birds-eye view is produced by executing the drawing processing operation in the above manner, in particular, the images cannot be produced in real time while the birds-eye view is scrolled.