Image data

SuperMap Realspce supports preprocessing image files in *.bmp, *.gif, *.jpg, *.png, and *.tiff formats to generate image data that can be loaded in a 3D scene. Image data can then be added on the globe surface in a 3D scene according to their coordinate information. The image data here include image datasets (sit), raster map datasets and corresponding 2/3D image cache data (sci, sci3d), and 2/3D map cache data (sci, sci3d). Because the size of image/raster map data is usually large, pre-caching is usually employed when a service containing this type of data is to be published, to improve the efficiency of loading and viewing the data. The idea is that, after pre-caching, the image data saved on the SuperMap iServer server has been preprocessed.

The processes of caching image/raster map data are: first partition the data and generate tile data; save the tile data into corresponding cache folders after naming the tile data based on certain rules; in the mean time, the system will generate an index file in *.sci3d format in the root cache directory. The tile data are usually in the PNG, GIF, or JPG format. The *.sci3d file records the cache information of the map data, i.e., it is the configuration index file of the cache data files of the image/raster map. When adding image/raster map cache data in a 3D environment, users can load data by loading the corresponding index file of the image/raster map cache. Thus the efficiency of loading and viewing massive image data and raster map data can be greatly improved by caching the image/raster map.

The figure below shows the cached image in image format after a remote sensing image and map data are loaded in a 3D scene and published as a realspace service.

To improve the data transmission speed and lower the data transmission volume, SuperMap Realspace considers the advantages and disadvantages of JPEG and PNG formats when deciding the format for the tile data in an image cache, then a caching scheme with mixed format is designed. The JPEG image format (*.JPG, *.JPEG) is a commonly-used image format. It is a format with lossy compression, and can compress an image to a size that fits in very small storage space. Thus it has very powerful compression capability. The PNG format (*.png) is the most popular format in WEB applications. It supports high level and lossless compression, and can produce compressed image files that are 30% smaller than GIF files without loss of data. Compared to lossy compression by JPEG, PNG offers a smaller compression ratio, but maintains the transparency of edges.

Considering the above characteristics, SuperMap 3D provides a mixed model of JPEG and PNG for caching. It allows users to use tiles in both JPEG and PNG formats for the same cache. Users can employ the JPEG format for tiles in the middle cache blocks to reduce the sizes of corresponding files, and employ the PNG format for the outer cache blocks (because the transparency of tiles in this part need to be maintained). This kind of mixed model has the advantages of both the JPEG and the PNG image formats. Cache files can benefit from the JPEG format and use less disk space. In the mean time the display of the entire image is not affected by the edges of cached tiles. The disk space used by cache files is effectively reduced, and the speed of image data transmission is improved.