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provide such data, without the need for real-time resampling.
That is advantages of pyramid model. If there
is no pyramid model, it is necessary to real-time
simplified based on the original terrain data and image
data for achieving the level of detail results. Despite an
increase the data storage space, the pyramid model can
reduce the time in mapping and speed up the rendering of
the scene.
B. Pyramid-based Texture Mapping
In 3D terrain visualization, the true reflection of the
terrain is an important part. Texture mapping is a major
technology to increase the authenticity. The Ground
Remote sensing image is a real record on ground floor
landscape; it contains a rich mount of information.
Therefore, take remote sensing image as texture image,
through texture mapping; 3D terrain model can have a
richer information and realism.
Texture mapping used in this paper, is a method which
is used in multi-resolution terrain LOD simplification.
Firstly, for the texture, establish a quadtree-based
pyramid and texture tree. Because the terrain geometry
and textures use in the same pyramid model, in real-time
terrain rendering, the appropriate resolution texture is
selected from the texture tree according the details of
terrain levels.
The terrain geometry and texture use in the same
pyramid model. When rendering the terrain geometry, in
accordance with corresponding relation, finding the
corresponding block, and then by way of texture of
texture mapping, the remote sensing image pictures was
draw to the terrain, thus increasing the real effect of the
terrain.
Ⅲ. VISUALIZATION OF VECTOR DATA IN 3D TRRRAIN
The basic idea that vector data visualized in 3D terrain
is, reading the vector data, raster 2D vector data into
texture image, and then using the standard texture
mapping method to draw, so the vector data can be draw
out in 3D terrain.
The 3D terrain visualization needs two kinds of data,
geometric terrain data (DEM) and surface remote sensing
image (DOM). Which are organized in the external
device in the form of layer and sub-block. But in this
paper, pyramid model was used which based on Quadtree.
The vector data take as whole, recorded regional
geographic information, not organized in the form of
block. Vector data can be visualized in 3D terrain by
raster into the form of texture. Texture block vector data
is composed of two parts: remote sensing image subblock
data, the vector data in the same region. As
showing in the Fig. 2, the terrain geometry and textures
use in the same pyramid model. When rendering to a
block of terrain geometry, according the corresponding
relation to find the corresponding block, and calculate to
get the location information that the remote sensing
image block represent, which according the pyramid tree,
and then read the vector data obtained in the region.
Using the render-to-texture, rendering the vector data to
remote sensing image block, and attributes of vector data
Figure 2. The schematic of vector data visualized in 3D terrain
such as color 、 fill-color, need to be determined in
advance of drawing vector texture mapping. So the new
remote-sensing texture block that obtain vector data was
got, which called “vector texture block”. Then the
ordinary texture mapping, texture mapping to the
topography of the block, making the three-dimensional
terrain rendered on vector data. As the vector data have
been drawn to the texture, therefore, the vector data can
be up and down with the terrain raising and falling.
Ⅳ. THE KEY TECHNOLOGY OF REAL-TIME RENDERING
A. The determination of details on different levels.
In this paper, the vector texture data is organized in
quadtree structure. According the distance of view point,
dynamically render the different LOD vector texture. In
the same distance, to determine the texture block that is
necessary in the current view of the area.
In order to real-time render the vector texture data. The
texture level detail was calculated by the distance that is
from the view point vertically to the ground. So the
texture level detail which the whole vector data rendered
is consistent. According to the features of quadtree, for
the four times differed between the neighbor level data, in
order to keep the texture resolution consistently which
displayed in screen. Switch the detail level when the
distance of the view point is twice.
The level of vector texture transits gradually, through
which the texture displaying scope controlled by the
1
Viewpoint
Detail level L
d
2
The distance between the different levels of detail
d
3
d
Figure 3. detail by switching levels
4
d
5
Ground
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