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distance of view point. Given that the distance of view
point was increasing, the vector textures need to become
thicker from the level of L to L+1. The L level is “OUT”
stage, and the L+1 level is “IN” stage. Conversely, when
the point of view gradually decreases, the vector texture
become thinner from L level to L-1 level, the L level is
called “OUT” stage, while the L-1 level called “IN”. As
shown in Fig. 3
B. The determination of target sub-block visibility
For the view point, most domains of the terrain are
invisible, so cutting off that before rendering to accelerate
the speed of terrain rendering. The four tree node consist
of one three-dimensional bounding box which contains
all of its own sub-tree ,its leaf nodes is an actual drawing
of the block bounding box and adjacent to the Terrain
area block of the bounding box. If the leaf nodes of the
bounding box are located partially or entirely within the
visual, then this terrain block can be marked as visible.
Traverse the entire four-node-tree, cutting and processing
from the root, Each block will be marked as visible or not
visible, thus can obtained a collection of plots which
should be rendered.
Target sub-block search in fact accompanied by
resolution of search. If adopt one resolution, then only
related to the regional level search and domain search.
For the use of View-dependent multi-resolution terrain
visualization techniques to simplify the system, Require
different regions corresponding to different resolution
topographic data. Therefore, in sub-block search process
needs resolution test. Fig. 4 is Resolution requirements
for topographic block diagram, the white rectangle that
intersects with the view frustum and the need to
participate in the sub-block data Terrain Rendering, The
smaller rectangular block that higher the resolution
First, determine whether the topography of the region
can be seen that within the data. Testing and visible
regions of the top-level sub-block overlap is to meet the
resolution requirements, if met, will continue to test subbranch
block, until the search out all the requirements to
meet the resolution of sub-blocks. Algorithm steps
described as follows:
[1] Judge whether the Regional and topographic data can
be seen overlapping or not, If the overlap, then enter
the Step 2, otherwise the end.
[2] One by one to judge whether the block to meet the
resolution of top-level requirements, if met, will
insert it into the draw list, or a branch of its four subblocks
A, inserted into the test list.
[3] Judge whether the list is empty, if empty, the
algorithm ended, or else turn to Step 4.
[4] One by one to judge whether the list one by one in
the sub-block is overlapped with the visible region, if
the overlap will insert it into the test list B, otherwise
discard them.
[5] Clear the test list.
[6] Test one by one to determine whether the sub-block
in list B is to meet the resolution requirement, if met,
will insert it into the draw list, or a branch of its four
sub-blocks A, inserted into the test list.
[7] Clear test list B, enter step 2.
When overlapping test, As long as the four corners of
points in the block at any point in the visible region, It
means that the sub-block overlap with the visible region.
For the resolution test, first, calculate the goal resolution
of the four corners, If the goal resolution values of the
four corners are greater than or equal to which in the
pyramid layer.
Ⅴ. EXPERIMENT
Based on the study above, this paper presents the
vector data in three-dimensional terrain visualization,
Experimental results are shown in follow Figure. Fig. 5
does not contain the vector data for the three-dimensional
scene, Fig. 6 for the vector data contained in the same
area of the three-dimensional scene. From the two
Figures, we can see that the vector data lines can be
drawn high-quality and clearly. From Fig. 6, we can see
that the path line (yellow line) of vector graphics and
which in remote sensing images accurate overlap, and
Always tightly attached to the surface of the earth.
Roaming, amplifying or minifying the scene, vector lines
will not change with the terrain changes. This is
consistent with the characteristics of vector graphics.
In our experiment, we choose terrain elevation and
remote sensing image data in Hangzhou, Zhejiang
Province as the main data. The centre of the coordinate is
located at 30.301ºE, 120.231ºN. The bounds are
29.7513ºE ~31.2358ºE and 119.8352ºN~121.3525º N.
Image texture data: a region-wide multi-resolution
remote sensing image data with the amount of 113.25MB;
DEM data: SRTM region-wide multi-resolution
elevation data with the amount of 23.905MB.
We use the vector data in the urban area, Hangzhou,
Zhejiang Province. The centre of the coordinate is located
Figure 4. Resolution demand of target-blocking
Figure 5. Terrain effects chart comparison with non-vector data (left)and invector
data(right)
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