Based on the experimental verification and dynamic three-dimensional rendering of GIS technology process LOD

For the shortcomings of existing vector map dynamic LOD rendering perspective projection, the paper from the objective demands rendered using the trapezoidal grid unifies and simplifies the process of subdivision and to study a simplified algorithm trapezoidal grid under perspective projection. After analyzing the factors affecting simplified map, eventually, for GPU rendering process is proposed based on three-dimensional GIS vector map grid trapezoidal dynamic LOD rendering method. On this basis, design visualization platform rendering engine to achieve the map rendering, design and the typical test case to verify the feasibility and applicability of the method.
1. The frame structure visualization platform
designed in this paper include map visualization platform organization module, operating module and the map rendering module to perform the three sub-modules. Geographic data, symbol libraries and data to determine the style map rendering style. Map data is organized in layer styles and geographic data rendering module for rendering vector map map design requirements, while adding a map operation module to support user operations. Visualization platform of the frame structure as shown in FIG.

(1) map module tissue
map organization module for visualization platform global tissue, comprising a layer class, a data source class, and class map rendering classes. Geometric vertex data source class contains data and vertex index data, to build a trapezoidal grid and add spatial index in the data source class, the user operates, retrieve the corresponding data area based on the map, to the rendering module for rendering. Layers comprising layer class ID, the proportion of the layer is visible range, the color layer, layer transparency, and other related properties, each layer corresponding to a class layer rendering module to render class, provided for the layer properties shader program is completed rendering of the map.
(2) map operation module
maps user operation module for realizing the basic operation of the map. Map operation module provides a number of interface includes acquiring the map data, the map set attributes such as: pitch angle, rotation angle, the level map, the map center point; map may be translated through the operation module such as the user, rotate, zoom, tilt, etc. map operation.
(3) rendering execution module
Rendering execution module performs the map rendering, the rendering performed in the vertex data organization module completes the transmission, the memory update EBO data, map symbols of rendering operations. Performing rendering module uses OpenGL 3.0 interface enables, solid fill trapezoidal grid line plane. First, the affine matrix to achieve the geographical coordinates to screen coordinates affine transformation, design the appropriate class for rendering layer layer class design corresponding shader program for the complete mapping layer styles.
2. Experiment 1: Examples of effects of the proposed method to draw
2.1 experimental data
select lines and polygons OpenStreetMap i.e. four data sets (land use, buildings, rivers and China natural elements) as the test data set, the data set comprising a total of 42 334 and 3040 polygons lines, including simple polygons, polygons with holes, polygonal with islands, single and multi-line. The four data sets are organized into four layers and filled with a solid color symbolized.
2.2 Experimental Design and Analysis
In order to verify the effectiveness of the method used in this experiment the solid fill map symbolize achieve zoom, pan function, rotation, tilt function, the function of flight map for the user to operate. This experiment obtained by simulating a user operation amplifying effect gradually enlarge the map, the following figure (a), (b), (c) is gradually enlarged planar geometric elements results can be observed on a planar map with a map geometric elements enlarged gradually showing more detail; FIG under (d), (e), (f) a linear geometric elements gradually enlarge the result can be observed linear geometric elements on the map as the map is gradually enlarged show more and more detail. Verified that the proposed method can achieve better LOD effect.

To verify the validity of the proposed method is simplified, without a map feature visibility test case, by simplifying the control of the magnification M of tolerance, and observe the effect of rendering mapping the number of nodes N in the global state. For M values 0,1,10 separately tested. FIG follows (a), (b), (c) , the number of rendering nodes following three cases are layers 2,066,480, 981,240, 335,032: below (d), (e), (f) , the drawing three cases the number of nodes offline layers were 394244,104860, 12160. Validated, this method can simplify the line, on the basis of geometric elements to ensure that the surface pattern drawn on the effect of reducing the number of nodes in the drawing.


At the same time, we design two kinds of solid and surface 3D rendering styles, 3D surface rendering styles to support the rendering of the building. Which render the results as shown below.

3. Experiment 2: Comparative experiment rendering efficiency
methods described herein are implemented using OpenGL3.0 for all tests were run under Windows 10 system, equipped with a 3.4GHz Intel Core 4 Quad CPU, 8GB RAM , and Nvidia GeForce GTX 960 driver version 10.6 on the PC to perform apple wood 1920 * 1080 resolution.
3.1 Experimental data
selected four OpenStreetMap line and polygon data sets (land use, buildings, natural elements and Chinese river) as the test data set, data set contains a total of 42,334 polygons and 3040 lines, including simple polygon, perforated polygons, polygons, single and multi-line with the island. The four data sets are organized into four layers and filled with a solid color symbolized.
3.2 Experimental Design and Analysis
In order to verify the efficiency of this method, and methods described herein without LOD conventional dynamic and efficient method of comparing a tile Mapbox vector (real slice) comparing rendering method, Mapbox tile vector (real slice) method Geojson data format to load, use geojson-vt libraries open source real-time slice, slice contains a process, hereinafter referred to as Mapbox
Vector drawing method tiles. Paper by simulating a user to pan, zoom, tilt, and four kinds of maps flight operating scenario, the same data were used for the proposed method and the drawing method Mapbox vector multiple tiles tested are averaged to the experimental results.
(1) translating the test
for the same data, using the methods described herein and methods for rendering Mapbox translation vector tile test pan is divided into small-scale test operation test pan, zoom operation with a scale and large scale pan pan test three test stages, during , its comparative overall efficiency as shown in FIG.

Analysis, the small scale (data is reduced to nearly invisible) are two methods of data is greatly simplified, drawing very little data, direct drawing method Mapbox vector using cached tiles tiles finish drawing, using caching methods described herein draw, drawing both efficiency approaching. In the scale (global data shown) to drag the map, the drawing method Mapbox vector tile has been completed this time slice, but the methods described herein may be dragging a partial update data. Large scale (local data show), the initial vector drawing method Mapbox tile not complete slice of the surrounding data, real-time slice may occur in the process of translating the map, and the higher the efficiency of the proposed method simplifies the comparison to Mapbox vector tile rendering rendering efficiency higher initial slice after slice Mapbox vector drawing method to complete the tile drawn below its efficiency, as shown in FIG.

In contrast to the overall rendering efficiency of vector drawing method Mapbox tile method described herein above. Rendering based on the above schematic seen, complete with Mapbox vector tile slices, the method described herein below rendering efficiency gradually rendering efficiency Mapbox method. But the average overall test time translation of this method is 2.65ms, under a large proportion of the local test mean 3.44ms, the proposed method simplifies real-time and rendering efficiency and achieves Mapbox tile vector drawing method near high mapping frame rate, to meet user demand for real-time viewing.
(2) scaling the test
for the same data, using the methods described herein and methods Mapbox vector drawing two successive tiles zoom operation, the overall efficiency comparison as shown in Figure 1.

Figure I

Figure II
FIG based on analysis shows that, when the first scaling Mapbox tile vector map rendering method of the ongoing level jump (a change scale) leads to a large number of features in the slicing operation during zooming, the red peaks seen in Figure 2, and presented herein simplified methods described herein when the map level jump (a change scale), which simplifies vector tile Mapbox efficient than rendering efficiency slice method, two peaks in the purple as shown in FIG. Both methods are employed drawn between cache levels, except that the tiles Mapbox vector drawing method using the static methods described herein vector pyramids dynamic buffer structure for storing data. Amplifying the initial process, when the slice Mapbox tile vector drawing method is much higher than the progressive long duration of this method; however, reduction process, the method of drawing a tile Mapbox vector slice has been completed, and gradual approach requires real-time simplify. Figure II according to the number of two amplification process produces a number of peaks found in the amplification process simplified methods described herein, as compared to tile rendering method of generating vector Mapbox more levels of detail.

FIG based on analysis, after the completion of slicing Mapbox vector tile method, the result is stored to the vector tile sections pyramid, slice data for direct use high efficiency plotted when drawing the drawing. However, the proposed method in real time to simplify the time-consuming relatively long. Mean time scaling method described herein is about 4.O5ms, Mapbox tile vector drawing method scaled mean time of about 2.38ms, but with a tile sections Mapbox vector drawing process is completed, the efficiency of drawing the vector drawing method tile Mapbox It will gradually increase until it is stable. As shown in the figure above, the mean vector Mapbox completed sliced tile consuming scaled drawing method is about 1.21 ms, the mean of this method is about 3.71ms.
However, the proposed method with a small memory footprint rendering efficiency can be obtained with methods similar tile Mapbox vector, and a simplified Scaleless adaptive method, levels between the smoother, higher rendering efficiency, to meet the user real-time view of demand.
(3) Test pitch
for the same data, using the methods described herein and Mapbox vector tile frame rendering method of any pitching test. Continuously varying the inclination of test data herein show the state of global, efficiency as shown in FIG.

Analysis, the large pitch angle situation, Mapbox vector tile DP algorithm rendering method using a simplified pyramid vector construct, which depends on the number of slice level map data and the visible range, the viewpoint is not affected by distance. This results in a slice in a manner where a large angle, the number of slices Mapbox vector drawing method tile surge, is extremely long time sliced, such as red peaks occurring above figure (this number represents the node time slice), so that the tile vector drawing method Mapbox rendering efficiency is lowered, which is the average operating time was pitch 77.33ms. Simplified and more efficient than the methods herein Mapbox tile vector drawing method according slice efficient and simplified view, the distance data that is maximally simplified method rendering high efficiency Mapbox vector with respect to the tile, which time average pitch operation 21.05 ms. Verifiable, with respect to the proposed method and rendering efficiency Mapbox tile vector drawing methods greatly enhance the tilting operation.
(4) Flight Test
map flight operation refers to leap from one position to another position between a process achieved by the two point positions between smoothing interpolation. Map is a flight operations include translation, rotation, scaling, combined operation of multiple operations such as pitch. For the same data, using the proposed method and Mapbox tile vector drawing method any frame test flight, flying a large-scale map at the end of a great dip. An efficiency as shown, CPU usage as shown in Figure 2, the memory occupancy table below.

Figure I

Figure II

analysis shows, tiles vector drawing method Mapbox long time at a large angle sections, a sharp increase in CPU usage, but after completion sectioned tile pyramid vector using cached results slice, plotted directly using the cache data, rendering efficiency as a whole is excellent, but the memory and CPU utilization is relatively large. The proposed method based on simplified viewpoint, the flight data led to the changing view map constantly streamline operations, drawing a long time. But with less memory and CPU utilization, efficiency can be achieved to draw about 50 frames per second, to meet the user real-time view of demand.
In summary the experiment results, the proposed method in a translation operation, scaling operation, a smaller memory footprint and CPU acquires Mapbox tile vector drawing method similar efficiency can be achieved than Mapbox drawing method of drawing vector tile operation of the pitch efficiency in flight operations can achieve rendering efficiency of about 50 frames per second. Further, due to the presence in the tilting operation using the cache drawing, since the greater inclination during flight, resulting in real-time changes in real-time viewpoint simplified so that the cache can not be used to draw such that the overall efficiency of the operation is less than the pitch of flight operation, the average time which is plotted in the following table shows.

4. Conclusion In
this paper, the data organization module map, the map drawing operation module and the operating module introduces three frame structures described herein rendering engine design. Second, by rendering comparative experiments, to verify that the proposed method can effectively ensure that the drawing in simplified vector data on the basis of the result verified the effectiveness of the method. By rendering efficiency comparative experiments, to verify the efficiency of the proposed method, compared with the drawing method can Mapbox vector tiles with smaller memory and CPU usage reaches just below Mapbox the rendering efficiency. At the same time, the proposed method is an adaptive method of the stepless scale, compared to the problems of the jump vector Mapbox method for fixing tiles to draw level with better fluency, we can ensure a high rendering frame rate.
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