Appendix 5 | Second Point Cloud Example (UAVs) | Virtual CRASH 3


A Note to Virtual CRASH 4 Users

With Virtual CRASH 4, one can directly import point cloud data and use the Easy Surface Builder tool to create a terrain mesh from point cloud data. To learn how to incorporate point cloud data in Virtual CRASH 4, please see Chapter 20.


Introduction

Virtual CRASH 3 allows the user to import dxf and 3ds objects into the simulation environment. By transforming an imported mesh into a terrain object, the vehicle tire force model can interact with these objects. This is a great way to incorporate forensic scene evidence directly into a simulation. Here we demonstrate a second method to import point cloud data. You can find a simulation video using this scene below:

Point Cloud File

In this example, our point cloud was generated using a photogrammetric process using a photo series obtained by unmanned aerial vehicle (UAV) [1]. A top-down orthographic view of the resulting point cloud can be seen in the next figure below.

A perspective view of the scene can be seen in next figure below.

The point cloud is visualized in the free application CloudCompare [2].

Trim Points

Using the same process described in Chapter 18, we first remove excess points from trees and power lines in order to help obtain a better surface fit. The trimmed point cloud is shown in next figure below.

Calculate Normals and Surface

With your trimmed point cloud selected, go to Plugins > RANSAC Shape Detection. Select all shape types and press ok. Next, go to Plugins > Poisson Surface Reconstruction. Finally, select the mesh object and trim away excess polygons.

Final Trim

Enter the top-down orthographic view. Select both the point cloud and the mesh surface simultaneously.

Next, using the trim tool, cut the inner section of both the point cloud and mesh simultaneously. This ensures a good match in the outer perimeter of both shapes.

Another useful tactic is to trim the upper left and lower right hand panel corners of the point cloud only to reveal a small amount exposed mesh. This will further improve your ability to align the image and mesh in Virtual CRASH.

Export Image and DXF

Using the top-down orthographic projection of the final trimmed point cloud go to Display > Render to File to save an image of the point cloud and mesh objects [3].

This will serve as the color image map which will be projected onto the wire mesh. Next, export the final trimmed wireframe object as a dxf file [4].

Import into Virtual CRASH 3

Import the mesh object dxf file into Virtual CRASH 3.

You will be prompted for the units. In this example, the mesh is in feet. 

Next import the image file. Ensure that the upper left and lower right hand corners of the exposed mesh lines from the image align with the dxf mesh.

Once you have good alignment, you can freeze the image

make the mesh a terrain object, and start working on your simulation.


Notes:

[1] The point cloud file used in this example can be found here: http://tinyurl.com/goj8my3

[2] CloudCompare can be obtained for free online at: http://cloudcompare.org/

[3] The image file can be obtained at: http://tinyurl.com/ztr6pqq

[4] The final dxf can be obtained at: http://tinyurl.com/jfrw9da




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