Photogrammetry with large structures: Experiments with Autodesk Recap 360, Meshlab and BuildIT

September 29, 2015 Onat Ekinci

Figure 1 – Workflow: Photogrammetry (Autodesk Recap 360) – Cleaning (Meshlab) – Analysis (BuildIT)

Photogrammetry, despite sounding rather complex, is basically  the stitching together of a series of photos to obtain a 3D model. In this post, we will analyze the results of a 3d model provided by Autodesk Recap 360. This post will just be a simple example; we will delve deeper in future posts for more detailed analyses.

Our general workflow can be summarized as follows (Figure 2):

Figure 2 – Photogrammetry workflow with related software websites: [http://www.apple.com/ca/ipad/] [https://recap360.autodesk.com/] [http://meshlab.sourceforge.net/] [http://www.builditsoftware.com/]

Taking Photos:

First, you have to take good photos. Please see addendum for ideal environmental conditions. For more information, please refer to [1]. In this example, our camera was an iPad camera, so the image quality is not the best possible. Also, the sky was cloudy and lighting was quite unstable, which created several blurry regions in the subsequent stitching steps.

Submitting Photos to Autodesk Recap 360:

Autodesk Recap is an online service that can create 3D models from the photos you submit (Figure 3). You have to go to the Autodesk Recap 360 website and log in. Autodesk Recap 360 is a subscription based application; for more information on their fees, please refer to the Autodesk website.

Figure 3 –  Submitting photos to Autodesk Recap 360

Once your model is created you can just right click and download the .obj model. (You can also create a .rcm model, but here we will focus on the .obj file type).

Cleaning the point cloud and model in Meshlab:

For our analysis purposes in BuildIT, we have to transform the .obj file into an .stl file and also take the vertices of the meshes as point clouds. This procedure can be summarized as follows:

  1. Import the .obj file in Meshlab (Figure 4).
Figure 4 – .obj model opened in Meshlab.
  1. File–> Export Mesh as… Import twice as .stl and .xyz files.
  2. Close the file and open the newly created .stl and .xyz files.
  3. Clean the unwanted faces on the .stl file:

Edit > Select faces in rectangular region > Select the faces you do not want > Delete the faces

  1. Clean and simplify the .xyz file:

Edit > Select vertexes > Select the vertexes you do not want > Delete the vertexes

 

Analysis in BuildIT

For demonstration purposes, we will conduct a very simple analysis in BuildIT. To start, we import the clean .stl and .xyz files into BuildIT (Figure 5).

Figure 5 –   .stl and .xyz files opened in BuildIT software

Our aim here is to analyze the surface deviations on the cylindrical region (Figure 6).

Figure 6 – Region to analyze on the imported mesh.

For this, we begin by filtering the points related to this region and extract a cylindrical surface feature. Then we carry out a surface deviation analysis on the extracted geometry. The results are shown in Figure 7. This kind of analysis can be useful in the preliminary inspection of a large structure.

Figure 7 – Surface deviation analysis results with BuildIT software


REFERENCES

  1. Autodesk Recap documentation, pages 25-26

ADDENDUM

Ideal Lighting conditions for Photogrammetry

You should avoid any environment in which the light creates strong contrast shadows over the object you photograph. Shadows are the enemy of the reconstruction process.

  • For interior shots:  The object must be placed in a room with diffuse light that does not produce contrast light or shadow.   Do not use the flash.
  • For exterior shots:  Avoid shooting at midday under strong sun. The best times of day to photograph for photogrammetry are early in the morning or later in the afternoon.
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