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Point cloud coloring and laser scan registration

 

Welcome to the 3DF Zephyr tutorial series.

In the following tutorial you will learn how to:

import a point cloud collected by using laser scanner equipment or any other external source, align that with the point cloud you generate with 3DF Zephyr and then color the original laser scanner cloud with the photogrammetry information, as well as pure laser scan clouds registration.

This is a tutorial for Zephyr 3.0 – the old tutorial for Zephyr 2.x can still be found here

The registration menu allows to handle point clouds registration. From this menu you can access the ICP function ( Iterative Closest Point (ICP) is an algorithm employed to minimize the difference between two clouds of point ) and see a point cloud difference report.
  • Point cloud registration (ICP)

Use this function when you need to align a laser scanned point cloud with a photogrammetry point cloud: this function consider translation, rotation and scale. Using this function you can then align a laser scanned point cloud which can then later, for example, be structured and colored with the photogrammetry obtained point cloud.

  • Multi view point cloud bundle adjustment (Multi ICP)

Use this function when dealing with many point clouds that are already in the correct scale system, for example multiple scans from a laser scanner device. This function does not apply any scale factor.

Please remember that:

  • the objects must be close to each other
  • You can used this tool to register multiple unstructured objects and structured ones
  • the adjustment and the minimization will be performed in a global way
  • if you need to register point sets with different scales, it is advisable to use the two view ICP dialog and enable scaling.

Importing and coloring an external point cloud

3DF Zephyr can import many file formats as well as some native laser scan formats. Native file formats (such as .fls and .fws) support can be installed by installing the correct plugin from this page.

For the first part of this tutorial we will generate a quick point cloud from few photos, import an external point cloud into Zephyr that will then be aligned and scaled and then colorized. The reference object can be either the external point cloud or the photogrammetry one.

download dataset Download 3DF Zephyr Tutorial Dataset – Mirandola (230MB)

  • Step 1 – generate a dense point cloud

Use the pictures provided in the example dataset to extract a dense point cloud (Urban/default in both the SfM and MvS steps will work) or open the Zep file, which is ready for step 2.

  • Step 2 – external point cloud import
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In order to import the external point cloud (in this specific case, mirandola_laserscan.ply – a laser scan point cloud saved as ply and simplified for tutorial purposes) simply drag and drop the desired file into the workspace or use the import menu.

When asked, click merge.

Zephyr will import the file in its workspace as an unstructured object. In order to make it structured, the two point clouds must be aligned first.

  • Step 3 – registration of the external point cloud and coloring
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laser_scan_photogrammetry_tutorial_zephyr_03
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The external point cloud will be out of scale, rotation and position. Use the Gizmo (1) to bring the two point clouds close togheter and approximatively of the same size.

This process is extremely easy since it’s a visual process, however can be tricky recognizing different point clouds. We will tackle this possible issue in later in this same tutorial. Make sure to select the correct object in the object selection menu (2).

When the point clouds are roughly aligned and scaled, click OK (3).

You are now ready to run the ICP wizard: from the Tools Menu, select Registration and then select Point cloud Registration (ICP).

The ICP wizard will appear. Follow the on screen instructions (in this case, you should be able to leave everything to default).

On the last page, you can setup the stop condition for the ICP algorithm. You may leave everything to default, however, make sure to check the “Enable scaling” checkbox and proceed with the computation.

When the computation has finished, a residual histogram will be shown onscreen. You may access this chart again from the Tools > Registration > Point Cloud Comparison menu.

All that is left to do now is to simply right click the unstructured object and then select “make structured” (4).

The Make structured dialog will appear: select the appropriate options (i.e. compute colors and compute normals) and then click OK.

The new structured point cloud can be then processed and manipulated like any other point cloud: all tools mesh extraction, contour line generation, etc. can work with any structured point cloud, regardless of its origin.

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Laser scan registration

Laser scan registration in Zephyr 3.0 has been made extremly easy and extremly accurate thanks to our brand new multi ICP algorithm. The process is very similar to the one described above, except that the laser scan registration usually does not have a scale factor to consider. The process can be summarized in this very short video tutorial:

The next tutorial will show how generate contour lines and sections. Proceed to the next tutorial.