Automating 3D Scanner Data Alignment Using Markers
Posted by Thomas Tong on Fri, Feb 24, 2012 @ 11:15 AM
In the previous blog post, Ways to Automate 3D Scanning, we discussed a few ways to automate 3D scanning to reduce scanning and processing time. Another way to speed up the process is to automate the alignment of 3D scanner data. This process is called photogrammetry data alignment using markers.
Markers are generally small, round stickers that you place either on the scan subject or on the surface the subject rests on. After scanning the subject and generating a mesh, the 3D scanner searches for the markers and attempts to align them with the previous scan. Here are some of the benefits to using markers for data alignment:
- Scanning with markers is a faster and more convenient way of aligning multiple scans. The alignment algorithm is greatly sped up by having far fewer reference points to search for and to match up for alignment.
- It is easier to align 3D scan data for objects that have no distinct geometry features (ie. cylindrical pipes) and/or large objects.
- This method yields more accurate data alignment results compared to other registration methods.
Photogrammetry Marker Setup
There are two ways to place the markers:
1) Direct Placement
For direct placement, the markers are placed directly on the subject. This is an appropriate strategy if the subject has enough smooth surfaces for the markers to stick to, and if the markers will not cover up any necessary fine details. This is most useful for creating a complete reconstruction of the subject from all angles because this does not require the subject to be fixed to a surface.
2) Indirect Placement
If the subject has a lot of fine detail that cannot be concealed, or is textured in such a way that direct placement will not work, then it is best to use indirect placement. This method works by placing the markers on the table or on a surface that the subject is resting on. This approach preserves maximum detail in the subject while still being able to use fast marker alignment. The subject must remain static relative to the markers so only the surface can be moved and never the subject itself (ie. placing it on a lazy susan or a rotary table). The drawback of this method is the inability to scan the underside of the subject without starting a new mesh fragment. Creating a complete reconstruction of a subject using indirect placement requires multiple mesh fragments to be created and then later aligned using mesh alignment.
Do you have any questions or comments on alignment using markers? Please post in the comments section.