Using AprilTags for Loop Closure in Dot3D

Using AprilTags for Loop Closure in Dot3D

AprilTags for Loop Closure provide a way for Dot3D to identify 3D points in the scene across different points in time.
They do not require any additional measurement or manual input and will never decrease your optimized data quality, so they are very safe to use in a scan (as long as you avoid duplicate tags in the scene!) and are therefore a recommended good practice.

Let's say you start your scan in one part of the scene, then continue to scan many other parts of the scene and then scan back to your starting point. In this case an AprilTag placed at the start/end point will help Dot3D "close the loop" in the scan by establishing identity of the start/end point.

When to use AprilTags for Loop Closure:
The Dot3D loop closure and registration engine is already quite robust even without the help of AprilTags.
There are three situations however in which using AprilTags can make a big difference:

a) When scanning stretches with a lack of geometric structure: Large walls, floors, asphalt, parking lots, anything that's flat and doesn't fit in the camera's field-of-view. In these cases we recommend putting one or more AprilTags on the flat structure in areas that are subject to loop closure (i.e. you start at a flat area, then scan other things then come back to it).

b) When scanning stretches with a lack of visual texture: Any element that is a single color, or a blend of colors changing slowly over the surface, and doesn't fit in the camera's field-of-view. White walls in a room would be a good example here. In these cases we recommend putting several AprilTags in the untextured area to provide the Dot3D algorithms with some added visual texture.

c) When scanning large scenes (entire apartments, residences, parking lots, large outdoor scenery). In those cases it is again recommended to use AprilTags, especially in the loop closure areas.

Basics of AprilTag Targeting for Loop Closure:
  1. Automatically improved accuracy within single scans
    1. Improved frame-to-frame registration via accurate loop closure
  2. Available for download at
  3. Can be printed to different sizes, but larger tags will be recognized more readily by the software
  4. Preparation:
    1. Place 1-3 AprilTags at the starting point of your scan
      1. If using more than one tag, place them at least 1.5 meters (5 ft) apart from each other.
      2. Plan to return back to the same area at the end of your scan
    2. Scatter additional AprilTags throughout the scan
      1. Quantity is up to you (typically 5-20 total depending on scene size)
      2. Should be evenly placed throughout scene at varying elevations
      3. Also can be placed in difficult areas (i.e. empty white walls) to assist with tracking
      4. Initial 1-3 are most crucial
AprilTag Targeting for Loop Closure Procedure:
  1. Begin your scan in the area of your first 1-3 tags
  2. Turn the AprilTags highlighted orange while scanning (detected)
    1. If not detected easily, move slowly around the tag while keeping in in the center of view. Moving closer can also help but try to avoid very close distances below 60 cm (2 ft).
    2. Turn all tags orange before moving on
  3. Pan through the scene in standard fashion
    1. Move about the scene in a loop to return back to the start tags
  4. Upon return to the start, turn tags orange again before finishing
AprilTags can also be utilized for scale bar and survey targeting, which do require additional input of known measurements/coordinates to add even higher accuracy/confidence to the data. Please click here for a video tutorial on these more advanced workflows.

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