What are AprilTags and what AprilTag size should I use?
TitleWhat is an AprilTag?
An AprilTag is a visual reference system or a visual target.
AprilTags can be detected by sensors such as cameras or lasers.
TitleHow to use AprilTags?
Sensors can identify AprilTags using AprilTag detection software. The detection software calculates the exact 3D position, orientation and identity of the tag relative to the camera position. AprilTags allow for very fast detection and processing – almost in real time.
AprilTags can be used for a variety of applications such as augmented reality, robotics and camera calibration.
TitleTest of LSE AprilTags with DotProduct
Versions for Android (v. 4.1.6) and iOS (beta, v. 0.16.(21)) were tested.
On Android, the following sensors were examined:
- PrimeSense Carmine 1.08,
- PrimeSense Carmine 1.09 (SR),
- Intel Real Sense D415.
The test measurements took place indoors in daylight.
TitleAprilTags detection with iOS, integrated camera and LiDAR (iPad)
Real-time AprilTags detection works indoors, but is only used during processing for improved results. Detection during file processing as well as measurement does not work yet in the beta version.
Basically all AprilTags are detected. For the small AprilTags with a size of 10x10cm, a smaller operating distance is required. The detection takes place almost simultaneously with the capture of the AprilTags.
For the test of the AprilTags with the iPad, a vehicle was additionally captured in daylight. Here, the advantages of LiDAR become evident: Even shiny objects (e.g. car) can be scanned, which is hardly possible with PrimeSense sensors. Scanning the vehicle in the shade worked without any problems.
TitleAprilTags detection with Android
Full functionality such as real-time detection, tagging, use for alignment and measurement is supported and working.
PrimeSense Carmine 1.08
Detection does not always occur at the same time as data capture. Although the area with the AprilTag has already been captured, the detection sometimes takes place with a delay. This might be due to the fact that the camera's images are not as high resolution (compared to the iPad) and fewer images are captured.
PrimeSense Carmine 1.09 (SR)
The detection works on much smaller distances compared to the above mentioned sensors, which is naturally caused by the sensor itself. Here, too, a delayed detection of the target in real time sometimes occurs.
Intel Real Sense D415
The detection works quite well and in real-time without any delay compared to PrimeSense sensors. This may be because the range of Real Sense is shorter and the overlap between images and data works better.
TitleTest results under different conditions
Targets with the following detection surfaces were tested at different angles:
- A4 AprilTag – 16x16cm
- Large AprilTag – 15x15cm
- Small AprilTag – 10x10cm
Detection distance at smallest angle (approx. 45°) – worst condition
| Camera sensor | AprilTag 16x16cm | AprilTag 15x15cm | AprilTag10x10cm |
|---|---|---|---|
| iOS (Beta) | < 2m | < 3m | < 1m |
| PrimeSense | < 3m | < 2m | < 1m |
| PrimeSense SR | < 2m | < 1.5m | < 1m |
| RealSense D415 | < 3m | < 2m | < 1m |
Erkennungsabstand bei senkrechter Aufnahme (Winkel ca. 90°) – ideale Bedingung
| Camera sensor | AprilTag 16x16cm | AprilTag 15x15cm | AprilTag10x10cm |
|---|---|---|---|
| iOS (Beta) | < 5m | < 5m | < 2m |
| PrimeSense | < 5m | < 5m | < 3m |
| PrimeSense SR | < 3m | < 3m | < 3m |
| RealSense D415 | < 5m | < 3m | < 3m |
Erkennungsabstand unter normalen Bedingungen (Winkel ca. 60°)
| Camera sensor | AprilTag 16x16cm | AprilTag 15x15cm | AprilTag10x10cm |
|---|---|---|---|
| iOS (Beta) | < 3m | < 3m | < 3m |
| PrimeSense | < 5m | < 4m | < 3m |
| PrimeSense SR | < 3m | < 3m | < 1.5m |
| RealSense D415 | < 3m | < 3m | < 3m |
TitleConclusion on the test measurement
Considering the angle of capture and AprilTag size as well as the short-range camera property, it becomes clear that the PrimeSense SR 3D camera sensor requires the shortest operating distances.
The test results of iOS, PrimeSense and Real Sense D415 are similar. The Real Sense D415 allows the longest operating distance at both a poor and optimal capture angle; however, under normal conditions, it detected AprilTags only at a shorter distance than iOS and PrimeSense in the test measurements.
For all AprilTags sizes, PrimeSense has the largest detection distance regardless of the angle of capture.
