Fiduciary markers, also known as fiducial markers, are reference points such as barcodes that are strategically placed in an environment. The purpose of these markers is to aid in precise positioning, orientation, and alignment during various applications.

Advantages of using optical labels

Using machine-readable optical labels as fiduciary markers offers several advantages, including:

  • Easy detection and decoding: Optical labels are designed to be easily recognized and processed by imaging devices and computer vision algorithms. This allows for efficient and reliable identification or tracking.

  • Versatility: Optical labels can be applied to a wide range of objects, surfaces, or environments. This makes them suitable for many applications and industries.

  • Resilience and accuracy: Many optical labels incorporate error correction techniques. Doing so makes them more resilient to damage or distortion. In addition, this will ensure accurate decoding even under challenging conditions.

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The two types of fiduciary markers

ArUco markers and AprilTags are both types of optical labels that are used as fiducial markers. They are specifically designed for computer vision applications, such as robotics, augmented reality, and camera calibration. These optical labels are square-shaped markers with black-and-white patterns that can be easily detected, identified, and tracked by computer vision algorithms.

Both marker types have been widely used due to their robustness, accuracy, and ease of implementation. Other marker types exist, but ArUco markers and AprilTags are the most common.

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ArUco markers

ArUco markers are a part of the ArUco (Augmented Reality University of Cordoba) library, an open-source project for augmented reality applications. These markers consist of a black square border surrounding an inner pattern of black and white squares.

The pattern encodes a unique identifier for each marker. This identifier is a number between 0 and 1,000. These identifiers allow a computer vision system to simultaneously recognize and track multiple markers.

Some key features of ArUco markers include:

  • Robust detection: ArUco markers can be detected under various lighting conditions. Also, their detection is relatively unaffected by occlusions or partial visibility.
  • Fast processing: The ArUco library is designed to be computationally efficient. This enables real-time marker detection and tracking on various devices, including smartphones and embedded systems.
  • Scalability: Aruco markers can be created in various sizes and resolutions, making them suitable for different applications and distances.

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AprilTags

AprilTags are another type of fiducial marker developed at the University of Michigan’s APRIL Robotics Laboratory. Like Aruco markers, AprilTags consist of a black square border with an inner pattern of black and white squares, encoding a unique identifier.

AprilTags are designed to be particularly robust and accurate, even under challenging conditions, such as low-resolution images or motion blur.

Some key features of AprilTags include:

  • High accuracy: AprilTags are designed to provide sub-pixel accuracy in their detection, allowing for precise positioning and orientation estimation.
  • Robustness: AprilTags are less sensitive to lighting conditions, image blur, and occlusions compared to other marker types. This makes them more reliable in various environments.
  • Error detection and correction: AprilTags incorporate error detection and correction techniques, allowing for accurate decoding even if the marker is partially damaged or obscured.
  • Multiple tag families: AprilTags support various tag families with different sizes, error correction capabilities, and data capacities, enabling users to choose the most suitable option for their application.

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Uses of fiduciary markers

Below are some typical example applications of machine-readable labels as fiduciary markers.

Medical imaging and image-guided surgery

Fiducial markers are often used in medical imaging, such as MRI or CT scans, to help radiologists and surgeons accurately localize and target specific areas of interest within the patient’s body during image-guided procedures.

Robotics and automation

In robotics, fiducial markers can be used for accurate positioning and orientation of robotic systems, enabling precise and reliable movement and interaction with the environment.

An example is Duckietown. This is an open-source, hands-on educational platform for teaching autonomous robotics. It uses small-scale robots called “Duckiebots” to navigate a miniature city environment. AprilTags are employed as landmarks for localization, navigation, and communication between the Duckiebots and the infrastructure.

Geospatial applications

In surveying and mapping, fiduciary markers can be used as reference points to ensure accurate and consistent measurements across different data sets or platforms.

Drone navigation

Both Aruco markers and AprilTags are used in drone navigation projects, where they serve as reference points for precise positioning, navigation, and control. The markers can be placed on the ground or other objects, and the drones use computer vision algorithms to detect, identify, and track them for localization and guidance.

Augmented reality applications

Both Aruco markers and AprilTags have been used in a variety of AR applications, ranging from educational tools to interactive marketing campaigns. These markers provide an efficient and accurate way to anchor digital content to the real world, enabling immersive and interactive experiences.

An example is AR.js. This is an open-source library for developing web-based AR applications. For this, AR.js uses ArUco markers to overlay digital content onto the physical world. Developers can create AR experiences that run in web browsers on smartphones, tablets, and other devices without requiring native apps.