When utilizing a handheld 3d scanner, you might notice distinctive circular black-and-white patches attached to the objects undergoing scanning. These are known as markers, but what exactly is their function?
Figure 1: iReal M3 3D Scanning with Markers
The use of a handheld scanner involves dealing with a limited measurement area during each scan. Consequently, aligning data from multiple scans becomes essential. While several methods exist, including feature-based, texture-based, and hybrid techniques, marker-based alignment is often the preferred choice for small to medium-sized scanners. The circular patches mentioned earlier serve as markers in this context.
In the realm of handheld 3D scanning, each scan captures only a limited measurement area. Therefore, accurately aligning data from successive scans is crucial. There are various methods to achieve this alignment, including feature-based, texture-based, and hybrid approaches. However, for small to medium-sized scanners, marker-based alignment is often the method of choice. The aforementioned circular patches are these very markers.
But what precisely are markers in 3D scanning?
Markers are commonly referred to as “reference points”, “target points”, or “tracking points”, terms that more accurately reflect their function in the scanning process. Specifically, ‘scanning marker points’ refer to a subset known as non-coded marker points. On the other hand, coded marker points exist, each bearing unique codes to further enhance the precision and efficiency of tracking. This exploration will initially focus on coded markers, dissecting their structure and utility in the 3D scanning process.
Coded markers are widely used in photogrammetric systems, which are based on the principle of capturing images to reconstruct three-dimensional objects in space.
Figure 2 illustrates the basic principles of camera imaging and triangulation, providing a reference for understanding the three-dimensional reconstruction process in photogrammetry.
Coded markers play a pivotal role in photogrammetric systems, which rely on capturing images to reconstruct three-dimensional structures in space. These systems utilize the foundational principles of camera imaging and triangulation, as depicted in Figure 2, offering a conceptual framework for understanding the 3D reconstruction process inherent in photogrammetry.
Figure 2: Camera Imaging Principle and Triangulation Principle
Aerial triangulation, based on photogrammetry, is a common method for three-dimensional reconstruction. Coded markers can serve as a means to calibrate the internal parameters of the camera. During camera calibration for aerial triangulation, multiple photos are taken from various angles, focusing on several coded markers. Each coded marker point corresponds to a unique code value, providing essential information for image recognition. The internal camera parameters, such as focal length, distortion, and offset, can be determined using triangulation algorithms.
A key application of photogrammetry is aerial triangulation, a method commonly employed for 3D reconstruction. In this context, coded markers are instrumental in calibrating the camera’s internal parameters. This calibration involves capturing multiple images from different angles, each focusing on various coded markers. Each marker is uniquely identifiable by its specific code value, providing crucial data for image recognition and processing.
The process utilizes triangulation algorithms to ascertain the camera’s internal parameters, including focal length, distortion coefficients, and principal point offset. Establishing these parameters is fundamental for accurate image analysis and 3D reconstruction.
In practical scenarios, after determining the camera’s internal parameters, it becomes feasible to discern the relative spatial relationships among the scanned objects. This understanding is pivotal for accurate mapping and 3D modeling.
Figure 3: Common Coding Markers
Figure 3 illustrates some typical examples of coded markers. These markers are designed with a central circle or cross to facilitate precise coordinate positioning. Surrounding this central point, circular bands encode specific information, decipherable through predetermined rules. Functionally analogous to QR codes, these markers contain pre-computed codes. When captured by the camera, these codes are decoded to identify each marker’s unique identifier, thus ensuring accurate and efficient 3D reconstruction and spatial analysis.
Advantages of coded markers include:
Unique Identification: Each marker has a distinct code, preventing confusion and ensuring data integrity.
High Precision: They offer exact positioning, crucial for industries like manufacturing and healthcare.
Automatic Recognition: Scanners recognize these markers automatically, boosting efficiency and reducing errors.
In summary, coded markers excel in precision, especially in large spaces, making them superior to non-coded markers in specific applications.
Non-coded markers, prevalent in 3D scanning with devices like handheld lasers or stationary scanners from Scantech 3D, are pivotal for accurate alignment. Known for their circular design, these markers are the go-to choice for ensuring precise marker point alignment in various scanning applications.
Figure 4: Common Non-coded Reflective Markers
Circular Shape Benefits: Non-coded markers are commonly circular to ensure consistent detection as ellipses by machine vision, regardless of angle. The ellipse’s center offers a stable reference point, crucial for precise center determination.
Evolution and Material Advancements: Initially, markers were simple, often homemade from printed dots or hand-drawn circles on paper, recognized by basic fixed scanners. However, this rudimentary approach led to unstable stitching and increased risks of misidentification, compromising point cloud quality. Modern printing advancements have led to markers with sharply defined edges, minimal shape deviation, and often, a reflective inner material. These reflective markers, though slightly costlier, offer significant benefits.
Advantages of Reflective Markers:
Enhanced Detection: Reflective markers allow cameras, even under low exposure, to detect markers while filtering out background noise. This minimizes misidentification, conserves computational resources, and boosts scanning efficiency and accuracy.
Variants and Specialized Uses: Market-available non-coded markers come in various diameters (e.g., 3mm, 6mm, 12mm) to suit different scanning distances and scenarios. Some markers feature free rotation, maintaining consistent orientation, while others possess magnetic properties for easy attachment to metal surfaces.
Principles of non-coded marker stitching:
- Marker Point Groups:Triads of markers (e.g., 1, 2, 3) are grouped, forming the basis for alignment.
- Alignment Process:When a new group (a, b, c) is detected, the scanner compares its geometry — lengths and angles — with existing groups (1, 2, 3). If they match, it’s inferred that the new group corresponds to a previously identified group.
- Establishing Correspondence:A one-to-one mapping is created between the new and known groups, ensuring they share the same coordinate system.
- Data Alignment:Leveraging this correspondence, the scanner merges the new data with the existing dataset, building a cohesive 3D model.
This method’s strength lies in its simplicity: it doesn’t require unique codes on markers but uses geometric similarity for marker identification. This can be particularly advantageous in dynamic environments where markers are frequently repositioned or added.
Innovative Use of Markers in Scantech KSCAN-Magic Series
Unique Marker Design:
Coded Markers: The KSCAN-Magic series use multiple non-coded markers to form a unique coded marker. This structure, distinct from traditional designs (as in Figure 3), ensures precise location determination with a central position for each coded point, showcased in Figures 5 and 6. Furthermore, these markers are crafted from magnetic rubber for effortless attachment to metal surfaces.
Non-Coded Markers: Reflective markers are preferred for their dependable performance in the non-coded category.
Figure 5: Coded Markers No. 751
Figure 6: Coded Markers No. 753
Synergistic Use of Coded and Non-Coded Markers:
Addressing the limitations of handheld scanners in large-area scanning, the KSCAN-Magic series employs a strategic combination of coded and non-coded markers. The system begins by pre-scanning the markers using internal photogrammetry, placing coded markers in conjunction with non-coded ones around the object. This approach leverages photogrammetry’s strength in controlling accuracy, enabling the precise reconstruction of non-coded points within the confines of photogrammetric precision. The scanner effectively mitigates the accuracy degradation typically associated with larger scanning spaces by globally adjusting the pre-scanned marker points. This not only ensures a uniform distribution of accuracy across the entire scan but also preserves the intricate details in localized areas, thereby maintaining high accuracy throughout the scanning process.
Figure 7: KSCAN-Magic Working Scene Using Photogrammetry
Grasping these fundamental concepts about markers is essential for a deep comprehension of the nuances in 3D scanning technology. Thank you for taking the time to engage with this content. Should you have further inquiries or require more detailed information, the team of experts at iReal 3D is always ready and eager to assist you.