3D scanner
From Wikipedia, the free encyclopedia
A 3D scanner is a device that analyzes a real-world object or environment to collect data on its shape and possibly color. The collected data can then be used to construct digital, three dimensional models that are used in a wide variety of applications. These devices are used extensively by industry in the production of such things as movies and video games. Other applications include industrial design and prototyping, computer vision and documentation of cultural artifacts.
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[edit] Functionality
The purpose of a 3D scanner is usually to create a point cloud of geometric samples on the surface of the subject. These points can then be used to extrapolate the shape of the subject (a process called reconstruction). If color information is collected at each point, then the colors on the surface of the subject can also be determined.
3D scanners are very analogous to cameras. Like cameras, they have a cone-like field of view, and like cameras, they can only collect information about surfaces that are not obscured. While a camera collects color information about surfaces within its field of view, 3D scanners collect distance information about surfaces within its field of view. The “picture” produced by a 3D scanner describes the distance to a surface at each point in the picture. If a spherical coordinate system is defined in which the scanner is the origin and the vector out from the front of the scanner is φ=0 and θ=0, then each point in the picture is associated with a φ and θ. Together with distance, which corresponds to the r component, these spherical coordinates fully describe the three dimensional position of each point in the picture, in a local coordinate system relative to the scanner.
For most situations, a single scan will not produce a complete model of the subject. Multiple scans, even hundreds, from many different directions are usually required to obtain information about all sides of the subject. These scans have to be brought in a common reference system, a process that is usually called alignment or registration, and then merged to create a complete model. This whole process, going from the single range map to the whole model, is usually known as the 3d scanning pipeline[1].
[edit] Technology
Two types of 3D scanners are contact and non-contact. Non-contact 3D scanners can be further divided into two main categories, active scanners and passive scanners. There are a variety of technologies that fall under each of these categories.
[edit] Contact
Contact 3D scanners probe the subject through physical touch. A CMM (coordinate measuring machine) is an example of a contact 3D scanner. It is used mostly in manufacturing and can be very precise. The disadvantage of CMMs though, is that it requires contact with the object being scanned. Thus, the act of scanning the object might modify or damage it. This fact is very significant when scanning delicate or valuable objects such as historical artifacts. The other disadvantage of CMMs is that they are relatively slow compared to the other scanning methods. Physically moving the arm that the probe is mounted on can be very slow and the fastest CMMs can only operate on a few hundred hertz. In contrast, an optical system like a laser scanner can operate from 10 to 500 kHz.
Other examples are the hand driven touch probes used to digitize clay models in computer animation industry.
[edit] Active
Active scanners emit some kind of radiation and detect its reflection in order to probe an object or environment. Possible types of radiation used include light, ultrasound or x-ray.
[edit] Time-of-flight
The time-of-flight 3D laser scanner is an active scanner that uses laser light to probe the subject. At the heart of this type of scanner is a time-of-flight laser range finder. The laser range finder finds the distance of a surface by timing the round-trip time of a pulse of light. A laser is used to emit a pulse of light and the amount of time before the reflected light is seen by a detector is timed. Since the speed of light c is a known, the round-trip time determines the travel distance of the light, which is twice the distance between the scanner and the surface. If t is the round-trip time, then distance is equal to 
. Clearly the accuracy of a time-of-flight 3D laser scanner depends on how precisely we can measure the t time: 3.3 picoseconds (approx.) is the time taken for light to travel 1 millimetre.
The laser range finder only detects the distance of one point in its direction of view. Thus, the scanner scans its entire field of view one point at a time by changing the range finder’s direction of view to scan different points. The view direction of the laser range finder can be changed by either rotating the range finder itself, or by using a system of rotating mirrors. The latter method is commonly used because mirrors are much lighter and can thus be rotated much faster and with greater accuracy. Typical time-of-flight 3D laser scanners can measure the distance of 10,000~100,000 points every second.
[edit] Triangulation
The triangulation 3D laser scanner is also an active scanner that uses laser light to probe the environment. With respect to time-of-flight 3D laser scanner the triangulation laser shines a laser on the subject and exploit a camera to look for the location of the laser dot. Depending on how far away the laser strikes a surface, the laser dot appears at different places in the camera’s field of view. This technique is called triangulation because the laser dot, the camera and the laser emitter form a triangle. The length of one side of the triangle, the distance between the camera and the laser emitter is known. The angle of the laser emitter corner is also known. The angle of the camera corner can be determined by looking at the location of the laser dot in the camera’s field of view. These three pieces of information fully determine the shape and size of the triangle and gives the location of the laser dot corner of the triangle. In most cases a laser stripe, instead of a single laser dot, is swept across the object to speed up the acquisition process. The National Research Council of Canada was among the first institutes to develop the triangulation based laser scanning technology in 1978[2].
[edit] Notes on time-of-flight and triangulation scanners
Time-of-flight and triangulation range finders each have strengths and weaknesses that make them suitable for different situations. The advantage of time-of-flight range finders is that they are capable of operating over very long distances, on the order of kilometers. These scanners are thus suitable for scanning large structures like buildings or geographic features. The disadvantage of time-of-flight range finders is their accuracy. Due to the high speed of light, timing the round-trip time is difficult and the accuracy of the distance measurement is relatively low, on the order of millimeters. Triangulation range finders are exactly the opposite. They have a limited range of some meters, but their accuracy is relatively high. The accuracy of triangulation range finders is on the order of tens of micrometres.
At a rate of 10,000 samples per second, low resolution scans can take less than a second, but high resolution scans, requiring millions of samples, can take minutes for some time-of-flight scanners. The problem this creates is distortion from motion. Since each point is sampled at a different time, any motion in the subject or the scanner will distort the collected data. Thus, it is usually necessary to mount both the subject and the scanner on stable platforms and minimize vibration. Using these scanners to scan objects in motion is very difficult.
Recently, there has been research on compensating for distortion from small amounts of vibration. Refer to Accurate 3D acquisition of freely moving objects, François Blais, et al.
[edit] Conoscopic Holography
In a Conoscopic system a laser beam is projected onto the surface and then the immediate reflection along the same ray-path are put through a conoscopic crystal and projected onto a CCD. The result is a difraction pattern, that can be frequency analyzed to determine the distance to the measured surface. The main advantage with Conoscopic Holography is that only a single ray-path is needed for measuring, thus giving an opportunity to measure for instance the depth of a finely drilled hole.
[edit] Structured light
Structured light 3D scanners project a pattern of light on the subject and look at the deformation of the pattern on the subject. The pattern may be one dimensional or two dimensional. An example of a one dimensional pattern is a line. The line is projected onto the subject using either an LCD projector or a sweeping laser. A camera, offset slightly from the pattern projector, looks at the shape of the line and uses a technique similar to triangulation to calculate the distance of every point on the line. In the case of a single-line pattern, the line is swept across the field of view to gather distance information one strip at a time.
An example of a two dimensional pattern is a grid or a line stripe pattern. A camera is used to look at the deformation of the pattern and a fairly complex algorithm is used to calculate the distance at each point in the pattern. One reason for the complexity is ambiguity. Consider an array of parallel vertical laser stripes sweeping horizontally across a target. In the simplest case, one could analyze an image and assume that the left-to-right sequence of stripes reflects the sequence of the lasers in the array, so that the leftmost image stripe is the first laser, the next one is the second laser, and so on. In non-trivial targets having holes, occlusions, and rapid depth changes, however, this sequencing breaks down as stripes are often hidden and may even appear to change order, resulting in laser stripe ambiguity. This particular problem was recently solved by a breakthrough technology called Multistripe Laser Triangulation (MLT). Structured light scanning is still a very active area of research with many research papers published each year.
The advantage of structured light 3D scanners is speed. Instead of scanning one point at a time, structured light scanners scan multiple points or the entire field of view at once. This reduces or eliminates the problem of distortion from motion. Some existing systems are capable of scanning moving objects in real-time.
[edit] Modulated light
Modulated light 3D scanners shine a continually changing light at the subject. Usually the light source simply cycles its amplitude in a sinusoidal pattern. A camera detects the reflected light and the amount the pattern is shifted by determines the distance the light traveled.
[edit] Passive
Passive scanners do not emit any kind of radiation themselves, but instead rely on detecting reflected ambient radiation. Most scanners of this type detect visible light because it is a readily available ambient radiation. Other types of radiation, such as infrared could also be used. Passive methods can be very cheap, because in most cases they do not need particular hardware.
[edit] Stereoscopic
Stereoscopic systems usually employ two video cameras, slightly apart, looking at the same scene. By analyzing the slight differences between the images seen by each camera, it is possible to determine the distance at each point in the images. This method is based on human stereoscopic vision.[1]
[edit] Silhouette
These types of 3D scanners use outlines created from a sequence of photographs around a three-dimensional object against a well constrasted background. These silhouettes are extruded and intersected to form the visual hull approximation of the object. With this kind of techniques some kind of concavities of an object (like the interior of a bowl) are not detected.
[edit] User Assisted (i.e. Image Based Modelling)
There are other methods that, based on the user assisted detection and identification of some features and shapes on a set of different pictures of an object are able to build an approximation of the object itself. This kind of techinques are useful to build fast approximation of simple shaped objects like buildings. Various commercial packages are available like iModeller, D-Sculptor or RealViz-ImageModeler.
This sort of 3D scanning is based on the principles of photogrammetry. It is also somewhat similar in methodology to panoramic photography, except that the photos are taken of one object on a three-dimensional space in order to replicate it instead of taking a series of photos from one point in a three-dimensional space in order to replicate the surrounding environment.
[edit] Reconstruction
The point clouds produced by 3D scanners are usually not used directly. Most applications do not use point clouds, but instead use polygonal 3D models. The process of converting a point cloud into a polygonal 3D model is called reconstruction. Reconstruction involves finding and connecting adjacent points in order to create a continuous surface. Many algorithms are available for this purpose.
[edit] Applications
[edit] Material processing and production
Laser scanning describes a method where a surface is sampled or scanned using laser technology. Several areas of application exist that mainly differ in the power of the lasers that are used, and in the results of the scanning process. Lasers with low power are used when the scanned surface doesn't have to be influenced, e.g. when it has to be digitized. Confocal or 3D laser scanning are methods to get information about the scanned surface.
Depending on the power of the laser, its influence on a working piece differs: lower power values are used for laser engraving, where material is partially removed by the laser. With higher powers the material becomes fluid and laser welding can be realized, or if the power is high enough to remove the material completely, then laser cutting can be performed.
Also for rapid prototyping a laser scanning procedure is used when for example a prototype is generated by laser sintering.
The principle that is used for all these applications is the same: software that runs on a PC or an embedded system and that controls the complete process is connected with a scanner card. That card converts the received vector data to movement information which is sent to the scanhead. This scanhead consists of two mirrors that are able to deflect the laser beam in one level (X- and Y-coordinate). The third dimension is - if necessary - realized by a specific optic that is able to move the laser's focal point in the depth-direction (Z-axis).
The third dimension is needed for some special applications like the rapid prototyping where an object is built up layer by layer or for in-glass-marking where the laser has to influence the material at specific positions within it. For these cases it is important that the laser has as small a focal point as possible.
For enhanced laser scanning applications and/or high material throughput during production, scanning systems with more than one scanhead are used. Here the software has to control what is done exactly within such a multihead application: it is possible that all available heads have to mark the same to finish processing faster or that the heads mark one single job in parallel where every scanhead performs a part of the job in case of large working areas.
[edit] Entertainment
3D scanners are used by the entertainment industry to create digital 3D models for both movies and video games. In cases where a real-world equivalent of a model exists, it is much faster to scan the real-world object than to manually create a model using 3D modeling software. Frequently, artists sculpt physical models of what they want and scan them into digital form rather than directly creating digital models on a computer.
[edit] Reverse engineering
Reverse engineering of a mechanical component requires a precise digital model of the objects to be reproduced. Rather than a set of points a precise digital model is typically represented by a set of surfaces such as a set of flat triangular surfaces, a set of flat or curved NURBS surfaces, or ideally for mechanical components a CAD solid which is composed of a CAD subset of NURBS surfaces. A 3D scanner can be used to digitize free-form or gradually changing shaped components as well as prismatic geometries whereas a coordinate measuring machine is usually used only to detemine simple dimensions of a highly prismatic model. These data points are then processed to create a usuable digital model.
[edit] Cultural Heritage
There have been many research projects undertook the scanning of historical sites and artifacts.
[edit] Michelangelo
In 1999, two different research groups started scanning Michelangelo's statues. Stanford university with a group led by Marc Levoy[3] used a custom laser triangulation scanner built by Cyberware to scan Michelangelo’s statues in Florence, notably the David, the Prigioni and the four statues in the The Medici Chapel. The scans produced a data point density of one sample per 0.25mm, detailed enough to see Michelangelo’s chisel marks. These detailed scans produced a huge amount of data (up to 32 gigabytes) and processing the data from his scans took 5 months. Approximately in the same period a research group from IBM, led by H. Rushmeier and F. Bernardini scanned the Pietà Palestrina acquiring both geometric and color details.
[edit] Monticello
In 2002, David Luebke, et al. scanned Thomas Jefferson’s Moticello[4]. A commercial time of flight laser scanner, the DeltaSphere 3000, was used. The scanner data was later combined with color data from digital photographs to create the Virtual Monticello, and the Jefferson’s Cabinet exhibits in the New Orleans Museum of Art in 2003. The Virtual Monticello exhibit simulated a window looking into Jefferson’s Library. The exhibit consisted of a rear projection display on a wall and a pair of stereo glasses for the viewer. The glasses, combined with polarized projectors, provided a 3D effect. Position tracking hardware on the glasses allowed the display to adapt as the viewer moves around, creating the illusion that the display is actually a hole in the wall looking into Jefferson’s Library. The Jefferson’s Cabinet exhibit was a barrier stereogram (essentially a non-active hologram that appears different from different angles) of Jefferson’s Cabinet
[edit] Cuneiform tablets
In 2003, Subodh Kumar, et al. undertook the 3D scanning of ancient cuneiform tablets[5]. Again, a laser triangulation scanner was used. The tablets were scanned on a regular grid pattern at a resolution of 0.025 mm.
[edit] “Plastico di Roma antica”
In 2005, Gabriele Guidi, et al. scanned the “Plastico di Roma antica”[6], a model of Rome created in the last century. Neither the triangulation method, nor the time of flight method satisfied the requirements of this project because the item to be scanned was both large and contained small details. They found though, that a modulated light scanner was able to provide both the ability to scan an object the size of the model and the accuracy that was needed. The modulated light scanner was supplemented by a triangulation scanner which was used to scan some parts of the model.
[edit] See also
[edit] References
- François Blais, Michel Picard, Guy Godin, "Accurate 3D acquisition of freely moving objects," Proceedings. 2nd International Symposium on 3D Data Processing, Visualization and Transmission, 2004, pp.422-429.
- Qian Chen, Toshikazu Wada, "A light Modulation/Demodulation Method for Real-Time 3D Imaging," Fifth International Conference on 3-D Digital Imaging and Modeling, 2005, pp.15-21.
- Brian Curless, "From Range Scans to 3D Models," ACM SIGGRAPH Computer Graphics, Vol. 33, Issue 4, Nov 2000, pp.38-41.
- Joseph P. Lavelle, Stefan R. Schuet, Daniel J. Schuet, "High Speed 3D Scanner with Real-Time 3D Processing," 2004 IEEE International Workshop on Imaging Systems and Techniques, 2004, pp.13-17.
- Katsushi Lkeuchi, "Modeling from Reality," Third International Conference on 3-D Digital Imaging and Modeling, 2001, pp.117-124.
- ^ Fausto Bernardini, Holly E. Rushmeier: The 3D Model Acquisition Pipeline. Comput. Graph. Forum 21(2): 149-172 (2002), (pdf).
- ^ Roy Mayer, Scientific Canadian: Invention and Innovation From Canada's National Research Council, Vancouver: Raincoast Books, 1999.
- ^ Marc Levoy, Jeremy Ginsberg, Jonathan Shade, Duane Fulk, Kari Pulli, Brian Curless, Szymon Rusinkiewicz, David Koller, Lucas Pereira, Matt Ginzton, Sean Anderson, James Davis, "The Digital Michelangelo Project: 3D Scanning of Large Statues," Proceedings of the 27th annual conference on Computer graphics and interactive techniques, 2000, pp.131-144. (pdf)
- ^ David Luebke, Christopher Lutz, Rui Wang, and Cliff Woolley, “Scanning Monticello,” 2002, http://www.cs.virginia.edu/Monticello.
- ^ Subodh Kumar, Dean Snyder, Donald Duncan, Jonathan Cohen, Jerry Cooper, "Digital Preservation of Ancient Cuneiform Tablets Using 3D-Scanning," Fourth International Conference on 3-D Digital Imaging and Modeling, 2003, pp.326-333.
- ^ Gabriele Guidi, Laura Micoli, Michele Russo, Bernard Frischer, Monica De Simone, Alessandro Spinetti, Luca Carosso, "3D digitization of a large model of imperial Rome," Fifth International Conference on 3-D Digital Imaging and Modeling, 2005, pp.565-572.
