One of the most time-consuming aspects of creating 3D virtual models is the creation of the geometric models of objects. This can be particularly frustrating when there is a real or physical version of the object. Fortunately, there are a variety of commercially available technologies that can be used to digitize objects from the molecular scale up to multistorey buildings. Many of the commercial products are in use at service bureau, so builders with smaller budgets or infrequent demands can avoid the capital and time investments. Many commercial vendors offer sensors, software and/or complete integrated systems.
The process of 3D digitizing basically consists of a sensing phase followed by a reconstruction phase. The sensing phase collects or captures the raw data and generates the initial geometry data, usually as a 2D boundary object, or a 3D point cloud. Sensing technologies are based on tracking, imaging, range finding or their combination. The reconstruction phase is the internal processing of the data into conventional 3D CAD and animation geometry data, such as NURBS and polygon sets. Sophisticated reconstruction software packages are available from scanner vendors and third-party software providers.
Tracking systems digitize by positioning a probe on the object and trigger the computer to record the location. The simplest tracker is a mechanical linkage or pantograph. Coordinate measuring machines (CMMs) are robust 3D mechanical trackers for manufacturing applications. Electromagnetic, ultrasonic, optical, gyroscopic and inertial trackers are also used in some commercial 3D digitizers. Trackers can suffer from interference problems, either mechanical or electromagnetic. Object and environment space and materials need to be considered. Manual tracking systems require a large amount of patient, skilled labour, but they can digitize an object directly into polygonal models, eliminating the need for the reconstruction phase. Automated probe tracking systems produce point cloud data that will require reconstruction. One form of automated tracker is the scanning probe microscope (SPM), which can be used to create 3D models of molecular-scale objects.
Imaging starts with the capture of one or more 2D images that are then used as input to image processing algorithms to create the initial geometry data. Some imaging methods create a point cloud, while others use feature extraction to create an initial topology model. Active imaging systems project a moire or grid pattern on the objects to provide known reference points and simplify the image processing. Passive imaging systems collect available light. There are some commercial software packages that can use stereoscopic satellite and aerial images to create terrain elevation and building models.
Another image approach to 3D scanning relies on using a series of slices sectioned through the object. These can be obtained by actually cutting the object and taking optical photographs of the slices or by using advanced sensors such as ultrasound, magnetic resonance imaging (MRI), X-ray computed tomography (CT) and confocal microscopes. The slices can be used to produce volumetric data (voxels) or feature extraction might be used on the images to produce contour lines. Both forms of data can be readily converted to polygonal and surface models. Microscopic and volumetric systems are generally very expensive.
Range finding is related to imaging as it usually results in a 2D array of range data (think Z buffer) that is then processed as an image. While 3D Pipeline Corporation (CA, United States) has developed an ultrasonic range finder system to digitize caves and other large structures, optical systems are the dominant range-finding technology. Both laser and white-light range finding are available. Some optical range scanners also capture object colour into an image for use as a texture map.
All imaging technologies require multiple images from different views in order to create a complete object scan. Some scanning systems acquire these by rotating the object, while others combine an imaging (or range-finding) sensor with a tracker device allowing the sensor to be moved around the object. Many of the laser or moire scanner vendors market their sensors for attachment to CMM mechanical arms.
Not all the 3D digitizing techniques can be used for medical imaging because of the different requirements for medical scanning and industry scanning. This chapter only discusses some medical scanning techniques and their processes.
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