3D Scanning 101 The Fundamentals of the Structured Light 3D Scanning Process

Transcription

1 3D Scanning 101 The Fundamentals of the Structured Light 3D Scanning Process

2 3D3 Solutions is a developer of innovative structured light 3D scanners. Based in Vancouver, Canada with more than 20 distributors worldwide.

3 Structured Light Technology Structured light 3D scanner projects a series of patterns onto the object. Based on the deformation of those patterns, it can construct a 3D scan. Video demonstration

4 Laser versus Structured Light Structured Light: Also known as White Light, Blue Light, Coded Patterns, etc. Laser: Laser Line / Laser Dot Lasers gather data a line or a point at a time Structured Light captures the entire area using multiple patterns and images Both are Optical, Light Based Measurement Techniques. Both are good for different applications

5 The Assumption People often perceive that 3D scanning is a fully automated magical process. Place the part in front of the 3D scanner, press a button, and out comes a digital 3D model.

6 The Reality Building a complete digital model of an object from all angles is a process of scanning multiple sides of an object, aligning the scans, and merging them to create a polygon mesh. From here the data can be used to do either 3D modeling or measurement applications.

7 Restrictions Highly reflective surfaces Transparent, translucent or emissive Highly occluded surfaces with no line of sight Hard edges and deep concave segments Great for Extremely complex surfaces Highly detailed organic surfaces Surfaces that can not be touched Automation with high acquisition speed Anything from 1cm to 3 meters in size

8 Work Flow Part Mounting and Part Preparation Alignment Aids 3D Scanning Scan Data Alignment Scan Data Cleanup and Analysis Export and Use

9 Part Mounting and Preparation Work up front = less work later This is the MOST important stage Bad decisions here become 10x worse as you progress Proper fixture and surface preparation will turn a difficult project in to an easy one. What is your goal?

10 Alignment Aids The most common technical issue is the in-ability to deliver properly aligned 3D scan data. Strategy to scan the part Adding Reference Geometry Photogrammetry

11 This is the easy part Point the scanner, press the button You will need to scan from multiple directions to get complete coverage 3D Scanning Between 20 to 50 scans for most objects Make sure your scanner is in calibration and tested Ensure critical areas are captured 100% coverage is very difficult / costly

12 Scan Data Alignment Most time consuming stage depending on your part size and complexity If you did not do the previous stages correctly, you will get bogged down here Poorly captured data will not align You may need to use separate 3D scan data processing packages

13 Cleanup and Exporting Once all the data is aligned, it may require hole filling Data will also require some editing, smoothing and refinement depending on target usage How much you need to do, really depends on what you re looking to do

14 Done! (Sort of) Target use will require very different data processing. CAD Another big topic beyond the scope of this talk Visualization (Games, Movies, Internet) Polygon Scan Data 3D Printing And many more

15 Product Profile 3D scanning 101 By Terence Whitty The technology to acquire complex shapes in three dimensions using digital scanning technology has developed rapidly of late and has now advanced to the level of micron accuracy. The applications of this technology have evolved in dentistry over the past 30 years and can now be employed to routinely map the human dentition, both directly in the mouth and more commonly, through indirect methods that involve scanning models and more recently, impressions. The degree of accuracy with which this can be accomplished is more than adequate for it to be used as the front end for the manufacture of various custom prosthetics... from crowns to chrome dentures. The equipment and software is now turnkey and relatively inexpensive so any lab can afford a solution... and solutions abound. All CAD/CAM systems require some way of creating a 3D object to manipulate (that s the CAD part) and eventually manufacture (CAM). Obtaining 3D objects in dentistry involves using a 3D scanner in some form that can convert something real - like a crown prep - into shape data that is then can compiled and manipulated using software. Various different types of scanning systems exist but they all do basically the same thing. 3D scanners measure the distance of various points on an object utilising a reference point and then compile this data to create an accurate virtual model on your computer. Contact scanners use a contact probe and measure through physical touch. This method can be very accurate however modification or damage to the scanned object can occur, especially when scanning fragile items. This method is also quite time consuming. Non-contact scanners use some kind of radiation or light source and a method of detecting its reflection such as a digital Figure 1. Basic 3D acquisition. Object, light source and camera. Figure 2. Structured light patterns (striping) on an object and the rendered acquisition. Figure 3. The Maestro 3D OrthoScan A20 advanced structured light dental scanner. Figure 4. Various techniques include the reference point to measure distances of the points on the surface of the object. Once the measurement is completed the resulting set of co-ordinate points is called a point cloud. 46 elaborate September/October 2010

16 Product Profile Fig 5 Basic 3D-triangulation is primarily designed to represent the triangulations of a set of points so the object can be visualized and rendered. How many triangles used directly affect the accuracy of the surface of the object. Figure 6. Scan from a laser scanner. Notice the interdental areas have not been captured well and the software interpolates the data. Figure 7. The rendered model from the scan in Figure 6. The rendering has been oversmoothed and lacks detail. Figure 8. The same model scanned by the Maestro 3D OrthoScan A20. The Scan is full of detail and is not oversmoothed. Figure 9. The laser scan showing triangular faces at full resolution. Only 290,000 triangles. Fig 10 Same model scan from the Maestro 3D OrthoScan A20 shows the resolution 3 times that of the previous example. Approximately 850,000 triangles. camera system. Common types of emission used include light, ultrasound and x- rays. There are various types of non-contact scanners but in dental, the most common and road tested types are Structured Light and a relatively new method called Conoscopic Holography. Structured light Structured-light 3D scanners project a pattern of light on the object being scanned and look at the deformation of the pattern on the object. The pattern may be one dimensional or two dimensional. An example of a one dimensional pattern is a line. The line is often projected onto the subject using a sweeping laser. A camera, offset slightly from the projector, looks at the shape of the line and uses a mathematical technique to calculate the distance of every point on the line. This is where the term laser scanner often comes from but it is still technically a structured light scanner. An example of a two-dimensional pattern is a grid or a line stripe pattern. Acameraisusedtolookatthedeformation of the pattern and an algorithm is used to calculate the distance at each point in the pattern. 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. Some existing systems are capable of scanning moving objects in real-time. Structured-light scanning is still a very active area of research with many research papers published each year citing major new developments. September/October 2010 elaborate 47

17 Product Profile Fig 11. Ten unit crown and bridge model ready to be scanned. Figure 12. Maestro 3D OrthoScan can acquire any type of dental model with a high level of accuracy. Shown is the scan of the 10-unit model shown in Figure 11. The scanner can acquire 7 dies simultaneously. Conoscopic holography In a conoscopic system, a laser is projected onto the surface of the object and the immediate reflection along the exact same path is then processed through a special cone shaped crystal then onto a camera. The data is computed and the distance from the object can be extrapolated. The main advantage of conoscopic holography is only a single path is needed for measuring. This allows, for example, the complete depth of a finely drilled hole to be scanned. The dental application touted for this type of acquisition is scanning of impressions, but at this time the speed and accuracy is still in it s infancy. Model or impression? The ability to scan impressions rather than stone models is a relatively new option so it is difficult to comment on the uptake of this by the dental technology community or how it will be utilised in laboratory workflow. The main issue is that a model still needs to be created from which to work, so it is just as easy to scan the model. Systems offering impression scanning also offer to manufacture (generally through milling, 3D printing or stereolithography) a model from the impression scan, however this adds an additional cost to the process at a time when labs are looking to reduce costs in any way possible to remain competitive. No doubt costs will reduce as demand and hence competition increases. Also most people are aware and in control of the compensations they make when fabricating prosthesis, allowing PVS to settle, setting expansion of die stones, relief, die spacing, investments, casting etc. If impressions are scanned it will be a new ball game of compensations. Also in most cases you have to pour an impression anyhow to see if there is any obvious distortion, it may not be so great to go through an expensive lengthy procedure to find your model is a dud, and if it is, who pays for the remake? Intraoral scanning is already available from Sirona (CEREC), D4D (E4D) and Cadent (itero) and rumours of many more systems being commercialised abound, so impression scanning may prove to be interim technology. Software No 3D system is complete without the software to drive it. Software tells the scanner where and how to measure, then how to put all these measurements together to form the 3D model. Consider a simple object such as a pyramid shaped block. If we measured various points on the pyramid s surface from a defined point some distance away we would see, for example the apex would be closer than the corner of the base (Figure 4). Measuring many points we would also have their relative distances from each other and could then compile a map of these points. Transfer this data to a computer screen with the points at the same relative distances from one another and join the points in the correct way to form a surface and you have a computed 3D object. This is a very simplistic explanation but in theory this is how it works. This mathematical collection of co-ordinate points is known as a point cloud and the processing of the points to visualise the model, termed rendering, uses complex mathematical algorithms. The better the algorithm the better your scanned model should be. Some companies use simple smoothing algorithms to make the surface look clean and smooth but often this lacks detail and it can also hide areas that data was not successfully acquired, so it has a best guess - termed interpolation - as to what should be there. When viewing models as a mesh of triangles, the inadequacies of interpolation can be obvious. Figure 6 show the interdental areas of this dental model are poorly scanned so the software makes up the suitable data to fill in the gaps. Figures 7-10 illustrate how the level of information obtained in a scan affects the accuracy of the 3D model. There are major differences between different brands of scanners and the software the companies make available to scan and manipulate the scanned data. How accurate scans need to be may be a contentious issue between manufacturers but logic deducts the more accurate is usually the better choice. About the author Terry Whitty is the Technical Editor of elaborate and the Australian and New Zealand agent for Maestro scanners. For more info, on the Maestro digital scanner, see or contact Truline Dental on (02) elaborate September/October 2010