What is a 3D scan?
3D scanning generates a digital representation of an object or person. The shape of the object is captured with a 3D scanner. Sensors collect data regarding the shape, depth and colour of the object and form a virtual representation from this. A point cloud representing the virtual image of the shape of the scanned object is generated during the scanning process. The 3D scan can then be processed with a 3D modelling application.
Why 3D scanning?
The combination of 3D scanner and 3D printer creates a kind of copier for three-dimensional objects. Theoretically, it is therefore possible to duplicate an existing object very easily with this technology. For complicated objects (e.g. organic/ free-form shapes), modelling using CAD software is an enormous challenge, whereas the operation of a 3D scanner can be learned quickly.
In the teaching context, for example, the hope is that the 3D scanner can be used to duplicate existing teaching materials. This could be a useful technique for old models that are no longer available for purchase. It is also conceivable to build a unique piece out of wood, cardboard, polystyrene, clay or similar and then scan it. In this way, a 3D model could be created without the need for extensive skills in 3D modelling.
3D scanner
For a long time 3D scanners were almost prohibitively expensive for the home user or for use in a school. Over time, however, prices dropped to several thousand euros. Devices are now available for well under €1,000.
Today, a whole range of 3D scanners can be used in two different modes:
- The 3D scanner is held in the hand and moved around the object to be scanned. The object must be scanned from every perspective – it must be completely circled. For example, a human being can be digitally scanned. Models of about 2m^3 in size can be scanned by many scanners in this mode.
- The 3D scanner is fixed on a kind of tripod. The object to be scanned is placed on a turntable at a defined distance from the scanner and rotated. This technique is useful for small objects. The objects to be scanned are usually the size of a golf ball or even a football. Depending on the resolution, the duration of the scan can range from just a few minutes to half an hour.
Currently, 3D scanners work with different technologies, none of which has yet become widely established. 3D scanners from the professional sector work with laser triangulation, while the affordable devices work with structured light. Fringe patterns are superimposed on the object to be scanned, and curvatures in the fringe pattern are created by the different heights of the object. A camera captures these height distortions in a large number of individual images. The software then uses these images to calculate the virtual 3D model. Devices are also available that work with a bright and structured flash light as well as light close to the infrared range. Some devices advertise a scanning accuracy of up to 0.05mm.
Post-processing of 3D scans
It is almost always necessary to post-process the 3D model in additional software after scanning. Unfortunately, small errors are always produced during scanning which must be corrected as far as possible.
It is best not to view the model with its coloured texture. This makes scanning errors less visible, which is why a 3D scan with texture looks much more accurate than it actually is.
There will always be unnecessary elements that have to be removed. When scanning, parts of the base on which the object has been lying are scanned in. These should be cut off before printing with a 3D printer.
For the post-processing of 3D scans, the free software MeshLab can be used. This open-source software is available for various platforms.
Photogrammetry as an alternative to 3D scanners
Instead of using a 3D scanner, a camera or even a smartphone can be used to get started. With the help of photogrammetry technology it is possible to create a 3D model based on photos. All that is needed is a large number of photos of the model from as many angles as possible and software to generate the 3D model.
When taking the photos, make sure that the lighting conditions are good and that the object to be scanned stands out well against a neutral background.
The more photos are taken, the more accurately the software can calculate the 3D model. Depending on the number of photos and how powerful the computer is, this process can take more than an hour.
Good results can be achieved, for example, with the free open-source software Meshroom.
After creating a 3D model with photogrammetry, it must always be scaled to the correct size, as the size of the object to be scanned is not recognised, unlike with 3D scanners.
Promising results can currently be achieved with the apps Polycam (available for iOS and Android) and RealityScan (available for iOS).
Problems and limitations of 3D scanning
Unfortunately, many models will not give a high quality result when scanned. This may be due to the geometry or surface of the model. Problems often occur for the following reasons:
– reflective or transparent parts
– very dark objects
– deep dark holes or shadows
– undercuts
– bifurcations
The results can then very often display major flaws, which can sometimes be corrected with a lot of reworking. However, it is possible that no model can be created at all due to problems with the tracking.
Example
The following model of a rubber hand was scanned with the Shining EinScan-SE scanner and printed using various 3D printers. The quality of the scan and also the print is excellent. But bear in mind that the model itself was ideal for scanning, which explains the quality.
Assessment
3D printing is a beneficial option for the production of models for teaching visually impaired pupils. Models can be printed with just a few clicks that would have had to be built by hand as unique pieces without 3D printing. However, more often than not, you are wanting to print models that you do not currently have. Therefore, using the combination of a 3D scanner and a 3D printer as a kind of 3D copier cannot be called a game changer. On the other hand, it can be useful to scan particularly small or large models and print them in scaled form. These would be impossible for a non-sighted person to handle in their original form due to the size. Unfortunately, problems often occur when scanning objects as described above, which is why this technology is still far from a plug and play solution. Instead of directly purchasing one’s own 3D scanner, it is advisable to first gain initial experience with photogrammetry or to search the internet for scanned objects to download. The necessary post-processing of 3D scans also requires a range of technical skills and extensive program knowledge.
