A marker is a reference point in the form of a white circle inscribed in a black one. The markers are glued to the surface of the object being scanned (or around the object), are read by the texture camera of the scanner and allow tracking of objects that do not have enough geometric data. 150 magnetic markers are shipped with every 3D scanner. These markers can also be printed on sticky paper using a regular office printer. DOWNLOAD THE FILE HERE.
While many 3D scanners use markers (or targets, as they are also commonly known) for “post-processing” alignment, Thor3D scanners use them for real-time tracking, allowing for fast and accurate scanning. This means that the embedded software recognizes and then analyzes each marker in real time in every frame at up to 10 frames per second. It records the exact location of each marker as they relate to each other and using that information aligns each frame to the following one.
Without markers, the device will lose its alignment during scanning and the software will not know how Frame1 is supposed to be aligned to Frame2, Frame2 to Frame3 and so on… The software will try to align all frames using the objects’ unique geometry. It has sophisticated algorithms which will understand that a human nose in Frame1, should be aligned to the same human nose in Frame2 and not to some other body part.
However, this guesswork doesn’t always produce great results and in many cases, especially on objects with large flat surfaces and no unique geometry, even the most sophisticated algorithm will fail. Attaching markers to the object and giving the software a “constant feature” to track ensures the highest accuracy. It also ensures repeatability and that a geometrical error will not accumulate over a long distance.
Difficulty attaining high accuracy on large objects
Imagine scanning a long pipe. It is 10 meters long and 1 meter in diameter. If you don’t use markers, over some distance, the scan of the pipe will appear to bend slightly. That is because optical scanners accumulate an error over distance. Markers solve this issue and provide you with the most accurate results.
Here are a few tips for scanning with Thor3D scanners and markers:
At any given time during scanning, the scanner needs to see a minimum of 3 markers in every frame. When it doesn’t, scanning will automatically stop.
Although 3 markers per frame is the minimum, the optimal number is 5. That is because in certain circumstances, even if the marker is visible to the camera, the software doesn’t recognize it as such. This could be due to glare, for example.
When affixing markers, apply them on flat areas of the object and not on sharp edges of the object. Also, remember, that the scanner might sometimes not see a marker when looking at it from an awkward angle. This means that when scanning around a large corner (like from the bumper of the car to the back door), try affixing more markers in the field of view.
Whether scanning with or without markers, the center of the frame of any optical device will produce better results than the edges of the frame. If you want a great scan of the entire object, make sure to scan each portion of that object with the center of the frame.
Thor3D scanners show, in real time, how many markers are in the field of view. If you are not sure about whether you’ve affixed enough markers on the object, turn on the scanner in preview mode and walk around the object. You will see how many markers the scanner sees at all times and if it’s less than 3, you’ll know where more markers are needed.