How Do 3d Scanners and Photogrammetry Work?

The first time anyone mentioned a 3d printer to me, I was amazed. The possibilities were endless! All you needed was to pair it with a 3d scanner and you could copy any object and make a copy. Scan a full-sized car, scale it down, and print out a model of it. Same with a person, or an airplane, or a horse! I was very excited.

3d scanners use probes and sensors to determine the position of the surface of an object. The probes can be physical or they can be laser light or projected light patterns. Software interprets the distance detections from the probe or emitter to the object, and calculates a cloud of dots representing the surface of the target. Photogrammetry works by analyzing a series of photographs from different angles of a target object and determining the surface of the object.

It’s taken quite a number of years for scanners and photogrammetry to come to the point where it can accurately create electronic copies of objects, but it is now good enough to produce passable objects at reasonable prices.

Styles of 3d Scanning

There are two distinct schools of 3d scanning. The first is a family of scanners that use some form of probe to judge the distance from the device to the surface of the target object. The second is to take a series of photos of a target object as input, and determine through software the surface of the object as an output.

Laser Triangulation

Laser triangulation uses a combination of a laser emitter and a laser sensor. It works a little like radar. The laser is sent out from the device, and bounces back, giving an exact distance to the object. Sent enough times, this method generates a data set of the object based on distance from the emitter.

Structured Light

Structured light uses a light pattern that is projected onto an object, such as a square grid. The distortion of the square grid, due to the object’s surface, provides information to the light sensor. This gives a good approximation of the surface of the target object.

Physical Contact Sensing

As the name suggests, a probe is used to sense the surface of the object, and the location is recorded by the device. Some systems use a series of probes, like a bed of nails-like sensor grid, that will deform to the object. The position of the probes can be recorded to gain an accurate position of the surface of the object.

Photogrammetry

This is some heavy math! Someone figured out that if you take enough photos of an object, from all sorts of angles, you can use math to create an accurate representation of the object. It analyses light distortion and edge detection to stitch together the images and gain a 3d understanding of the object.

What Resolution Can You Get From a 3d Scanner?

Consumer grade scanners can scan at a dot density of around one dot per 0.1mm. That’s good enough for most objects in the world. For example, if you used a 3d laser triangulation scanner on a full-sized statue, you would generate a lot of data, but a reasonably accurate dot cloud. Now, if you shrank that object down to 10% of its size, your resolution goes effectively up to 0.01mm. Now you have a good density for a 1/10th scale model of the statue. Convert this to an STL file and you’ve got a great model!

Prosumer or entry-level commercial scanners can scan at 0.05mm cloud density, or over four times the amount of data. You will get more accurate scans, but you’ll also generate a lot more data. The problem gets worse with professional scanners. They can generate dot clouds at 0.025mm. That can be more than 16 times the data of the 0.1mm scan, but surfaces will be modeled very accurately and textures will be apparent and far more detailed.

How Much Do 3d Scanners Cost?

You can purchase an entry level laser 3d scanner for small objects. Here’s an example from SOL. It can scan at 0.1mm dot density. It includes a turntable to rotate the object so that all sides of the object can be scanned in sequence. It includes software to allow you to export the model as an STL or OBJ file (and a few other formats). https://amzn.to/2W3qTGK

Contrast this with the Peel2, a professional grade scanner than can be used as a hand-held device. It can scan to 0.025mm resolution, and will pick up texture and color to give you a very accurate 3d model. Its price is dramatically higher, but it’s not always available. Software is also included to export the model as an STL or OBJ file.

Problems with 3d Scanning

The scanning process generates a cloud of dots that represent the surface of the object being scanned. Stray dots can creep into the process, giving incorrect surface data for the object. So for many use cases, you not only have to scan the object, but you must then take that object into a 3d design or sculpting software to clean it up.

The advantages are that you’re starting with a very accurate shape of the object, but you will need to clean up the errant dots. You will want to smooth out the surfaces to match the reality of the target object, and sharpen any fine details that weren’t that well captured. It’s nowhere near as simple as scanning, then printing the device, at least not for good quality outputs. It will take a lot of post-processing. I liken it to using Photoshop on a photo to tweak it to get exactly what you want before printing it.

Using Photogrammetry At Home

For creating a mesh of dots, then creating a usable STL file, you’ll need some software. Much of this software is still in the experimental stage, so it’s not as polished as you’re likely used to for home applications. Some will have to be downloaded from GitHub and compiled before running.

The advantage is that they tend to support all the popular computer types out there, and the software isn’t ready for prime time, so it’s free while they work on it. Here is a list of four free photogrammetry softwares that are available for Windows, Mac and Linux.

Remember, these are, what I would call, works in progress. They will do what you need, but you have to go into them with the same attitude that you did with your 3d printer – you’re going to have a somewhat steep learning curve and you’re going to need to experiment until you get the results you’re after. None of them is as simple as pushing a button and out pops the result.

Still, free and powerful are two great attributes, so it’s worth exploring. You’ll have to do some cleanup of the final product to remove stray points and smooth out flat surfaces, but it’s magic-in-a-can, as far as I can tell.

MicMac	https://micmac.ensg.eu/index.php/Accueil	Windows, Mac, Linux
ColMap	https://colmap.github.io/index.html	Windows, Mac, Linux
Regard3d	http://www.regard3d.org/index.php	Windows, Mac, Linux
OpenMVG	https://github.com/openMVG/openMVG	Windows, Mac, Linux