Different types of motion capture

Different types of motion capture

When deciding to use motion capture technology, it’s important to know which variety you need. Each has its own unique properties that work well with specific tasks, and some will be better for your project than others.

Optical motion capture: passive

Passive optical systems are the most widely used type of motion capture. It’s mainly used in film and television, but other use cases include video games or ergonomic research.

If you’ve ever seen behind-the-scenes footage of a film, where the actors are wearing suits covered in dots, you’ll know what optical motion capture looks like. The dots are markers that pinpoint parts of the body to gather data from. They’re retro-reflective, meaning they bounce infrared light back to specialized cameras, allowing them to exclusively track the markers instead of anything else that might be going on in the shot. 

This type of motion capture requires a multi-camera setup to create a full field of vision within a designated area. The subject needs to stay in that set space in order for their motion to be recorded by the cameras. While this limits the actor to a set space, this multi-camera operation ensures optimal accuracy and reduces the risk of clunky movements in processing software. A minimum of three cameras are used to triangulate a point and pick up on every movement, but to ensure good quality data you need at least six.

Optical motion capture: active

We’re back to the dotted suits again, but this time the markers do a slightly different job. Instead of bouncing light back to the camera, the suit emits its own light from LEDs, which the cameras pick up on. These lights have to be bright enough so that the camera only focuses on the markers, and not the actor. 

To see your results, all camera footage must be put through processing software, which then generates 3D coordinates for each marker. The combination of markers, footage, and coordinates can then create a 3D model of the subject, which means a more efficient post-production process despite the longer setup time.

Markerless motion capture

Powered by AI and dedicated software, markerless motion capture does away with the need for suits. Instead, AI picks up on movements from a setup of one or more cameras, given you have the correct lighting that clearly exposes the subject.

Using the footage, AI generates a 3D model of the subject. This type of motion capture technology, however, isn’t as accurate as the others, because rather than pinpointing exact markered locations the AI is interpreting motion, essentially creating a “best guess” model.

Because less equipment is needed, AI motion capture is relatively easy to set up and calibrate. However, the lack of markers makes the output less accurate. Nuanced motions of the body and smaller details, such as hands, are not up to the standards required for major projects at this point, bringing the need for inertial motion capture gloves to be added into the mix. This leads to more data cleanup and post-processing in the long run.

Inertial motion capture

Unlike the other methods of motion capture, inertial technology doesn’t require a setup of cameras, instead it relies on its own built-in technology. Gyroscopes send data to processing software that establishes their orientation, and an accelerometer tracks magnetic north so that each sensor is faced in the correct direction. The inbuilt nature of inertial motion capture makes the technology entirely portable while maintaining pinpoint accuracy.

There’s no need to collate data before sending it to a computer, either. Instead, the data received from the sensors is streamed directly into processing software, allowing for real-time visualization. Inertial motion capture can be used anywhere and doesn’t require a dedicated studio, unlike optical or markerless motion capture, which makes it very flexible.

High-end inertial motion capture

The typical downside of inertial motion capture is magnetic interference. Due to the electromagnetic fields around metal objects – which includes computers – the data can be of lower quality. High-end inertial motion capture, however, is immune to magnetic interference, producing optical-quality data.

This combination of portability, ease of use, and accuracy makes for a highly adaptable and precise method of recording movement – the choice of many AAA game studios and VFX houses.

Combining strengths: Xsens

Comprising a lightweight suit or a set of 17 wearable sensors, Movella’s Xsens system is the gold standard in motion capture technology. And it’s fast: Xsens takes around 15 minutes to set up, whereas the optical counterpart can take 45 minutes or longer to do the same – assuming you’ve already got cameras in place. 

Xsens inertial sensors track the subject’s exact movements, which are then translated to a 3D model in near real-time. The comfort of the wearables makes for more authentic movement, which then creates highly accurate data.

Xsens’ high-end inertial motion capture system in particular boasts electromagnetic shielding, which uses proprietary software to block disruptive signals from nearby electronics. Magnetic immunity enables motion capture from anywhere, no matter the surroundings, which serves as a more practical alternative to optical or AI motion capture. 

To take a look into Xsens’ inertial motion capture units.