Motion capture is a technology that allows you to record the movements of a person or object, and reproduce them in a virtual environment. It was invented at the end of the 20th century. It was in the 1970s that the military and scientific institutions became interested in this subject and this technology. The first tests were conducted through the study of the movements of a galloping horse, which were projected on screen. This new technology has greatly contributed to the study of human movement, allowing advances in health and anatomy.
Today, we use this technology to compare and validate our analysis results after each activity. We have invested in this sophisticated equipment: the Opti Track, a motion capture system.
How does it work?
In most cases, a person is used as a source of data that will be transformed into animation (but it can be also an animal, a puppet or an object). This way, the movements of the virtual subject will be very close to reality.
There are several steps to motion capture. The first step is the “Studio Set-up” (the use of four to eight cameras generally, with a maximum of 24 for multiple captures), i.e. the arrangement of a room for motion capture. Then it is necessary to calibrate the capture area, then to capture motion. Once the data has been recorded, it must be processed in order to be analyzed. This data will then become usable and comparable scientific data.
What are the different methods?
To perform motion capture, it is necessary to place a multitude of sensors on the actor at strategic places on the body, such as joints. The movements of the sensors will be recorded and then transcribed virtually. This style of motion capture is the optical capture. It is the most used. The cameras specially designed for motion capture will send infrared rays, which will be reflected by the sensors placed on the subject. It is these reflections that allow the transcription of the actor’s movements on a computer. The main problem with this method is its sensitivity to obstacles between sensors and cameras. Infrared rays will not be able to pass through the obstacle and therefore, will not be reflected by the sensors.
Movie studios and others are turning to gyroscopic motion capture. The principle is the same as for the previous method, but the sensors will be replaced by gyroscopic and inertial sensors. They allow for capturing in real time the angle and position of the part of the body where they are placed in real time. A transmitter connected to the computer will re-transmit the information. There is no obstacle problem with this method, but it is less accurate than optical capture.
This method works thanks to an exoskeleton built around the element to be captured. The exoskeleton is a structure whose angles are angular sensors. These angles are connected by wires to a computer. Each sensor knows its position in relation to the others, which allows the computer to know the movements of the entire structure. This method is very popular because it is very accurate and calculates the data recorded by the sensors. But the need to be connected by wires to the computer reduces the range and imposes mechanical constraints.
This method uses sensors placed on the actor’s body to measure the low-frequency magnetic field generated by an emitter. The sensors, as well as the transmitter, are connected to a computer that will report the positions of the sensors in the magnetic field. A software program will then transcribe the position and movements of the sensors in a 3D space. If several actors use this method at the same time, the sensors from the different actors will interfere with each other. The recorded data will be distorted and therefore unusable.