Motion capture
Motion capture (sometimes referred as mo-cap or mocap, for short) is the process of recording the movement of objects or people. It is used in military, entertainment, sports, medical applications, and for validation of computer vision[3] and robots.[4] In filmmaking and video game development, it refers to recording actions of human actors and using that information to animate digital character models in 2D or 3D computer animation.[5][6][7] When it includes face and fingers or captures subtle expressions, it is often referred to as performance capture.[8] In many fields, motion capture is sometimes called motion tracking, but in filmmaking and games, motion tracking usually refers more to match moving.
In motion capture sessions, movements of one or more actors are sampled many times per second. Whereas early techniques used images from multiple cameras to calculate 3D positions,[9] often the purpose of motion capture is to record only the movements of the actor, not their visual appearance. This animation data is mapped to a 3D model so that the model performs the same actions as the actor. This process may be contrasted with the older technique of rotoscoping.
Camera movements can also be motion captured so that a virtual camera in the scene will pan, tilt or dolly around the stage driven by a camera operator while the actor is performing. At the same time, the motion capture system can capture the camera and props as well as the actor's performance. This allows the computer-generated characters, images and sets to have the same perspective as the video images from the camera. A computer processes the data and displays the movements of the actor, providing the desired camera positions in terms of objects in the set. Retroactively obtaining camera movement data from the captured footage is known as match moving or camera tracking.
The first virtual actor animated by motion-capture was produced in 1993 by Didier Pourcel and his team at Gribouille. It involved "cloning" the body and face of French comedian Richard Bohringer, and then animating it with still-nascent motion-capture tools.
Motion capture offers several advantages over traditional computer animation of a 3D model:
Non-optical systems[edit]
Inertial systems[edit]
Inertial motion capture[40] technology is based on miniature inertial sensors, biomechanical models and sensor fusion algorithms.[41] The motion data of the inertial sensors (inertial guidance system) is often transmitted wirelessly to a computer, where the motion is recorded or viewed. Most inertial systems use inertial measurement units (IMUs) containing a combination of gyroscope, magnetometer, and accelerometer, to measure rotational rates. These rotations are translated to a skeleton in the software. Much like optical markers, the more IMU sensors the more natural the data. No external cameras, emitters or markers are needed for relative motions, although they are required to give the absolute position of the user if desired. Inertial motion capture systems capture the full six degrees of freedom body motion of a human in real-time and can give limited direction information if they include a magnetic bearing sensor, although these are much lower resolution and susceptible to electromagnetic noise. Benefits of using Inertial systems include: capturing in a variety of environments including tight spaces, no solving, portability, and large capture areas. Disadvantages include lower positional accuracy and positional drift which can compound over time. These systems are similar to the Wii controllers but are more sensitive and have greater resolution and update rates. They can accurately measure the direction to the ground to within a degree. The popularity of inertial systems is rising amongst game developers,[10] mainly because of the quick and easy setup resulting in a fast pipeline. A range of suits are now available from various manufacturers and base prices range from $1000 to US$80,000.
Mechanical motion[edit]
Mechanical motion capture systems directly track body joint angles and are often referred to as exoskeleton motion capture systems, due to the way the sensors are attached to the body. A performer attaches the skeletal-like structure to their body and as they move so do the articulated mechanical parts, measuring the performer's relative motion. Mechanical motion capture systems are real-time, relatively low-cost, free from occlusion, and wireless (untethered) systems that have unlimited capture volume. Typically, they are rigid structures of jointed, straight metal or plastic rods linked together with potentiometers that articulate at the joints of the body. These suits tend to be in the $25,000 to $75,000 range plus an external absolute positioning system. Some suits provide limited force feedback or haptic input.
Magnetic systems[edit]
Magnetic systems calculate position and orientation by the relative magnetic flux of three orthogonal coils on both the transmitter and each receiver.[42] The relative intensity of the voltage or current of the three coils allows these systems to calculate both range and orientation by meticulously mapping the tracking volume. The sensor output is 6DOF, which provides useful results obtained with two-thirds the number of markers required in optical systems; one on upper arm and one on lower arm for elbow position and angle. The markers are not occluded by nonmetallic objects but are susceptible to magnetic and electrical interference from metal objects in the environment, like rebar (steel reinforcing bars in concrete) or wiring, which affect the magnetic field, and electrical sources such as monitors, lights, cables and computers. The sensor response is nonlinear, especially toward edges of the capture area. The wiring from the sensors tends to preclude extreme performance movements.[42] With magnetic systems, it is possible to monitor the results of a motion capture session in real time.[42] The capture volumes for magnetic systems are dramatically smaller than they are for optical systems. With the magnetic systems, there is a distinction between alternating-current (AC) and direct-current (DC) systems: DC system uses square pulses, AC systems uses sine wave pulse.
Stretch sensors[edit]
Stretch sensors are flexible parallel plate capacitors that measure either stretch, bend, shear, or pressure and are typically produced from silicone. When the sensor stretches or squeezes its capacitance value changes. This data can be transmitted via Bluetooth or direct input and used to detect minute changes in body motion. Stretch sensors are unaffected by magnetic interference and are free from occlusion. The stretchable nature of the sensors also means they do not suffer from positional drift, which is common with inertial systems. Stretchable sensors, on the other hands, due to the material properties of their substrates and conducting materials, suffer from relatively low signal-to-noise ratio, requiring filtering or machine learning to make them usable for motion capture. These solutions result in higher latency when compared to alternative sensors.