The motion capture system is essentially a positioning system, which usually requires positioning equipment to be placed on the target for tracking. The motion capture system based on the principle of infrared optics mainly consists of optical lenses, motion capture software, reflective marking points, POE switches, and several accessories. Among them, the reflective marking point (Marker point) is a type of marker placed on the target. special sphere.
Motion capture systems based on infrared optical principles are divided into active and passive types. One of the differences lies in the reflective marking points. The reflective marking points of the active principle need to be connected to a power source to actively emit light, so that the optical camera can obtain its spatial position. The reflective marking ball in the passive optical principle is a small ball with a surface coated with reflective material. It has no connecting wires, no power supply, and no electronic components inside. It is fixed on the surface of the captured object and reflects the infrared light emitted by the lens. , and then received by the sensor matrix on multiple lenses, and the three-dimensional coordinates of the center of the sphere are obtained. This article will introduce the reflective marker points (Marker points) in the passive optical principle motion capture system.
Different types of reflective marking points (Marker points) are adapted to different application scenarios. For example, when the size of the captured object is small, a smaller Marker is needed to adapt to it, and when the surface is very smooth, a double marker is needed. The reflective marking points (Marker points) of the surface adhesive base are fixed.
The size selection of reflective marking points is usually related to the type and size of the captured object:
- When capturing the whole body movement of the human body, reflective marking points (Marker points) with a diameter of 12 to 15mm are commonly used
- When capturing the fine movements of human hands, feet, and faces, small reflective marking points (Marker points) with a diameter of less than 8mm are often used.
- Indoor positioning and pose capture of multi-agent objects such as drones and robots, which are common in the direction of automation, usually use 8 to 12mm reflective marking points (Marker points)
- If the scene space and the size of the captured object are both relatively large, use a marker with a diameter greater than 15mm.
Fixing method
In order to ensure the normal use of the system, the reflective marking points (Marker points) need to be stably fixed on the surface of the captured object. The reflective marking points (Marker points) are mainly fixed through three methods: hook surface base, flat base and no base.
Hook surface base: It is a Velcro method, and the hook surface is attached to the adhesive Velcro on the surface of the object to be measured.
Flat base: Double-sided tape is usually used to fix the base on a solid and smooth surface of the object to be measured, or it is used to capture small movements of parts of the human body.
No base: The reflective marking points (Marker points) are directly fixed to the surface of the mechanical structure, acrylic plate, carbon fiber rod and other materials through screws, hot melt glue, etc.
There are usually the following precautions when attaching reflective marker points (Marker points):
① In order to ensure that the system establishes rigid body and multi-rigid body models for the captured object, at least three reflective marker points (Marker points) need to be set on the surface of the captured object. To define a model to obtain its 3D coordinates.
② The two reflective marking points (Marker points) cannot be too close, otherwise the system may not be able to correctly identify them.
③ The reflective marking points (Marker points) need to be arranged in a non-linear and asymmetrical manner so that the system can correctly identify them. direction and distinguish corresponding actions.
In different application directions, such as indoor positioning of robot drones, film and television motion capture special effects production, large-space VR training, human gait analysis, etc., there are also many types of corresponding point posting methods.
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In the field of motion analysis,
there are usually special human body point templates, such as the Helen Hayes model, etc. The reflective marking points (Marker points) are attached to the human skin surface or tights according to fixed positions. -
In the field of film and television animation production
, we usually use motion capture clothing to attach points. Generally, the number of points posted on the whole body is usually greater than or equal to 50 reflective marking points (Marker points). Each limb segment is guaranteed to have 3 to 5 Markers attached. , the sticking points are usually at the joints of bony landmark points and the centers of limb segments to ensure that the occlusion of the points during movement will not affect the stability and smoothness of the motion capture effect.