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--- visual3d:documentation:marker_based_motion_capture [2026/05/26 19:26] – [Forces] wikisysop
+++ visual3d:documentation:marker_based_motion_capture [2026/06/09 13:26] (current) – wikisysop
@@ Line 1: / Line 1: @@
-====== Marker-Based Motion Capture Overview (Under Construction) ======
+====== Marker-Based Motion Capture Overview ======
+Marker-based motion capture systems can be split into passive and active systems.
+**Passive marker-based systems** generally use retroreflective markers placed on the body in accordance with a particular marker set. Cameras are set up to record the near infrared light reflected by these markers and estimate the 3-dimensional position of these markers throughout the movement. A model, essentially a mapping from markers to the skeleton, provides these marker positions with biomechanical meaning and is defined by the user. This model is applied to the recorded 3D marker positions during dynamic movements of interest to produce pose estimates.
+**Active marker-based systems** generally use markers that emit light, which are placed on the body. Cameras record the light emitted by these markers and estimate their 3-dimensional position throughout the movement.
 ===== Marker Sets =====
-Before data collection using markerbased motion capture, you should decide on the appropriate [[visual3d:documentation:modeling:marker_sets:marker_sets_overview|Marker Set]] for your research question. See the [[visual3d:documentation:modeling:marker_sets:marker_set_guidelines|Marker Set Guidelines]] and read more on [[visual3d:documentation:modeling:marker_sets:marker_placement|Marker Placement]].
+Before data collection using marker-based motion capture, you should decide on the appropriate [[visual3d:documentation:modeling:marker_sets:marker_sets_overview|Marker Set]] for your research question. See the [[visual3d:documentation:modeling:marker_sets:marker_set_guidelines|Marker Set Guidelines]] and read more on the specific considerations of [[visual3d:documentation:modeling:marker_sets:marker_placement|Marker Placement]].
 In general, the marker set will comprise of:
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 // Fig. 1. Visual of anatomical and tracking markers on a left tibia. The segment coordinate system is defined at the proximal end of the segment //
+Marker sets commonly fall into three families:
+  - Plug-in Gait ([[visual3d:documentation:modeling:marker_sets:pig_ll_original|Plug-in Gait Original Marker Set]], [[visual3d:documentation:modeling:marker_sets:plug-in_gait_clavicle|Plug-in Gait Clavicle Marker Set]], [[visual3d:tutorials:modeling:plug-in_gait_lower_limb|Plug-in Gait Lower Limb]])
+  - IOR Gait ([[other:ior_gait:ior_gait_overview|IOR Gait Overview]])
+  - OptiTrack ([[visual3d:documentation:modeling:marker_sets:optitrack_biomech_57_marker_set|OptiTrack Biomech 57 Marker Set]])
-Some common marker sets include:
-  * The [[visual3d:documentation:modeling:marker_sets:optitrack_biomech_57_marker_set|OptiTrack Biomech 57 Marker Set]]
-  * The [[visual3d:documentation:modeling:marker_sets:pig_ll_original|Plug-in Gait Original Marker Set]]
-  * The [[visual3d:documentation:modeling:marker_sets:plug-in_gait_clavicle|Plug-in Gait Clavicle Marker Set]]
-  * The [[visual3d:tutorials:modeling:ior_gait_full_body_model|IOR Gait Full Body Model]]
 ===== Data Collection =====
-A [[visual3d:documentation:definitions:static_trial|static trial]] of the subject is required to define the segment coordinate system and build the model. Subsequent Dynamic Trials should contain the movement of interest to be analyzed.
+A [[visual3d:documentation:definitions:static_trial|static trial]] of the subject is required to define the segment coordinate system and build the model. The position of the subject will depend on the marker set you selected and the activity you want to look at. Subsequent Dynamic Trials should contain the movement of interest to be analyzed.
 ===== Model Building =====
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 ==== Segments ====
 Segments are defined using 3 or more noncolinear markers attached to a rigid body. The longitudinal axis of the segment (default Z) runs from the distal to proximal end.  The frontal plane is defined by 3 or 4 targets / landmarks (default XY plane). The segment ends are described by targets or landmarks. Due to the [[visual3d:documentation:definitions:right_hand_rule|Right Hand Rule]], the segment coordinate systems are not mirrored for right and left sides of the body. The segment coordinate system is always defined at the proximal end of the segment.
 See the 4 methods to define a segment on the [[visual3d:documentation:modeling:coordinate_system|Segment Coordinate System]] page. See the [[visual3d:documentation:modeling:segments:segment_overview|Segment Overview]] for more information.
@@ Line 45: / Line 49: @@
 //Fig. 3: The segments are represented by the grey volumes. Target markers shown in grey, and landmarks in teal.//
 For more guidance on model building, see the [[https://www.youtube.com/playlist?list=PLe3xyjLUkDOOtZbx2jzBU_X0fPywJzh6T|Model Building Playlist]] on the HasMotion youtube or check out the [[visual3d:documentation:modeling:modeling_faq|Modeling FAQ]].
 ===== Pose =====
@@ Line 54: / Line 59: @@
 ==== The 6 DOF Model / Segment Optimization ====
-This method of pose estimation tracks each segment independently. Here, we use 3 translational (X, Y, Z) and 3 rotational (qx, qy, qz) components to describe the rigid body (6 DOF) relative to the global coordinate system. One marker defines 3 DOF (X, Y, Z), the second marker adds two more DOF (qx, qy), and the third (non-colinear) marker adds 1 more DOF (qz). A fourth marker adds no additional information (the system is overdetermined). Learn about the [[visual3d:documentation:kinematics_and_kinetics:six_degrees_of_freedom|6 Degrees of Freedom]] pose estimation, or complete the [[visual3d:tutorials:modeling:building_a_6_dof_model|Building a 6 DOF Model Tutorial]]
+This method of pose estimation tracks each segment independently. Here, we use 3 translational (X, Y, Z) and 3 rotational (qx, qy, qz) components to describe the rigid body (6 DOF) relative to the global coordinate system. One marker defines 3 DOF (X, Y, Z), the second marker adds two more DOF (qx, qy), and the third (non-colinear) marker adds 1 more DOF (qz). A fourth marker adds no additional information (the system is overdetermined). Learn about the [[visual3d:documentation:kinematics_and_kinetics:six_degrees_of_freedom|6 Degrees of Freedom]] pose estimation, or complete the [[visual3d:tutorials:modeling:building_a_6_dof_model|Building a 6 DOF Model Tutorial]].
 ==== IK Model ====
 Inverse Kinematics (IK) offers an alternative approach to pose estimation from the 6 DOF. The IK method is also referred to as Global Optimization, since segment positions and orientations are solved in order to minimize a global loss function while respecting certain joint constraints. Learn about the theory of [[visual3d:documentation:kinematics_and_kinetics:inverse_kinematics|Inverse Kinematics]], then check out the [[visual3d:tutorials:modeling:building_an_ik_model|IK Model Building Tutorial]].
-===== Marker Data in V3D =====
+===== What Next? =====
-==== Filtering ====
+To visualize your motion data, check out the [[visual3d:tutorials:reports:visualizing_data|Visualizing Data]] tutorial.
-==== Forces ====
-If you collected force data along with your markerbased data, you can [[visual3d:documentation:kinematics_and_kinetics:external_forces:force_assignment|assign those forces]] to the model that you built in Visual3D and compute [[visual3d:documentation:kinematics_and_kinetics:inverse_dynamics|Inverse Dynamics]]. See the [[visual3d:documentation:kinematics_and_kinetics:external_forces:force_overview|Force Overview]] page.
-==== EMG ====
-Another method of biomechanical data collection is [[visual3d:documentation:emg:emg_overview|electromyography (EMG)]], which may be collected synchronously with markerbased data.