# Markers

These routines facilitate the calculation of 3d movement kinematics for marker-based video recordings.

## Functions

### Optical Systems

- :func:’kinematics.analyze3Dmarkers’ ... Kinematic analysis of video-basedrecordings of 3D markers
- `kinematics.findTrajectory()` ... Calculation of point trajectory, from initial position + sensor position/orientation

## Details

Utilities for analyzing movement data recorded with marker-based video systems.

`markers.``analyze3Dmarkers`(MarkerPos, ReferencePos)

  Take recorded positions from 3 markers, and calculate center-of-mass (COM) and orientation Can be used e.g. for the analysis of Optotrac data.

  Parameters: | MarkerPos (ndarray, shape (N,9)) – x/y/z coordinates of 3 markers ReferencePos (ndarray, shape (1,9)) – x/y/z coordinates of markers in the reference position
  Returns: | Position (ndarray, shape (N,3)) – x/y/z coordinates of COM, relative to the reference position Orientation (ndarray, shape (N,3)) – Orientation relative to reference orientation, expressed as quaternion

  Example

      >>> (PosOut, OrientOut) = analyze3Dmarkers(MarkerPos, ReferencePos)

`markers.``findTrajectory`(r0, Position, Orientation)

  Movement trajetory of a point on an object, from the position and orientation of a sensor, and the relative position of the point at t=0.

  Parameters: | r0 (ndarray (3,)) – Position of point relative to center of markers, when the object is in the reference position. Position (ndarray, shape (N,3)) – x/y/z coordinates of COM, relative to the reference position Orientation (ndarray, shape (N,3)) – Orientation relative to reference orientation, expressed as quaternion
  Returns: | mov – x/y/z coordinates of the position on the object, relative to the reference position of the markers
  Return type: | ndarray, shape (N,3)

  Notes

  \[\vec C(t) = \vec M(t) + \vec r(t) = \vec M(t) + {{\bf{R}}_{mov}}(t) \cdot \vec r({t_0})\]

  Examples

      >>> t = np.arange(0,10,0.1)
      >>> translation = (np.c_[[1,1,0]]*t).T
      >>> M = np.empty((3,3))
      >>> M[0] = np.r_[0,0,0]
      >>> M[1]= np.r_[1,0,0]
      >>> M[2] = np.r_[1,1,0]
      >>> M -= np.mean(M, axis=0)
      >>> q = np.vstack((np.zeros_like(t), np.zeros_like(t),quat.deg2quat(100*t))).T
      >>> M0 = vector.rotate_vector(M[0], q) + translation
      >>> M1 = vector.rotate_vector(M[1], q) + translation
      >>> M2 = vector.rotate_vector(M[2], q) + translation
      >>> data = np.hstack((M0,M1,M2))
      >>> (pos, ori) = signals.analyze3Dmarkers(data, data[0])
      >>> r0 = np.r_[1,2,3]
      >>> movement = movement_from_markers(r0, pos, ori)