1. SS5305 – Motion Capture Initialization 1

2. Objectives Camera Setup Data Capture using a Single Camera Data Capture using two Cameras Calibration Calibration using Qualisys 2

3. Motion Capture Workflow 1. 2. 3. 4.5.6.7. automated by most mocap systems

4. Camera Setup & Input Field-of-view determines the volume of an individual camera. Collection of cameras define a capture volume. More cameras = Less occlusion = Large capture volume = High accuracy Video input must record capture data from all cameras simultaneously.

5. Data Capturing using a single camera One camera is not sufficient to record depth.

6. Two views can uniquely determine a 3D point in space. Intersection of lines in space through the image points gives three dimensional position of the marker. Data Capturing using two cameras

7. Motion Capture is sensitive to error Assume d = 3 meters (camera distance to marker) Assume camera = 640 pixels Assume field of view = 42 degs Then error is: e = d tan ( ang ) ang = 42 / 640 = 0.065 degrees / pixel e = 3 * 0.001145 e = 0.003145 meters e = 3.145 millimeters per pixel If camera distance (d) doubles… Error e doubles If camera resolution is halved… Error e doubles If pixel error doubles… Error e doubles Error is compounded with multiple cameras.. creating an error volume.

8. Camera Calibration Problem: Determine the exact position and orientation of virtual cameras to match real world cameras? Generally solved using Direct Linear Transform (DLT). System of equations with 11 unknowns. Requires at least 6 known non-coplanar points. To provide known axes, a calibration targer or wand is used. 11 unknowns Camera position Xo, Yo, Zo Camera direction Uo, Vo Camera angles Xr, Yr, Zr Camera scaling Sx, Sy Camera distance d

9. Remember: The camera information must be known exactly c0 = camera location, V = camera rotation, P = projection matrix Direct Linear Transform (DLT) uses the colinearity condition. Points C,I,O must be colinear by definition of projection. Given known points in 3D space, we can construct a system of equations that solves for C. http://www.kwon3d.com/theory/dlt/dlt.html

10. 3. Pose Calibration Establishes the 3D marker locations when the joint system is in the T-pose position. Joint centers are not the marker centers. Pose calibration allows the system to match marker locations to joint distances in the resting position

11. Calibration For 3D measurement, the system needs to be calibrated. A wand is simply moved around in the volume while a stationary reference object in the volume defines the coordinate system for the motion capture. The calibration is finished after approximately 10-20 seconds. X Y Z X Y Z Y X Z Origo = 0, 0, 0

12. Calibration Review results: Number of points and Average residual for each camera Standard deviation of wand length < 1 mm X Y Z X Y Z