Graph Plots reference

This section provides further details about options and modifiers found within the Graph Plots view.

For a description of all channel types and the channel sources within each type, see Channel types.

For information on the calculations used in Graph Plots, including modifiers, see the relevant topic:

• Derivatives
• Rotation unwrapping
• Magnitude

For a more general introduction, see Graph your data.

Channel types

Selected channel types

These are channels that require a channel source, for example, a device, subject, segment, or marker.

Cameras

• Camera Position The position of a calibrated camera. Use this with the Difference Between channel type and the Magnitude toggle to plot ranges (the distance between a camera and a tracked subject or marker).

  

Relative Measurements

• Angle Between The angle ABC is defined by the positions A, B and C (subjects, labeled markers, unlabeled reconstructions or segments, or if you select a subject, a subject's root segment).

• Difference Between The difference between two positions (subjects, labeled markers, unlabeled reconstructions or segments, or if you select a subject, a subject's root segment).
• Relative Position The position of subject B relative to subject A (in the frame of reference of subject A).
• Relative Rotation The rotation of subject B relative to subject A.

Subjects

• Labeled Fraction The fraction of markers labeled for a subject. Use this to investigate subject occlusion or potential tracking issues.
• Position The position of a subject segment or subject bone, or if you select a subject, the root segment (from the solving skeleton or the retarget skeleton) relative to the volume origin.
• Rotation The rotation of a subject segment or subject bone, or if you select a subject, the root segment (from the solving skeleton or the retarget skeleton) relative to the global coordinate system axes.
• Position and Rotation Displays the Position and Rotation channels.

Trajectories

• Marker Position The marker position relative to the volume origin.
• Reconstruction Position The unlabeled reconstruction position relative to the volume origin.

Global channel types

These are channels that do not require a source to be selected in order to display data.

Subjects

• Enabled Count The number of subjects enabled in the Tracking panel.
• Tracked Count The number of subjects currently tracked.

Trajectories

• Labeled Marker Count The number of object markers currently tracked.
• Unlabeled Reconstruction Count The number of marker positions reconstructed from camera centroids but not labeled as part of an object.

Derivatives

Default derivatives

For most channel types, the derivatives are computed using a central finite difference approximation.

First order (velocity):

x'(t + 1/2) = x(t + 1) - x(t)

Where x is the channel sample value and t is the channel sample number.

Note that first-order derivatives are offset by half a sample with respect to the input channel.

Second order (acceleration):

x''(t) = x(t + 1) - 2x(t) + x(t - 1)

Rotation derivatives

First-order rotational derivatives (rotational velocity) are computed using the relative rotation between successive samples:

R'(t + 1/2) = R^-1(t)R(t+1)

Second-order rotational derivatives are computed using the central finite difference of the first order rotation derivative:

R''(t + 1) = R'(t + 3/2) - R'(t + 1/2)

Rotation unwrapping

The angle range (-π, π) radians is sufficient to describe all orientations. However, as an object continues to rotate in the same direction and approaches these boundaries, a flip in orientation (and sign) can be observed to keep the angle within the (-π, π) radians range. Rotation unwrapping enables you to strategically add factors of 2π to the angle to generate equivalent representations of the same orientation. This helps to remove any discontinuities in the data.

For example, an object rotating through an angle of 4π might result in a trace like this:

Note that unwrapping doesn't necessarily tell you the trajectory of orientations the object went through to get from A to B, and the results are not easy to interpret if the axis of rotation changes. Gaps in the object tracking can also cause the rotation unwrapping to restart.

Rotation unwrapping is enabled by default, but you can disable it in user preferences (Settings > Preferences > User Preferences > Graph Plots section > Enable rotation unwrapping).

Magnitude

Magnitude can be used to compute the resultant of a vector quantity, or the absolute value of a scalar quantity. Some common use cases include:

• Magnitude of position = distance from origin
• Magnitude of rotation = angle rotated about the rotation axis
• Magnitude of difference between or relative position = distance between
• Magnitude of velocity = speed