Theia3D automatically performs gap-filling and smoothing of the 3D poses for all people tracked within processed movement trials. This is achieved using the Generalized Cross-Validation Spline method, which is equivalent to a double Butterworth filter with some additional advantages.

For all processed movement trials, Theia3D saves both the unfiltered 3D poses and filtered 3D poses of all tracked individuals and either version of the tracked pose can be rendered in Theia3D (see Render Smooth IK in the Preferences window). When .c3d files are saved for a processed movement trial using Save Skeleton Poses, two separate .c3d files containing the filtered and unfiltered versions of the 3D poses are saved.

The default GCVSPL cutoff frequency is 20 Hz, however it is recommended that this is adjusted based on the movement contained in the trial; slower movements should be processed with a lower cutoff frequency, and faster movements with a higher cutoff frequency. The default 20 Hz cutoff frequency provides some smoothing and noise reduction while generally being sufficiently high to avoid ‘over-smoothing’ most movements.