9.6. Extracting angles

We already know how to create homogeneous transforms based on known angles, which is easily done using Kinetics Toolkit’s ktk.geometry.create_transform_series function. In biomechanics, it is often useful to do the inverse: computing frames and transforms from measured kinematics (e.g., using markers or inertial measurement units), and then extracting joint angles from those transforms.

Unfortunately, extracting tridimensional angles from a given homogeneous transform is notoriously difficult to understand and is often overlooked. Understanding how to communicate angles is very important because similar angles can lead to completely different poses if the rotations are not performed in the same order or around the same axes. This section will tackle this concept and show how to work with 3D rotations, starting with 2D rotations.