**2. Model of Visual–Inertial Odometry**

In this section, we present the model of a keyframe-based VIO system consisting of the motion model, the keyframe management, and the measurement model. The motion model was formulated in form of classical rigid-body kinematics with the modified Rodrigues parameter representing the attitude. The keyframe management was mainly inspired by Engle's DSO system with some simplifications. The measurement model was formulated in the form of the simple frame-to-frame alignment with the intensity as the measurement.
