I see preintegration in IMU-fused SLAM literatures, which mention that preintegration is useful in avoiding to recompute the IMU integration between two consecutive keyframes.
However, when I see some open-sourced SLAM code, e.g. OrbSLAM3, I haven't found anything special in preintegration because it just looks like an ordinary integration (with same time-intervals, same interpolation, same full caculation over multiple intervals), and I don't see an expected pre-calculated value that is reused time and time again.
So my question, is preintegration just an alias of the integration of IMU, or else how to correctly understand "pre"?
Finally I've made clear of Preintegration.
It is a misunderstanding to think of Preintegration as a pre-calculated value that is reusable in each process for calculating any new integration.
In fact, Preintegration actually means that people temporarily calculate integrations in a rough way for incoming any time instant (say, instant 0, 1, 2, ..., k-1) and, at a key instant k, a bundle adjustment can be performed on the pre-integrations over time instants [0, k] without recomputing the integration on time instant 0, 1, 2, ..., k-1.