I'm looking for more information on how to use drift-correction correctly (using Unity SDK).
On the Tango website it says "Drift-corrected frames come through the Area Description reference frame", that the frame pair Start Of Service -> Device "does not include drift correction" and for Area Description -> Start Of Service that it "provides updates only when a localization event or a drift correction occurs".
The way I'd like to use a drift-corrected pose is like in the TangoPointCloud prefab, where depth points are multiplied by a matrix startServiceTDevice which results from the frame pair SoS -> Device. Assuming that the drift-corrected frame is in the AD frame, I'd need SoS -> AD. Since only AD -> SoS is available, I tried with this one and its inverse. The resulting pose is too small though to make any sense (even if using it the wrong direction, the translation shouldn't be close to zero if I had been walking around). Then I considered that the AD frame might actually be something like a drift-corrected Start of Service, but then again I can't find any significant/visible difference between AD -> Device and SoS -> Device, definitely no loop closures in it. I'm requesting and applying poses after finishing my scan, so drifts should have been detected by then.
On the Tango website it's further said that "There will be a period after Startup during which drift-corrected frames are not available.", yet the AD -> SoS pose is available (and valid) from the beginning and I couldn't yet produce a situation where it wasn't (e.g. no motion, rapid motion...).
Is drift correction working at all? Or am I using it all wrong?
PS: On the latest stackoverflow post it sounds as if drift correction would be for relocalization after tracking loss only. However, I find this hard to believe since the Tango website describes drift correction as "When the device sees a place it knows it has seen earlier in your session, it realizes it has traveled in a loop and adjusts its path to be more consistent with its previous observations.".
Drift correction is working as experimental features at this moment, there's corner cases that it will break. I will go into more details later.
In order to use drift correction pose, you will need to use ADF_T_Device frame pair (ADF is base frame, Device is target frame). In the example of using drift-correction pose to project points into world space, you don't need to do Adf_T_ss * ss_T_device transform, instead, all you only need to use ADF_T_device frame directly. If this is in Unity, you can just check the use area description pose on PointCloud prefab.
Corner cases that breaks drift-correction:
Under the hood, drift correction is constructing a more dense but more accurate version of ADF. If user covers camera or shake device at the very beginning, that will cause that no ADF (or features) being saved in the buffer. Thus the API could get into a state that never gives any valid pose from ADF_T_Device frame pair.
This is similar to the first case. If user moved to a new space without relocalizing after lost tracking, device will never relocalized, thus no valid pose will be available through ADF_T_device frame.
Drift correction API is still experimental, we are trying to address above issues from API level as well.