I asked the two smartest people I know
James Strawson at ModalAI (implemented VIO on VOXL2)
Alex Klimaj (created the ArkFlow board)
Would like to note that BOTH indicated that the best outcome would be a combination of both, and Alex indicated there are people flying with both today.
James
Optic flow requires an accurate measurement of the distance to the ground and also assumes a planar surface. If both of those are met then optic flow can be just as accurate and more reliable than VIO. Ideally one would use a combination of both. it’s more robust due to having fewer states, VIO is very complicated with a lot of states.
Yes, one could contrive some scenarios where a camera could see texture that could be tracked with patch-based optic flow but does not have point features, but our experience flying high up outdoors is that the real world almost always has features to track.
This reminds me of the Multi-IMU / Multi EKF2 thing in PX4. It’s all based on the idea of “what would happen if an IMU fails”, yet I’ve never once in 12 years of making drones had an IMU fail in flight. Sensor choice should be based on specific environments not wild guesses.
Alex
Probably a combination of the two is best. Downward facing optical flow gives you a planar velocity thats corrected for angular velocity with the gyro. Then you can fuse external vision odometry on top of that. I think they each have their place. You can’t do any obstacle avoidance or mapping with optical flow.
I know a number of companies using my flow with VIO as well. At the very least the flow allows you to hold position when the VIO falls apart
Takeaway
These two are not that different. Just like anything, both of these need cameras to understand localization. There are instances where optical flow can be more reliable, assuming accurate height and a flat surface. Probably great for low flying drones indoors. Like most things, VIO is more complex, but you can do more with it…especially use it in more outdoor settings as backup incase of GPS loss.