Flow deck v2 — how do I obtain flow divergence from the sensor raw data?

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shreeram
Beginner
Posts: 4
Joined: Wed Dec 15, 2021 7:12 am

Flow deck v2 — how do I obtain flow divergence from the sensor raw data?

Post by shreeram »

Hi everyone!

I’ve been working with the flowdeck v2 to try and figure out how to obtain the flow divergence in the vertical direction of the optical flow vectors that are sensed from the camera. The flow divergence is equal to vz/z, where vz is the vertical downward velocity and z is the height above the ground. In other words, I’m looking for an output from the flow sensor which correlates with vz/z.

Looking at the variable groups that can be logged from the flowdeck, I can see that there’s deltaX and deltaY in flow/fr that I can log. I can also see that there’s motion.Rawsum that gives me the raw sensor data from the flow deck.

Here’s what I have tried so far:
  1. I’ve tried taking the hypotenuse of the quantities deltaX and deltaY to see if it correlates with vz/z. I don’t think this is working.
  2. I’ve tried logging motion.Rawsum and seeing whether its derivative correlates with vz/z.
So my doubts are:
  1. What are the units of the number (unsigned int 8-bit) that’s being logged from motion.Rawsum? Is it number of pixels?
  2. Is there a way I can extract the value of flow divergence — vz/z — from the optical flow sensor alone? (I don’t want to use the TOF sensor or anything else.)
Thank you in advance!
kimberly
Bitcraze
Posts: 1050
Joined: Fri Jul 06, 2018 11:13 am

Re: Flow deck v2 — how do I obtain flow divergence from the sensor raw data?

Post by kimberly »

Hi!

For units it is perhaps best to look at the datasheet of the flow sensor. I see that it is still on our old wiki so we would need to change that still to our website. But you can find it here: https://wiki.bitcraze.io/_media/project ... public.pdf

I believe with divergence is that you would need to have access to the individual flow factors per pixel and until now I haven't heard somebody achieving this with this particular sensor.
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