Something strange I found today on my videos from my Mavic Air ...

At the beginning of the video you can see the shadow in the lower left quarter, he moves towards the vines and becomes a bright spot when it flys over it. No backlight, the light comes from behind. Other reflection? Rather unlikely ... At the end of the clip the spot is in the middle, this means that this is no scratch on the lens.

This Video is out of the box (original and unfinished from the sd card of the mavic air). A bright spot passed through the video where the drone's shadow was supposed to be. Nearby may be quite helpful, for the distance remains a visible bright spot ... Currently I can not explain it other than that the software of the Mavic looking for a shadow of the drone and retouched ...

Did you already have that?

I’m sure that is just reflection from the angle of the drone to the sun.

A CW Posted at 2018-3-16 14:33
I’m sure that is just reflection from the angle of the drone to the sun.

That's what I thought ... but somehow I doubt it, I don't know ...

lens flare. polarized ND filters will fix.

Bob Brown Posted at 2018-3-17 09:26
lens flare. polarized ND filters will fix.

Thanks Bob!
Lens flares from behind, is that right? Is it mirrored by the front lens? ... I know lens flares when it is backlit or glancing light ... Ok, I'm still waiting for the ND filters, I' m curious

No not 'lens flare'! This is a physical phenomenon of sun light on vegetation and any surface material that 'back scatters' specular reflections*. Basically every surface reflects light even 'Vantablack' back to it's origin or source. You usually don't see this until you are at altitude either from an airplane window, drone POV,  or up a mountain - as in the famous 'Brockengespenst' / 'Brocken spectre', where the similar effect comes from refections off water droplets. The more common sight of this backscatter effect is because the sunlight reflection angle is at or near zero relative to the observer/camera, so that self shadowing and insident shadows are 'hidden' from that POV - therefore the light appears to be brighter compared to the surounding parts of the image. Your video shows this really well if you look carefully at the vine poles .... hidden shadows.
Separately the drone ground shadow - can only be 'removed' by using a 3rd party image compositing application (Aftereffects / Nuke etc.) that can track and mask the area out of the video. Likewise the spectral 'hot-spot' would also need to be tracked and masked out with a similar graduated luminance mask.

*The technical term is called 'Bidirectional Reflectance Distribution Function' (BRDF). It is used to monitor crop/vegitation health too.

Lamplighter55 Posted at 2018-3-18 07:26
No not 'lens flare'! This is a physical phenomenon of sun light on vegetation and any surface material that 'back scatters' specular reflections*. Basically every surface reflects light even 'Vantablack' back to it's origin or source. You usually don't see this until you are at altitude either from an airplane window, drone POV,  or up a mountain - as in the famous 'Brockengespenst' / 'Brocken spectre', where the similar effect comes from refections off water droplets. The more common sight of this backscatter effect is because the sunlight reflection angle is at or near zero relative to the observer/camera, so that self shadowing and insident shadows are 'hidden' from that POV - therefore the light appears to be brighter compared to the surounding parts of the image. Your video shows this really well if you look carefully at the vine poles .... hidden shadows.
Separately the drone ground shadow - can only be 'removed' by using a 3rd party image compositing application (Aftereffects / Nuke etc.) that can track and mask the area out of the video. Likewise the spectral 'hot-spot' would also need to be tracked and masked out with a similar graduated luminance mask.

Thank you very much for the great explanation! This all sounds very plausible, even though the Mavic Air is a rather small drone, I can imagine the reflection behavior at a certain angle in the form as in the video ... What you do not have to watch out for when the drone is in the air!

And many thanks for the tip with the tracking Mask, I will give it a try!

If it is possible, could you please upload the original video to Dropbox and provide me with the link for a better confirmation? Thank you.

Amazing, I needed to watch it twice for spotting it.

Wachtberger Posted at 2018-3-19 00:50
Amazing, I needed to watch it twice for spotting it.

Backscatter - get it all the time esp in the wet winter months when the sun is low:
A right pain as it draws the eye.

Michael_G Posted at 2018-3-18 22:44
Thank you very much for the great explanation! This all sounds very plausible, even though the Mavic Air is a rather small drone, I can imagine the reflection behavior at a certain angle in the form as in the video ... What you do not have to watch out for when the drone is in the air!

And many thanks for the tip with the tracking Mask, I will give it a try!

You're welcome! One thing that may help to reduce the' backscatter hot spot' is to use a polarising filter, which might cut the reflected component of the light. That said, it would also cut the same reflected light (and more so) from the off-centre light coming back to the drone - so increasing the contrast in the image. (Another thing I'll try some tests with when the weather is sunny again.)

Michael_G Posted at 2018-3-18 22:44
Thank you very much for the great explanation! This all sounds very plausible, even though the Mavic Air is a rather small drone, I can imagine the reflection behavior at a certain angle in the form as in the video ... What you do not have to watch out for when the drone is in the air!

And many thanks for the tip with the tracking Mask, I will give it a try!

Not sure you fully understood? What you are seeing is a physical phenomenon of sunlight being reflected from the ground when seen from a particular angle - when the sun is directly behind you (the camera). It is not a refection in the glass of the camera lens. So to answer your question ... in order to not see this effect you will need to have the sun always off to one side - probably outside a cone angle of 45 degrees behind the drone camera's POV. Hope that makes sense?

Lamplighter55 Posted at 2018-3-19 05:43
Not sure you fully understood? What you are seeing is a physical phenomenon of sunlight being reflected from the ground when seen from a particular angle - when the sun is directly behind you (the camera). It is not a refection in the glass of the camera lens. So to answer your question ... in order to not see this effect you will need to have the sun always off to one side - probably outside a cone angle of 45 degrees behind the drone camera's POV. Hope that makes sense?

Many thanks Lamplighter55!
But yes, I think that I have roughly understood it, at least I know now that it is not advisable to film from that highs with the sun from behind for two reasons ... because of the drone shadow (a black spot) or because of the back scatter effect (a lighten spot). To build and track a mask is a little tricky for me as a beginner in resolve, to increase the contrast is helpfull but not the best.
From today you can buy ND filters from the web store in germany, and I almost ordered it. But now I think it will be ND-PL from PolarPro, with all the traps (moves to left and right for example) that a pol filter has. But a light spot is also a un-nicely thing ...
Thanks again!

RocketChap Posted at 2018-3-19 04:52
Backscatter - get it all the time esp in the wet winter months when the sun is low: https://www.youtube.com/watch?v=uUXghpgCZ8Q
A right pain as it draws the eye.

Aha, one more with back scatter ... are you working out the spot in the editing? Or do you want to buy a pol filter?

Michael_G Posted at 2018-3-19 06:12
Aha, one more with back scatter ... are you working out the spot in the editing? Or do you want to buy a pol filter?

I'll give my polarizing filter a try next time next time I fly (when its stops gales/rain/snow/freezing here).

RocketChap Posted at 2018-3-19 09:20
I'll give my polarizing filter a try next time next time I fly (when its stops gales/rain/snow/freezing here).

Yes, you're right, it's currently not a good time to fly extensively, rather sporadically when the sun shows up for once ... but spring comes soon!

Wachtberger Posted at 2018-3-19 00:50
Amazing, I needed to watch it twice for spotting it.

Ditto:  I did too.

I think similar and I have a few videos where shadows must be, but there's not and in the place where it should be for sure, there is this brighter feathered spot. And I think that it super functionality.

It is quite common, and it has a name, Heiligenschein, here is more about it: https://en.wikipedia.org/wiki/Heiligenschein