Extreme bowling effect - better images/new flight needed?

Hey,

I computed about 190 12MP-images from an archeological site again – I had done this already 2 years ago with “Maps made easy”. The result is very similar, I get an extreme bowling effect:
Bildschirmfoto 2021-10-22 um 19.47.52
Bildschirmfoto 2021-10-22 um 19.51.18
Since MME and WebODM deliver the same distortion of the surface, I think there is something wrong with the images and/or I the way I’ve taken them.
Would the only way to improve this to fly another mission like it is explained in the tutorial of the documentation: “fly two patterns separated by 20°, and rather than having a nadir (straight down pointing) camera, use one that tilts forward by 5°.”?
Or is there another way, by tweaking something in the settings?
Here is access to all the files: Hub

Cheers,
Sören

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Give me a day to take a look.

Do you remember your flight plan settings?

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Hey,

I had a look in Map Pilot: (I flew that mission with it)
Altitude 70m → Resolution 2.5 cm/px, Overlap 80/80, Max. Speed 5.0 m/s.
Hopefully I didn’t change anything after the mission.
I think I left the camera on auto exposure…

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Although I’m early in my journey through photogrammetry techniques, I think the wisdom around camera auto-calibration (in structure-from-motion stage) goes like this:

  • shoot features at varying angles (not just nadir)
  • shoot at different elevations

You could also check the setting that you’re running the processing task with. I think the default “auto” setting auto-calibrates (optimises) over a “brown” camera model…but you could try some other manual settings here.

If you have some other dataset that you processed and got good results with (no doming), you could try grabbing the camera parameters file from that project and use them in your currently ‘domed’ project.

Alternatively do a bench-calibration of your camera? This is what DJI do (at factory) for the P4 RTK and probably the Agri drones; I have a P4 RTK, and the camera distortion parameters that it bakes into the JPG images is based on a “brown” model (with 3 radial and 2 tangential coefficients).

Cheers

Tom

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This page might help.

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Created on: 10/23/2021, 11:21:33 PM
Processing Node: Lightning (manual)

Options: auto-boundary: true, cog: true, crop: 0, debug: true, dem-gapfill-steps: 4, dem-resolution: 1, dsm: true, dtm: true, matcher-neighbors: 16, mesh-size: 300000, min-num-features: 16000, orthophoto-resolution: 1, pc-classify: true, pc-ept: true, pc-filter: 1, pc-geometric: true, pc-quality: high, use-3dmesh: true, verbose: true

Average GSD: 2.53 cm
Area: 69,681.22 m²
Reconstructed Points: 23,198,894

image

Not too bad, but still bowling a bit.

GCPs would likely help correct that, as well as potentially not moving while taking shots as Rolling Shutter Correction artifacts can sometimes cause bowling if not corrected for.

As above, overly-aggressive self-calibration also seems to be a factor here, so another way to potentially save this would be to use the camera calibration JSON from another more carefully collected dataset for this to see if it helps reduce it further.

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Thanks for the all the answers. I’ll try another mission with two flight paths, one 20 degrees rotated and 5 degrees camera angle…
@Saijin: Hey, fantastic, thank you! Looks much better! Yeah, the rolling shutter – I thought about it… I’ve only got a Mavic Air, not the best drone for mapping purposes. The next drone I’ll buy should have a mechanical shutter. And good to know that GCPs could also help. It would be very nice if I could download your results from dronedb.
Cheers,
Sören

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Does your Mavic Air let you stop for photos? I think Pix4D Capture allowed it for most supported sUAS.

If so, then rolling shutter isn’t a big deal anymore.

I’ll PM you the results a bit later, if that’s okay. Not at my PC.

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mavic air has a global shutter

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I believe it has the same camera as the original pro, which is rolling shutter. Also, this bowling I’m consistent with rolling shutter.

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Indeed! But if they are not moving during capture and we assume the scene being imaged is static then Rolling Shutter distortion won’t be a big deal for that collection.

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Yup, static shots are an ideal workaround for rolling shutter.

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Well, like I mentioned, I use Dronelink also for my mapping missions…
It’s mapping component has no option to stop for every single photo, I think.
Of course I could make a feature request to the developers – that would make sense.
I don’t think I will use Pix4D Capture, because I’m already quite familiar with Dronelink and it enables you to put totally different motion paths in one mission. (waypoint paths, orbits, mapping grids and so on…)
So my solution must be to just fly at a slow speed.

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It seems even some people from DJI are not so sure about the type of shutter:

I believe it is a rolling shutter, because they describe the mechanical shutter of the Phantom 4 pro as an advantage on their website…

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Aha, yes: there are camera manufacturers that claim a global shutter for a sufficiently fast CMOS sensor. This is… not uncommon and somewhat deceptive for things like airborne photogrammetry.

We can take it a step further: not all mechanical shutters are a good choice. A leaf shutter is superior to a focal plane shutter, as it opens on the entire sensor area simultaneously.

So, usually from a photogrammetry perspective, when we say global shutter, what we mean is a mechanical leaf shutter. And for ODM, which doesn’t yet have a correction for rolling shutter, this matters a lot.

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If we talk about shutters here… There a rumors that the upcoming Mavic 3 will have a mechanical shutter:
https://twitter.com/JasperEllens/status/1441091686654906370?s=20
Could be a good choice for my next drone, but I’ll wait until it’s compatible with Dronelink.
(Which means when they will release the SDK)

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Well, that’s exciting. It eliminates the remaining problems with the Mavic, which is otherwise a fantastic platform.

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Generally yes, but a camera with a focal plane shutter will have a shortest usable exposure with a flash- that is the fastest exposure where the entire focal plane/sensor is all exposed at once, and might typically be 1/250, such as on my old Nikon FE2s.
At 1/500th sec exposure, only half the sensor is exposed at once, as both curtains move across together, with a half-sensor gap. At higher speeds up to 1/4000, the gap is very narrow, and effectively approximates the CMOS rolling shutter.
See my profile pic for what a rolling shutter does to a bicycle wheel when riding over a rough road!

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Ha! Yes, that picture should be added to docs.opendronemap.org to a section about rolling shutter. That’s :ok_hand:

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Here it is in all its glory :wink:

front-wheel

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