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Author Topic: Help with Camera Alignment  (Read 3739 times)

a.f.

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Help with Camera Alignment
« on: April 21, 2022, 07:15:55 PM »
Hello,

I am currently working on a project trying to reconstruct an urban-coastal area of a city in Metashape, but I am struggling to align the photos properly. What I have is a set of 2000+ high resolution (8192x5464, 72dpi, 24bit) photos, about 1/3 of them has GPS data. They were taken at multiple days and there are variable objects (people, cars...). What I try to create is a reconstruction of the street, houses and beach, the general landscape. I loaded the different sets into different Camera Groups. I assigned a Calibration Group for each set. I ran quality estimation and disabled all photos lower than 0.6. Then I aligned the first set (medium accuracy, generic+source preselection, 100k key and 10k tie points). Then I added more sets, one at a time, adding masks and markers to help the alignment and eliminate clouds, sky and water. I end up with only a mediocre product, a couple dozen cameras dont align properly and I also have some point clouds where nothing should be. Has someone experience with what I am trying to achieve? I would be thankful for any tips on how I can improve my process and start building a dense cloud.

Thanks
« Last Edit: April 21, 2022, 08:28:25 PM by a.f. »

Paulo

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Re: Help with Camera Alignment
« Reply #1 on: April 21, 2022, 11:34:39 PM »
hello af,

we would need more info to help you:
  • what camera dd you use seems to be a full frame?
  • what overlap did you fly at (forward/side)?
If you can share a set of photos, we could look at aligning ourselves.....
Best Regards,
Paul Pelletier,
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a.f.

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Re: Help with Camera Alignment
« Reply #2 on: April 22, 2022, 12:46:32 AM »
Hello Paulo,

a Canon Eos R5 was used and there was no flight but walking through the scenery and taking pictures at ground level and in 5m hight with a tripod, rotating it about 10 degrees.
I cannot share any of the photos unfortunately.

Paulo

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Re: Help with Camera Alignment
« Reply #3 on: April 22, 2022, 01:50:31 AM »
OK,

could you just show a representative image of your dataset... does it have a lot of sky?

What about Image stabilization? on? c0uld be problematic

Exposure time may have rolling shutter effect

« Last Edit: April 22, 2022, 01:53:35 AM by Paulo »
Best Regards,
Paul Pelletier,
Surveyor

a.f.

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Re: Help with Camera Alignment
« Reply #4 on: April 24, 2022, 06:28:28 PM »
Hey Paulo,

thanks for your reply! Here is a 2 representative images of my dataset. As you can see some have a lot of sky. But I went through all of them and masked out the sky on most and applied mask to all tie points.

About Image Stabilization, I do not know since I wasnt there taking the photos, but I will ask.

Why could the rolling shutter effect be problematic? The images have all different Shutter, from 1/60 to 1/1000. Would turning on Rolling Shutter Compensation in Camera Calibration help?
« Last Edit: April 24, 2022, 06:34:08 PM by a.f. »

Bzuco

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Re: Help with Camera Alignment
« Reply #5 on: April 24, 2022, 08:15:56 PM »
I would try few things:
-take photos from more stations(and less photos on each station(rotate camera at about 30 or 45 degree), this could help metashape to have data from more angles with less "jumps" instead of few angles and big jumps.
- if there is lot of traffic and people, wait a little bit between taking photos so the traffic and people completelly changes and metashape evaluate them as invalid feature points.
-taking photos in 360 degree can be useful in your project, but I would also try taking photos from sea line on beach towards buildings, proceed in strips. Buildings gives you plenty of stationary points and you don't need to care about sea and you can control to not have sky in camera view. This way you could be successful with beach surface. For roads and sidewalks can be used another similar technique and you can always wait for clean view.
-I always plane the route where I will be taking photos, so I will ended up with not so much nor few photos which are continuously connected.
- automatically disabled photos after quality estimation could be very missing in some area...I would manually disable only blured images with low exposure time and keep those which have maybe only blured corners, but useful center of image.

Some questions:
-how many photos are not aligned?
-misaligned photos are evenly spreaded acros whole area or are you missing some smaller area completelly and rest is fine?
- do you need detailed pointcloud for your project, or it is enough to recognize objects with certain precision. Do you need all objects, or you can omit some small objects like trash can, trees crown, benches,...?

I'm curious how you will manage all this, as I only do projects on my feet without using such a tall stick  :)

Paulo

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Re: Help with Camera Alignment
« Reply #6 on: April 24, 2022, 08:55:59 PM »
AF,

yes the rolling shutter could have an effect especially if you were moving while taking photo at slow shutter speed ....

IS is not good as the sensor moves during exposure and thus complicates the photogrammetry....
Best Regards,
Paul Pelletier,
Surveyor

a.f.

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Re: Help with Camera Alignment
« Reply #7 on: April 26, 2022, 08:45:00 PM »
Hey guys,

thanks for your tips. I have tried to apply them, but unfortuatly with only a mediocre result. I cannot retake the photos and have to work with the data sets I have.
I managed to align almost all photos with markers and realigning, but still ~10% of the 2000 photos are not aligned. They are not spread evenly across the area, but occur in certain areas, where I cant apply good markers, because its mostly facades with windows and reference points are missing. So metashape creates `floaters` and `sinkers` from them, basically points that are under ground or float high above the photographed area.
I dont need a detailed end product, for example I dont need any trash cans, park banks, etc. only the essential objects that define the area.
Also I've build a sparse point cloud and end up with ~4 mio. points, then I reduced with gradual selection tools to about 55k points, and built a dense cloud, but there is a lot of noise as you can see in my attachment. How can I reduce it?

Thank you

Bzuco

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Re: Help with Camera Alignment
« Reply #8 on: April 26, 2022, 09:10:37 PM »
For cleaning noise I am using free cloudcompare software and it's SOR filter. This filter can remove points which are far from their neighbors. Maybe metashape pro has some similar function.
4M to 55k sparse cloud seems to me drastic reduction, but maybe it is necessary. I am ending usually with ~200k sparse points cloud on project with ~250 photos. It can be different of course from project to project.
« Last Edit: April 26, 2022, 09:15:00 PM by Bzuco »

a.f.

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Re: Help with Camera Alignment
« Reply #9 on: April 27, 2022, 09:51:33 PM »
Hello guys,

I have all the cameras aligned almost perfect now. But the main problem I have now is, that a lot of them create point "swirls" under groud, where there should be no points at all. If I look at the photos that create the tie points in question, they are all aligned well. Are my aligning setting the problem maybe? I chose high accuracy, generic preselection, source preselection, 60k key point limit, unlimited tie point limit and ignore stationary tie points. Should I try different settings? Which do you suggest?

dpitman

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Re: Help with Camera Alignment
« Reply #10 on: April 28, 2022, 12:29:12 AM »
When you set unlimited tie points, you are setting yourself up for many poor tie points which will be worse for reconstruction than fewer good quality tie points.  Have you tried by using the default (4000) ?  More is not better in this case.