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Author Topic: "Zero Resolution" Error  (Read 3928 times)

MaKo

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"Zero Resolution" Error
« on: August 04, 2020, 02:04:06 PM »
Hi all,

I’m currently working on building an orthomosaic of a 4,7ha big agricultural field (wheat) with Agisoft Metashape 1.6.2.
The images were taken with an XT2 (12MP, 8mm focal length, 9.5mm sensor width) at an altitude of
70m, leading to a GSD of about 2.5cm. In total 425 images were taken with a side and frontal overlap
of 80%. Image quality of each image – according to Metashape – is bigger than 0.8. The image
reference was deleted and the reference of the chunk was set to the right one (EPSG: 31256) in order
to avoid any complications.
Throughout the generation process I came up with 3 different approaches to do so. I will try to lead you through my workflow:
Initially (Approach 1), after aligning the images (accuracy: high, key point limit: 40.000, tie point limit:
4.000, generic preselection) only about 105 images were actually aligned. But using the “align
selected cameras”, when right-clicking the previously non-aligned cameras, aligned all cameras –
resulting in a tie point cloud of about 340.000 points.
The RMS reprojection error of the point cloud is at 0.275081 (1.53462 pix), max reprojection error at
1.40366 (46.6136 pix), mean key point size at 5.34687 pix, and the average tie point multiplicity at
2.64979. The maximum amount of valid matches ranges from 299 – 1300, depending on the image.
I then set 12 GCP’s leading to an RMSE of 0.71m – which is completely fine for this field, since the
orthomosaic only needs an approximate reference. The tie point cloud looks good and the position
and rotation of the field also seem to fit but  it seems like Metashape thinks, that the images were
taken from below the ground (“Screenshot CameraPosition”).   

If I then want to calculate the dense point cloud, I get the “Zero resolution” Error.

Then I went in the Forum and searched for solutions but could not find any that worked for me. I
already tested changing the alignment accuracy (to medium and highest), unchecking
generic preselection and changing the tie point limit (10.000 & 40.000) (Approach 2).
When increasing the tie point limit to 10.000 and deactivating generic preselection (no other
alignment parameters were changed), I got a tie point cloud with 465.000 points, RMS reprojection
error of 0.215251 (1.4669 pix), Max reprojection error of 0.719796 (63.9647 pix), mean key point size
of 5.10988 pix, and an average tie point multiplicity of 2.75229. I then set 5 GCP’s to just see if it
works.
With that tie point cloud I am able to calculate the dense point cloud but it looks like “Screenshot
DenseCloud”, which obviously is rubbish. Nevertheless, I’m able to calculate a DEM and an
Orthomosaic ("Screenshot OM"). On first sight it looks good (except for the edges), but if you zoom in
you can see, that for example the lanes of the tractor are offset (“Screenshot OM-Zoom”).
I then tried to repeat the second approach (Approach 3) with an even higher tie point limit (40.000) but again get the “Zero resolution” error, when trying to calculate the Dense Point Cloud.

Do you have any help?
Am I using an insufficient amount of photos?
Do you think setting more GCP’s on Approach 2 could improve the Orthomosaic?

The storyline of my post might be confusing now – and I’m sorry for that – but I just wanted to inform you about the things I already tried.

I used this type of workflow and parameters in a couple of other projects and never had any problem
like that.

Thank you very much!
Matthias

James

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Re: "Zero Resolution" Error
« Reply #1 on: August 04, 2020, 05:13:16 PM »
it seems like Metashape thinks, that the images were taken from below the ground (“Screenshot CameraPosition”).

Where metashape thinks the images were taken from is the 'free' end of the black lines, i.e. not the end with the blue rectangle. The blue rectangle just shows the direction. It's not clear in your screenshot if the 'free' ends of the black lines are above or below ground. if you turn the mousewheel while holding either shift or ctrl (i can't remember which) it will scale the camera symbols which might help it look more like what it ought to.

With that tie point cloud I am able to calculate the dense point cloud but it looks like “Screenshot DenseCloud”, which obviously is rubbish.

I think that the zero resolution error is to do with insufficient data within the bounding box or 'region'.
https://agisoft.freshdesk.com/support/solutions/articles/31000148432-what-do-some-error-messages-in-metashape-interface-mean-

If your bounding box was not set properly it might not contain all of the data, and would explain why your screenshot shows a dense cloud of very few bits, which just happened to fall inside the misplaced 'region'. Your first screenshot, showing the cameras, seems to show the bounding box/region set appropriately, but it may have been subsequently moved, or left in place while all the data moved, somehow, maybe. You can use the 'reset region' command somewhere in the 'model' menu, i think, to reset it to include most of the data, which might allow you to build a proper dense cloud.
« Last Edit: August 04, 2020, 05:18:42 PM by James »

MaKo

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Re: "Zero Resolution" Error
« Reply #2 on: August 05, 2020, 10:57:51 AM »
First, thanks James for your fast response!

Where metashape thinks the images were taken from is the 'free' end of the black lines, i.e. not the end with the blue rectangle. The blue rectangle just shows the direction. It's not clear in your screenshot if the 'free' ends of the black lines are above or below ground. if you turn the mousewheel while holding either shift or ctrl (i can't remember which) it will scale the camera symbols which might help it look more like what it ought to.

I looked into that and it seems like the "free" end is sometimes above and sometimes below the tie point cloud (See Screenshots).

I think that the zero resolution error is to do with insufficient data within the bounding box or 'region'.
https://agisoft.freshdesk.com/support/solutions/articles/31000148432-what-do-some-error-messages-in-metashape-interface-mean-

If your bounding box was not set properly it might not contain all of the data, and would explain why your screenshot shows a dense cloud of very few bits, which just happened to fall inside the misplaced 'region'. Your first screenshot, showing the cameras, seems to show the bounding box/region set appropriately, but it may have been subsequently moved, or left in place while all the data moved, somehow, maybe. You can use the 'reset region' command somewhere in the 'model' menu, i think, to reset it to include most of the data, which might allow you to build a proper dense cloud.
Since, this was one of the first solutions i found on the forum, I tried that for each of the Approaches and it did'nt solve the problem (except for Approach 2).
Does this in return mean, that we took too few photos?
Or Is it possible, that not enough data was generated in Agisoft, since the field we observed was covered in 1-1.5m high wheat plants and there was too much movement (because of wind) between the photos?

Thank you very much!
Matthias

James

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Re: "Zero Resolution" Error
« Reply #3 on: August 05, 2020, 11:55:37 AM »
In that case my guess is it's probably something like this https://www.agisoft.com/forum/index.php?topic=9968.0

If the focal length was wrongly estimated then that could potentially compress the z values of everything so even an undulating or badly aligned surface could come out flat and the camera altitude could also be compressed, leading to positions on or close to the surface. At least that's how i explain it to myself without being very aware of the maths involved!

If you look in the camera calibration window, and compare the initial and adjusted focal lengths, they should be fairly similar, assuming the initial estimate was sensible, but if metashape has estimated a really dodgy focal length it could be wildly different. My guess from the numbers you gave is that it should be about 3500px.

In that case perhaps if you have a previous project that worked well with this drone/camera you could export the camera calibration from it to use here, or otherwise i believe it is possible to manually enter and fix the focal length. I'm a bit out of practice so don't remember the exact procedure, and don't have the software handy to look at - i normally just trial and error these things anyway!

Other things to try might be enabling rolling shutter correction (i found it worked best enabling this at the alignment stage, rather than only in subsequent optimisation), if your drone camera is of that type. Also maybe unchecking/discarding camera orientation (yaw/pitch/roll) values in the reference pane if they exist and are likely to be inaccurate at all (i'm not sure if that helps but i'd try it if it was me!)
« Last Edit: August 05, 2020, 11:57:43 AM by James »

MaKo

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Re: "Zero Resolution" Error
« Reply #4 on: August 05, 2020, 05:30:33 PM »
It was the camera calibration! This was totally wrong estimated/calibrated in the approaches. Like you suggested, I used a prior calibration and now it worked perfectly.

Thank you very much!