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Author Topic: Cylindrical Orthomosaic Generation for long think wall scan  (Read 699 times)

mitchgalea

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Cylindrical Orthomosaic Generation for long think wall scan
« on: January 24, 2022, 02:16:27 PM »
Hello,

We have been capturing some datasets of a port quay wall underwater. The datasets are single strips going across approximately 60m in length. Due to the high distortion of our lens and the data being captured underwater the resulting models expresses the 'bowl effect'. This is not too much of an issue as we are not interested in a model but of an orthomosaic. We tried generating a planar orthomosaic but it resulted in the edges being quite dark due to the curve of the model. I have been experimenting with the cylindrical orthomosaic generation process and have read through the following article https://agisoft.freshdesk.com/support/solutions/articles/31000153736-orthomosaic-generation-cylindrical-projection-
This was quite helpful to give background but I haven't been able to get it to work. I've been trying to use the marker method. To do this I have added IMU readings to align the model, then used a circle fitting algorithm with the data to get the center of the cylinder so that I can generate an axis. When creating markers and giving them the center axis coordinates and trying to generate the orthomosaic i get the follwing error 'location of marker is undefined'.

I have attached an image of the model curve, the mesh and planar orthomosaic.

What would be the best method to generate an orthomosaic of this model?

Thanks in advance
« Last Edit: January 26, 2022, 05:47:41 PM by mitchgalea »

Alexey Pasumansky

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Re: Cylindrical Orthomosaic Generation for long think wall scan
« Reply #1 on: January 25, 2022, 09:43:56 PM »
Hello mitchgalea,

I think that correct approach would be to fix the bowl effect (if the surface should be more or less straight), then generate the orthomosaic in planar mode.

Bruteforce approach to fix the problem for this project is the following: copy adjusted F value to Initial tag in the Camera Calibration dialog, fix F value, manually add or subtract rather small value (from tens to couple of hundreds pixels) from that F value, then optimize the camera alignment and see, if the bowl-effect is reduced - repeat until it is fixed up to acceptable results. Then build mesh and generate orthomosaic in planar mode.
Best regards,
Alexey Pasumansky,
Agisoft LLC

3create

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Re: Cylindrical Orthomosaic Generation for long think wall scan
« Reply #2 on: January 25, 2022, 10:25:56 PM »
Another approach for non-planar surfaces (in this case unintended) would be to create a simplyfied UV-mapped mesh model (in 3d software) and use the textured (bent) model as source for transferring its texture onto this new one (in Metashape). The resulting texture represents an accurate unwrapping of the source, as far as the  UV-coordinates of the simplyfied mesh are dimensionally correct (i.e. not stretched).

Guy

mitchgalea

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Re: Cylindrical Orthomosaic Generation for long think wall scan
« Reply #3 on: January 26, 2022, 05:48:25 PM »
Thanks for the feedback. I manually found an appropriate value for f and it flattened out the model. Tried on multiple datasets and worked a treat. Thankyou!