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Author Topic: Lens not detecting chess board  (Read 361 times)

Dave1

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Lens not detecting chess board
« on: July 27, 2022, 06:50:56 PM »
Good afternoon,

I am working with images collected from a submersible to reconstruct an underwater scene. Sadly, there is no GCPs or navigational information available due to the sort of extreme environment, only scale. I am trying to calibrate a camera lens to help alleviate any potential doming effects - which so far doesn't appear (obviously), probably to many oblique camera angles.

We have images of a chess board pattern, however the program is not picking the pattern out. This is likely because the board is too far away from the camera - despite good image clarity. I do however have some valid calibration points from the same scene, but the camera was zoomed in at 24mm, rather than the 18mm used for the reconstruction. So I have two questions,

Is there a way to manually calibrate the lens - trying to place points at the cross points of the checkers board?
If not, can i use calibration information from a different zoom? (I suspect not...)

Finally, I guess one other question. I have read that the automatic "calibration" when optomizing the camera positions is very good - so is it OK to not manually calibrate the lens?

Kindest regards,
David

Kiesel

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Re: Lens not detecting chess board
« Reply #1 on: July 28, 2022, 08:48:58 AM »
Hi David,

Quote
We have images of a chess board pattern, however the program is not picking the pattern out. This is likely because the board is too far away from the camera - despite good image clarity.

you can play with the tolerance value and maximum residual value in the 'Detect Markers' menu  for 'Cross (non coded)' targets and eventually use the 'Ignore masked image regions' to avoid detecting markers in unwanted regions.

Quote
can i use calibration information from a different zoom? (I suspect not...)

Not a good idea.

Regards,

Kiesel

SimonBrown

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Re: Lens not detecting chess board
« Reply #2 on: July 28, 2022, 10:25:08 AM »

Is there a way to manually calibrate the lens - trying to place points at the cross points of the checkers board?
If not, can i use calibration information from a different zoom? (I suspect not...)

Finally, I guess one other question. I have read that the automatic "calibration" when optomizing the camera positions is very good - so is it OK to not manually calibrate the lens?

The attempts I'm aware of - to manually calibrate a lens underwater using a chessboard - have proved unsuccessful.

Some observations:

Using a differing zoom to calibrate...will not ensure the path of light through the glass is consistent with the as-shot images used for reconstruction. Not recommended.

Metashape is very good with automatic calibration. There are some instances when manual calibration really pays off - we have provided consulting services to one massive civil engineering project client in North America on this - but the use case here sounds different.

ROV cameras can be a mixed bag...some good...some good at providing a visual feedback to the operator...but not all are going to deliver a result for photogrammetry.

So...some questions:

What camera is the lens attached to?
Are you shooting stills?
Or extracting frames from video?
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