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Author Topic: Use of markers and marker corrections  (Read 2100 times)

Cyberworld

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Use of markers and marker corrections
« on: April 11, 2021, 12:56:05 AM »
For a project I'm working on we use either non-coded or coded targets. After initial alignment of photos, we use the detect markers feature to auto-detect the targets on the images and place markers (and later scale bars).

Question 1: If I set markers manually, instead of using the detect markers feature, does it make any difference if my initial alignment already has a good RMS?

When the detect markers process finishes, it successfully finds the targets between the photos and places markers. However, on some photos the markers are placed a bit off the centre of the target and thin black lines indicate their residual errors. We then go over each photo to move markers back to the correct centre of the associated target hitting the Update button after all processes are done.

Question 2: Does manually moving the markers back to the correct centre of the target help increase the accuracy of the model? If yes, what does this action do exactly? Does it re-align the specific camera where the correction was applied?

Question 3: Instead of hitting just the Update button on the Reference pane, do we also need to run Optimise cameras?

The reason for questions 2-3 is because we don't really see much improvement in the RMS errors (initial RMS is around 0.25-0.3 (0.22-0.25 pix), after correction of marker/targets they stay about the same), and were wondering whether all the trouble of manually correcting each target is actually worth it.

Thank you for any clarifications.


JMR

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Re: Use of markers and marker corrections
« Reply #1 on: April 11, 2021, 10:00:31 PM »
Manual target placement accuracy depends on human factors and also on image quality, while automatic (computer) placements is supposed to be very reliable and less prone to bias.
If you find blue auto-markers misplaced in some photos there must exist a reason and they need to be corrected.
You have two options: to remove projection or to move manually to a better location. If your preferred option is to move, I also recommend you to remove first and place later. This way you'll see the expected location of a marker (gray flame) and perhaps it gives you some keys to understand the bias.
Removal or movement of a few marker projections to different image coordinates makes little difference in the alignment if you only update transform. Affected cameras wont vary their relative orientation but only global scale and rotation may change very slightly so the answer is you must OPTIMIZE to make your corrections meaningful.
Check individual pixel error per marker projection by using marker info tool, and check which projections are worse placed.

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Cyberworld

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Re: Use of markers and marker corrections
« Reply #2 on: April 15, 2021, 04:20:01 PM »
Thank you JMR for the clear response. As suspected it doesn't make a world of difference in model accuracy. We tried both just hitting Update and with Optimise Cameras, but the resulting corrections are negligible. Thank you again for your time. It is a useful clarification and maybe should be added to the User Manual, as the part regarding Markers does not fully or at least clearly explain what happens when you correct marker positions.

Regarding the biases for auto-detection, it has to do with markers in the non-focused parts of the image (we use prime lenses, hence focus to infinity not an option), where the targets are a bit blurry, which I guess confuses the software.