Lens Distortion Models In Layman's Terms

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14 comments

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    Götz Echtenacher

    Hi Steven,

    another valuable contribution, thank you!

    I've asked about the different models before but got not much reply.

    I find it is not very well covered out there, so you know more than I on the subject.

    If what you say on tangiental is true, it should be the standard setting, at least for, like you say, cheap but also zoom lenses.

    Do you have any idea what K means?

    To be honest, I tried around a bit with the different models but the differences seemed to be marginal, at least judging by the numbers of median and mean errors...

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    Steven Smith

    I forgot about K. I believe it is keystone, perspective, or skew

    and I believe this one can not be calculated. If no value is supplied then Reality Capture will use a default value of one. This is the least common distortion.

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    Götz Echtenacher

    Hey Steven,

    thanks, but already way over my head!

    Do you mean least or most?

    Anyway, why would RC not be able to calculate it and what does 1 mean???

    Plus, what kind of scenario are we talking about here? Optical axis not 100% perpendicular to the sensor?

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    Steven Smith

    I want to do a series of posts like these. The idea is to word the explanations at a 5th grader level with illustrations. It should help people with English as a second language and non-professionals alike have a better grasp on what these values and settings mean. 

    I do hope the mods participate, I'm positive my assumptions are either completely wrong or at least flawed in some respect. I do not want misinformation being spread. The goal is to get the correct understanding and make Users and DEVs lives a little easier. 

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    Steven Smith

    In depth Youtube videos was a consideration as well, but they are a lot of work. Maybe in the future depending on the community participation. 

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    Steven Smith

    Gotz, 

    I believe I read that K must be specified by user other wise a default value of 1 (no K distortion) will be applied. 

    "Plus, what kind of scenario are we talking about here? Optical axis not 100% perpendicular to the sensor?"

    I do believe this is how it would happen.

    As for the least common, I don't think it would be an issue unless you build your own home made camera or used a tilt shift lens or something. Otherwise this distortion would be at a sub pixel level that doesn't matter in calculations. 

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    Götz Echtenacher

    Ok, now I can follow it, too!  :-)

    But I do think that it can be calculated, otherwise it would not make any sense at all to offer it as an option. And sub pixel do count for RC, and the errors tend to accumulate...

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    Steven Smith

    I think you might be right Gotz. After re-reading, I think it is saying if using Brown 3 as opposed to k+brown 3, then K is defaulted to 1. 

    If you select K +...., then RC will try to calculate the K value.

    I don't even think RC will let you specify a K value, now that I'm trying to find it.

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    Steven Smith

    Now I have even more questions. What are the principal points? I see Skew and Aspect ratio now. 

    What is the difference in setting distortion model under the camera priors vs alignment settings?

     

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    Tom Foster

    So great to have such old puzzles explained "at a 5th grader level with illustrations" - on top of someone having actually worked it out!

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    Steven Smith

    Don't take as gospel. I'm not sure what parts are correct. I wish the Mods would confirm or correct whats wrong..

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    Tom Foster

    Quite. But until then ...

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    RockinAkin

    Seriously though... why aren't the Devs more active in these forums?

    I'd really like some hard answers about the finer / complex settings in RC without having to spend days of trial and error.

    This is a great topic for starters.

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    Steven Smith

    I've givin up on help from Reality Capture support really. I don't think they care. They have a figure it out for your self kinda of attitude.

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