creative checkerboard positioning

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max72
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creative checkerboard positioning

Post by max72 »

I would like to share this picture of a checkerboard I found today.
I went to do some photogrammetry. No laserscanning needed, already done..
After seeing this checkerboard I decided to do a couple of scans, just in case.
Am I paranoid?

Massimo
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ing. Massimo De Marchi - +39 347 32 17 049 - www.studiodemarchi.net
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Leandre Robitaille
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Re: creative checkerboard positioning

Post by Leandre Robitaille »

Dont post checkerboards on curved surfaces. That's not how they work. :evil:
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Re: creative checkerboard positioning

Post by VXGrid »

Leandre Robitaille wrote: Tue Feb 21, 2023 10:57 pm Dont post checkerboards on curved surfaces. That's not how they work. :evil:
I'm intrigued.
Why not?

The point fitting in the software should find the spot where the black and white crosses.
If I wrap that around something cylindrical, the cross point should be identical from any view, or am I missing something?
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Scott.Warren
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Re: creative checkerboard positioning

Post by Scott.Warren »

you can use them on rounded surfaces, but I find I need to re-pick the center on most of them. They will often fall outside the center, into the curved object. Looking good when facing front, but inset when looking side on and not resting on the paper's face.

If I have to put a target on a round surface, its going to be close enough (or use detail scans) to all the scans that see it that I can reasonably manually pick the center when in the office.

If you do put targets on curved surfaces, always check any auto-targeting, its more often than not incorrect. The smaller the radius on objects with paper targets, the larger the possible mis-targeting by the software.
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Re: creative checkerboard positioning

Post by TMillington »

"I'm intrigued.
Why not?"

My, admittedly basic, understanding is that for most softwares whilst intensity values from the target scan are used to locate the centre of the chequerboard, the centre will be placed on a plane generated through the target. Hence chequerboards placed on curved surfaces generally result in a centre point inside the feature it's fixed to. The discrepancy will be especially noticeable if the targets are also fixed with, for instance, a total station to geo-reference the scan.
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Re: creative checkerboard positioning

Post by VXGrid »

TMillington wrote: Wed Feb 22, 2023 3:28 pm "I'm intrigued.
Why not?"

My, admittedly basic, understanding is that for most softwares whilst intensity values from the target scan are used to locate the centre of the chequerboard, the centre will be placed on a plane generated through the target. Hence chequerboards placed on curved surfaces generally result in a centre point inside the feature it's fixed to. The discrepancy will be especially noticeable if the targets are also fixed with, for instance, a total station to geo-reference the scan.
I thought they just estimate two lines from the black to white fields and where they intersect is the middle point.
Of course this might create issues on rounded surfaces, namely if the lines are not lines (straight) but splines.
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Leandre Robitaille
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Re: creative checkerboard positioning

Post by Leandre Robitaille »

VXGrid wrote: Wed Feb 22, 2023 11:34 am
Leandre Robitaille wrote: Tue Feb 21, 2023 10:57 pm Dont post checkerboards on curved surfaces. That's not how they work. :evil:
I'm intrigued.
Why not?

The point fitting in the software should find the spot where the black and white crosses.
If I wrap that around something cylindrical, the cross point should be identical from any view, or am I missing something?
Software doesn't identify a single point that is ''on the middle''. Knowing most scans done are at 6.1mm @ 10m you would have room for +-3mm error on the center of the checkerboard. Most likely you do not have a point exactly in the middle
Some kind of interpolation needs to be done. You guessed it right, it assumes the checkerboard is on a flat surface and traces a line that is best fitted from the clashes of different intensities on the checkerboard. The point derived is the cross position of 2 lines, if you have a curved surface you will not be as acurate.

I might be wrong here but that's what I have always been lead to believe and to me it makes a lot of sense. Some software might have a spline algorythm but I do believe Scene has a straight line algorithm. I would just play it safe and always place checkerboard on flat surfaces. Also with proper glue so it doesn't get all warpy lol
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Re: creative checkerboard positioning

Post by pipingdesigner »

Dont post checkerboards on curved surfaces. That's not how they work.
Can you hang them from a string?
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gsisman
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Re: creative checkerboard positioning

Post by gsisman »

pipingdesigner wrote: Fri Feb 24, 2023 7:44 pm
Dont post checkerboards on curved surfaces. That's not how they work.
Can you hang them from a string?
Yes, just don't breathe near them or allow any breeze blowing through the space

And hang a plumb bob to weight them

And don't use a flat target- use a sphere

Lol
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Re: creative checkerboard positioning

Post by jedfrechette »

VXGrid wrote: Thu Feb 23, 2023 8:33 am I thought they just estimate two lines from the black to white fields and where they intersect is the middle point.
Of course this might create issues on rounded surfaces, namely if the lines are not lines (straight) but splines.
I'd be willing to bet that most software is extracting checkerboard centers using entirely 2D image analysis very similar to this approach:

https://theailearner.com/2021/10/14/fin ... -accuracy/

Both Faro Scene and Agisoft Metashape ship with OpenCV so I wouldn't be surprised at all if they're actually using the exact function that author describes.

That procedure is going to give you a subpixel 2D target center from which you're going to need to extract the 3D coordinates. If I was doing that I would interpolate the 3D position based on a small number of the immediately surrounding pixels, in which case, putting a checkerboard on a curved surface might not be as bad as you would initially expect compared to trying to do a full 3D plane fit from the beginning.

I don't use checkerboards much and when I have they've been flat so I don't know if that actually works out in practice, but it would be interesting to test out.
Jed
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