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Point Zero Marker

The place for questions, problems, comments and tips regarding the camera calibration.

Point Zero Marker

Postby MagWeb » Mon Jun 22, 2009 8:28 am

Hi, Simon and Sven or anybody who knows:

point 0 is calculated as the middle in X between two white markers at the bottom left (double ring).

With a normal calibpoints.dat file, this center has to be 147mm (*scalefactor) from the origin in X.
So far so good.

Think since 2.0 you use smaller markers for the printout. Now I noticed another difference to the pattern used before:
In the previous version with the fat markers, the right one of the two white markers was horizontal centered to the black markers in the column right.
Now the right edge of the black circle around the white marker is aligned to the right edge of the black markers. So the white marker changed its relative position to the markers above.

So my question:
Is the position of the white markers in relation to the ones above of any importance as long as the center between fits to the coordinates in the calibpoints.dat?

(Doing a calibration I often find this point to own the biggest error - green line, but that maybe caused by the wide cam angle as well)

Gunter
Attachments
point0.jpg
There´s a little error in centering the middle in old - ignore it, sorry
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Re: Point Zero Marker

Postby Sven » Thu Jun 25, 2009 9:18 am

MagWeb wrote:So my question:
Is the position of the white markers in relation to the ones above of any importance as long as the center between fits to the coordinates in the calibpoints.dat?


No. Only the center between the two is important.
Also it is important that they are very close to each other and "horizontally" beside each other for two reasons:
1. DAVID can detect this pair as the two white markers with the smallest distance.
2. The closer they are together, the less distortion happens here.

MagWeb wrote:(Doing a calibration I often find this point to own the biggest error - green line, but that maybe caused by the wide cam angle as well)

Yes, there is a slightly imprecise assumption we have put into DAVID: The center between two 3D points is not necessarily at the precise center in the camera image... :oops: It gets worse: The center of a (black or white) circle in 3D is not necessarily seen in the center of the corresponding ellipse in the camera image.
The error is very small, but we are thinking about improving the calibration.

I'm sorry Gunter, some of your self-made calibration patterns have a quite large distance between the two. If we had thought about that when we saw it, we would have told you of course. :oops: The more wide-angled the camera is, the more error is caused by this.
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Re: Point Zero Marker

Postby MagWeb » Thu Jun 25, 2009 10:38 am

Hi Sven,

thanks for pointing that out.

Sven wrote:some of your self-made calibration patterns have a quite large distance between the two


Yes, I often wondered about this, but I always tried to keep the ratio 13/60=z for " new horizontally distance" * z....... :oops:
So the bigger the horizontally distance, the bigger this distance too. :roll:
But from now on I´m glad to marry these lovers.... :D

thanks
Gunter
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