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DAVID-Laserscanner Forum • View topic - Calibration Parameters
Dear DAVID Community,

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Calibration Parameters

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

Calibration Parameters

Postby MagWeb » Sun Jan 10, 2010 1:39 am

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Re: Calibration Parameters

Postby Bongobat » Sun Jan 10, 2010 3:01 pm

DealExtreme red line laser, Microsoft Lifecam HD-5000
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Re: Calibration Parameters

Postby Sven » Mon Jan 11, 2010 10:48 am

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Re: Calibration Parameters

Postby MagWeb » Mon Jan 11, 2010 11:16 am

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Re: Calibration Parameters

Postby Sven » Wed Jan 13, 2010 11:59 am

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Re: Calibration Parameters

Postby MagWeb » Wed Jan 13, 2010 3:34 pm

Hi Sven,

what I´m going to try is e.g:

Imagine a rather flat with only a little topology.
I cann´t scan it in one scan (for I would loose much of the details this way). So I do several close ups (like a chess board with overlaps)
At present : The partial scans have to be aligned in Shapefusion. For the little topology DAVIDs automatics will patch each scan over, not beside the other. The only way here will be manual align, which is difficult too, for your eye has also the problem to find significant matching points.

So my idea is to put the surface into a machine like a 3 axis CNC and to put a small scanning rig (calibrated and ready to do a repetition scan) instead of the router. Now I can move the cam/laser - combo to precise coordinates.
Now I do a first scan using the "normal" calibration result.(to make it simple, which will never happen):

<ax>A</ax>
<ay>B</ay>
<az>A</az>
<px>X</px>
<py>Y</py>
<pz>X</pz>

(close David to save the calibration)

Now I move the rig exactly "S" mm to the right AND modify the coordinates of p in the config.xml:
<ax>A</ax>
<ay>B</ay>
<az>A</az>
<px>X + Sqrt((S*S)/2)</px>
<py>Y</py>
<pz>X - Sqrt((S*S)/2)</pz>

(open David to load the modified calibration)
do a second scan
(close David to save the calibration)

Now I move the rig exactly "S" mm to the right AND modify the coordinates of p in the config.xml again:
<ax>A</ax>
<ay>B</ay>
<az>A</az>
<px>X + 2*Sqrt((S*S)/2)</px>
<py>Y</py>
<pz>X - 2*Sqrt((S*S)/2)</pz>

(open David to load the modified calibration)
do a third scan
a.s.o

Now loading the three scans to Shapefusion, they shouldn´t appear as usual one over the other, but already one besides the other - registrated right (without doing any aligning in Shapefusion)

This 1D example can be transfered to a 2D motion easily and could be done also with controlled movements of the system in 3D (modifying the a - values too)...

Think that would help a lot scanning objects with little topology and also shorten the time spent in Shapefusion with "normal" objects...
Hope to´ve explained more precisely

Gunter
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Re: Calibration Parameters

Postby jantje » Thu Jan 14, 2010 12:32 am

Gunter
sounds interesting
I'm curious to hear about the results you will get.
Best regards
Jan
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Re: Calibration Parameters

Postby Sven » Fri Jan 15, 2010 11:01 am

Hi Gunter,

That's a nice idea but not supported by DAVID very comfortably...
When you change the camera position like that, but the camera is fixed to the laser motor, you will also have to change the laser poses in the laser_motion.xml as well. Because all coordinates are "absolute" (relative to one common coordinate system). When you work with "UseRememberedPlanePoses" you will have to change all those <plane> entries in the laser_motion.xml. Otherwise only the <estimation>.

Or when you scan with background, you will have to change the calibration planes in the advanced settings.

When you rotate also, it won't be enough to change the A values, you will have to change N and/or O too, so that (N,O,A) is a right-hand orthogonal coordinate system (pairwise orthogonal, all length 1).

An alternative would be to move the OBJ files. You could write a little tool that changes all the "v ..." lines in an OBJ file.
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Re: Calibration Parameters

Postby MagWeb » Fri Jan 15, 2010 12:50 pm

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Re: Calibration Parameters

Postby Bolte83 » Tue Oct 16, 2012 1:38 pm

HI

Is it possible to use more than one Parameter for the radial lens distortion. I calibrate my camera with another Software and I got four different Parameter for kappa.
First: a symmetric Parameter called A1, its nearly the same as kappa
Second: A1 and A2 for both directions
Third: A1 A2 B1
Four: Chebichew Model with 12 different Parameter.

So my question is, can I change the .cal File from DAVID Software, so that it can readin all Parameters ?

And by the way, is the uper Left corner the Origin from the Picture (important to calculate the center of a optical distortion) ?

Thanks
BOLTE
Student: Geodäsie & Geoinformation / DAVID-CAM-1-M / Red & Green FocusLaser
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Re: Calibration Parameters

Postby Sven » Wed Oct 17, 2012 6:06 pm

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Re: Calibration Parameters

Postby Bolte83 » Mon Nov 12, 2012 12:54 pm

Hi

I made some clibrations with DAVID Software and some with another Software used at our Photogrammetrie Labor in University. Aim was to verify the accuracy of the DAVID Calibaration and it works except for the Image focal point. For the other 3 Parameters variation is less then 0,0005 [ unit ]. The Variation of the Image focal point are cx ~8 and cy~ 7 [Pixel] in same direction. I already calculatet the Offset for the same Coordinate origin, because Photo Software uses Image center (1024 x 768 ) /2 and DAVID Software set up the Origin in the upper left Corner, but for both Y is downwards and X is right.
Example: cx = 525 cy = 366 (DAVID) // cx = 5 cy = -11 (Photogrammetrie) so for the Image focal point set up to the Origin in the center of image the correction from DAVID is

cx = 525-(1024/2) = +13
cy = 366-(768/2) = -18

for more details relating to Calibration Parameter: viewtopic.php?f=1&t=4175#p25032

QUESTION:
Does anyone have an idea why the difference is up to 7Pixel, but the rest fit so well ? How does DAVID Calculate the Image focal point ( I know the TSAI model quite well), is the an approximation in the DAVID algorithmus ?

QUESTION: Can i find somewhere the Coordinates for the CalibartionPoints (of the new pattern) like in the calibpoint.dat for the old pattern ? Because i want to another Calibration with my own Software, but I need the true Coordinates of the calibration points.
Thanks a lot for your help

Bolte
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Re: Calibration Parameters

Postby Sven » Tue Nov 13, 2012 11:18 am

Hi Bolte,

All camera models are only an approximation of the real lens. When you compare two models which have different parameters (maybe yours has some higher degree radial distortions or tangential distortion parameters), the values of mutual parameters may be different. I think a difference of 8 pixels (less than 1% of the image width) is low.

Also keep in mind that your calibration corner will never be perfect. This always has a small impact on calibration values, depending on the model...

DAVIDs parameter values are the result of a numerical optimization which minimizes the difference between measurement and model.

The 3D coordinates of the calibration markers: You will have to compute them yourself. It's quite easy actually, just look at the PDF pattern. The origin (0,0,0) is shown in the middle. Y is upward, X points to the right (lies in the right plane), Z to the left. The distance is always "scale"/4. The distance between the corner and the first columns is half as much.

Sven
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Re: Calibration Parameters

Postby Bolte83 » Sat Nov 17, 2012 1:54 pm

Hello again,

Thanks for your quick answers Sven.
But I got another question, i read in some papers that the TSAI kalibration need some input. Information about Pixel Size in the Image, real CCD Chip size in [mm] and the nominal focal length. Can i find some where (perhaps in the xml files, but i already had a look there) the Parameters DAVID Software uses for the DAVID-Cam-1-M ?

To calculate the orintation i use the information from the Data Sheet of the Sony Chip.
By the way i made my own calibration with MatLab and i got quite good terms for the inner Calibration, especially for the main Image Point -> difference 2 Pixel in X-Y direction. So the variation i got with the other software, my appear from the different Parameters and the other kalibration pattern.

Thanks for your held
Bolte
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Re: Calibration Parameters

Postby Sven » Mon Nov 26, 2012 11:15 am

Our Tsai implementation does not need these inputs. Therefore the focal length f is not computed in [mm], but in [units]. It does not know the chip size, it's not important.
The external 3D position, orientation etc. are still correct and at absolute scale in mm.
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