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Painting Calibration Panels on Boards

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

Painting Calibration Panels on Boards

Postby eliscio » Mon Sep 07, 2009 7:09 pm

Hello All,

I have printed out a very large sheet of calibration panels at the local Kinko's, but after a few days, there is some sagging since I just taped it onto the panels. I guess I could have asked them to mount it on foam board, but I was curious if anyone has actually painted their calibration panels white and then drawn in the dots with a circle template?

I think I could do this quite and then always ensure my panels are going to be "flat", but I was curious who out there has tried this and are there any things to consider?

Also, I thought about modifying the calibration layout to better fit the panels that I am using because it would give me a bit more area and coverage...any suggestions there?

Each panel is approximately 60x122cm.

Thanks,

Eugene
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Re: Painting Calibration Panels on Boards

Postby MagWeb » Mon Sep 07, 2009 9:28 pm

Hi,

it´s no problem if you paint the pattern accurate by hand at a big scale - there will be errors, sure. But you´ve to see the proportion of the errors to the size the pattern appears in the cam's picture.... I did it several times till now up to a dimension of 2m in height.

Distorting your pattern is no problem, too. But you´ve to put the new coordinates of the points into a modified calibpoints.dat file. If you need the correct size of the object too, pay attention to the scale value you' ll put in in the calibration step..

Gunter
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Re: Painting Calibration Panels on Boards

Postby eliscio » Tue Sep 08, 2009 2:31 pm

Hi Gunter,

I was reading about the window calibration and how the two sides of the calibration panel are moved farther apart from each other, but the relative position of each set of points on each side of the calibration panels seems to stay the same. So, let's say that I do everything very carefully by hand and then go back and find that a few points are very slightly off. Can I make corrections for specific points?

I believe that I could get the error down to less than 0.4mm over the whole calibration panel and I think that would be good enough for what I am trying to do. I might just try it on a smaller setup first to see how hard it is going to be to and see if this makes any improvement to my scans. My smaller calibration panels are also taped, but it doesn't seem to have moved much over the past month.

What is the best resource for learning about the calibration panels and modifying their setup? (You guys always seem to know the links to the previous posts!)

Thanks,

Eugene
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Re: Painting Calibration Panels on Boards

Postby MagWeb » Tue Sep 08, 2009 4:08 pm

Hi,

about modifying the patterns see: http://www.david-laserscanner.com/forum/viewtopic.php?t=242
(BTW: We try to gather important topics here: http://www.david-laserscanner.com/forum/viewtopic.php?f=7&t=1118)

your setup owns a problem:

Your boards are rather dark, while your printout is on white. This might make some trouble getting a well adjusted cam picture: Either the beam is visible on the darker board but oversaturated on the white paper, or set right on the paper but invisible on the planes. Try to give them an equal color - that will make the cam settings much easier.

Gunter
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Re: Painting Calibration Panels on Boards

Postby eliscio » Tue Sep 08, 2009 7:29 pm

Gunter,

You are right about the colors of the background, but I was using the Mask tool and it usually calibrated fine. However, I just painted the surface of the panels all white and am going to draw or paint on the black dots.

What I have read so far:

1. I noticed that there were some issues when the dots were too large and so I am going to keep them relatively similar to what I have now.
2. Most people are sliding the panels farther apart to one another (i.e. from the vertical center of the calibration panels). So, there are only a few rows that need to be adjusted.
3. However, my question...is there anything stopping me from placing the points in a best fit position on my background panels without keeping relative scale from one point to another? So, let's say I just painted the dots by "eye" and said, that was close enough.

The points would not have a scale or relative pattern to one another so I would have to enter the X,Y,Z coordinates for each point, right? However, what happens when I enter the scale in DAVID? what two points is is looking for to fix the scale?

Thanks,

Eugene
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Re: Painting Calibration Panels on Boards

Postby eliscio » Tue Sep 08, 2009 8:11 pm

It's now a fresh start! Each panel is approximately 25cm wide x 30 cm high.

What do you think as far as layout? Just stretch the calibration points to near the edges?
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Re: Painting Calibration Panels on Boards

Postby eliscio » Tue Sep 08, 2009 11:26 pm

Well, I decided I should make a test before I actually paint anything. So, I stuck some paper on the panels and spaced them out better.

I was able to calibrate, but it will not scan because it says the left and right laser is not on a plane.

So, I am I messed up the Calibpoints.dat file.

Here is the information:
Scale as measured on Paper - 54.6mm (I had to print on 8.5x11)
Actual distance of Right side Panel: 80.17mm (i.e. distance moved from center of panels to cut line)
Actual distance of Left Side Panel: 13.01mm

I believe the Calibpoints.dat file is based on a scale of 60mm so I added a scaled factor of 88.0989 nd 14.2967 mm respectively to push the left and right sides.

Any input would be appreciated.

Eugene
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Re: Painting Calibration Panels on Boards

Postby MagWeb » Wed Sep 09, 2009 8:31 am

Hi,

IF you use a rightangled corner, AND the value C is equal on the left and the right side (same height) AND the value D is equal for the upper and lower marker on the right side, AND the value E is equal for the upper and lower marker on the left side AND you use the posted formulas ---> it should work.

The calibpoints.dat is written in scale 60 in a ratio 1:1. So whenever you put in the real values, measured from your printout, you may simply use scale 60, which means 1:1.
For some other variables (e.g the backgroundFilter) use the scale value too, you should put values in into your calibpoints.dat in a ratio to 60:
means:
Code: Select all
 60/measured value of a * measured value (of either a,b,c,d,e or x).


The distances are center to center. X is the distance of the center between the white markers ( of the double ring marker, point zero) from the vertical of the right column.
Hope that helps
Gunter

Edit: I think it´s better to spread the markers more equal. Means: instead of moving the ready made printouts, you could also use different distances for a and b. This way, you can leave the distance of the inner columns d and e (at about 40) and equal. Now you only have to double the distance a on the left printout to 2a. See 4:3 pattern in the topic linked above.
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Re: Painting Calibration Panels on Boards

Postby eliscio » Wed Sep 09, 2009 3:39 pm

I will check my numbers shortly and then get back to you with the results. Thanks!
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Re: Painting Calibration Panels on Boards

Postby eliscio » Wed Sep 09, 2009 4:17 pm

OK, everything looks in order and here is the first scan. (First one has no smoothing or anything, second one has some smoothing)

Can you tell me if the "lines" going across some of the points on the calibration screen are something like residual errors?

I am going to study this a bit more and see what other options are available to me.

Thanks for all your help Gunter.
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Re: Painting Calibration Panels on Boards

Postby MagWeb » Wed Sep 09, 2009 9:24 pm

Hi,

are you sure to have calculated point zero, the double ring marker, right? The green line seems to be a little too long.

Well, the markers might hide some area of the line on the planes. So it might get too short to be detected, if the object covers a big part of the planes too. You could paint or print and fix the markers on separate boards. This way you can remove them while scanning. If you do this, you´ve to put in the thickness of the boards ( * scalefactor (see above)) everywhere the value is equal zero in the calibpoints.dat.

Gunter
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Re: Painting Calibration Panels on Boards

Postby eliscio » Thu Sep 10, 2009 12:59 am

Gunter,

I don't understand the value for point zero. Where is d+2a-x actually located? All the other points are easy because they are the center of the targets, but where is point zero supposed to land?
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Re: Painting Calibration Panels on Boards

Postby MagWeb » Thu Sep 10, 2009 7:54 am

Hi,

The point, that is described by the double ring marker (point0), is the middle of the distance between the centers of its rings.

Gunter
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Re: Painting Calibration Panels on Boards

Postby eliscio » Thu Sep 10, 2009 3:29 pm

Aha! Now it's clear. This is what did not make sense to me. I'll have to change my number for point zero then.

I think the answer to my other question is that the lines we see on the calibration panels is an offset or error distance on where David thinks the point should be located and where it is actually measured to be??

You have really helped my understanding...Thank you.

Eugene
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Re: Painting Calibration Panels on Boards

Postby eliscio » Thu Sep 10, 2009 8:02 pm

Here is a corrected calibration with the right X value.

I wish to get the other errors down low too so I am going to spend some time with tweaking the next few days.

Regards,

Eugene
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