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checking calibration panels

Problems, solutions and discussion about scanning with a video projector and stripe patterns.

checking calibration panels

Postby avogra » Thu Jul 21, 2016 2:44 pm

Hi,

I have finished my SLS setup some weeks ago, played around with David4 and now I'm waiting for David5. One of the trickiest parts was to build calibration corners with a perfect 90° angle. As planes I use mirrors with the patterns printed on self-adhesive paper or on plain paper and attached with spray-on glue. To mount them in a 90° angle I 3D-printed some brackets, quite similar to the David ones. In the end, the angle was good but not perfect. I tried to measure the true angle (which is quite a challenge by itself) and came around to 89,8°, so off by only 0,2°! I'm not super confident in my measurement, but after I entered this deviation in the David settings, alignment instantly improved a lot. So the angle seems to be highly critical.

I then had an idea how to check the angle between the panels: After calibration, my first scan was a try-square in the same orientation as the calibration corner. In the scan, the slim edges should be perfectly square, if the calibration corner and compensation value are correct. Instead of trying to measure the actual angle of the calibration panels one could step by step alter the compensation until the scanned try-square has exactly 90°.

But how do you best measure the angle of the scan? My first solution was to insert two blocks in meshmixer, so that one side of each block coincides with each arm of the try-square, aligning them by eye. Then I used MeshLab to measure the angle between the two blocks. This takes quite some time and is somewhat error prone, so do you have a better idea?
Projector: Acer K11 with Achromat f=400mm | Cameras: 2x Logitech 9000 Pro with custom C-Mount body | Lenses: 1x Computar M1214-MP, 1x Computar M5018-MP, 1x Schneider-Kreuznach Cinegon CNG 1.4/12
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Re: checking calibration panels

Postby drndadoo » Fri Jul 22, 2016 9:57 am

The problem with "non 90" degree angle is that your coordinate system will be distorted. So everything within that distorted coordinate system will be also distorted.
The solution is not to give more tolerance to alignment process, then to fix your calibration corner to be as good as it gets.
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Re: checking calibration panels

Postby Alfa » Fri Jul 22, 2016 10:48 am

avogra wrote:Hi,
and came around to 89,8°, so off by only 0,2°!


Do you think that these 02 degrees can affect?
Do you think that the original DAVID system is accurate to 90 degrees? I doubt.
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Re: checking calibration panels

Postby drndadoo » Fri Jul 22, 2016 12:40 pm

Someone already posted accuracy scans of DAVID Calibration Panels, hope I will not make him angry if I repost that here.

horiz_16deg_240mm.JPG


horiz_over_15deg_240mm.JPG


horiz_under_15deg_240mm.JPG
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Re: checking calibration panels

Postby Alfa » Fri Jul 22, 2016 1:48 pm

Hi drndadoo,
cool!!!
With the program gives such analysis?
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Re: checking calibration panels

Postby avogra » Fri Jul 22, 2016 3:41 pm

Alfa wrote:Do you think that these 02 degrees can affect?
Do you think that the original DAVID system is accurate to 90 degrees? I doubt.

I did some math and came to the conclusion, that accuracy of the panel angel is really important. Just a scenario: You want to scan an object with a diameter of 100mm und rotate it 8 times. Then you align each scan to the previous. The missing 0,2° will add up for each pairwise alignment, so in the end, the first and the last scan will be off by 1,6°. For an object of 100mm this means almost 2,8mm displacement between first and last scan! Depending on the object, global alignment will have a difficult time to correct this.

drndadoo wrote:The problem with "non 90" degree angle is that your coordinate system will be distorted. So everything within that distorted coordinate system will be also distorted.
The solution is not to give more tolerance to alignment process, then to fix your calibration corner to be as good as it gets.

In David advanced settings, there are those fields to tell david the deviation of left and right panel. I found it somewhere explained in the wiki or in the forum but can't find it anymore. I understand that if my panels aren't exactly 90° but I know, how much they are off, I can use these fields to compensate for the error. Or is it not the same as using perfect panels in the first place?
If I got that right, you still need to measure the exact deviation. To do that, I would like to use a known precision-reference (the try-square). When I scan that and the scanresult has exactly 90°, then I know that David's coordinate system is perfect.
Projector: Acer K11 with Achromat f=400mm | Cameras: 2x Logitech 9000 Pro with custom C-Mount body | Lenses: 1x Computar M1214-MP, 1x Computar M5018-MP, 1x Schneider-Kreuznach Cinegon CNG 1.4/12
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Re: checking calibration panels

Postby Alfa » Fri Jul 22, 2016 7:22 pm

I think it's really logical that David must have some possibility to level error of the angel.
Especially if it's so important. And once again I'm sure that DAVID original panels have not 90 90° angle. It is easy to see, Davids panels construction can not provide such standard.
I am planning now to build myself calibration panel. Today I went to the OBI store and looked 5-6 kinds of ordinary angles (7-15 EUR), so most of them were not coincide with each other. In the distance sometimes was of 20 cm aprox. 0.1-0.2 degrees error ...
Yes, it is really the high-quality tool according DIN is more expensive. For example 200 x 130 mm about 30 EUR
http://www.messmittel.tools/Messmittel- ... IN-875-00/
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Re: checking calibration panels

Postby avogra » Thu Jul 28, 2016 6:19 pm

I found the software used for those measurements: It is Geomagic Verify Viewer. Great free tool and that also answers my initial question of how to measure the angle of my scans :)
So here are some test results:
I first tried to measure the angle of my calibration corner with a digital angle gauge, and read 89,8°, but if i touched it here or there it easily changed by 0,1°.
Next I did a calibration with the corner. Sidenote: does it matter if the corner is standing or laying on its side? I used it laying, like y-axis is horizontal.
Then I did a scan of my try-square which was in the same orientation as the corner. I cleaned the scan and exported it. Then I imported it in Geomagic Verify viewer.
There I measured the angel:
trysquare1.png

Seems like my corner is 0,369° off. So in David advanced settings -> Calibration -> CalibrationPanels -> RightCalibrationPattern -> rot_y I entered 0.369
Next step I did a new calibration, and thereafter a new scan of the try-square.
Export the scan, import in Geomagic and measure the angle again:
trysquare2.png

Not perfect but a huge improvement :)
Projector: Acer K11 with Achromat f=400mm | Cameras: 2x Logitech 9000 Pro with custom C-Mount body | Lenses: 1x Computar M1214-MP, 1x Computar M5018-MP, 1x Schneider-Kreuznach Cinegon CNG 1.4/12
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