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Scan Region and Size vs Accuracy

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

Scan Region and Size vs Accuracy

Postby RudeDog » Mon Jan 19, 2015 9:39 pm

My initial results with David SL scanning have been fantastic. However, now I am running into distortion issues when scanning some objects. I believe my calibrations are good. I may need to understand more about where to place my scanned objects for the best results.

Please refer to figure 1), where I have defined some regions related to calibration panel geometry, for discussion.

My question is, can I use regions 4, 5 and 6 at all without expecting distortion? Where is the best location for the center of a turn table? It seems the more I squeeze objects back into region 1), the smaller the image, therefore less resolution.

Advise on turntable placement from more experienced members is welcome.

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Re: Scan Region and Size vs Accuracy

Postby micr0 » Fri Feb 13, 2015 4:19 pm

Sorry this is such a late response but I thought it might be helpful even if only for future reference. I've been using inverted (outside) calibration panels (150 to 250mm) and have made a fixture to mount them consistently to my rotary. I have the panels located such that if you found the midpoint of a line from the apex of the triangle to the midpoint of the hypotenuse, that midpoint would be coincident with the axis of rotation of the rotary. This seems to work very well in terms of accuracy, and I have had very little trouble with the alignment of the resulting scans, even as oblong shapes move away from the ideal scanning regions.

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Re: Scan Region and Size vs Accuracy

Postby TheChosenOne » Mon May 02, 2016 10:09 am

2016-05-02_120658.png (3.76 KiB) Viewed 3234 times
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Re: Scan Region and Size vs Accuracy

Postby WP2016 » Thu Jun 23, 2016 5:26 pm

This is a great question! I also want to know, especially how the triangulation works in David.

I do not think these regions make any difference in terms of scanning result, the issue should be about the sharpness of the captured projected pattern. This is because the triangulation allows each X1 Y1 of the camera sensor to correlate with X2 Y2 of the projector to calculate X, Y, and Z of the object (the correlation is achieved during calibration).

If my understanding is correct, the issue of accuracy depends on how accurate can the system acquire X1, Y1, and X2, Y2 (sensor recognizing the projector pattern). Once calibrated, the full frame projected area will provide similar, if not same accuracy so far as the object receives a clear structure light pattern and the projected area is within the depth of view of the camera. Thus separating the regions may only be relevant when depth of field is considered.

Based on my understanding (because David's code or algorithm is secret), an optimized scanning for resolution (and accuracy) would require the object filling the view as much as possible (to use as many pixels of the sensor as it can), matching the structure light pattern to the view (to best achieve the pattern resolution, as limited by the projector's own resolution), and setting up a depth of view to enclose the object surface to be scanned (recess surface being the far plane of acceptable circle of confusion, and the nearest surface being the near plane of acceptable COC). But if my presumption was mistaken, such setup would not be necessary.

I am questioning my understanding, because I have seen scanning not tuned to "zoom in" for high resolution and it seems a good result is still produced.

If we can just leave the object to occupy a small portion of the calibration size (say about 20% of width and height of the sensor leaving at the center), then the setup is more versatile and the scan region issue is more relevant. Any comment or clarification is appreciated.
Projector: Acer K132 + 1/2/4/10 X closeup dai. 52 mm.
Camera: Grasshopper 3 GS3-U3-28S4C-C (@ 1600 X 1200, 30 fps); Lens: 8-50 mm F/1.4.
Computer: USB 3.0, CPU: 2.6-3.5 GHz, GPU: 3G NVidia 970M, 32G RAM.
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