Forum

Author Topic: Camera calibration  (Read 6153 times)

AVisser

  • Newbie
  • *
  • Posts: 4
    • View Profile
Camera calibration
« on: August 17, 2015, 06:36:01 PM »
I recently did a survey using a precalibrated camera. When processing in Photoscan, however, when I fix the calibration under Tools->Camera calibration the results are way worse than when I do not fix the calibration. The difference in error is about one order of magnitude.
I suspect this is because if you fix the calibration Photoscan has less degrees of freedom moving things around to obtain a proper fit. Thus, is it better to precalibrate and get large errors, or do not precalibrate and basically do not know what changed?

I just processed the last of the test flights and the result is again interesting. Not fixing calibration gives 1.9m X error, 2.3m Y error and 3.4m Z-error, but when I fix the calibration the errors become 4.6m, 4.5m and 56m. This decreases the flight altitude from 113m to 61m as well, and the fly path was set to be 120m, thus the first is much more likely, considering the terrain is about 7m at its highest elevation.
« Last Edit: August 18, 2015, 11:55:53 AM by AVisser »

nadar

  • Full Member
  • ***
  • Posts: 130
    • View Profile
Re: Camera calibration
« Reply #1 on: August 18, 2015, 03:49:36 PM »
did you check if the calibration parameters are correctly interpreted by Photoscan ? I know from (painful) experience that you can find alpost as many conventions than existing software... Check if the naming of the parameters are OK (K1, K2... can be very confusing), how the parameter is measured (for instance, some measure decentering offset from the center, other from top left corner...) Check also if units are correct (sometimes mm, sometimes micron, sometimes pixels !)

May be worth doing an independanrt calibration procedure in Photoscan and compare if the value of the parameters are roughly similar.

Good luck.

AVisser

  • Newbie
  • *
  • Posts: 4
    • View Profile
Re: Camera calibration
« Reply #2 on: August 18, 2015, 05:07:25 PM »
I calibrated the camera using Agisoft Lens, exported that as an .xml file, which I imported into Agisoft Photoscan. I did not rename anything, since I assume Agisoft wrote their programs to work together.

James

  • Hero Member
  • *****
  • Posts: 733
    • View Profile
Re: Camera calibration
« Reply #3 on: August 19, 2015, 11:35:12 AM »
If your camera was focused at infinity for the flight, and focused closer for the calibration, then that could explain the problem to some extent.

AVisser

  • Newbie
  • *
  • Posts: 4
    • View Profile
Re: Camera calibration
« Reply #4 on: August 19, 2015, 03:42:49 PM »
If your camera was focused at infinity for the flight, and focused closer for the calibration, then that could explain the problem to some extent.

If this is true, that'd mean calibrating your camera using Agisoft Lens has to be done at the same range as you intent to take photos. Wouldn't Lens loose its purpose then? Since I would have 8 pixels or so on my monitor if I'd take a picture 120m away (assuming I could actually put my monitor outside and still photograph it)
« Last Edit: August 19, 2015, 03:44:57 PM by AVisser »

James

  • Hero Member
  • *****
  • Posts: 733
    • View Profile
Re: Camera calibration
« Reply #5 on: August 20, 2015, 12:27:08 AM »
You don't have to focus on the screen, or put it 120m away. You can keep focus at infinity and use smaller aperture to increase depth of field, the images may still be blurry but it should give better results than focusing close on the screen as this changes the internal orientation parameters.

Not tried it myself, however!

Hello Nathan,

Wide angle lens calibration using AgiSoft Lens can be indeed problematic as the camera needs to be placed very close to the monitor. As the camera calibration parameters depend also on the focusing distance, it may be very hard to get sharp pictures of the screen. Focusing the camera on the screen in undesirable, as it will affect the calibration.

You may find the following recommendation useful:
1. Try to use larger screen when possible.
2. Use higher aperture setting. This may also require using a tripod and higher exposure setting.

Please note, that even this recommendation may be insufficient to capture the calibration photos using infinite focus setting. In this case only approximate calibration values will be obtained, but they should be sufficient for a proper operation of PhotoScan. Just make sure that "Fix calibration" flag is not set during photo alignment, so that PhotoScan can refine the obtained calibration.