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Author Topic: What units PS use for distortion parameters?  (Read 12666 times)

BronislavKoska

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What units PS use for distortion parameters?
« on: June 12, 2011, 07:36:49 PM »
Hi,
I have a guestion, what units use PS for distortion parameters.
More preciselly, what units are used for radial and tangencial distortion parameters because it seems clear fx, fy, cx, cy are stated in pixels.

For k1, .., k3 are usually use (mm^-2,mm^-4,mm^-6) or (pixel^-2,pixel^-4,pixel^-6) but it is not the case of PS.
I would like to use calibration parameters from PhotoModeler (where mm are used) and I'm not sure about conversion.
Thanks
B. Koska

Diego

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Re: What units PS use for distortion parameters?
« Reply #1 on: June 12, 2011, 07:45:49 PM »
Hello,

AgiSoft Lens performs lens calibration using Brown's distortion model, and I have understood that the units are in pixels.

Regards,

Diego

BronislavKoska

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Re: What units PS use for distortion parameters?
« Reply #2 on: June 12, 2011, 11:44:28 PM »
Hi,
thanks for quick answer, but Im pretty sure you are wrong about units.
As I wrote before it uses pixel for fx, fy, cx, cy but k1, k2, k3 , p1, p2 have different units.
I know it because its value are very big (usually about 1e-2) to be in mm or pixels.
May be they are in pixels internally but they used some constant to multiply its value before showing it/export/import.
Any other idea?
B.
« Last Edit: June 12, 2011, 11:46:10 PM by BronislavKoska »

gEEvEE

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Re: What units PS use for distortion parameters?
« Reply #3 on: June 13, 2011, 10:52:04 AM »
Hello Bronislav,

I am also working on this problem. As you already mentioned: only the focal length and principal point are expressed in pixels.  I used different calibration software packages and the tangential and radial distortion parameters I get there are way off of those calculated in PS. I mailed the PhotoScan people already about this. I will now look into the camera models these other packages use. It is certainly something to pay attention to.

Cheers,

Geert

Alexey Pasumansky

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Re: What units PS use for distortion parameters?
« Reply #4 on: June 14, 2011, 02:46:41 PM »
Hello Bronislav,

Distortion model used in PhotoScan is the same as described in the last chapter of AgiSoft Lens manual - http://www.agisoft.ru/lens/help/en/lens.pdf

K1, K2, K3 are used in units of focal length and to convert them into mm or pixels their values need to be divided by fx(in mm or pix)^2 for K1, fx^4 for K2 and fx^6 for K3. I think it could be a solution if you are sure about distortion model used in Photomodeler.
Best regards,
Alexey Pasumansky,
Agisoft LLC

BronislavKoska

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Re: What units PS use for distortion parameters?
« Reply #5 on: June 16, 2011, 05:45:03 PM »
Hi,

thanks for answer, you are right about units, but there is one more thing to be mentioned.
PS uses opposite sign for k1, ..., p1 values.
I have tried to compare distorition results from PS and PhotoModeler. They seem pretty simmilar, but the signs are inverted.
I have been looking for answer. PS has differen definition of distortion values than is usual.
PS:
http://www.agisoft.ru/lens/help/en/lens.pdf (last page)
and usual definition from wikipedia:
http://en.wikipedia.org/wiki/Radial_distortion
Usualy distorition values are defined in direction from distorted image to undistorted image.

Regards
Bronislav

mirco

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Re: What units PS use for distortion parameters?
« Reply #6 on: May 12, 2012, 03:13:11 PM »
hello,

how can i use the tangential coefficients P1 & P2?? Are they used in the units of focal length, too? How i?ll convert them to use it in pixels or mm??

have you any idea??

ciao mirco