motion capture system - 3d calibration flaw

[Wagner De Godoy]

Dear Subscriber,

I am posting this message to render aid to the Gait Analysis Laboratory of Curitiba - CHR, in Brazil.
This laboratory uses a Motion Analysis Co. system, 6 cameras Hawk, and software of capture Cortex.
Calibration flaws are happening continuously, and the causes of the problem could not be determined, thus, the laboratory stays inactive.

A summary of the accomplished procedures, and some images, are available in the web site below:

http://analisedemarcha.com/fotos/index_mocap_cal.php

We will be thankful for any support for the solution of this problem.
Please, the e-mail for contact with the gait laboratory of Curitiba is:
Dr. Alessandro G. Melanda
melanda@uel.br

Thank you very much for your attention.
Best Regards,

Wagner de Godoy
HIAE - Brazil

[Samantha L. Winter]

Re: motion capture system - 3d calibration flaw

Dear Wagner,
We have a similar system, when we see the kind of problem you are describing it generally means there is a reflection that is being picked up by one of the cameras. All of the frames including this reflection are then discarded and the system can't calibrate. You may be blocking the reflection with your body when performing the seed calibration and intermittently during the wand calibration - we have found this to be the case when the seed (L frame) calibration works and some frames are retained from the wand calibration. If you can't find the reflection we have also sometimes solved the problem by adjusting the settings of the affected camera by changing the intensity of the ring lights or the threshold for marker detection.
Hope this helps!

[John Greaves]

Re: motion capture system - 3d calibration flaw

Dear Wegner,
Thank you for the detailed explanation and pictures of the problems you are having with your Hawk system. It looks like the cameras might not be in focus correctly. If that is the case, especially for the cameras with the higher numbers (residuals) in the 2D display are higher, like you show in one of your slides. To check that the cameras are in focus, get the Camera Focus Card that came with your system, place it in the middle of your volume and adjust the focus while you are looking at the full screen 2D display in Cortex for that camera. Use the large dots on the Camera Focus Card to get oriented, then adust the focus so that you get the smallest bright dots in the smaller markers. If you cannot find the Camera Focus Card, use some 7 to 10 mm markers near the center.

If you send your email and the link to your biomech posting to support@motionanalysis.com, we will ask you to post your calibration and a capture onto our ftp site. When we have the data, we can see more clearly what the possible problems are and how you can proceed to get good quality calibration and captured data.

Hope this helps,
John Greaves, Customer Support
Motion Analysis

[Wagner De Godoy]

Re: motion capture system - 3d calibration flaw

Dear Dr. Samantha Winter and Mr. John Greaves.

Thank you very much for you important help. I sent the two answers to the gait analysis laboratory of Curitiba (Mr. Rodrigo Faucz, engineer, and Dr. Alessandro Melanda, physician). All to information and support are priceless to determine the flaws that happen in the system calibration.

Thank you again.
Best regards from Brazil.
Wagner de Godoy
Hospital Albert Einstein - HIAE