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Motion Analysis / Cortex: ghost markers and marker shaking issues.

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  • Motion Analysis / Cortex: ghost markers and marker shaking issues.


    In our current lab set-up we are using 8 Osprey cameras set on a 180 degree space around the front of the subject at various heights in order to track 2 mm markers placed on the phalanxes of index and middle fingers, hand, and wrist. The hand lays supine on a table and the subject performs two tasks: a hand open (MCP, PIP, DIP extension) and a pinch (MCP flexion; PIP, DIP extension).

    After data collection we proceed to analysis and find that two issues happen very often:
    1) ghost markers appear very close to real markers and cause the tracking to shift the position of the marker from the real one to the ghost.
    2) sometimes two markers that are close to each other start shaking.

    Our objective is to track angular motion, so you can imagine how 'pretty' it looks the graph later when the start/end points of those vectors are dancing all over the place. The usual manual tracking of the data helps, but I am much more concerned that the data is overall affected negatively by this because of the amount of linear/cubic joining that needs to be done.

    I've tried to adjust the configurations on Cortex to compensate for marker size (2 mm) and prediction error (20 mm/frame). The whole process happens over the span of 10 seconds, during which the markers move for 5 seconds. The largest motion (of the distal phalanx marker) is in the order of 80-100 mm... to no avail. Data looked exactly the same after the adjustments.

    I have thought about a few of the reasons why this could be happening:

    1) Too many cameras. Cameras are creating doubles of the markers when seeing them from both sides.
    2) Camera resolution (640x480) is too low. The camera that is farthest away from the markers sits at a high point, about 4 ft from the hand. The cameras are seeing two markers on the same pixel.
    3) Markers are too close when the joints achieve extension. There is a separation of ~5 mm between joint markers from opposing phalanxes (e.g. MP distal/DP proximal). The cameras are seeing both markers as one.

    These seem to me like sound hypotheses, but there are a few things that do not seem to add up. For example, markers that get too close to each other sometimes shake and have a ghost marker too. This would only make sense if 1, 2 and/or 3 are happening at the same time.

    I am considering changing the camera set-up to have 4 cameras positioned each at a 45 deg. angle from the sagittal plane, two higher up, two higher down, so as to create a pyramidal focus on the hand and fingers. I believe this could eliminate the ghost markers, but I do not have extensive experience with this equipment yet (Motion Analysis).

    Any thoughts on this?

  • #2
    Re: Motion Analysis / Cortex: ghost markers and marker shaking issues.

    Hi Gustavo,

    I do not have Motion Analysis equipment, but this sounds like a general issue with camera settings not being optimized for your environment/markers. You should place a few of your markers in the area you intend to collect data in and then adjust the threshold settings on each camera so that you're getting the strongest possible signal from your markers, without any erroneous signals. I don't think having "too many" cameras can be a problem, so long as they are all calibrated properly. It is possible that resolution/small markers/small separation distance could be an issue but I'd try optimizing all settings before reaching that conclusion.



    • #3
      Re: Motion Analysis / Cortex: ghost markers and marker shaking issues.

      Hi Gustavo,

      I very much doubt the volume of cameras you have is an issue, as Kyle mentioned the set up could be though. Without details of your calibration it can be very hard to tell.
      There are a few things you could try. First recalibrate and check you have a good calibration, particularly in the area in which you're tracking. MAC systems usually come with a card used to check the focus/aperture hardware settings. It has a series of reflective markers of different sizes which you can use to ensure your cameras can detect the marker size you wish to track. Double checking your set up for each individual camera could help confirm the hardware set up in addition to the software light and threshold settings.
      Once your line of sight(physical position of the cameras), aperture, focus, light and threshold settings are correct; consider the tracking options you have in cortex. If you are using the default 'minimum two cameras for tracking' option try increasing this to three cameras, this may well help remove the ghost markers.
      Finally review the models themselves. In our lab we had many issues with various models which were solved by increasing the number of links between markers.

      I presume you have also sent a query to the support team at MAC? The guys there are very good and helpful. No doubt they will be able to offer better (and more reliable) help than I can offer.



      • #4
        Re: Motion Analysis / Cortex: ghost markers and marker shaking issues.

        Dear Dr. Heidner.,
        please, could you send some illustrations of biomechanical model and hands with the markers?
        - 2mm markers seem too small
        - are these markers hemispherical or flat?- for an accurate calculation of the marker center is necessary that several pixels the CCD camera are filled by the marker. Thus, small marker (2mm) with 0.3Megapixle cameras seems to be a motion capture condition very complex.
        I realized some motion capture of fingers and wrist but for use in animation movies. Only two phalanges of each finger were used. The markers were 6 mm (flat) and were made with an office punch in reflective tape.
        The capture system had 10 cameras of 4 Megapixel, but the capture volume was the same used for clinical gait analysis (a room of 12 x 7 x 3.5 meters).
        Maybe I can try to help if I can see some pictures.
        I apologize for translation failures.

        Best regards,

        Wagner de Godoy


        • #5
          Re: Motion Analysis / Cortex: ghost markers and marker shaking issues.

          Hi Gustavo,

          The others have definitely provided good suggestions and covered important information. I just wanted to add that what you're describing with the ghost markers is exactly what I see on my Motion Analysis system when I haven't calibrated before use. Even when all of my cameras are attached to the wall and shouldn't have moved, I find that I need to recalibrate (refine calibration) if a day has passed since my last calibration. I'm not sure if there are small subtle movements of the building or what causes this, but for my system I have found the calibrating within an hour of testing eliminates the problem that you're describing. I would start here because it's easy to test, but obviously if you're already doing this then you should move on to the others' suggestions.