Manually Inserting Markers in Vicon Nexus

[Julie Rekant]

I am processing a trial in Vicon Nexus. One of my markers is missing for the first 20 frames. Because it does not have a predecessor, I am unable to use the built in gap fill functions. However, the marker in question does not move much throughout the trial. Is there a way I can set the position of the marker in frame 21 as the position also for the first 20 frames?

[Scott Dueball]

Re: Manually Inserting Markers in Vicon Nexus

PM sent. I'd be happy to help you out if you no longer have a support contract.

[Edmund Cramp]

Re: Manually Inserting Markers in Vicon Nexus

There are two simple ways to edit the C3D file contents and insert or edit marker positions. If you have a lot of data to modify or edit then our C3Deditor from Motion Lab Systems is probably the way to go - you can download a trial of this software from the Motion Labs website but you have to purchase the software ($495) before it will edit your files. Use this option if you need to perform repetitive edits for large numbers of files or if you need to do complex tasks like resetting force plate parameters, changing analog or video sample rates etc.

The second option is to install the C3Dserver - this is an SDK for C3D files that allows users to write programs that access the C3D file structure and data via Visual BAsic, C++, C# and MATLAB etc. You probably don't want to have to do this to fix your problem but the C3Dserver includes as an example application, a simple C3D file editing application written in Visual Basic (including the source code too) - you could easily edit a few marker locations using this application.

The C3Dserver and the C3D file editing application can be downloaded for free - you can buy the full speed version (works identically but faster) but the free version is fully functional and would probably solve your problem if you only have a few files to edit.

Before you start editing C3D files you might want to read about the C3D file structure - there is a user guide that details all of the C3D file structure available at the C3D web site. You can either download the PDF version or use the online version of the manual.

[Robert D. Catena]

Re: Manually Inserting Markers in Vicon Nexus

Julie,

I have recently switched from MotionAnalysis to Vicon. Both have their benefits and pitfalls. This was done with MotionAnalysis by either creating a virtual marker for those first 20 frames or doing a fill based on other marker locations. I was told that this was not as easy in Vicon Nexus and that you essentially need another software (BodyBuilder) to do any virtual marker work. Nexus 1.8 I hear is coming out soon. Hopefully this is an addition.

Please post a response if you figure this problem out. I am sure many of us would be interested in a new way to do this.

[Scott Dueball]

Re: Manually Inserting Markers in Vicon Nexus

Julie,

I was told that this was not as easy in Vicon Nexus and that you essentially need another software (BodyBuilder) to do any virtual marker work.

Someone really smart told you that

Nexus 1.8 I hear is coming out soon. Hopefully this is an addition.

Please post a response if you figure this problem out. I am sure many of us would be interested in a new way to do this.

I would be shocked if this were added and Body Builder replaced. Nexus and its predecessors were designed for clinical applications. That means that allowing the user to edit data based on their model is not ideal. Creating virtual markers is risky business. If there were "easy" methods I would be concerned with their validity because you would not have control over the assumptions made. Assumptions can have negative impacts to models (knee and ankle markers in PiG for example). Body Builder or any method is limited by the marker set and the marker missing.

Certainly, other options like C3D Editor and Visual3D exist that can help as well. However, Body Builder can do this in 6 lines of code (my Matlab scripts were closer to 3 times as long).

Also, check your raw data. If you see the reflection in some of the camera views, you can get that marker reconstructed. Just need to know the tricks

[James Dunne]

Re: Manually Inserting Markers in Vicon Nexus

If you have matlab running on your system you can download the pecs plugin for Nexus and run scripts to create virtual markers.

If you know the 3-d coordinate in the global and the frame numbers in which you need to insert them then its a simple one line function.

If you want to go this route I can send you some example code.

regards,

-James

[Robert D. Catena]

Re: Manually Inserting Markers in Vicon Nexus

Yes, a very smart guy that I would recommend to anyone having a Vicon issue! I also agree virtual markers left in the wrong hands can be dangerous, but it would be nice to have a little bit of flexibility.

In Julie's situation she may just need a filler when the subject was just standing still before the trial. I have needed something like that in the past because my Matlab code required all columns to be full of data in every row. I know that I can just go into the TRC and manually copy up from the first row of real data or I can just the first 20 frames off of the data, but it is just another step in the already long process of data processing. It is the poor grad student working at 9pm on a Friday night that suffers.

More was the case that I wanted to create a virtual marker based on other markers on the segment to identify a joint center or a segment COM assuming the segment was rigid. I have not had the chance to use Bodybuilder. One of these days I would like to. I have relied on Visual3D to do this, but the Visual3D process can be time consuming if this is all you want to do and again means another step in the process before I have a usable file for my Matlab code for data processing.