Number of markers per rigid body

Discussion in 'HAPTIX' started by David Kluger, Jan 22, 2016.

  1. In an effort to eliminate occlusion of the 3 IR markers on the hand, I attempted to add a few more IR markers to the existing wrist tracker skeleton. I then redefined the "hand" rigid body in Motive to include the existing 3 markers plus the ones I added. Motive handles this very well. The software only needs to see 3 of the total number of markers in order to track the rigid body through space once you change a setting in the body's advanced options (Rigid Bodies -> Rigid Body Settings -> Advanced -> Min Marker Count = 3). The added markers help (at least somewhat) prevent obstructions from compromising wrist tracking in Motive.

    However, when I tried to load MuJoCo with the project.tpp file with the novel hand trackers, I got an error while loading MuJoCo saying "Expected 3 markers per body," and I cannot track anything in the VRE thereafter.

    Would it be possible to remove this limitation and allow more markers per body? Removing this limitation for the wrist is the most important since the wrist markers are the most likely ones to become occluded. Furthermore, this tracker has the most control over the virtual hand, and therefore a person's control of objects in the VRE. I see adding this feature as a good way to improve the VRE's interface, assuming the VRE's handling of extra markers won't sacrifice too much processing power.
    Last edited: Jan 22, 2016
  2. Emo Todorov

    Emo Todorov Administrator Staff Member

    MuJoCo does its own rigid-body tracking using 3 markers. This is partly because at the time Motive wasn't very reliable, and partly because MuJoCo needs to know where the hand is relative to the markers, while the Motive body coordinate frame is somewhat arbitrary. Changing this is a significant rewrite.

    How much improvement are you getting from the extra markers? The reason I settled on 3 markers at the time was because extra markers increased the likelihood that 2 of the markers fall on the same line of sight from the camera and overlap, in which case they are both are lost effectively.
  3. Yes, I noticed this problem too. Even though Motive can do a slightly better job tracking the wrist position with a 6 choose 3 marker set, overlap causes other issues for tracking wrist orientation.

    3 markers is probably the best option, but adding more cameras from different angles would probably be the best solution to avoid loss of tracking due to marker occlusion. Can MuJoCo HAPTIX handle more cameras? Is this something worth investigating?
  4. Emo Todorov

    Emo Todorov Administrator Staff Member

    More cameras will indeed improve things. Good luck getting Doug Weber to pay for them though :)

    I have only used the Optitrack V120:Trio, but my understanding is that the Motive software works will all their camera systems through the same interface. So I am guessing that MuJoCo HAPTIX will just work and not even notice that you have more cameras. Unfortunately you cannot add cameras on top of the V120:Trio. Instead you have to get their stand-alone cameras and assemble your own system.