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Arm tracking problems
https://forum.ipisoft.com/viewtopic.php?f=6&t=6122		 Page 1 of 1
Author:  kreeg [ Fri Jun 15, 2012 3:41 pm ]
Post subject:  Arm tracking problems
Hello,
I just got the trial working on a machine after some trial and error. Did some basic motion cap sessions with one kinect and have gotten some ok results. Actually not bad for right out of the box.


My question is, are there any special consideration for tracking arms, I find that the rig in ipi tracks ok but the arms tend to lose tracking quite a bit.

Second, is there a preferred distance from background for the subject you are capturing or does this matter?
I would think you would want maximum separation from everything but I could be wrong.
Author:  Greenlaw [ Fri Jun 15, 2012 4:21 pm ]
Post subject:  Re: Arm tracking problems
It depends on what the arms are doing. Sometimes if arm movement is too quick, the tracker can lose it, or if an arm crosses the body very closely, it might 'blob' into the body. I think this is because of the Kinect's fairly low resolution. With practice, you can perform to avoid such problems without looking unnatural. For example, if I need to bring my arms to my chest, I may only bring my arm close to my chest without actually touching. This isn't critical but it may help if you run into exceptionally difficult problems.

Most of the time,I don't need to do much correction to the arms...but of course we may be capturing very different kinds of motions. When these errors do pop up (as they inevitably do,) you can fix them a number of ways.

The first thing I will try is to track past the error, correct the arm position and click Pose Refit, and then track backwards through the error. This often works if the problem isn't too vague for the tracker. (Remember to bump Trajectory Filtering to smooth out any curve mismatches.)

If it still has trouble, you may have to step through while manual adjusting and clicking Pose Refit at each problem frame. It may sound tedious but it's actually a quick process and you rarely have to do this for a lot of frames.

Finally, if Pose Refit fails, you will need to manually pose as closely to the volume as you can. Step through to check the smoothness. This is usually a last resort and I rarely need to do this.

You might want to enable show paths or trajectory or whatever it's called (sorry, I can't look at the program right now.) This may help you analyze the smoothness of the 'hand keyed' motion.

If the motion looks a little shakey, don't worry about it--Configurable Jitter Removal should clean that up. With experience you'll be able to recognize what type of jitter you can ignore and leave for CJR to handle when you're finished tracking.

Hope this helps.
Author:  Greenlaw [ Fri Jun 15, 2012 4:31 pm ]
Post subject:  Re: Arm tracking problems
Regarding the background, I assume you mean a wall. In general, you should avoid getting too close to walls and furniture because the performer's volume may get 'blobbed' into the item's volume and confuse the tracker.

Elsewhere in these forums, a suggested distance was mentioned but it's not hard to figure out the 'safe range' through practice.

Of course, sometimes you may actually want to come in contact with items, like a chair or a prop. In this case, it's best to use furniture or props that don't have enough volume to occlude the body. For example, a stool or a simple folding chair works best for sitting on instead of a solid box or a sofa. In our current project, we have a character playing a guitar, and on set we used a Nerf Sword as a stand in because it's very thin but can be handled like a guitar. Motion was synced by performing to video playback of the actual guitar performance recorded for the soundtrack.

G.
Author:  kreeg [ Fri Jun 15, 2012 6:31 pm ]
Post subject:  Re: Arm tracking problems
Great advice, thank you very much.
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