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Choosing appropriate marker placements

Markers have to be placed carefully on moving bodies to ensure that motion is captured effectively.
12.6
We can show this box because it has all the different sizes.
19
Well, we have many different sizes of markers that we use for different reasons. They’re quite large ones, which we usually use for tracking large-scale body movements. Smaller ones, which might be used for marking instruments, for instance. And very tiny ones, which are useful for marking fingers or faces.
48.6
Here, I also think we have, actually, the gum that I will speak about. OK, for attaching.
59.4
This is what I used to place markers on the fiddle. And it can also be used on the face. [LAUGHING] Probably need a bit more. When we are doing recordings of participants, we often use these suits. And here is the jacket.
83.6
And the suits are supposed to be quite close-fitting. Because when you’re taking somebody’s arm, you want to get, actually, the motion of the arm and not the motion of their clothes moving around. So the arms should be quite close. And then in the torso, it should be quite close. And they’ve also got these covers for the hands, with Velcro in the back of the hands so that you can attach– You can attach the markers to the back of the hands this way. And the whole suits can be– yeah, you can have those markers attached with Velcro anywhere. We also have pants and hats. The hat’s useful for, of course, tracking head movements. Yeah.
130.8
OK, so when you’re placing the markers on your participants, is there anything in particular that you– Yes. –you think about? Well, when you’re looking at musicians, your placement of markers depends on what type of movement you want to capture. For instance, when I have looked at pianists or string players, it’s useful to capture some of the movements that are involved in sound production, which is mainly the arms. So this is one of the jackets that we might use. And so I would place markers, for instance, up the arms, at the wrists, and the elbow, and the shoulders. On the joints, right? On the joints. And possibly, also, on the hands.
178.8
For some of the more expressive and communicative motion, I often look at the head. So in this case, I would usually place three markers on the top, the front, and one side, which would allow you to get also information about which direction the person is facing. And also, for expressive movements, the upper back, just at the top of the back is a very common place to measure this gets, kind of the body’s way that musicians do in time with the music. And what do you do when you look for your fiddlers? What kind of movements do you capture? Yeah, so I was interested in, well, mainly, actually, foot tapping and bowing movements. But I actually use full-body motion capture.
226.4
So I did the same. I had markers on the joints. But I also have the upper body and the legs. And because I’m not entirely sure how he would tap his foot if it’s the heel or the toe. I had to have both markers on the heel and the toe. And since I was interested in bowing motion, I was interested in how it moved in relation to the fiddle. So I also had markers on the fiddle. And so I did the same thing that you did with a head. So I had the three markers, or maybe I actually had four on the fiddle so I could create a plane of the fiddle. And so I had this bow marker.
277.4
And then I could calculate after, like the distance from the fiddler, from the fiddle. [LAUGHING] And how do you attach the markers to a sensitive instrument? Yeah. So the instruments that they play on is actually a very particular fiddle. It’s a Hardanger fiddle. And it’s very ornamental, and it’s a really nice instrument. It’s beautiful, actually. Yeah, it’s beautiful. But then you have this– it doesn’t sound so good– but it’s like chewing gum. [LAUGHING] But it doesn’t leave any marks. So that’s what I actually used for the fiddle For the bow, I actually used– I attached it, and I think I tied it, or something. OK. Yeah. What did you use? Did you have any markers on the fiddle?
326.8
Yes, well, I had some on the bow. I used a marker that was attached to a long piece of Velcro, a small marker. So I took some athletic tape, sort of this athletic gauze, and wrapped it around the bow so nothing would get scratched. And then just wrapped this piece of Velcro around. And so everything stays in place. So the thing with these markers, they’re quite light. But I think they can actually still feel it when you attach it to the bow. But that’s one of the nice things with these markers, that the performers, they don’t really notice it so much. Well, they haven’t complained.
365.5
But I believe that, in my experience, some people are very sensitive to changes in these things. And then this affects the quality of data that you get, of course, because now they cannot move the way they normally move. Yeah. So I guess that is important, also, to make sure that they don’t do something completely different. [LAUGHING] Yeah, and that they have some time to get used to the new position of the markers. We’ve also had some challenges in placing markers when I’ve done motion capture with eye tracking glasses.
399.5
In this case, sometimes I try to track the position of the glasses, because this allows us to kind of recalculate the position of the gaze, where the person’s looking in the capture volume. And in order to do this, you need to know the position of the glasses. And so you have to put markers very close together, attached to the glasses. Which are, again, very close to the markers that are already on the head. And when you have multiple musicians, then you have to make, like you said, distinctive arrangements for each person. So that can be a little bit challenging. [STRING INSTRUMENTS PLAYING]
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[INSTRUMENTS STOP] [APPLAUSE]

Markers have to be placed carefully on moving bodies to ensure that motion is captured effectively. How do you decide where to place your markers? What marker sizes should you use? How do you attach them to a body so that they don’t fall off?

In this video, Laura and Mari talk about placing markers to capture different kinds of motion. They discuss some of the challenges that they have had with marker placement in their research.

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Motion Capture: The Art of Studying Human Activity

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