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Visiting the Mocap lab

Alexander Refsum Jensenius and Kristian Nymoen demonstrate an infrared marker-based motion capture system in the University of Oslo.
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Now we are standing in the music-related motion capture laboratory here at the Department of Musicology. And in this lab we are studying music-related body movements with different types of technologies and methods. There are different methods when we study music-related body movement. We typically work with camera-based systems and also with sensor-based systems. And today we’re going to show you different types of these systems– how we work with them, and how we also analyse the data. First, now, we’re going to look at one system that we call an optical marker-based motion capture system. And here we have one of these cameras. We have many of these cameras around the space. And they each send out infrared light being reflected on the marker.
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Something like this that Kristian has on. So when we work with this, we typically take on a suit, as we’re going to do, and then we work in the space and record the body movement, and then we analyse it afterwards. So let’s try. So the idea, now, is that we put these reflective markers on all the body joints so that we are able to track them separately. And then from the front and the back, it’s also important to be able to differentiate the different sides. So we typically put, for example, one here, but also a symmetrical one so it’s possible to see that this is the backside of the body. This optical motion capture system is extremely sensitive to motion.
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so that means that we need to calibrate it each time we use it to really get good results. So this is where my reference system will be because I record the x, y, and z positions in space. So there are three axes that we record, and they are based in this frame. And then I have this wand with markers on it and with a very specific length, so it’s possible to measure exactly how good and precise the system is at any point in time.
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So now we are ready to start real recording. Here we can see all the images from each of the cameras, and I also have a 3D representation of the space in question. And then we are ready to record. 1, 2, 3, go.
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And as we see here in this overview, we get all the dots from all the different cameras, and then they can be combined into a 3D image like this. And the nice thing about such a system like we have here, we’re able to move around and look at the movements of the body from different angles and perspectives, and even zoom in on details.
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So now we’re looking at the recording of your movements. And if I start the playback here, we can take a look at the different markers. And the nice thing about such a motion capture system is that we’re able to rotate around and look at this from different perspectives. Another thing is that we can also turn on trajectory so we can actually look at how the movements are changing over time.
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And this also makes it easy to isolate out, say, the vertical component of movement if we’re interested in that, and also to calculate the velocity and acceleration rotation. Let’s try to do that. So we can, for example, take out one of these markers and look at a plot of the set position, which is the up and down movement of this particular marker– where you can see this repetitive pattern over time in the up and down moment. So to try to summarise then, from your experience, what are the good things about working in a lab like this, and are there any bad ones?
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Well the main advantage of working with this type of motion capture system is the high accuracy you get from the recordings. You’re able to record the markers down to a 0.1 millimetre position, which is state of the art, really. And also at very high speeds. Very high speeds. Up to 500 Hz is no problem. Even more is actually possible. The downside is really that you need a big space to be able to do it– preferably a fixed location where the motion capture system can be attached because it takes a long time to set it up, and to calibrate, and also even to put on the suit like this.
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So to try to summarise then, this type of motion capture lab is, in many ways, the best you can do when it comes to getting very, very good and accurate results, but if we want to be more flexible when we work with music-related body movement, we need to use other techniques.

Alexander Refsum Jensenius and Kristian Nymoen demonstrate an infrared marker-based motion capture system.

This lab at the University of Oslo contains an advanced system from Qualisys with 13 cameras hanging around the walls and in the ceiling. The researchers use the system for studying performers and perceivers. Since the system can run in realtime, it is also used in the development of new interfaces for musical expression.

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