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Calibration demonstration

Calibration allows an infrared motion capture system to establish the distances between cameras and the distance between cameras and the floor.
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So now we’re going to talk about the calibration of the system. So what do we actually do when we calibrate? Calibration is a process of allowing the system to get an idea of first, how the cameras are situated relative to each other, and then, how the cameras are situated relative to the floor. And so this is done through kind of a process of reverse triangulation compared to what happens during the actual take. So we use the calibration wands. And the system knows the distance between these two markers, and so it’s able to use its knowledge of that distance to triangulate the position of all of the cameras relative to each other.
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And this ground plane tells the system where the ground is so it can figure out how the cameras are situated relative to this plane. And it also have another function for the system because it’s creating the virtual coordinate system. So this is actually what we are recording when we are recording. Where is my marker?
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So when we are doing recordings, what we actually get is numbers for each marker. And these numbers, they relate to this coordinate system that we just made. So here are the axis. So you have like, one dimension, the first dimension, the second dimension, and the third dimension up here. And then the system measure where this marker is and in millimetres relative to this axis. Very precisely. Yeah, very precisely. So to take the process of calibration, the first thing you want to do is to clear the space of any other possible markers. Yes, so I have to remove this [LAUGHS] and hide it so it can’t be seen by any of the cameras.
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The reason for this is that any other markers or reflective spots that are visible during the calibration process will cause an interference, and usually the system will complain that it can see other markers and you need to move them before you can calibrate. Do you want to demonstrate your technique? [LAUGHTER] So we also set the calibration space. So it’s important that you move about because if I just stand here, then I will actually occlude these markers– or I mean, occlude that camera. So it’s important that I move around.
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And also make sure if the recording will also happen quite high up, that’s important. The floor, you can make a dance if you want.
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And so as she’s doing this, the cameras are constantly collecting samples at a very– well, at the frame rate that you’re intending to capture at. And so she’s moving around and trying to show the wand to all of the cameras in as much of their visual field as she can.
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And if you need a large space, you can also set the calibration time. It can have a longer calibration time. [VOCALISING] [LAUGHS] When I’m capturing musicians, usually I’m quite careful about the orientation that I use when I place this. For instance, if I’m capturing pianists who are moving quite a lot back and forth and forwards and backwards, then I would set this axis so that it’s parallel to the direction where I expect them to move so that it’s easy to isolate that movement afterwards.
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And also because I sometimes need quite large volume, I sometimes extend– so it’s often like a default setting or duration of a calibration, and I quite often extend that a bit so I have some more time to get a really good calibration and to get the whole volume good calibrated. And I guess for the things that I do, I also find it very important to calibrate a much larger area than I anticipate using. In the past, I’ve had difficult situations when there was a lot of musicians.
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For instance, with a string quartet where we had a kind of concert situation and there was not a whole lot of space around them, we had a bit of an issue where some of them were very much at the edge of the capture space of the calibrated area. And then you end up getting a lot of noise and losing some of the markers so– Yeah, because there’s more noise like in the– –towards the edge of the calibrated area. But what do you do when you– when you are recording pianists? So do you remove the instrument before calibrating? Yeah. I would take everything out of the area.
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If you have the piano, that’s if you’re– people are going to play piano and you have the piano sitting here, then that’s space that you cannot calibrate. And it’s especially important if you are expecting that the people are going to be very near to that space. For instance if you’re tracking their hands, and then you’re not going to calibrate the space where their hands are going to be, then that’s pretty dangerous. So I would take everything away and bring them back after everything’s calibrated. Yeah. And then I guess you also get not so many– that you don’t have to mask so much because there’s not so many things that– Yeah, actually pianos are really shiny, especially grand pianos.
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And so you would end up marking– actually masking a lot of the space where the people are expected to move, especially on the keys. The keys are very shiny. So then you would be disqualifying some of the space that you want to capture in. Yeah. And do you have these problems as well with the dancers and the violinists Not so much with the dancers but sometimes with the instruments. But yeah, I also get rid of everything before I calibrate. And chairs and– yeah. And do you have any problems with calibrating when you have these kind of setups where the cameras are focused on different locations? No.
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I just have to– you have to remember that when you calibrate so you really get a good calibration of the space that you were going to use. Yeah. I found it very good with the OptiTrack system. Because while you’re calibrating, then you can see for each camera individually how many samples have been taken and which part of the visible space for each camera has been calibrated. But not all systems give you this kind of real-time feedback. Yeah, true. Yeah, you’re really painting with a– with a wand. Yeah.
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That’s good. That’s it. Right? [LAUGHTER] That’s all we need to say about that.

Calibration allows an infrared motion capture system to establish the distances between cameras and the distance between cameras and the floor. It also establishes the origin of the capture volume.

In this video, Mari and Laura explain why calibration is important and how it is carried out. They also answer these questions:

  • How do you calibrate a motion capture system?
  • What are the wand and ground plane tools used for?
  • How are the X, Y and Z axes established?
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