The NaturalPoint cameras as well as your typical Vicon and Motion Analysis systems are what are known as Optical Motion Capture Systems. More specifically, in their more common configuration, they’re Retroreflective Optical Motion Capture Systems. Though, they can also be configured as active marker systems as well. Its just less common.
Diffuse Bounce, Reflectivity and Retroreflectivity
Wikipedia has a page on these different types of reflected light (doesn’t it always?). However, its a bit dense. I’ll summarize and provide context.
There are plenty of potential light sources in your mocap space. It can come through a window. It can come from light bulbs. It can come from the LED ring around the lense of your cameras. When light hits the surface of an object, you tend to think about it as a whole bunch of individual rays generally coming from the same direction if it comes from a single light source, and generally having the same angle (orientation). Anyhow, when the light strikes the surface, lots of different things happen to it. For example, some of the light can be absorbed. The resulting energy needs to go somewhere and can become heat, light, electricity etc. This is how most pigments work. Most of the light is usually not absorbed however. Its either reflected or refracted. A simplified explanation of refracted light, is that it passes through the object, like say, glass. Reflected light however, is what we’re more concerned with.
Simple reflection or specular reflection, is what you find in a mirror. The light ray bounces off a surface as per the law of reflectance. More important than any one ray following the law of reflectance, in a material that has high specularity, most if not all the rays follow the law and end up having a similar angle after being reflected. Hence an image as seen in a highly specular material maintains its general appearance. It doesn’t blur or distort beyond recognition. This is true of a mirror as an extreme example. Its also true of say, car paint. You can see things reflected in car paint and as such, it can be said that a significant number of light rays hitting car paint exhibit a tight specular reflection. Or you could say car paint has high specularity (not as high as a mirror).
Diffuse bounce light is another form of reflection. Diffuse bounce light is the light that you see when looking at a matte object, such as say, concrete or paper. In the case of diffuse light, the incoming rays still respect the law of reflectance. However, the material is rough enough, that its highly faceted at a microscopic level. That is to say, at any given point on the surface, its orientation or surface normal is somewhat random. So while individual rays reflect, as a whole, they scatter all over the place because the material doesn’t exhibit a single smooth uniform surface for all the rays to bounce the same direction off of. The appearance and general characteristics of such a surface can generally be predicted through Lambert’s Cosine law. Hence, why in 3d animation, we’re often applying "Lambert" shaders to objects for their diffuse component. Diffuse bounce light makes up the majority of light you see when looking at objects in our world. Anything that’s sorta matte finish, is putting out a lot more diffuse bounced light than other types of light.
Retroreflected light is light that manages to reflect directly back at the light source. Retroreflection doesn’t usually happen naturally all that much. However, it is incredibly useful for optical motion capture and safety. "Reflective" paint on the road at night, and roadsigns are examples of man made retroreflective materials used for safety. Also, those strips of "reflective" material you put on haloween costumes are good examples. Notice these materials are marketed as "reflective" when in reality its not their simple reflective characteristics that make them desirable. Its their retroreflective characteristics, a subset of reflectivity, that make them work. Marketing often isn’t concerned with being succinct. Technically, a roll of masking tape is reflective tape. Its just mostly diffuse reflection is all. And it probably wont alert anyone driving a car as to its presence.
What does this have to do with Mocap?
So, how do we use this knowledge to get our mocap cameras to see markers and nothing else? Hence making the task of tracking those markers easier? Well, its generally a matter of contrast. If you can make your markers brighter than anything else in the frame, you can adjust your exposure and threshold the image to knock everything else out of contention, leaving you with a mostly black image, with little gray and white dots that are your markers.
Its probably worth noting that this is not the only way to accomplish the task of tracking markers. Another approach would be pattern recognition. A system based on pattern recognition would probably count as an optical mocap system but doesn’t fall into the historical category of an optical system as used in the entertainment industry.
Anyhow, back to contrast. The task of making your markers brighter than everything else. Simple specular reflectivity makes some pretty bright highlights. You could theoretically conceive of a scenario where you know where your light source is and if you catch a reflection in a marker in a camera, you could solve for the marker. In reality though, this isn’t useful. Its rare that you’ll catch a reflection of a light source in a camera. You’d need way too many light sources to make it common enough to use. Its possible you could take this to an extreme and set up a colored dome and then use the color of the dome reflected in a marker to track the ray back to its source location, but again, this is speculative and the kind of setup you’d need to do is is expensive and quite disruptive on the shooting environment. Remember, one of the goals of viable mocap systems is to be able to be used in parallel with principal photography on a movie set.
Diffuse light is potentially useful. However, fact of the matter is, most things are fairly diffuse. Things that are white, or light gray are highly diffuse. A diffuse object can only put out as much light as it takes in. Its not possible to be SO much more efficient than a white piece of paper. So instead, approaches to using diffuse light to generate contrast go the other direction. You try to make everything in your environment matte black (full absorption, no diffuse bounce). That way, your markers show up bright by contrast. Again, this solution isn’t ideal. The room, the cameras, the people, everything but the markres must be matte black to get contrast this way.
As you might imagine, the solution here is retroreflection. Again, retroreflection is light that reflects back at the light source. So its super bright like specular reflection, but unlike specular reflection, its easy to pick up. You know exactly where its going, right back to the source. All you need to do is make sure your light source is also your camera lense (or close enough). This is of course, why NaturalPoint cameras and optical mocap cameras in general, tend to have LED rings around the lense. NP camera LEDs show up a dull pink when they’re active but don’t let this fool you. They are actually putting out a ton of light. Its just infrared… about 850 nanometers in wavelength. According to Jim Richardson, the CMOS sensors in the cameras are actually more responsive to visible light than IR. However, IR l
ight is usually used in mocap because a) we can’t see it, so it doesn’t distract us. b) motion picture film and video cameras already filter it out because they are mimicking our own visual response. This way, the mocap system’s lighting doesn’t interfere with human vision based imaging.
If you’ve got your light source and camera all set up to pick up retroreflective light, then all thats left to do is make sure your marker actually is retroreflective. There are typically two ways this is done by contemporary humans.
Firstly, we can use "corner reflectors." An example of a corner reflector is a bicycle reflector. Corner reflectors are made by butting three mirrors together at right angles. A bicycle reflector often has hundreds of little mirrors set up in triplets in this manner. Believe it or not, this does actually work. I have to cover up my bicycle all the time when I use cameras in my apartment. I have looked into getting a bunch of small 1" bicycle reflectors to use as markers and in some situations, they may actually be useful. Though, there are better solutions.
The second retroreflective material is whats known as 3m scotchlite. Pretty much any retroreflective material you can think of besides corner reflectors comes back to 3m and scotchlite. Even those reflective paints on the road are made with materials bought from 3m. I have a can of "reflective" spray paint from Rustoleum. They bought their materials from 3m. Scotchlite is based on glass beads and can be bought in many forms, from raw beads (sand like) to textiles to tapes to paints. Scotchlite comes in different grades and colors. Generally though, the best retroreflectivity comes from scotchlite products in which the beads have been bonded to a material by 3m, rather than bonding done by other parties. So, buying 3m tape or textile is your best bet for mocap. The material that NaturalPoint sells in their own store is actually the highest quality material I’ve come across. Markers built from that material perform better than some of the "hard" markers in their store, that clearly had the material sprayed on by a 3rd party.
You may have noticed that to this point, we’ve been talking about generating contrast on materials that are bouncing light from a separate light source. However, its possible that a marker could emit its own light. Generally, these types of markers are known as active markers. I have actually constructed active markers in the past and will probably do so again within the year. NaturalPoint actually sells wide throw 850nm LEDs in their store for this kind of application. Mocap systems by PhaseSpace also work off of active LED markers. Active markers have benefit and detriment. They often put out a lot more light than a retroreflective maker will and therefore are really easy to track. They are however, expensive, and they do require mounting electronics on your mocap talent. This can be problematic in some cases. In some cases, they heat up quite a bit, though this problem can be designed away.
Hopefully some of this has helped give an understanding of what is going on in your mocap volume. You can use this information to help get better quality captures. Throwing your cameras into grayscale mode and looking at the enivironment as the camera sees it, will let you see these concepts in action. It should also give you a better idea of how to go about optimizing your mocap environment and exposure settings for capture.