Part 3 in our camera review series. Click below to read our posts on other systems:
The next multi-camera system in this review series is produced by NaturalPoint and is called the OptiTrack Prime Color camera.
Components: A typical system consists of 8 prime color cameras and lenses, a high-bandwidth power over ethernet switch, a 10 G Nic, and a high-power computer.
Setup: Like most high-speed cameras, a single ethernet cord goes from each camera directly to the switch. The switch is then connected to the computer’s dedicated NIC via a high-bandwidth cord (cat 6a). So, in other words, the cameras all need to be connected to the switch and they are not daisy-chained, and these cords provide both data transfer and power to the cameras. From an accessory perspective, NaturalPoint sells a few different lens options, all with fixed focal lengths, which for the most part will be appropriate for most volumes. However, if this is not the case, the standard mounting brackets on the cameras allow for other lenses to be used.
Calibration: For this system, the calibration is performed by Theia and not within the collection software. Similar to the Sony system, it requires a checkerboard that Theia ships with the system. Since there are multiple lens options, there is an additional lens calibration step that needs to be done once when you receive the cameras. After this, only the camera positions need to be computed when the cameras are moved, and this is performed using the same checkerboard.
This seems like the most confusing part of the process, so I want to spend a bit of time here to describe it. Once the cameras are fixed in their location, the checkerboard is waved in the volume showing groupings of cameras on the board. We use this information to determine the camera positions relative to one another. At the end (or beginning) of this trial, we suggest placing the board on the ground, which makes the final, absolute global reference frame (where the blue corners intersect). For this frame, if you are collecting with force plates, it’s important that the board is positioned on a force plate in an accurate way, because this will be the basis for determining the force plate location (in Visual3D, you will subtract the board border and thickness and use this to define the force plate corners). If this final position is important to you, we also recommend that at least 3 cameras are positioned to see the board when it is placed horizontally. This may sound obvious, but in a lot of setups the camera view is parallel to the floor, so it isn’t usually possible to see the floor very well. It sounds kind of confusing, but after running a couple calibrations it will become second nature. More details on this calibration can be found here.
Benefits: The spec sheet on these cameras really says it all. They can collect full 1080P at 250 FPS, and 540P at 550 FPS. These types of capture rates are really exceptional. So for fast movements, they are really an excellent choice.
To continue on this point, NaturalPoint also provides compatible eStrobes that light the capture volume at the same moment that a particular frame is collected. These can improve the lighting and allow very subtle movements to be measured during fast actions.
Need to know: The one predominant challenge with this system is the post collection conversion. Unfortunately the raw video collected from Motive is not compatible with Theia, so an additional, time-consuming conversion step is needed to get the data in a format that Theia3D can read. On the bright side, it’s all automatic. On the less so bright side, it can add a significant amount of time, and an additional step, to the processing workflow.
Another less important caveat is that the cameras are white balanced to quite a bright light (4000K), so, sometimes the video can appear to be a little bit darker or appear slightly discolored. Though this does not seem to affect our algorithms, it may affect the type of lighting selection you make for other purposes.
Overall, the OptiTrack Prime Color video cameras are a go to camera for activities that require a large resolution with a high capture rate. Furthermore, they may be the only option if needing to go above 500 FPS, which is sometimes required for high velocity motions.