Meant to be Seen

I think I might be getting my degrees-of-freedom tracking terminology mixed-up somewhere. I've always understood 3DOF to be (pitch, yaw, roll) and 6DOF to be (3DOF, position xyz). It didn't really seem like there was much left after that, so talk of 9DOF trackers threw me for a bit of a loop.

As a guess based on what I've been reading lately, it seems that people are talking about 6DOF as (pitch, yaw, roll, acceleration x, acceleration y, acceleration z) and 9DOF as (6DOF, position xyz). Is that the case? It seems a bit weird to me since you could derive acceleration from position and time. Doing that wouldn't give you the direction of gravity, but gravity isn't really a "degree of freedom..."

Am I missing something? Are these terms that have been muddied by marketing? Help a guy out who's just waking from a long sleep away from this stuff?

It's a silly marketing term that really means "9 sensor values".
3 axis accelerometer
3 axis gyroscope
3 axis magnetometer

In theory you could use any one of those sets to derive 6 degrees of freedom (yaw, pitch, roll, x, y, z) but in practice they each have fundamental weaknesses. When fused together the sensors offset each other's weaknesses to yield better results.

Now the next thing is going to be integrated digital compasses so maybe we will have 10 DOF sensors soon.

9DoF is kind of a misnomer, but it typically refers to combining 3 axes each of accelerometer, magnetometer, and gyroscope. The actual number of degrees of freedom for that system depends on how you define degrees of freedom, what environment you are in, and what you are using your data for. I blame SparkFun for the popularity of the term.

Similarly, 6DoF is a misnomer that often refers to 3 axes of accelerometer and gyroscope, but it can also somewhat more accurately refer to a system of tracking that resolves 3 axes of rotation as well 3 degrees of positioning in space.

Basically, all of the terms mean many things, so it is always useful to be specific when you are describing a device/algorithm/program.

Well I guess it can mean different things depending on the field or what you are using it for. 3DOF almost always refers to orientation (yaw/pitch/roll) but can use different methods to obtain that reading. Personally, I've always understood 6DOF to mean orientation and position, at least that's how I've seen it used in terms of VR. Then you have 9DOF marketed devices that really aren't 9 different degrees, it just combines different readings from multiple sensors. So in regards to VR its a little bogus, but whatever.

Okay, got it. It's good to know that I wasn't completely off my rocker... I'll just have to be specific when I'm talking about sensor capabilities and go look up the specs if someone else uses the term on me.

Thanks, everyone!

Correct term for 6DOF is Rotation and Translation tracking, at least in the programming world