1 | initial version |

I don't know if there's a built in function to do this. But the relationship between the w element of a quaternion and the angle it's rotating by is fairly simple.

```
w = cos(angle / 2)
```

therefore:

```
angle = 2 x arccos(w)
```

So you can calculate this yourself using this equation from the w element.

2 | No.2 Revision |

I don't know if there's a built in function to do this. But the relationship between the w element of a quaternion and the angle it's rotating by is fairly simple.

```
w = cos(angle / 2)
```

therefore:

```
angle = 2 x arccos(w)
```

So you can calculate this yourself using this equation from the w element.

**Update:**

This is mathematically equivalent to the more complex form shown on the Wikipedia page here. See their curves here:

angle = 2*atan2(sqrt(1-w^2), w)

Note: I've replaced `sqrt(qi^2 + qj^2 + qk^2)`

with `sqrt(1-qr^2)`

since this expression can more easily be calculated using just the w value.

ROS Answers is licensed under Creative Commons Attribution 3.0 Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.