Oolite Bulletins

Makara wrote:

Ahhhh - so you are saying that the original quaternion (1 0 0 0) was a rotation about an undefined axis.

No, (1 0 0 0) is simply the normal, unrotated quaternion.

Commander McLane wrote:

Makara wrote:

Ahhhh - so you are saying that the original quaternion (1 0 0 0) was a rotation about an undefined axis.

No, (1 0 0 0) is simply the normal, unrotated quaternion.

To be specific: given an axis [x, y, z] and a twist angle α, the quaternion is (cos α, x sin α, y sin α, z sin α). Not rotating an object is equivalent to rotating it 0 ° about an arbitrary axis. cos 0 ° is 1, and sin 0 ° is 0, so the identity rotation is (1, 0, 0, 0) regardless of what axis you’re rotating 0 ° about. In other words, it is a rotation about an undefined axis, but it’s also explicitly a rotation of 0 °, so the axis doesn’t matter.

Oolite normalizes all quaternions it reads in, which means scaling all its components by the same value has no effect. (1, 0, 0, 0) is therefore equivalent to (47, 0, 0, 0) or (0.1, 0, 0, 0).

The obvious question is why you’d use such a brain-bending representation. I’m glad you asked!

• Rotating a vector by a quaternion only requires multiplication and addition/subtraction, since the trig functions are already baked in.
• Normalizing a quaternion only requires multiplication, addition and one square root.
• Every normalized quaternion represents a rotation, and every rotation is represented by exactly one normalized quaternion. Quaternions which can’t be normalized, such as (0, 0, 0, 0), can reasonably be treated as the identity rotation.

The quaternion page on EliteWiki provides examples, and quaternions and spacial rotation on Wikipedia explains the maths (or, alternatively, makes your brain dribble out of your ears).

Ahruman wrote:

Oolite normalizes all quaternions it reads in, which means scaling all its components by the same value has no effect. (1, 0, 0, 0) is therefore equivalent to (47, 0, 0, 0) or (0.1, 0, 0, 0).

To ask a somewhat tangential side question then, how does the rotational velocity key for subentities work? That's defined as a quaternion, so is the rotation speed there fixed, or relating to the pitch/roll/yaw settings for the sub-entity, or something else?

Thargoid wrote:

To ask a somewhat tangential side question then, how does the rotational velocity key for subentities work? That's defined as a quaternion, so is the rotation speed there fixed, or relating to the pitch/roll/yaw settings for the sub-entity, or something else?

The rotational velocity key is rotation per second.

For instance, if you want to rotate once per 20 seconds around the Z axis:
α = 360 ° / 20 = 18 °
[x, y, z] = [0, 0, 1]
sin α ≈ 0.30902
cos α ≈ 0.95106
Q = (0.95106, 0, 0, 0.30902)

Given the normalization step, this can equally be written as (3.07768, 0, 0, 1), where 3.07768 = 0.95106 / 0.30902. In fact, in this form, the first number is the cotangent (1/tangent) of α, and the other three numbers are a unit vector. This works with any unit vector. How convenient. :-)

In practical terms, you could thus use 3 0 0 1 as the rotation parameters, and fiddle with the 3 until it looks right (higher values mean slower rotations).

Note: because this uses an orientation representation to represent rotation per second, it’s not possible to specify rotations faster than one rps. On the other hand, simply rotating a model isn’t a very good way to represent a propeller, because of temporal aliasing.

Thargoid wrote:

Happily I can confirm that if I make the 0 0 0 0 to 1 0 0 0 change on the cmounts using my jump gates, the Caddie Omega works fine with them rather than just exploding like it did before.

Does it also work after several times of loading oolite / flying around without flushing the cache in between?

That's where my problem with the said fix is: I can fly through fuel stations, but after some time I cannot until I flush the cache again

Screet

Thargoid wrote:

Happily I can confirm that if I make the 0 0 0 0 to 1 0 0 0 change on the cmounts using my jump gates, the Caddie Omega works fine with them rather than just exploding like it did before.

This is under 1.73, and also under 1.74 v2727 (which does the modification itself on the fly).

Eric did some interesting testing work on this problem (in this thread) and came up with some useful results. The popping problem still exists even without the cmounts being present - although they do seem to guarantee an explosion.

btw - yours aren't the only caddy-killers in the Ooniverse, Thargoid, just the most widespread

Greetings,

Anyone able to post a working link for this OXP please, i would very much appreciate it.

Hi Cobramac, and welcome to 'the friendliest forum this side of Riedquat'.

I think that the link will be restored in the next few days... ClymAngus had some problems with his hosting company, which he said will be sorted by the end of the month.

Thanks for the quick reply and the warm welcome

I thought his site has been down for some time and may take some further time to establish at the new hosts, and in light of my eager anticipation of adding the OXP, was hoping someone would be kind enough to provide an interim link if at all possible

Thanks again for the warm welcome !

Thank you very much for hosting the oxp Thargoid, much appreciated