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# combine / convert rotations (Quaternion and Vector3)

Hi, i've been trying to convert / combine two rotation transforms into one to stop them overriding each other.

```
//----- tilts character to surfaces -----//
transform.up = Vector3.Lerp(transform.up, surfaceNormal, Time.deltaTime * xRotSpeed);
//----- tilts character to surfaces -----//
//----- tilts left and right ------//
transform.eulerAngles = Vector3(0, curAngle, 0);
//----- tilts left and right ------//
```

I'm trying to either multiply / scale two together, to allow an X rotation to occur based on the surfaceNormal, or somehow convert the current Vector3.Lerp to allow me to extract a value to put into the X axis of the EulerAngles, as my character only has movement on the x and y (x for turning towards surfaces and y to rotate yaw based on forward velocity)

I hope you can help this hasgot me raging alot lately, tried so many Quaternion types and Vector scales and lerps its driving me nuts!

Thanks! Renn

**Answer** by Owen-Reynolds
·
Jun 27, 2012 at 03:30 PM

This is an untested variant of a working "angled with ground, but my spin" merged with your code. The way `facingQ`

is created/used is probably the most interesting part:

```
// your lerp to smooth into normal:
Vector3 newUpV=Vector3.Lerp(transform.up,surfaceNormal,Time.deltaTime*xRotSpeed);
// convert new up into a rotation:
Quaternion newUpQ=Quaternion.FromToRotation(Vector3.up, newUpV);
// your spinning:
Quaternion facingQ=Quaternion.Euler(0,curAngle,0);
// combine head tilt and spin:
transform.rotation = newUpQ * facingQ;
```

The problem with setting `transform.up`

then `transform.eulerAngles`

is that there's no good way to say "keep the work from the last instruction." Building quaternions for each local spin lets you combine them with `*`

. In this case, `newUpQ`

is the tilt that takes from facing North, head up, to still facing north-ish, but head tilted to the target. Then `*facingQ`

adds a local y-spin.

Thanks Owen, this worked perfectly, what you said about "building quaternion for each spin" only brings another question which is what is the difference between a quaternion and a vector (is it one is x,y,z, and one is x,y,z,w?)

Its something i will have to learn properly but a quick explanation would be grand, seeing as you understand that process clearly (as it worked a charm)

Thanks again, i now know you can multiply quaternions together to allow rotations on multiple axis ^_^

The x/y/z of Quats is just book-keeping stuff, with no useful meaning. A Quaternion is a complete "rotation/facing," ready to be assigned to your rotation. You could think of a Quat as a magic wand waved over the vector you want to face, plus the extra "spin" around it (the wand part makes it count as a rotation.)

A vector *could* be a way you want to be facing. If you use `Quaternion Q=Quaternion.LookRotation(V)`

, then `Q`

*is* that facing (with your head spun "up".)

The x/y/z/w quaternion notation has no simple/obvious "real world" representation, most notably the "x/y/z" is not directly related to actual X/Y/Z axes/rotations, it's just a way of accessing the Quaternion's members. You *can* convert a Quaternion into an axis+angle representation (where x/y/z would define the rotation axis, and w the angle around that axis), but that's not how quaternions are stored in Unity, and there is no trivial conversion (it involves asin/acos and sqrt).

If you *must* know, quaternions are represented in a 4-dimensional complex space :-P (and even that isn't the mathematically sound interpretation).

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