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# Rotating Steps in Quaternions

Hello,

I’m using a quaternion (obtained by a simple ArcBall system) to rotate an actor. Everything is working great!
(Important note: It doesn’t work very well if I convert it to Euler before applying the rotation )

But problem arises as soon as I try to make it rotate only by 10 degrees.

I tried using a FRotator:

``````

FQuat MyRotation = GetMyArcBall()->GetRotationQuat(); // My code, get the rotation quaternion
// ...
const int rotationStep = 10; // TODO: Add a setting for this
FRotator CurrentRotator = MyRotation.Rotator();

float YawMod = FMath::Fmod(CurrentRotator.Yaw, rotationStep);
if (YawMod > (rotationStep / 2))

``````

This works nicely. But if I try the exact same code for Pitch, the rotation freaks out! (with the obvious replacement, below)

``````

``````

I’m trying to fix this for a gooooood while now

From what I understood, I’d need to modify the x,y,z part of the quaternion, without touching the w part. Still, I’m having a hard time to understand how to do that properly.

Can someone give me a hand in this?

Thank you very much in advance!

I don’t understand why you’re creating an FRotator only to pass it to the FQuat constructor, just use directly a quaternion.
You can declare a quaternion like this:

``````
// World space

// Actor space, GetActorUpVector() if derives from AActor

``````

For pitch rotation:

``````
// World space

// Actor space

``````

This was my previous code:

``````

``````

With so many different attempts to find a solution, my code changed a lot and ended up with a FRotator.

Still, it works with Yaw, but in Pitch, it keeps “flickering” around.
I just tried with GetMyArcBall()->GetActorRightVector(), but didn’t work either

Are you trying to create a kind of angle snapping system?

Exactly

Well, it depends on what kind of rotation you want to perform. A very simple method is applying rotation and then round to the nearest 10 each component of the FRotator, something like:

``````
void RSnap(FRotator &OutRotation, float SnapValue = 10.f)
{
const float Pitch = round(OutRotation.Pitch / SnapValue) * SnapValue;
const float Yaw = round(OutRotation.Yaw / SnapValue) * SnapValue;
const float Roll = round(OutRotation.Roll / SnapValue) * SnapValue;

// I don't really remember components' order in constructor
OutRotation = FRotator(Pitch, Yaw, Roll).GetNormalized();
}

``````

But I’m not sure it’s what you want. Try it.

That was what I initially tried, and refined a bit to get to the code I pasted in the original topic. While it seems it will work, when it gets close to 0/90/180/270 degrees, the rotation gets wonky and starts to look bad.

Before, I converted the angle to euler and did all the math, it was great, but had singularity issues (i.e. gimbal lock, objects flipping, etc.), thus I converted the rotation to quaternion.
It works great, but I can’t find a way to apply rotation steps // angle snapping directly to quaternions.
All the attempts I did using euler angles did have problems

I’m hoping someone can aid me with this (dense) math required to manipulate it directly in quaternions

Thank you for your attempts! Hopefully you can help me reach a solution

I’m bumping this topic in hopes more people can help

``````
// set the direction of A to direction of B.
// rotator
FRotator UWatsUp::QuatRotation(UArrowComponent *Obj, FRotator RotA, FRotator RotB) {
FQuat QuatA = RotA.Quaternion();
FQuat QuatB = RotB.Quaternion();
FQuat FinalQuat = QuatA.Inverse() * QuatB;
FinalQuat = QuatA * FinalQuat;
//	Obj->SetRelativeRotation(FinalQuat);	// works fine
return FinalQuat.Rotator();			// works fine
}
//Directional Unit Vector
FRotator UWatsUp::QuatUnitVec(UArrowComponent *Obj, FVector VecA, FVector VecB) {
FQuat QuatA = VecA.ToOrientationQuat();
FQuat QuatB = VecB.ToOrientationQuat();
FQuat FinalQuat = QuatA.Inverse() * QuatB;
FinalQuat = QuatA * FinalQuat;
//	Obj->SetRelativeRotation(FinalQuat);
return FinalQuat.Rotator();
}

``````
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