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Convert Quaternion to Eulerangles

Hello, in my game I have to compare two y-rotation values of an object before and after an action.

Little bit of my code:

 Function Update()
 
 var firstRotation = pivotPoint.transform.localEulerAngles.y; // pivotPoint is the gameObject wich I need the y-rotation values off
 
 // after the action
 
 var secondRotation = pivotPoint.transform.localEulerAngles.y;
 
 rotationValue = firstRotation - secondRotation;

This works great except when the secondrotation is less or equal to 360. I've looked it up in the script refecence and it says : Don't increment Eulerangles, as they will fail when the angle exceeds 360 degrees. That explains the problem.

If I use the Quaternions I get 4 strange numbers. Is there any way I can convert these 4 numbers to the y-rotation value?

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asked Nov 06, 2011 at 06:46 PM

BarkShark gravatar image

BarkShark
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Look up Quaternions on the Wikipedia, it gives a detailed explanation of what those 4 numbers actually mean. Needless to say, they don't really translate that well to things you'd be used to. Quaternions are to imaginary numbers what imaginary numbers are to real numbers!

Nov 07, 2011 at 03:29 PM syclamoth
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2 answers: sort voted first

eulerAngles and localEulerAngles aren't real variables - they actually are calculated by Unity based on transform.rotation and transform.localRotation, using the method you're looking for. But there are several 3-axes combinations that result in the same quaternion, thus Unity chooses one of them when returning the eulerAngles: a 180 degrees rotation around Y may result in (180, 0, 180), for instance, thus the after-before angle difference may give very weird results. On the other hand, assigning angles to eulerAngles or localEulerAngles always works fine. If you need to know the exact angles all the time, you could do the contrary: have your own eulerAngles variable, rotate them in degrees by math and assign them modulo 360 to transform.eulerAngles - something like this version of Rotate:

var myEuler: Vector3;

function Start(){ myEuler = transform.localEulerAngles; // start with the initial rotation }

function Rotate(ax: float, ay: float, az: float){ myEuler = myEuler + Vector3(ax,ay,az); // rotate angles // assign the new angles in modulo 360: transform.localEulerAngles = Vector3(myEuler.x%360,myEuler.y%360,myEuler.z%360); } This is a local Rotate; if you use transform.eulerAngles, the rotation is world referenced. SIDENOTE: Any 3-axes rotation can be converted to one single rotation around a specific axis, and that's what a quaternion encodes in its mysterious four elements (x, y, z, w): the w component means cos(angle/2), and x, y and z are the axis direction multiplied by sin(angle/2). Unity has functions to convert a quaternion to its angle axis representation and vice versa, thus you don't need to make this calculation by hand.

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answered Nov 06, 2011 at 11:02 PM

aldonaletto gravatar image

aldonaletto
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This is an awkward aspect of Unity. The zero crossing condition for euler angles is a pain. As it stands trying to compare one objects rotation to another objects is way more code than it needs to be. Using mod 360 is helpful, but it does not help with the zero crossing issue. I thought a lot about this and an elegant approach did not come to me. If there is some code that handles general case elegantly, I would like to see it.

I coded around my issue by using a different control interface. It turned out to be way better for me this time, but the euler angle zero crossing issue is still a pain.

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answered Nov 07, 2011 at 04:50 PM

sdmodiano gravatar image

sdmodiano
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3-axes rotation is redundant: there are several, maybe infinity combinations that lead to the same result. That's why converting a quaternion back to 3-axes (Euler angles) gives so many weird results - Unity has to use some internal criteria to choose one of them. When the rotation is done around one root axis only (X, Y or Z) you may (not without pain and tears) create some correction algorithm; if the rotation affects all 3 axes, however, forget about it: the euler angles show absolutely unpredictable values! The only way is to do the inverse thing, like suggested in my answer.

Nov 07, 2011 at 10:21 PM aldonaletto
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asked: Nov 06, 2011 at 06:46 PM

Seen: 8622 times

Last Updated: Nov 07, 2011 at 10:21 PM