Results 1 to 2 of 2

Math Help - How to find rotation angle given output of cos?

  1. #1
    Junior Member
    Joined
    Feb 2010
    Posts
    30

    How to find rotation angle given output of cos?

    I have a unit quaternion q to represent a rotation around an axis. q = (cos(rotation-angle/2), unit vector of rotation axis)

    My q has come out to be (-1/sqrt(6), -1/sqrt(6)i, 1/sqrt(2)j, -1/sqrt(6)k).

    I don't know how to find the angle passed into cosine given that the output of cosine is -1/sqrt(6).

    How would I use arccos to solve this? If I only had x and y I would think to just use the i or j from the vector as the opposite or adjacent...but how do I extend that to work with 1/sqrt(6) in the z direction?

    Is there a different and/or easier way than using arccos?

    Thanks for any help.
    Last edited by SterlingM; September 9th 2012 at 06:09 PM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Junior Member
    Joined
    Feb 2010
    Posts
    30

    Re: How to find rotation angle given output of cos?

    Ugh okay its just arccos (-1/sqrt(6))...thats all there is to get the angle passed in...sorry to waste time..
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 0
    Last Post: January 14th 2012, 09:02 PM
  2. Replies: 3
    Last Post: May 29th 2011, 07:48 AM
  3. Angle from 2x2 rotation matrix
    Posted in the Advanced Algebra Forum
    Replies: 5
    Last Post: September 27th 2010, 07:22 PM
  4. Up vector and angle to rotation matrix
    Posted in the Geometry Forum
    Replies: 0
    Last Post: March 16th 2010, 07:21 AM
  5. Angle of rotation in a matrix
    Posted in the Advanced Algebra Forum
    Replies: 4
    Last Post: February 6th 2009, 02:23 AM

Search Tags


/mathhelpforum @mathhelpforum