Euler angle to Quaternion then Quaternion to euler angle

Question

I'm using lib glm (http://glm.g-truc.net/) for test quaternion but I've a problem; when I convert euler angle to quaternion then immediatly quaternion to euler angles, my result are totally different from my initial euler angles. Is this normal? Could it be because the rotations are not communative?

Code test:

#include <glm\quaternion.hpp>
#include <math.h>

#define PI M_PI
#define RADTODEG(x) ( (x) * 180.0 / PI )
#define DEGTORAD(x) ( (x) * PI / 180.0 )

int         main( void )
{
    float RotX = 90.f;
    float RotY = 180.f;
    float RotZ = -270.f;

    if ( RotX || RotY || RotZ )
    {
        std::cout << "Init: x= " << RotX << ", y= " << RotY << ", z= " << RotZ << "\n";
        glm::quat key_quat(glm::detail::tvec3<float>(DEGTORAD( RotX ),
                                                     DEGTORAD( RotY ),
                                                     DEGTORAD( RotZ )));
        glm::detail::tvec3<float> v = glm::eulerAngles(key_quat);

        /*  // the result is even worse with this code here
        RotX = RADTODEG(v.x);
        RotY = RADTODEG(v.y);
        RotZ = RADTODEG(v.z);
        */

        RotX = v.x;
        RotY = v.y;
        RotZ = v.z;

        std::cout << "Final: x= " << RotX << ", y= " << RotY << ", z= " << RotZ << "\n";
    }
    return (0);
}

Result:

Init: x= 90, y= 180, z= -270
Final: x= -90, y= -3.41509e-006, z= -90

thank you in advance o/

Answer 1

Yes, it is normal. There are 2 ways to represent the same rotation with Euler angles.

I personally don't like Euler angles, they mess up the stability of your app. I would avoid them. Plus, they are not very handy either.

Answer 2

If you end up needing quaternion's to Euler angles, but you need an arbitrary rotation order, I came across a site with conversion code. Sometimes the trick is just finding the right rotation order. (Btw, the orders that have the same letter twice, like XYX, are proper Euler angles, but the ones like XYZ are Tait-Bryan angles).

Here's the link: http://bediyap.com/programming/convert-quaternion-to-euler-rotations/

And here's the code:

///////////////////////////////
// Quaternion to Euler
///////////////////////////////
enum RotSeq{zyx, zyz, zxy, zxz, yxz, yxy, yzx, yzy, xyz, xyx, xzy,xzx};

void twoaxisrot(double r11, double r12, double r21, double r31, double r32, double res[]){
  res[0] = atan2( r11, r12 );
  res[1] = acos ( r21 );
  res[2] = atan2( r31, r32 );
}

void threeaxisrot(double r11, double r12, double r21, double r31, double r32, double res[]){
  res[0] = atan2( r31, r32 );
  res[1] = asin ( r21 );
  res[2] = atan2( r11, r12 );
}

void quaternion2Euler(const Quaternion& q, double res[], RotSeq rotSeq)
{
    switch(rotSeq){
    case zyx:
      threeaxisrot( 2*(q.x*q.y + q.w*q.z),
                     q.w*q.w + q.x*q.x - q.y*q.y - q.z*q.z,
                    -2*(q.x*q.z - q.w*q.y),
                     2*(q.y*q.z + q.w*q.x),
                     q.w*q.w - q.x*q.x - q.y*q.y + q.z*q.z,
                     res);
      break;

    case zyz:
      twoaxisrot( 2*(q.y*q.z - q.w*q.x),
                   2*(q.x*q.z + q.w*q.y),
                   q.w*q.w - q.x*q.x - q.y*q.y + q.z*q.z,
                   2*(q.y*q.z + q.w*q.x),
                  -2*(q.x*q.z - q.w*q.y),
                  res);
      break;

    case zxy:
      threeaxisrot( -2*(q.x*q.y - q.w*q.z),
                      q.w*q.w - q.x*q.x + q.y*q.y - q.z*q.z,
                      2*(q.y*q.z + q.w*q.x),
                     -2*(q.x*q.z - q.w*q.y),
                      q.w*q.w - q.x*q.x - q.y*q.y + q.z*q.z,
                      res);
      break;

    case zxz:
      twoaxisrot( 2*(q.x*q.z + q.w*q.y),
                  -2*(q.y*q.z - q.w*q.x),
                   q.w*q.w - q.x*q.x - q.y*q.y + q.z*q.z,
                   2*(q.x*q.z - q.w*q.y),
                   2*(q.y*q.z + q.w*q.x),
                   res);
      break;

    case yxz:
      threeaxisrot( 2*(q.x*q.z + q.w*q.y),
                     q.w*q.w - q.x*q.x - q.y*q.y + q.z*q.z,
                    -2*(q.y*q.z - q.w*q.x),
                     2*(q.x*q.y + q.w*q.z),
                     q.w*q.w - q.x*q.x + q.y*q.y - q.z*q.z,
                     res);
      break;

    case yxy:
      twoaxisrot( 2*(q.x*q.y - q.w*q.z),
                   2*(q.y*q.z + q.w*q.x),
                   q.w*q.w - q.x*q.x + q.y*q.y - q.z*q.z,
                   2*(q.x*q.y + q.w*q.z),
                  -2*(q.y*q.z - q.w*q.x),
                  res);
      break;

    case yzx:
      threeaxisrot( -2*(q.x*q.z - q.w*q.y),
                      q.w*q.w + q.x*q.x - q.y*q.y - q.z*q.z,
                      2*(q.x*q.y + q.w*q.z),
                     -2*(q.y*q.z - q.w*q.x),
                      q.w*q.w - q.x*q.x + q.y*q.y - q.z*q.z,
                      res);
      break;

    case yzy:
      twoaxisrot( 2*(q.y*q.z + q.w*q.x),
                  -2*(q.x*q.y - q.w*q.z),
                   q.w*q.w - q.x*q.x + q.y*q.y - q.z*q.z,
                   2*(q.y*q.z - q.w*q.x),
                   2*(q.x*q.y + q.w*q.z),
                   res);
      break;

    case xyz:
      threeaxisrot( -2*(q.y*q.z - q.w*q.x),
                    q.w*q.w - q.x*q.x - q.y*q.y + q.z*q.z,
                    2*(q.x*q.z + q.w*q.y),
                   -2*(q.x*q.y - q.w*q.z),
                    q.w*q.w + q.x*q.x - q.y*q.y - q.z*q.z,
                    res);
      break;

    case xyx:
      twoaxisrot( 2*(q.x*q.y + q.w*q.z),
                  -2*(q.x*q.z - q.w*q.y),
                   q.w*q.w + q.x*q.x - q.y*q.y - q.z*q.z,
                   2*(q.x*q.y - q.w*q.z),
                   2*(q.x*q.z + q.w*q.y),
                   res);
      break;

    case xzy:
      threeaxisrot( 2*(q.y*q.z + q.w*q.x),
                     q.w*q.w - q.x*q.x + q.y*q.y - q.z*q.z,
                    -2*(q.x*q.y - q.w*q.z),
                     2*(q.x*q.z + q.w*q.y),
                     q.w*q.w + q.x*q.x - q.y*q.y - q.z*q.z,
                     res);
      break;

    case xzx:
      twoaxisrot( 2*(q.x*q.z - q.w*q.y),
                   2*(q.x*q.y + q.w*q.z),
                   q.w*q.w + q.x*q.x - q.y*q.y - q.z*q.z,
                   2*(q.x*q.z + q.w*q.y),
                  -2*(q.x*q.y - q.w*q.z),
                  res);
      break;
    default:
      std::cout << "Unknown rotation sequence" << std::endl;
      break;
   }
}