Converting matrix to vectors

How do I convert an OpenGl matrix to vectors?

I am using Bullet (sweet) in my render routine fetching the matrix like so :

Code:


sio2PhysicRender( sio2->_SIO2physic, sio2->_SIO2window->d_time, 1 );         
sio2TransformBindMatrix( _myObject->_SIO2transform );
_myObject->_btRigidBody->getWorldTransform().getOpenGLMatrix( _myObject->_SIO2transform->mat );

I want to update the SIO2transform so I have a current vector based transform.

I have extracted pitch and roll angle from the matrix but matrices are hard work, vectors would be much easier.

Any ideas gratefully accepted ....



[code]

The sio2ObjectRender already update the _SIO2transform->loc for you... scale is not supported in Bullet and for the rotation you just have to convert euler to angle.

But of course SIO2 has done the hard work already. I should have known. Many thanks

But wait, I don't need location (yet) so sio2ObjectRender doesn't provide anything I don't have with my existing code. I presume I must convert from the OpenGL matrix to get the rotation angles; there is no rotation Euler available?

So there is no easy way. I have to calculate the angles ... which I've done ... but it looks ugly.

C'est la guerre

Here's the ugliness I mentioned:

Code:

// Calculate angles from matrix
   angle_y = -asinf( _myObject->_SIO2transform->mat[ 2 ] );   // Calculate Y-axis angle
   tmpc = cosf( angle_y );
   if( _myObject->_SIO2transform->mat[ 0 ] < 0 )                        // Quadrant 2 & 3
   {
       if( _myObject->_SIO2transform->mat[ 2 ] > 0 )
      angle_y = -acosf( _myObject->_SIO2transform->mat[ 2 ] ) - DEGREES90_IN_RAD;   // Quadrant 3
       else
      angle_y = acosf( - _myObject->_SIO2transform->mat[ 2 ] ) + DEGREES90_IN_RAD;   // Quadrant 2               
   }
   angle_y *= RAD_2_DEGREES;

   if( fabs( tmpc ) > 0.005 )                           // Gimball lock?
   {
   // No, so get X-axis angle
   angle_x  = atan2( - _myObject->_SIO2transform->mat[ 6 ]  / tmpc,
                _myAObject->_SIO2transform->mat[ 10 ] / tmpc ) * RAD_2_DEGREES;
               
   angle_z = atan2( - _myObject->_SIO2transform->mat[ 1 ] / tmpc,
               _myObject->_SIO2transform->mat[ 0 ] / tmpc ) * RAD_2_DEGREES;
   }
   else                                          // Gimball lock has occurred
   {
      angle_x = 0.0;                                 // Set X-axis angle to zero & calculate Z-axis angle
      angle_z = atan2( _myObject->_SIO2transform->mat[ 4 ],
               _myObject->_SIO2transform->mat[ 5 ] ) * RAD_2_DEGREES;
   }


Now if there's a better way I'd like to hear it.

_myObject->_SIO2Transform->rot->z
rot is a vec3

But _myObject->_SIO2Transform->rot vector is not updated by eg sio2ObjectRender, just _SIO2Transform->loc. I need the vector (and rotation angles) for calculating the forces and velocity on my object.

cgen2: Are you sure that the code above work? Anyway since bullet send you an un-scaled matrix I think that the following code would work:

_SIO2transform->rot->x = acosf( _SIO2transform->mat[ 0 ] ) / SIO2_PI * 180.0f;
_SIO2transform->rot->y = acosf( _SIO2transform->mat[ 5 ] ) / SIO2_PI * 180.0f;
_SIO2transform->rot->z = acosf( _SIO2transform->mat[ 10 ] ) / SIO2_PI * 180.0f;

SIO2: I was sure until I looked at the delta with your suggestion. The deltas are significant (> 0.5 degrees at some angles). I didn't calc the absolute error but my ugly complex routine is probably guilty. It certainly has more scope for error.

I ever so slightly enhanced your suggestion removing the run time divide by substituting SIO2_RAD_TO_DEG for / SIO2_PI * 180.0f.

Now I need to make this work over a full rotation, +/- 180 degrees ...

Taking the SIO2 suggestion a little further I propose to use code something like this to extract pitch, roll and yaw angles over a full +/- 180 degree range:

Code:

// Calculate object pitch, roll and yaw
            angle_x = acosf( _object->_SIO2transform->mat[ 0 ] ) * SIO2_RAD_TO_DEG;
            if ( _object->_SIO2transform->mat[ 8 ] < 0.0f )
               angle_x = - angle_x;            
            _FSobjectdescriptor->Pitch = angle_x;
            angle_y = acosf( _myobject->_SIO2transform->mat[ 5 ] ) * SIO2_RAD_TO_DEG;
            if ( _myobject->_SIO2transform->mat[ 9 ] < 0.0f )
               angle_y = - angle_y;
            _FSobjectdescriptor->Roll = angle_y;
            angle_z = acosf( _myobject->_SIO2transform->mat[ 10 ] ) * SIO2_RAD_TO_DEG;
            if ( _myobject->_SIO2transform->mat[ 6 ] < 0.0f )
               angle_z = - angle_z;            
            _FSobjectdescriptor->Yaw = angle_z;


But the yaw (z axis) is not right. Why?