Rotate Camera With Quaternions

Question

I am attempting to make a camera that can rotate when arrow keys are pressed, however, when I turn the camera and attempt to rotate it, it does not rotate around the correct axis.

For example, my up vector is ( 0, 1, 0 ). I can rotate my camera left and right perfectly because this vector is constant. Where my camera fails to work is when I attempt to rotate around its right axis. If I turn my camera left or right and rotate around the right axis it often treats the axis in a strange direction that is not correct. This should not be the case as I can still move my camera right perfectly, it just does not rotate correctly.

Below is all the relevant code. I'm not sure why rotation works around the y-axis and not any others.

// m_rotation is a Quaternion and rotation is a 4x4 view matrix

// Rotate around y-axis when left arrow pressed
// This works perfectly
if ( Input::IsKeyDown( Input::KEY_LEFT_ARROW ) ) {
    Rotate( Vector3<float>( 0, -1, 0 ), m_sensitivity );
}

// When down arrow is pressed, rotate around right axis
// This rotates around strange axis
if ( Input::IsKeyDown( Input::KEY_DOWN_ARROW ) ) {
    Rotate( m_rotation.GetRight( rotation ), m_sensitivity );
}

// Rotate methods
void Camera::Rotate( const Vector3<float> &axis, float angle ) {
    Rotate( Quaternion( axis, angle ) );
}

void Camera::Rotate( const Quaternion &quaternion ) {
    m_rotation = Quaternion( ( quaternion * m_rotation ).Normalized() );
}

// Quaternion code
Quaternion( const Vector3<float> &vect, float angle ) {
    float sinAngle = sinf( angle / 2.0f );
    float cosAngle = cosf( angle / 2.0f );

    ( *this )[ 0 ] = vect[ 0 ] * sinAngle;
    ( *this )[ 1 ] = vect[ 1 ] * sinAngle;
    ( *this )[ 2 ] = vect[ 2 ] * sinAngle;
    ( *this )[ 3 ] = cosAngle;
}

inline Quaternion operator*( const Quaternion &quat ) const {
    Quaternion ret;

    ret[ 3 ] = ( ( *this )[ 3 ] * quat[ 3 ] ) - ( ( *this )[ 0 ] * quat[ 0 ] ) - ( ( *this )[ 1 ] * quat[ 1 ] ) - ( ( *this )[ 2 ] * quat[ 2 ] );
    ret[ 0 ] = ( ( *this )[ 3 ] * quat[ 0 ] ) + ( ( *this )[ 0 ] * quat[ 3 ] ) + ( ( *this )[ 1 ] * quat[ 2 ] ) - ( ( *this )[ 2 ] * quat[ 1 ] );
    ret[ 1 ] = ( ( *this )[ 3 ] * quat[ 1 ] ) + ( ( *this )[ 1 ] * quat[ 3 ] ) + ( ( *this )[ 2 ] * quat[ 0 ] ) - ( ( *this )[ 0 ] * quat[ 2 ] );
    ret[ 2 ] = ( ( *this )[ 3 ] * quat[ 2 ] ) + ( ( *this )[ 2 ] * quat[ 3 ] ) + ( ( *this )[ 0 ] * quat[ 1 ] ) - ( ( *this )[ 1 ] * quat[ 0 ] );

    return ret;
}

inline Vector3<float> GetRight( const Matrix4<float> &viewMatrix ) const {
    return Vector3<float>( rotation[ 0 ][ 0 ], rotation[ 1 ][ 0 ], rotation[ 2 ][ 0 ] );
}

Any help or advice is greatly appreciated. Thanks.

Answer 1

After a few days of trying all sorts of different things I found the problem. I forgot that Quaternions are non-commutative when multiplied. This was causing the camera to rotate in weird ways. So to fix the problem all I had to do was change this code.

void Camera::Rotate( const Quaternion &quaternion ) {
     m_rotation = Quaternion( ( quaternion * m_rotation ).Normalized() );
} 

To this...

void Camera::Rotate( const Quaternion &quaternion ) {
     m_rotation = Quaternion( ( m_rotation * quaternion ).Normalized() );
} 

I wanted to post this in hopes it helps someone in the future.