rotating a quaternion on 1 axis?

Question

I have a model rotated by a quaternion. I can only set the rotation, I can't add or subtract from anything. I need to get the value of an axis, and then add an angle to it (maybe a degree or radian?) and than re-add the modified quaternion. How can I do this? (answer on each axis).

Answer 1

You can multiply two quaternions together to produce a third quaternion that is the result of the two rotations. Note that quaternion multiplication is not commutative, meaning order matters (if you do this in your head a few times, you can see why).

You can produce a quaternion that represents a rotation by a given angle around a particular axis with something like this (I'm going to write it in c++ but it's about the same for Java):

Quaternion Quaternion::create_from_axis_angle(const double &xx, const double &yy, const double &zz, const double &a)

{

// Here we calculate the sin( theta / 2) once for optimizationdouble factor = sin( a / 2.0 );

// Calculate the x, y and z of the quaterniondouble x = xx * factor;

double y = yy * factor;

double z = zz * factor;

// Calcualte the w value by cos( theta / 2 )double w = cos( a / 2.0 );

return Quaternion(x, y, z, w).normalize();

}