I have a system where an object rotates about its centre of mass however its centre of mass can change.
How do I maintain the direction (ie it doesn’t change its angle of rotation) when I change the centre of mass, most helpful solution would be just involving a change in the angle of rotation.
The answer is going to depend on exactly how you’re implementing everything. I don’t see why changing the center of mass would automatically change the rotation. Could you put together a small runnable example that shows what you’re talking about?
A pure rotation always rotates around the origin. Meaning, that the center of rotation is always (0, 0, 0) of whatever local coordinate system the things you are rotating are defined in. Just like when you scale an object, the center of that scaling is also always (0, 0, 0).
In order to rotate an object around another center of rotation other than the origin (0, 0, 0), you gonna have to transform (i.e. translate) your desired center of rotation to that origin first, do the rotation, and then translate back.
So, no matter how you do it exactly, (e.g. a sequence of glRotatef() calls, encoding those rotations in a matrix or in a quaternion or in an axis-angle, or hell, even if you use 2D and Java 2D’s AffineTransform, it is exactly the same:
T^-1 x R x T
where R is your rotation and T is the translation to bring your desired center of rotation to the origin. Likewise, T^-1 denotes the “inverse” translation of T.