Why is my 3D cube distorting when rotating?

The problem is that when rotating my 3D cube on more than one axes, it distorts weirdly roughly halfway through. I am using the JOML math library for matrices.

// This is the model matrix for the rotation of a textured cube
Matrix4f model = new Matrix4f();
model.identity();
model.rotate((float)(glfwGetTime() * Math.toRadians(50.0f)), new Vector3f(0.5f, 1.0f, 0.0f), model);

// Other matrices for coordinate system
Matrix4f view = new Matrix4f();
view.identity();
view.translate(new Vector3f(0.0f, 0.0f, -3.0f), view);
Matrix4f projection = new Matrix4f();
projection.identity();
projection.perspective((float)Math.toRadians(45.0f), 800.0f / 600.0f, 0.1f, 100.0f); // FOV is 45

This is a gif of the distortion:

The main problem is that your rotation axis (0.5f, 1.0f, 0.0f) is not unit/normalized, (as is also required by its JavaDoc)

Parameters:

axis - the rotation axis (needs to be normalized)

To solve the problem and still use a matrix, you simply need to normalize the rotation axis.

Also (but this is irrelevant to the error):

• JOML matrices are identity by default after instantiation - you do not need to call identity() on them)
• you can omit supplying model as the argument to the dest parameter of rotate()

So:

// This is the model matrix for the rotation of a textured cube
Matrix4f model = new Matrix4f();
model.rotate((float)(glfwGetTime() * Math.toRadians(50.0f)),
             new Vector3f(0.5f, 1.0f, 0.0f).normalize());