In Unity, rotating character spine keep going back to original rotation

Question

this is the code for rotating spine in my character model:

Transform spine;
float horzMovement;

void Awake() {
    spine = transform.Find("Character/Healthmale/Root/pelvis/spine_01");
}

void Update() {
    horzMovement = Input.GetAxis("Mouse Y");
}

void LateUpdate() {
    spine.localRotation = Quaternion.Euler(
        spine.localEulerAngles.x + horzMovement,
        spine.localEulerAngles.y,
        spine.localEulerAngles.z
    );

    print(horzMovement + " / " + spine.localRotation.eulerAngles.x + "    " + Time.time);
}

Code is quite simple, I'm just rotating "X" axis from y mouse movement. The problem is when running this code, it seems work but it keeps rotating back to the original rotation.

This is the log that I printed into the console. Note that spine's original x rotation is about 27.62928.

0 / 27.69298    4.510662
0 / 27.69298    4.527147
-0.1 / 16.33139    4.543715    // minus movement detected
-0.1 / 16.3314    4.560284
-0.1 / 16.33139    4.57685
-0.1 / 16.33139    4.593714
-0.1 / 16.3314    4.609998
-0.1 / 16.33139    4.626548
-0.1 / 16.33139    4.643115    // but it seems not much changed, why?
-0.15 / 10.51222    4.659858
-0.15 / 10.51222    4.676248
-0.15 / 10.51222    4.692952
0 / 27.69298    4.759083       // this is when I stopped the mouse. It's reset.
0 / 27.69298    4.775648

As you see the above log, first time, it prints 27.692.. and it's original x rotation of spine.

Next, I moved my mouse so the "horzMovement" value was updated to minus, and it updates x localRotation also.

But when I stopped move mouse, it just return to initial x rotation 27.69298!

There is no code or functionality to "rotating back" to original rotation of spine. I don't know why this is happening. Wham am I missing?

Any advice will very appreciate it!!

Answer 1

I solved this problem by follow DMGregory's saying: "Accumulate the desired rotation yourself rather than counting on it persisting frame to frame unchanged after animation".

This is the code:

Transform spine;
float horzMovement;
float prevSpineX;

void Awake() {
    spine = transform.Find("Character/Healthmale/Root/pelvis/spine_01/spine_02/spine_03");
    prevSpineX = spine.localEulerAngles.x;    // Save initial X rotation of spine
}

void Update() {
    horzMovement = -Input.GetAxis("Mouse Y");    // Invert direction so that aiming down results look down!
}

void LateUpdate() {
    spine.localRotation = Quaternion.Euler(
        prevSpineX + horzMovement;    // Calculate new x rotation by adding mouse movement and previous spine's X rotation
        spine.localEulerAngles.y,
        spine.localEulerAngles.z
    );

    prevSpineX = spine.localEulerAngles.x;    // Update previous x rotation to current rotation
}

I hope answer might help your problem. Special thanks for DMGregory who gaves me a good advice!