How do I add an "offset" to LookAt?

Question

I have created a Third Person Controller. The camera is behind the player:

I would like to make it so that the player aims at the mouse pointer position.

To do that, I use the following code to rotate the chest towards the position:

        var mousePos = Input.mousePosition;
        mousePos.z = 10; // Make sure to add some "depth" to the screen point 
        var aim = Camera.main.ScreenToWorldPoint(mousePos);
        Chest.LookAt(aim);

The hands do not perfectly straightly point forwards, so I need to add an "offset" to the LookAt.

I tried this approach:

var mousePos = Input.mousePosition;
mousePos.z = 10; // Make sure to add some "depth" to the screen point 
var aim = Camera.main.ScreenToWorldPoint(mousePos);
aim += new Vector3(ChestAimCenterOffsetForLookatBecauseChestIsNotStraightForward, 0, 0);

Chest.LookAt(aim);

Of course I do all rotation patching in LateUpdate(). In Update(), it would simply be erased by the animation itself.

It does work, but when the model is rotated around the Y axis, something goes wrong: The offset is "wrong", it doesn't have the same effect.

This surprises me because all I do is rotate the entire model.

So I think this line is not correct:

aim += new Vector3(ChestAimCenterOffsetForLookatBecauseChestIsNotStraightForward, 0, 0);

Is this not the way to offset the LookAt results?

Answer 1

If an easy-to-read way to offset the LookAt is required, then the following will work:

Chest.LookAt(aim);
Chest.rotation *= Quaternion.Euler(ChestAimCenterOffsetBecauseHandsDontPointStraightForward.x, ChestAimCenterOffsetBecauseHandsDontPointStraightForward.y, ChestAimCenterOffsetBecauseHandsDontPointStraightForward.z);

According to DMGregory's lesson however this is a more elegant and thus preferred solution with only 1 setter:

Quaternion lookAt = Quaternion.LookRotation(aim - Chest.position);
Quaternion correction = Quaternion.Euler(ChestAimCenterOffset);

Chest.rotation = lookAt * correction;

ps: I'm now using a Vector3 instead because I noticed that the up-down also needs an offset.