How to get the correct Input direction based on the camera angle to use in a rootmotion based third person controller

Question

I move my character based on the rootmotion of their animation using a blend tree that uses 2D Freeform Cartesian input (Vertical, Horizontal)

if Vertical = 1, the character moves forward based on the direction he is facing

if Horizontal = 1, the character turns right based on the direction he is facing

and the rest of the system follows, here is the current Blend Tree:

Currently, if i press forward, the character move forward no matter where the camera is facing:

But in this example, what i want is for the character to slightly make a turn left to face the camera direction and then start walking forward:

When working on a game that moves the character by code (without rootmotion), something like this would solve the issue:

_inputAxis = new Vector3(Input.GetAxis("Horizontal"), Input.GetAxis("Vertical"), 0);
_inputCam = Quaternion.Euler(0, _cam.transform.eulerAngles.y, 0) * _inputAxis;

And then use _inputCam to move the character, but for some reason this isn't working, and these two variables are always equal to each other, what am i doing wrong ? also if there is a "standard" solution for this problem (am sure it is) please link it (doesn't matter if its Unity or not as long as the code is readable)

Thanks!

Answer 1

You have two steps here.

1. Convert your controller-relative input into a desired direction in the world, based on the camera perspective.

2. Convert your desired direction in the world to a steering direction, based on the object orientation.

The code you've shown is one way to do step 1.

Myself, I like to use a different version that can also handle slopes:

Vector3 CameraRelativeFlatten(Vector3 input, Vector3 localUp)
{
    Transform cam = Camera.main.transform; // You can cache this to save a search.

    Quaternion flatten = Quaternion.LookRotation(
                                        -localUp, 
                                        cam.forward
                                   )
                                    * Quaternion.Euler(-90f, 0, 0);

    return flatten * input;
}

We can use that to do step 1 like so:

// If you want to handle slopes, use the character's up vector,
// or a terrain normal you get from a raycast here instead.
Vector3 groundNormal = Vector3.up;

Vector3 controllerSpaceInput = new Vector3(Input.GetAxis("Horizontal"), 
                                           0f,
                                           Input.GetAxis("Vertical"));

Vector3 worldSpaceInput = CameraRelativeFlatten(input, groundNormal);

Now that we have an arrow in the world pointing where we want to go, we want to look at that from the character's point of view - to know whether it's pointing left or right based on which way they're facing. That's step 2:

Vector3 localSpaceInput = characterTransform.InverseTransformDirection(worldSpaceInput);

Now localSpaceInput.x contains the amount of input pointing to your character's right, and localSpaceInput.z contains the amount pointing directly ahead in their forward direction.