Godot Billboard "2.5D" Node

FacingSide.cs

public enum FacingSide : sbyte
{
    None = 0,
    Left = -1,
    Right = 1,
}

Node25D.cs

using System.Runtime.CompilerServices;

public partial class Node25D : Node3D
{
    [Export]
    public Camera3D Camera;

    [Export]
    protected bool mirror = true;

    [Export]
    public FacingSide FacingSide { get; private set; } = FacingSide.Right;

    [ExportGroup("Billboard", "billboard")]
    [Export]
    bool billboardEnabled = true;

    [Export(PropertyHint.Range, "0,1")]
    float billboardWeight = 0.5f;

    bool lookUseModelFront = true;
    Vector3 lookModelUp = Vector3.Up;

    public void SetFacingSide(FacingSide side)
    {
        if (FacingSide == side) return;
        FacingSide = side;
        ApplyFacingSide();
    }

    public override void _Ready()
    {
        if (!IsInstanceValid(Camera))
            Camera = GetViewport().GetCamera3D();

        ApplyFacingSide();
    }

    public override void _PhysicsProcess(double delta) => LookAtCamera();

    void ApplyFacingSide()
    {
        var basis = GetBasis();

        if (FacingSide is FacingSide.Left)
        {
            basis.X = Vector3.Left;
            lookUseModelFront = false;
            lookModelUp = Vector3.Down;
        }
        else
        {
            basis.X = Vector3.Right;
            lookUseModelFront = true;
            lookModelUp = Vector3.Up;
        }

        SetBasis(basis);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    void LookAtCamera()
    {
        if (!billboardEnabled) return;
        var targetX = Mathf.Lerp(GlobalPosition.X, Camera.GlobalPosition.X, billboardWeight);
        Vector3 target = new(targetX, GlobalPosition.Y, Camera.GlobalPosition.Z);
        LookAt(target, lookModelUp, lookUseModelFront);
    }
}