shmodot/scripts/orbits/OrbitSystem.cs

using Godot;
using System;

public class OrbitSystem : Node, IMassive, ILocation
{
    [Export] private NodePath _a;
    [Export] private NodePath _b;
    [Export] private NodePath _barycenter;

    private readonly PointMass[] _pointMasses = new PointMass[2];

    public Vector3 Position => Barycenter;

    [Export]
    public Vector3 Barycenter
    {
        get
        {
            var p0 = _pointMasses[0].Position;
            var p1 = _pointMasses[1].Position;

            return p0.LinearInterpolate(p1, .5f);
        }
        set => _ = value;
    }

    public float Mass { get; } = 1;

    private Orbit _orbit;

    private ImmediateGeometry _orbitGeometry;

    public override void _Ready()
    {
        InitPointMasses();
        InitGeometry();
    }

    public override void _Process(float delta)
    {
        DrawOrbit();
    }

    private void InitOrbit()
    {

    }

    private void DrawOrbit()
    {
        int steps = 100;

        _orbitGeometry.Clear();
        _orbitGeometry.Begin(Mesh.PrimitiveType.LineLoop);
        _orbitGeometry.SetColor(new Color(1, 0, 0));

        var ellipse = new Ellipse(1, .5);

        for (int i = 0; i < steps; i++)
        {
            float t = i / (float)steps;
            float a = t * Mathf.Tau;

            var v2 = ellipse.Focus0 + ellipse.GetPosition(a);

            _orbitGeometry.AddVertex(new Vector3
            {
                x = (float)v2.X,
                z = (float)v2.Y
            });
        }
        _orbitGeometry.End();
    }

    private void InitGeometry()
    {
        _orbitGeometry = new ImmediateGeometry();
        var m = new SpatialMaterial();
        m.VertexColorUseAsAlbedo = true;
        m.FlagsUnshaded = true;
        _orbitGeometry.MaterialOverride = m;
        AddChild(_orbitGeometry);
    }

    private void InitPointMasses()
    {
        var a = GetPointMass(_a);
        var b = GetPointMass(_b);

        if (a.Mass > b.Mass)
        {
            _pointMasses[0] = a;
            _pointMasses[1] = b;
        }
        else
        {
            _pointMasses[0] = b;
            _pointMasses[1] = a;
        }
    }

    private PointMass GetPointMass(NodePath path)
    {
        var spatial = GetNode<Spatial>(path);
        //var massive = node.GetChild<IMassive>(0);

        return new PointMass(spatial, 1f);
    }
}