Switch-Toolbox/Switch_Toolbox_Library/OpenGL/OpenGLHelper.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using OpenTK;
using OpenTK.Graphics.OpenGL;
using System.Drawing;

namespace Toolbox.Library
{
    public class OpenGLHelper
    {
        public static Point convertScreenToWorldCoords(int x, int y, bool isTopDown = false)
        {
            int[] viewport = new int[4];
            Matrix4 modelViewMatrix, projectionMatrix;
            GL.GetFloat(GetPName.ModelviewMatrix, out modelViewMatrix);
            GL.GetFloat(GetPName.ProjectionMatrix, out projectionMatrix);
            GL.GetInteger(GetPName.Viewport, viewport);
            Vector2 mouse;
            mouse.X = x;
            mouse.Y = y;
            Vector4 vector = UnProject(ref projectionMatrix, modelViewMatrix, new Size(viewport[2], viewport[3]), mouse, isTopDown);
            Point coords = new Point((int)vector.X, (int)vector.Y);

            return coords;
        }

        public static Vector4 UnProject(ref Matrix4 projection, Matrix4 view, Size viewport, Vector2 mouse, bool isTopDown)
        {
            Vector4 vec;

            vec.X = (2.0f * mouse.X / (float)viewport.Width - 1);
            vec.Y = -(2.0f * mouse.Y / (float)viewport.Height - 1);
            vec.Z = 0;
            vec.W = 1.0f;

            if (isTopDown)
            {
                vec.Y = 0;
                vec.Z = -(2.0f * mouse.Y / (float)viewport.Height - 1); 
            }

            Matrix4 viewInv = Matrix4.Invert(view);
            Matrix4 projInv = Matrix4.Invert(projection);

            Vector4.Transform(ref vec, ref projInv, out vec);
            Vector4.Transform(ref vec, ref viewInv, out vec);

            if (vec.W > float.Epsilon || vec.W < float.Epsilon)
            {
                vec.X /= vec.W;
                vec.Y /= vec.W;
                vec.Z /= vec.W;
            }

            return vec;
        }
    }
}