Picking without colliders

[Idomeneas1970]

This has come up many times on discord. I'm relaying some of the posts I made there and Nicogos's on github, so it can be found easier and be discussed if needs be.

This code gets a Ray object based on screen Position. You can use it when you capture mouse clicks like this:

                     ...
  foreach (var pointerEvent in Input.PointerEvents)
                       {
                               CameraComponent camera = Game.Services.GetService<SceneSystem>().GraphicsCompositor.Cameras[0].Camera;
                               Ray ray = GetPickRay(pointerEvent.Position, camera);
                             ...

public static Ray GetPickRay(Vector2 screenPos, CameraComponent camera)
        {
            Matrix invViewProj = Matrix.Invert(camera.ViewProjectionMatrix);

            // Reconstruct the projection-space position in the (-1, +1) range.
            //    Don't forget that Y is down in screen coordinates, but up in projection space
            Vector3 sPos;
            sPos.X = screenPos.X * 2f - 1f;
            sPos.Y = 1f - screenPos.Y * 2f;

            // Compute the near (start) point for the raycast
            // It's assumed to have the same projection space (x,y) coordinates and z = 0 (lying on the near plane)
            // We need to unproject it to world space
            sPos.Z = 0f;
            var vectorNear = Vector3.Transform(sPos, invViewProj);
            vectorNear /= vectorNear.W;

            // Compute the far (end) point for the raycast
            // It's assumed to have the same projection space (x,y) coordinates and z = 1 (lying on the far plane)
            // We need to unproject it to world space
            sPos.Z = 1f;
            var vectorFar = Vector3.Transform(sPos, invViewProj);
            vectorFar /= vectorFar.W;
            Vector3 dir = vectorFar.XYZ() - vectorNear.XYZ();
            dir.Normalize();
            Ray ray=new Ray(vectorNear.XYZ(),dir);
            return ray;
        }

Now if you have a mesh and you know its points you can use this below to get the hit; I use it for my terrain editor, might be slower but it guarantees precise hits:

public static bool IntersectsRay(this Heightmap heightmap, 
     Ray ray, out Vector3 point,float m_QuadSideWidthX=1.0f,
     float m_QuadSideWidthZ = 1.0f)
 {
     //point = ray.Position;
     //check each quad
     int quadnumx = (heightmap.Size.X - 1), quadnumz = (heightmap.Size.Y - 1);
     BoundingSphere sphere = new BoundingSphere(Vector3.Zero, 1.5f);
     int x, z;
     float mindist = 1000000000.0f;
     point = Vector3.Zero;
     bool foundit = false;
     for (z = 0; z < heightmap.Size.Y; z++)
     {
         for (x = 0; x < heightmap.Size.X; x++)
         {
             sphere.Center = new Vector3(x* m_QuadSideWidthX, 
                 heightmap.GetHeightAt(x, z), z* m_QuadSideWidthZ);
             if (sphere.Intersects(ref ray, out Vector3 pt))
             {          
                 //get nearest hit
                 float dist = Vector3.Distance(pt, ray.Position);
                 if (dist < mindist)
                 {
                     mindist = dist;
                     point = pt;
                     foundit = true;
                 }
                 //return true;//gets the first hit, replace out Vector3 pt with out point and comment the above
             }
         }
     }            
     return foundit;
 }

The corresponding code when you have colliders is below (this is by Nicogo).

public static HitResult ScreenPositionToWorldPositionRaycast(Vector2 screenPos, CameraComponent camera, Simulation simulation)
 {
     Matrix invViewProj = Matrix.Invert(camera.ViewProjectionMatrix);

     // Reconstruct the projection-space position in the (-1, +1) range.
     //    Don't forget that Y is down in screen coordinates, but up in projection space
     Vector3 sPos;
     sPos.X = screenPos.X * 2f - 1f;
     sPos.Y = 1f - screenPos.Y * 2f;

     // Compute the near (start) point for the raycast
     // It's assumed to have the same projection space (x,y) coordinates and z = 0 (lying on the near plane)
     // We need to unproject it to world space
     sPos.Z = 0f;
     var vectorNear = Vector3.Transform(sPos, invViewProj);
     vectorNear /= vectorNear.W;

     // Compute the far (end) point for the raycast
     // It's assumed to have the same projection space (x,y) coordinates and z = 1 (lying on the far plane)
     // We need to unproject it to world space
     sPos.Z = 1f;
     var vectorFar = Vector3.Transform(sPos, invViewProj);
     vectorFar /= vectorFar.W;

     // Raycast from the point on the near plane to the point on the far plane and get the collision result
     return simulation.Raycast(vectorNear.XYZ(), vectorFar.XYZ());
 }

//Use :
 var hitResult = ScreenPositionToWorldPositionRaycast(Input.MousePosition, Camera, this.GetSimulation());

Feel free to discuss as needed.
Cheers