XNA renderTarget, how to convert mouse coordinates to world position when upscaling

Question

I'm currently developing a monogame game with a tile engine.

I have been experimenting with different tiles and sprites sizes : 16px, 32px, 64px I have settled for 32x32 pixels tiles rendered at a 64x64px size on screen.

I have read that a good way to do so is to use a RenderTarget2D. I've done so. It works well. I'm using SamplerState.PointClamp to keep the pixel aspect clean.

So for a 1440x900 resolution I use

 renderTarget = new RenderTarget2D(graphics.GraphicsDevice, 720, 450);

My question : when using the RenderTarget2D to upscale the screen, it logically messes up the camera I had working, and even more so the conversion of mouse coordinates to world coordinates. How should I modify the camera and the "mouse coordinates" to world position function. Is it even possible to convert properly mouse coordinates when you are upscaling everything with a renderTarget ?

Screenshot

This answer hints that it may be problematic, can somebody explain to me why ?

From the link : so if your graphics aren't dependant on other graphical features like a mouse, then I would use a RenderTarget

I can't find any resources about this. I always find a solution by myself, but this one is too much for me. Linear algebra is not my strong suit. Thank to anybody who can help.

My transform Matrix and Worldposition conversion

public Matrix Transform(GraphicsDevice graphicsDevice)
{
    m_Transform =

      Matrix.CreateTranslation(new Vector3(-m_CameraPosition.X, -m_CameraPosition.Y, 0)) *
         Matrix.CreateRotationZ(Rotation) *
         Matrix.CreateScale(new Vector3(Zoom, Zoom, 0)) *
         Matrix.CreateTranslation(new Vector3(ViewportWidth * 0.5f, ViewportHeight * 0.5f, 0));

    return m_Transform;
}

public Vector2 Worldposition(GraphicsDevice graphicsDevice, Vector2 position)
{
    //Invert camera
    Matrix m_InvertTransform =
    Matrix.CreateTranslation(new Vector3(-(m_CameraPosition.X + ViewportWidth * 0.5f), 0, -(m_CameraPosition.Y + ViewportHeight * 0.5f))) *
    Matrix.CreateRotationZ(Rotation) *
    Matrix.CreateScale(new Vector3(Zoom, Zoom, Zoom)) *
    Matrix.CreateTranslation(new Vector3(ViewportWidth * 0.5f, 0, ViewportHeight * 0.5f));
    m_InvertTransform = Matrix.Invert(m_InvertTransform);
    //Transform the position
    Vector3 newPos3 = Vector3.Transform(new Vector3(position.X, 0, position.Y), m_InvertTransform);
    return new Vector2((newPos3.X - 720) , (newPos3.Z - 450) );
}

Edit :

In my Update() method :

Vector2 vector = new Vector2((int)player.X, (int)player.Y);
camera.ChangePosition(GraphicsDevice, vector);

The ChangePosition function in my Camera Class public void ChangePosition(GraphicsDevice graphicsDevice, Vector2 newposition) { m_CameraPosition = newposition; }

Answer 1

So just to answer my own old question. I had already done what I was asking for in my prototype, and found my solution when looking back in my git repository. Only thing missing from the previous code was the division by 2 of the position in the transform Vector.

public Vector2 Worldposition(Vector2 position)
{

  float ViewportWidth = 720;
  float ViewportHeight = 450;

  //Invert camera
  Matrix m_InvertTransform =
  Matrix.CreateTranslation(new Vector3(-(m_CameraPosition.X + ViewportWidth * 0.5f), 0, -(m_CameraPosition.Y + ViewportHeight * 0.5f))) *
  Matrix.CreateRotationZ(Rotation) *
  Matrix.CreateScale(new Vector3(Zoom, Zoom, Zoom)) *
  Matrix.CreateTranslation(new Vector3(ViewportWidth * 0.5f, 0, ViewportHeight * 0.5f));
  m_InvertTransform = Matrix.Invert(m_InvertTransform);
  //Transform the position
  Vector3 newPos3 = Vector3.Transform(new Vector3(position.X / 2, 0, position.Y  2), m_InvertTransform);
>  return new Vector2(newPos3.X - ViewportWidth / 2, newPos3.Z - ViewportHeight  2);
}