How to get mouse position relative to the map?

Question

I'm creating a game in XNA similar to Minecraft - Minecraft 2D

It looks like this

Here's how it works:

• All blocks are generated once with fixed x,y coordinates and just re-drawn.

      //generate world
      for (int i = 0; i < 25; i++)
      {
          blocks.Add(new Block("top", i * 32, 32 * 7, Color.White));
          for (int j = 8; j < 20; j++)
          {
              blocks.Add(new Block("dirt", i * 32, 32 * j, Color.White));
          }
      }
  

• When user clicks on a block, I just browse through all blocks and check whether block exists.

  if ((mouseCoordinate.X - (mouseCoordinate.X % 32)) == block.x && (mouseCoordinate.Y - (mouseCoordinate.Y % 32)) == block.y) {
        //destroy block
  }
  

• Currently, without any camera movements, everything works. When I move my camera just about 32px to the right, for example, it's all broken. I can't do previous step anymore.

• What I need: I need to get mouse position relative to the map, not to the game window. How can I get mouse position relative to the map? Basically, I want mouse position 0,0 on the point I have circled here

How do I calculate it?

EDIT: here's my camera class (yes, Matrix)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using System.Reflection;
namespace MojePrvniHra
{
    public class Camera2d
    {
    protected float _zoom; // Camera Zoom
    public Matrix _transform; // Matrix Transform
    public Vector2 _pos; // Camera Position
    protected float _rotation; // Camera Rotation

    public Camera2d()
    {
        _zoom = 1.0f;
        _rotation = 0.0f;
        _pos = Vector2.Zero;
    }// Sets and gets zoom
    public float Zoom
    {
        get { return _zoom; }
        set { _zoom = value; if (_zoom < 0.1f) _zoom = 0.1f; } // Negative zoom will flip image
    }

    public float Rotation
    {
        get { return _rotation; }
        set { _rotation = value; }
    }

    // Auxiliary function to move the camera
    public void Move(Vector2 amount)
    {
        _pos += amount;
    }
    // Get set position
    public Vector2 Pos
    {
        get { return _pos; }
        set { _pos = value; }
    }

    public Matrix get_transformation(GraphicsDevice graphicsDevice)
    {
        _transform =
          Matrix.CreateTranslation(new Vector3(-_pos.X, -_pos.Y, 0)) *
                     Matrix.CreateRotationZ(_rotation) *
                     Matrix.CreateScale(new Vector3(Zoom, Zoom, 1)) *
                     Matrix.CreateTranslation(new Vector3(graphicsDevice.Viewport.Width * 0.5f, graphicsDevice.Viewport.Height * 0.5f, 0));
        return _transform;
    }

    }
}

and here's how I start my spriteBatch

       spriteBatch.Begin(SpriteSortMode.BackToFront,
                BlendState.AlphaBlend,
                null,
                null,
                null,
                null,
                cam.get_transformation(graphics.GraphicsDevice));

Answer 1

Solution

Is your camera simulated using a view matrix? If so, all you need to do is:

Vector2 worldPosition = Vector2.Transform(mousePosition, Matrix.Invert(viewMatrix));

And if you're not using a view matrix... you should :-)

Explanation

The view matrix transforms coordinates from world space into view space. The inverse of this matrix does the opposite - it transforms coordinates from view space back into world space.

Since your mouse coordinates are defined in view space, all you need to do is transform them with the inverse view matrix to convert them into world space.

Answer 2

The concept here is unprojecting a mouse coordinate to eye-space. When dealing with graphics, there are several different coordinate systems used to represent objects. I'll explain all of them, but the way you have set your rendering up cancels the need for the coordinate spaces:

• Everything is generally defined by vertices relative to (0, 0). This is called local space.
• If you want to position an object in the world, you need to convert local space to world space. You do this with what is called a model matrix, which allows for movement, rotation, and scaling of the object. The way you've set up your rendering does this, or a simple version of this by just providing the X and Y location of the Block in the world.
• If you want to move around the world, you need to convert world space to eye space. This is done with a view matrix. You can move, rotate, and scale the camera in any way you want. In your case, you're either using a view matrix or just storing the location of the camera and offsetting everything by that position.
• DirectX uses a default coordinate system of -1 to 1 on the X and Y axes (0 to 1 on the Z, but that's not important). Everything that is rendered must exist within that range, and to convert to that coordinate system, you need one last transformation - the projection matrix. In this case, you've set up your projection matrix to convert pixel coordinates to the default coordinate system. In this case the projection matrix doesn't make a difference when unprojecting, as it'll just give you the same mouse coordinate back.

On to the actual solution, you've removed the need for the model and projection matrices, so you can set up a view matrix from your camera position if you don't already have one. From there you're just using Vector2.Transform as suggested by the previous answer.

If you want the most direct solution that you can implement very quickly and without matrices, you can just offset your mouse coordinates by the camera position before aligning it to the nearest block like this:

Vector2 cameraPos = /*The position of your camera*/;
Vector2 clickCoord = mouseCoordinate + cameraPos;
if ((clickCoord.X - (clickCoord.X % 32)) == block.x && (clickCoord.Y - (clickCoord.Y % 32)) == block.y) {
      //destroy block
}