How to convert isometric screen space coordinates to top-down world coordinates in Unity?

Question

I have 2d grid in a 3d world facing the world up. My camera is setup so it shows an isometric view of the grid.

I want to get the player's input in screen space (cyan) and find the same point on the grid (purple) ignoring the z position which will ultimately select the nearest tile (red square) from the input. I want to do this without using colliders as there could be hundreds of tiles in the same scene.

I want to know how I can project the screen space point into the 2d grid in a 3d world. The image below summarizes what I want to achieve.

Answer 1

You can add one collider which covers the whole grid. You can then use a usual mouse-into-scene raycast. The resulting RaycastHit object has a member Vector3 point which is the point in world space where the collider was hit.

Assuming your tiles are in a two-dimensional array, each tile is 1 unit in size and the origin of your tilemap is at world coordinates 0:0, you should be able to do something like this:

 Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
 RaycastHit hit;
 if (Physics.Raycast(ray, out hit)) {
       // check if we actually hit the tilemap and not something else
       if (hit.collider.gameObject == myTilemap) {
           // convert world coordinates into tilemap coordinates
           // This might get a bit more complicated if the assumptions above do not apply
           int tileX = Mathf.FloorToInt(hit.point.x);
           int tileY = Mathf.FloorToInt(hit.point.z);
           Tile hitTile = tilemap[tileX][tileY];

           /* process hitTile */
       }
 }