<commit_before><commit_msg>Created a new unity article.<commit_after># Screen World Positions

It is quite useful to transfer the screen position (2D position shown on screen) to a world position (3D position in game world). Most of the useful methods are located in `Camera` class, since it holds the necessary information to do the transform (several matrices I suppose).

The method signatures are listed below:

  ```csharp
public Vector3 ScreenToWorldPoint(Vector3 position);
public Ray ScreenPointToRay(Vector3 position);
  ```

The second one is especially useful for collision detection, which could be used to implement e.g. the mouse click selection function.
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