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Question by ColeBKray · Dec 22, 2011 at 10:35 AM · nullreferenceexception

NullReferenceException error

Can anyone help me with this? I have the following code:

  var dmg :int = 4;
 var hp: int = 5;
 var target: GameObject;
 var creatureLocX  = 0;
 var creatureLocY = 0;
   
 function OnGUI() {
 
 var location : Vector3 = Camera.mainCamera.WorldToViewportPoint(target.position);
 creatureLocX = location.x;
 creatureLocY = location.y;
 GUI.Label(Rect(creatureLocX,creatureLocY,120,60), "HP:" + hp + " DMG:" + dmg);
 }

When I run the game I get the following message: "NullReferenceException: Object reference not set to an instance of an object" for the following line: "`var location : Vector3 = Camera.mainCamera.WorldToViewportPoint(target.position);`"

For target I've assigned an object in the inspector.

What can I do about this?

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Answer by Statement · Dec 22, 2011 at 12:34 PM

Try using Camera.main.WorldToScreenPoint which return screen pixel coordinates. Camera.main.WorldToViewportPoint would return values between 0 to 1 in each axis.

Once you got the screen point, you can convert it to GUI space by flipping the y component since GUI has 0, 0 in upper left and screen point has 0, 0 in lower left.

Here's an example. To try it out, just put the script on any game object and text will appear on it.

 var text : String = "Example";
 
 function OnGUI() {
     var g : Vector2 = WorldToGUIPoint(transform.position);
     var r : Rect = Rect(g.x, g.y, 100, 20);
     GUI.Label(r, text);
 }
 
 function WorldToGUIPoint(world : Vector3) : Vector2 {
     var s : Vector3 = Camera.main.WorldToScreenPoint(world);
     var g : Vector2 = Vector2(s.x, Screen.height - s.y);
     return g;
 }

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