Getting texel on a gameobject

Hi.

I am trying to draw to a texture when clicking on a game object - but after getting the local position of the click I can't figure out how to get the values I should draw to, as I am getting values that does not represent the texture (and 0.0 is in the middel of the gameobject I am clicking).

So far I have this:

 Vector2 input = new Vector3(Input.mousePosition.x,Input.mousePosition.y); 
         RectTransform t = this.gameObject.GetComponent<RectTransform>();
         Vector2 output;
         bool wasHit = RectTransformUtility.ScreenPointToLocalPointInRectangle(t,input,Camera.main, out output); 
         Vector2 localpoint = output;
         Debug.Log ("out:"+input.ToString() +" local: "+localpoint.ToString());
 
         Vector2 uv; 
         //= value * scale * textureSize;
          uv.x = localpoint.x * 1*512  ;
          uv.y = localpoint.y* 1*512  ;
     int texelX = Mathf.FloorToInt(uv.x );
     int texelY = Mathf.FloorToInt(uv.y );

However the texel values are clearly wrong, with 0.0 still being in the middel of the gameobject instead of the bottom right corner as I want. I seem to be missing some math understanding here, so any help would be very much appriciated!