Camera zoom while keeping an off-center object at the same screen co-ordinates

Question

Camera zoom whiloe keeping an offcenter object at the same screen co-ordinates.

Im doing a zoom by dragging on the highligted red ball below.

How do I get the ball to stay at it's screen coords while the rest of the view zooms in?

Thanks in advance

James

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Answer 1

Answer by robertbu · Feb 15, 2013 at 07:54 PM

I've not got time right now to write the test code, but here is the idea:

Use Camera.main.WorldToViewportPoint() before making the move (or even in Start()) to get the position in viewport space of the object.
Move the camera in for the zoom.
Get the new Viewport coordinates for the new position of the object.
Using the new distance between the camera and the board, calculate the world positions for the two viewport positions using Camera.ViewportToWorldPoint();
Subtract the two to get how far to translate the board or the camera.

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robertbu · Feb 15, 2013 at 08:57 PM 1

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Got a chance to test it in code:

 public class ZoomWithOffCenterObject : $$anonymous$$onoBehaviour {
     
     public GameObject goTrack;
     private Vector3 v3ZoomFactor = new Vector3(0.0f, 0.0f, 0.05f);
 
     void Update () {
         if (Input.Get$$anonymous$$ey ($$anonymous$$eyCode.UpArrow))
             ZoomCamera (true);
         if (Input.Get$$anonymous$$ey ($$anonymous$$eyCode.DownArrow))
             ZoomCamera (false);
     }
     
     private void ZoomCamera(bool bIn) {
         Vector3 v3OrgPos = Camera.main.WorldToViewportPoint(goTrack.transform.position);
         
         if (bIn)
             transform.Translate (v3ZoomFactor);
         else
             transform.Translate (-v3ZoomFactor);
         
         v3OrgPos.z = $$anonymous$$athf.Abs (Camera.main.transform.position.z - goTrack.transform.position.z);
         v3OrgPos = Camera.main.ViewportToWorldPoint(v3OrgPos);
         transform.Translate (goTrack.transform.position - v3OrgPos);
     }
 }

Jimmythesaint82 · Feb 16, 2013 at 03:33 PM 0

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Looks really good, and works in a clean scene...

However my camera tracks in Y and I can't for the life of me get this to handle that...

Any ideas?

Jimmythesaint82 · Feb 16, 2013 at 03:34 PM 0

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As in y is up

robertbu · Feb 16, 2013 at 05:07 PM 0

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The code above looks down the Z axis, so Y is up. Are you saying your camera is looking down the Y axis at the XZ plane?

Jimmythesaint82 · Feb 17, 2013 at 01:26 AM 0

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yes, thats what I meant :)

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Camera zoom while keeping an off-center object at the same screen co-ordinates

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