How to convert between local / world coordinates?

Questions & Answers
1

I’ve been checking the script reference for: http://unity3d.com/support/documentation/ScriptReference/Transform.TransformPoint.html

But I can’t seem to make it work.

Here is my setup:

I have a large tilebased map (dynamically generated). I have my levelcam and some objects moving along in this map. But sometimes I need to take the object from the view (which has the camera hierachy as parent) and convert its local relative position into a world/level coordiate so I can freeze it in the map instead of moving it around with the cam.

As this happens on user-event, I would like to see how to make the calls if anyone can tell me?

I have the following GameObjects:

  • goLevelCam
    • goObjectToFreeze (has goLevelCam as parent until frozen)

And in the tilebased map:

  • goLevelMap

I thought something like:

Vector3 freezePos = goLevelMap.transform.TransformPoint( goObjectToFreeze.transform.position );
goObjectToFreeze.transform.parent = goLevelMap;
goObjectToFreeze.transform.position = freezePos;

But this makes the object jump to 0,0,0 (which is its normal coord in the hierachy as its child-child-child of a bigger setup I have tried to simplify for example usage.

Really hope this makes any sense…

Let me know if you need more explanations.

3

It seems you are totally confused about all the Transform stuff:

Note that a transformation is not just a position offset. It’s also rotated and scaled according to the parent.

This line in your solution:

Vector3 newPos = goObjectToFreeze.transform.TransformPoint( Vector3.zero );

is the same as

Vector3 newPos = goObjectToFreeze.transform.position;

Also in most cases you don’t need to convert much. When you change the parent of a Transform it will keep it’s world position (and rotation / scale). It’s automatically converted. If you want to change the parent of an object and move the object to the parents position do either:

childObject.parent = parentObject;
childObject.localPosition = Vector3.zero;

or

childObject.parent = parentObject;
childObject.position = parentObject.position;

The first case is the usual way.

2

Ohhh, I think I’ve got it now.

You need to use the transform of the OBJECT you want to convert into Worldspace, not the destination parent.

So its:

Vector3 newPos = goObjectToFreeze.transform.TransformPoint( Vector3.zero );
4

I just wish to chime in here as I realised something that was not immediately obvious to me when I was working with this.

I had some vector2 Positions that I needed to convert to local positions. (This was due to the fact that in my game the player can rotate the camera and therefore the lerping animation I was doing had to be done within the transform not the world)

My solution was to do this:

Vector2 localStartPos = transform.InverseTransformPoint(_StartPosition);
Vector2 localEndPos = transform.InverseTransformPoint(_EndPosition);

May be just my tired brain but thought I’d mention it as it wasnt immediately obvious to me that I could simply declare it that way.