Find anchoredPosition of a RectTransform relative to another RectTransform

Hello,

The thread title might look confusing, so let me make an example.

EXAMPLE
I have two RectTransforms, A and B. A has its anchor set to the top-left of the screen, B to the center. Now, I would like to move B to the same exact position as A, eye-wise, so that they overlap, but without changing its original anchor offset etc: just by giving it a new anchoredPosition.

So the question is, how would I calculate the correct anchoredPosition for an object, so that it perfectly overlaps another object, even if they have different offsets, anchors, and maybe even parents?

Thanks,
Daniele

 

One thing you have to remember is that the anchors position is relative to the RectTransform's pivot point.
So in that scenario you might be better trying to align them based on their pivot point and the RectTransforms dimensions / size delta

 

@SimonDarksideJ I know, but I really need to be able to align them only by changing the anchoredPosition, and still can't find the right way to do that. Any help there?

 

I would like to know that as well. I need to visually align 2 ui elements but they have different parents. What equations should I use?

 

After many trials and errors, I managed to create a method just for that:

Code (csharp):

1.  
2. /// <summary>
3. /// Converts the anchoredPosition of the first RectTransform to the second RectTransform,
4. /// taking into consideration offset, anchors and pivot, and returns the new anchoredPosition
5. /// </summary>
6. public static Vector2 SwitchToRectTransform(RectTransform from, RectTransform to)
7. {
8.    Vector2 localPoint;
9.    Vector2 fromPivotDerivedOffset = new Vector2(from.rect.width * 0.5f + from.rect.xMin, from.rect.height * 0.5f + from.rect.yMin);
10.    Vector2 screenP = RectTransformUtility.WorldToScreenPoint(null, from.position);
11.    screenP += fromPivotDerivedOffset;
12.    RectTransformUtility.ScreenPointToLocalPointInRectangle(to, screenP, null, out localPoint);
13.    Vector2 pivotDerivedOffset = new Vector2(to.rect.width * 0.5f + to.rect.xMin, to.rect.height * 0.5f + to.rect.yMin);
14.    return to.anchoredPosition + localPoint - pivotDerivedOffset;
15. }

 

Thanks, worked like a charm.

Just a small change to make it return coordinates which respect the pivots of the two RectTransforms:

Code (csharp):

1.  
2. private Vector2 switchToRectTransform(RectTransform from, RectTransform to)
3.   {
4.   Vector2 localPoint;
5.   Vector2 fromPivotDerivedOffset = new Vector2(from.rect.width * from.pivot.x + from.rect.xMin, from.rect.height * from.pivot.y + from.rect.yMin);
6.   Vector2 screenP = RectTransformUtility.WorldToScreenPoint(null, from.position);
7.   screenP += fromPivotDerivedOffset;
8.   RectTransformUtility.ScreenPointToLocalPointInRectangle(to, screenP, null, out localPoint);
9.   Vector2 pivotDerivedOffset = new Vector2(to.rect.width * to.pivot.x + to.rect.xMin, to.rect.height * to.pivot.y + to.rect.yMin);
10.   return to.anchoredPosition + localPoint - pivotDerivedOffset;
11.   }
12.  

 

Great stuff, will see about adding these functions to the UI Extensions project, with a few variations

 

Highfive to you for the addition, @Brave Michael! And woah, didn't know the UI Extensions project. Cool!

 

I did a tool called "Rect Transform Extended" for doing exactly this kind of stuff, take a look:
https://www.assetstore.unity3d.com/#!/content/41927

 

FYI - I don't think the pivot offset above is actually necessary for this. From what I can tell, Transform.position is already going to give you exactly where the pivot is in world space (regardless of whether the pivot is in the center of the Rect or not). So, I think it's enough to just do this:

Code (CSharp):

1. Vector2 localPoint;
2. Vector2 screenP = RectTransformUtility.WorldToScreenPoint( null, from.position )
3. RectTransformUtility.ScreenPointToLocalPointInRectangle( to, screenP, null, out localPoint );
4.  

 

How come WorldToScreenPoint is not documented in RectTransformUtility. Am I reading the docs wrong?

 

It is a script utility.
It is documented in the script part of the documentation, it does not have a page in the "manual" section of the Unity documents.

 

For my particular scenario, I had a position relative to a certain RectTransform Anchored space and had to convert it to another RectTransform's space. So I updated the function provided above to take in a position and convert it from it's current RectTransform space into a target RectTransform space.

Code (CSharp):

1. private Vector2 ConvertPosToTargetPivotSpace(Vector2 fromAnchoredPos, RectTransform fromPivotSpace, RectTransform toPivotSpace)
2. {
3.     Vector2 convertedPos;
4.  
5.     Vector2 fromPivotDerivedOffset = new Vector2(fromPivotSpace.rect.width * fromPivotSpace.pivot.x + fromPivotSpace.rect.xMin, fromPivotSpace.rect.height * fromPivotSpace.pivot.y + fromPivotSpace.rect.yMin);
6.  
7.     Vector3 fromWorldSpace = fromPivotSpace.TransformPoint(fromAnchoredPos);
8.     Vector2 screenP = RectTransformUtility.WorldToScreenPoint(null, fromWorldSpace);
9.     screenP += fromPivotDerivedOffset;
10.  
11.     RectTransformUtility.ScreenPointToLocalPointInRectangle(toPivotSpace, screenP, null, out convertedPos);
12.     Vector2 pivotDerivedOffset = new Vector2(toPivotSpace.rect.width * toPivotSpace.pivot.x + toPivotSpace.rect.xMin, toPivotSpace.rect.height * toPivotSpace.pivot.y + toPivotSpace.rect.yMin);
13.  
14.     return toPivotSpace.anchoredPosition + convertedPos - pivotDerivedOffset;
15. }

Hope this helps anyone else who was seeking this type of solution, or even myself if I run into the same issue in the future.

Be sure that you're providing the CORRECT fromPivotSpace RectTransform, initial tests, I was not and thought I was going down the wrong direction.

Goodluck!

 

Code (CSharp):

1. Vector2 fromPivotDerivedOffset = new Vector2(fromPivotSpace.rect.width * fromPivotSpace.pivot.x + fromPivotSpace.rect.xMin, fromPivotSpace.rect.height * fromPivotSpace.pivot.y + fromPivotSpace.rect.yMin);

Pretty sure adding this directly to a world position is wrong.
Aren't you forgetting to take scaling or rotation into account there ?

 

Well it's not working at all guys.

 

I think what @IronHelmet means is ... in the Unity scripting docs this method you used DOES NOT EXIST - https://docs.unity3d.com/ScriptReference/RectTransformUtility.html

(I came here in the hope that someone else had implemented the obvious missing API call. I was a bit stunned to find the API call isn't missing, just some muppet at Unity deleted it from the docs - it's a public method in the class, no reason for it to be absent)