Align Parent object using child object as point of reference

Question

Hierarchy:

Game object 1 with child transform (empty gameobject as a child)
Game object 2 with child transform (empty gameobject as a child)

Goal: Align game object 2's position and rotation with the game object 1 transform. Game object 2 needs to be aligned via its child transform (empty game object with only a transform). Also want to end up with the same hierarchy as we started with as the final result.

Current Approach - hoping for more optimized solution or easier to read / shorter:

make child of game object 2 the 'parent' of game object 2 (it keeps its relative position this way)
move the now 'parent' to game object 2 to the destination game object 1 transform (via transform.position and rotation - easy enough)
make the game object 2 the 'parent' of its original child object transform

Obviously changing the parent / child several times gets old to type (i can make it a function yes) but it still seems slow / not required. Also consider that both game object 1 and 2 are child objects to yet another object in the scene (for both organization and movement). I tried Transform.Translate as it seemed to be the correct thing, but it didn't provide the correct results when testing, I could have just had something misplaced.

I can provide REAL code if required, this is a c# project but I can totally understand javascript, post whatever you feel is appropriate - THANKS!

Edit Image Attached Note that if half the rod is missing (as shown in the image) the overlap is visible in the objects i'm using, thus why i mention the visual issue (besides potential collider, center of mass and other physics issues)

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Answer 1

Best Answer

Answer by robertbu · May 20, 2013 at 05:31 PM

Here is a bit of code that does what I think you want:

 // tr1P - Game Object 1 parent transform
 // tr1c - Game Object 1 child transform
 // tr2P - Game Object 2 parent transform
 // tr2C - Game Object 2 child transform
 void Alignment(Transform tr1P, Transform tr1C, Transform tr2P, Transform tr2C) {

     Vector3 v1 = tr1P.position - tr1C.position;
     Vector3 v2 = tr2C.position - tr2P.position;
     tr1P.rotation = Quaternion.FromToRotation(v1, v2) * tr1P.rotation;
     tr1P.position = tr2C.position + v2.normalized * v1.magnitude;
 }

Note I cannot think of anything particularly wrong with the way you are currently doing it.

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Vance Walsh · May 21, 2013 at 10:27 AM 0

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Thanks for this!

Owen-Reynolds · May 21, 2013 at 04:26 PM 0

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I think this assumes that, when connected, A, the connectors, and B are always in a straight line. In other words, every object's connector always faces directly away from the center of the object (which is the case in the picture.)

Think would need more math if, say, there was an off-center up-facing socket on the flat part of the disc (but a center-socket on the disc, should work fine.)

Vance Walsh · May 21, 2013 at 05:09 PM 0

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I haven't tested your proposed theory, but in my tests its working with my 'plug in here' game object transforms. because its all based on those positions / rotations. And size I keep z forward, it works fine for me. But I can see what you are getting at. I'll make sure I don't have a bug in the works!

robertbu · May 21, 2013 at 10:41 PM 0

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No more math needed. Regardless of the relationship of the empty game object to the parent, this code will align the objects. There is a question of the relative rotation of the two visible object along the axes that connects them.

Note, if the socket is always forward, you can simplify the code just a bit. v1 would be -tr1.forward. v2 would be tr2.forward. You would still need the magnitude of v1, but you could calculate it or hard code it. Unless your are going to be doing a lot of these kinds of changes per frame, the simplifications are not necessary.

Owen-Reynolds · May 22, 2013 at 01:31 PM 0

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"...question of the relative rotation..." Yes. Typically, the rotation of a mount point is meant to affect the rotation of the connected object.

The child of the rod might be rotated 90 degrees (but not moved) to indicate it only connects to the disc like a "b" or a duck-foot (rotation of the disc mount point would say which one.)

Answer 2

Answer by EthanF4D · Oct 02, 2019 at 07:06 AM

The following do not need a source parent. and target, targetChild don't have to be direct parent/child. can be ancestor/descendant in the hierachy

 public static void Align(Transform target, Transform targetChild, Transform source)
 {
     if (target && targetChild && source)
     {
         target.rotation = source.rotation * Quaternion.Inverse(Quaternion.Inverse(target.rotation) * targetChild.rotation);
         target.position = source.position + (target.position - targetChild.position);
     }
 }

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Jon_Olive · Apr 21, 2020 at 11:03 AM 0

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This thread helped me solve a problem that was plaguing me for ages - namely lining up coordinate systems of two different 3d tracking systems - thanks a lot! I used EthanF4D's method ultimately - see my reply to this question: https://answers.unity.com/questions/1493924/align-htc-vive-controller-with-optitrack-rigid-bod.html

EthanF4D Jon_Olive · Jan 05, 2022 at 04:35 PM 0

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i am glad it helped out

unity_0bE92MX1w5VaiA · Jan 05, 2022 at 03:21 PM 0

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This solution was exactly what I needed!

Answer 3

Answer by ljdp · Sep 04, 2018 at 07:43 PM

I know this is an old question, but I came to a different solution. This also works well when tweening to position.

 void Alignment(Transform t1P, Transform t1C, Transform t2) {
     t1P.rotation = t2.rotation * Quaternion.Inverse(t1C.localRotation);
     t1P.location = t2.position + (t1P.position - t1C.position);
 }

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Chambers88 · Mar 22, 2019 at 06:41 PM 0

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You mean position instead of "location" right? Or would it be localPosition?

Either way, didn't work here :/ perhaps because my t2 is scaled?

DaedricWolf19 · Jul 13, 2019 at 07:57 PM 0

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I'm sorry to revive this, but could you explain mathematically how you came to this solution? I'm stuck at how i would actually go about taking it into consideration myself, let alone write my own solution.

Align Parent object using child object as point of reference

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