GameObject position precision loss when parent to an object not at the origin.

Hello everyone, bonjour !

I use Unity 2018.3.9f1 Personal.

I noticed something strange when programmatically parenting a GameObject to a parent that is not at the origin.
If I set the world position of the child object, it seems that when we query the word position, this queried world position is not perfectly equal to the desired world position.

This happens when these two methods are used :

Code (CSharp):

1. var childObj = GameObject.Instantiate(Resources.Load<GameObject>("ChildPrefab"), desiredChildWorldPosition, Quaternion.identity, parentObj.transform);

And

Code (CSharp):

1. var childObj = GameObject.Instantiate(Resources.Load<GameObject>("ChildPrefab"), parentObj.transform, true);
2. childObj.transform.position = desiredChildWorldPosition;

The tests are :

Spoiler: Tests detail

Code (CSharp):

1.     [UnityTest]
2.     public IEnumerator GOInstanciate_ParentTo10_10_10()
3.     {
4.         var parentWorldPosition = new Vector3(10f, 10f, 10f);
5.         var desiredChildWorldPosition = new Vector3(-23.2f, 0.5833334f, -7f);
6.  
7.         var parentObj = new GameObject();
8.         parentObj.transform.position = parentWorldPosition;
9.  
10.         var childObj = GameObject.Instantiate(Resources.Load<GameObject>("ChildPrefab"), desiredChildWorldPosition, Quaternion.identity, parentObj.transform);
11.  
12.         Assert.AreEqual(0f, Vector3.Distance(childObj.transform.position, desiredChildWorldPosition), "desired world position and real world position should be equal.");
13.         Assert.AreEqual(desiredChildWorldPosition, childObj.transform.position, "child object position should be the world position set.");
14.         yield return null;
15.     }

desired world position and real world position should be equal. Expected: 3,576279E-07 == 0

Code (CSharp):

1.     [UnityTest]
2.     public IEnumerator TransformSetPosition_ParentTo10_10_10()
3.     {
4.         var parentWorldPosition = new Vector3(10f, 10f, 10f);
5.         var desiredChildWorldPosition = new Vector3(-23.2f, 0.5833334f, -7f);
6.  
7.         var parentObj = new GameObject();
8.         parentObj.transform.position = parentWorldPosition;
9.  
10.         var childObj = GameObject.Instantiate(Resources.Load<GameObject>("ChildPrefab"), parentObj.transform, true);
11.         childObj.transform.position = desiredChildWorldPosition;
12.  
13.         Assert.AreEqual(0f, Vector3.Distance(childObj.transform.position, desiredChildWorldPosition), "desired world position and real world position should be equal.");
14.         Assert.AreEqual(desiredChildWorldPosition, childObj.transform.position, "child object position should be the world position set.");
15.         yield return null;
16.     }

desired world position and real world position should be equal. Expected: 3,576279E-07 == 0

The difference between the queried position and the desired one tends to increase as the distance of the parent object from the origin increases.
With a desired world position of (-23.2f, 0.5833334f, -7f) I have the following results :

1. Parent world pos = (0,0,0) -> distance delta = 0 --- TEST OK
2. Parent world pos = (10f,10f,10f) -> distance delta = 3,576279E-07 --- TEST KO
3. Parent world pos = (10000f,10000f,10000f) -> distance detla = 0,0003792614 --- TEST KO

Has anyone already encountered this ?
To me, this seems to not be the normal behavior of world positioning as I have not found any precision in the docs (https://docs.unity3d.com/ScriptReference/Object.Instantiate.html) and I expect that the world position that I set and the one that I query are identical.

Or maybe I am doing something wrong ?

I noticed it because I am using this position for geometry calculations that are thus erroneous.

I enclosed the project with the tests.
Also available here for quick look : https://github.com/ldalzotto/Parent...ter/Assets/Test/PrentingPrecissionLossTest.cs

Thanks.

 

Positions are Vector3's, which are three 32-but floating point numbers. That's not very exact. See https://0.30000000000000004.com/

 

32-but....It shouldn't, however, it made me chuckle!