Unity3d: Find which gameObject is in front - unity3d

Question

I have two gameObjects A and B. They are rotated at 90 degrees, which makes its local y axis face forward.
1st Case
In this case, the local y position of B is ahead of local y position of A
2nd Case
Even though their global position is same as the 1st case, we can observe here that local y position of A is ahead of local y position of B.
I tried using A.transform.localPosition.y and B.transform.localPosition.y to find which is greater but it doesnt work. What can I do to find which is front in these two different cases?

Answer 1

Vector projections are your friend here. Project both positions onto a line and compare their magnitude (or square magnitude, it's faster).
Case 1:
Vector3 a = Vector3.Project(A.position, Vector3.up);
Vector3 b = Vector3.Project(B.position, Vector3.up);
if (a.sqrMagnitude > b.sqrMagnitude)
{
// a is ahead
}
else
{
// b is ahead
}
Case 2: Project both positions onto Vector3.left.
Maybe you can even always simply project the two positions onto one of the two objects' forward vector (A.forward or B.forward assuming they're rotated equally).
Hope this helps.

Answer 2

You could compare Vector3.Dot(A.position, A.forward) and Vector3.Dot(B.position, B.forward) to find the one in front in relation to their forward.
The object with the bigger Dot product is in front, and this works in all rotations, including 3D ones.
You can use the following snippet to test for yourself:
// Assign these values on the Inspector
public Transform a, b;
public float RotationZ;
void Update() {
a.eulerAngles = new Vector3(0, 0, RotationZ);
b.eulerAngles = new Vector3(0, 0, RotationZ);
Debug.DrawRay(a.position, a.right, Color.green);
Debug.DrawRay(b.position, b.right, Color.red);
var DotA = Vector2.Dot(a.position, a.right);
var DotB = Vector2.Dot(b.position, b.right);
if (DotA > DotB) { Debug.Log("A is in front"); }
else { Debug.Log("B is in front"); }
}