Unity3d: Find which gameObject is in front

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?

What are you after exaclty? In front, but with regards to what? — Vaillancourt
– Vaillancourt ♦, Commented Jul 20, 2019 at 23:09
In 1st case, Imagine they are running straight, and in 2nd case, they are running at the left direction. The global postions are same in borh cases, but in 1st case B is ahead and in 2nd case, A is ahead — MrRobot9
– MrRobot9, Commented Jul 20, 2019 at 23:15
Yes, and so you want to find out which one is ahead of the other, assuming they'll always be facing the same direction and turning at the same time? — Vaillancourt
– Vaillancourt ♦, Commented Jul 20, 2019 at 23:34

DMGregory · Accepted Answer · 2019-07-21 05:10:02Z

5

You can check simple ahead/behind relationships using the dot product:

Vector3 displacement = B.transform.position - A.transform.position;

float dot = Vector3.Dot(displacement, A.transform.forward);

If dot is greater than zero, then B is ahead of A along A's forward vector. (The arrow from A to B has a component in the same direction as A's forward)

If dot is less than zero, then B is behind A along A's forward vector. (The arrow from A to B has a component in the opposite direction as A's forward)

If dot is zero, then B is somewhere directly to the side of A, neither in front nor behind. The A→B arrow points perpendicular to A's forward axis.

answered Jul 21, 2019 at 5:10

DMGregory♦

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\$\begingroup\$ FWIW, this approach will work even if B is in a different orientation from A -- it will just tell you whether B is in front or behind A, relative to A's orientation. N.B. that if the two orientations are different, then it is possible for A to be ahead of B along A's path, and B to be simultaneously ahead of A on B's path. \$\endgroup\$

A C
– A C

2019-07-21 05:49:40 +00:00
Commented Jul 21, 2019 at 5:49

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