Random point within circle with Min/Max radius

I know this question has been asked before, but most of the time it is for 3D, and they don't take into account the center point.

I have an enemy that burrows and unburrows at a point around the player. I don't want it to unburrow too close, or too far away.

Currently I am using (essentially):

Code (CSharp):

1. animator.transform.position = Random.insideUnitCircle * radius + player.transform.position;

 

haii

Give this a whirl

Code (CSharp):

1.         if ( Vector2.Distance(a, b) < rangeMin && Vector2.Distance(a, b) > maxRange )
2.         {
3.                 TryAgain();
4.         }

Or use

Vector2.Distance

to find which the enemy is closest to them move them in that direction

On second thoughts what I said is arguably terrible, what you're trying to do is much smarter.

 

Thanks for the quick reply.
I would prefer as little rerolling as possible, as that can cause a lot of unnecessary lag on the system. I'm going for a more mathematical solution.

 

Found someone here who wrote a script for finding a random position in a torus shape, I won't copy over the script but here's the post https://answers.unity.com/questions/1259394/finding-random-position-in-torus.html

Sorry I wasn't able to help more. Best of luck.

 

Again, I thank you for your reply, but I believe I came across this before, and found it to be far more complicated than I needed. Also, I was looking for a more 2D solution. Sorry for being difficult.

 

So, for those interested in the subject, here is an article that helps get closer to what I want. When I figure it out, I will post my solution.

https://programming.guide/random-point-within-circle.html

 

Ah, here we go. Found a thread asking almost exactly what I am. Surprising how are it was to find this...

https://answers.unity.com/questions/856819/generate-a-random-point-with-an-anullus.html

 

I couldn't find an easy solution so i made one myself, for anyone stumbling upon this:

Code (CSharp):

1. public Vector2 RandomPointInAnnulus(Vector2 origin, float minRadius, float maxRadius){
2.  
3.         var randomDirection = (Random.insideUnitCircle * origin).normalized;
4.  
5.         var randomDistance = Random.Range(minRadius, maxRadius);
6.  
7.         var point = origin + randomDirection * randomDistance;
8.  
9.         return point;
10. }

TestClass

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class RandomAnnul : MonoBehaviour
6. {
7.     public float minRadius = 3f;
8.     public float maxRadius = 7f;
9.     public int pointAmount = 100;
10.     public GameObject prefab;
11.     void Start()
12.     {
13.         var origin = transform.position;
14.  
15.         for (int i = 0; i < pointAmount; i++)
16.         {
17.             var pointToSpawnAt = RandomPointInAnnulus(origin, minRadius, maxRadius);
18.  
19.             Instantiate(prefab, pointToSpawnAt, prefab.transform.rotation);
20.         }
21.  
22.     }
23.  
24.     public Vector2 RandomPointInAnnulus(Vector2 origin, float minRadius, float maxRadius){
25.  
26.         var randomDirection = (Random.insideUnitCircle * origin).normalized;
27.  
28.         var randomDistance = Random.Range(minRadius, maxRadius);
29.  
30.         var point = origin + randomDirection * randomDistance;
31.  
32.         return point;
33.     }
34. }
35.  

 

Just do

Code (CSharp):

1. Vector2 point = Random.insideUnitCircle.Normalized * Random.Range(MinRadius, MaxRadius);

 

I find it really silly that I did something like this about a month ago, forgot about it, and spent a couple hours trying to do something similar in my current project
smh
thank you for this

 

Worked like a charm, thank you!

 

I think, what you need is this:

Code (CSharp):

1. private Vector3 GetRandomSpawnPosition()
2.     {
3.         Vector3 randomPosition = (Vector3)Random.insideUnitCircle;
4.         return transform.position + randomPosition.normalized * spawnRadiusMin + randomPosition * (spawnRadiusMax - spawnRadiusMin);
5.     }

 

I'd say that none of these solutions offer a random position in an annulus that is uniformly distributed and gives an answer in one pass. (So without rejecting and trying again.)

The solution of Random.insideUnitCircle and then rejecting in the inner circle is uniformly distributed, but not in one pass. All others break the uniform distribution.

All in all the math is not that difficult. You'd start with a random direction and distance, but you'll have to adjust the distance to make it uniform. (Else if the minimum radius is 1 and the maximum radius is 2 for example, the points will be twice as dense at the minimum.)

Code (csharp):

1.  
2. public Vector2 RandomPointInAnnulus(Vector2 origin, float minRadius, float maxRadius) {
3.     Vector2 randomDirection = Random.insideUnitCircle.normalized; // There are more efficient ways, but well
4.     float minRadius2 = minRadius * minRadius;
5.     float maxRadius2 = maxRadius * maxRadius;
6.     float randomDistance = Mathf.Sqrt(Random.value * (maxRadius2 - minRadius2) + minRadius2);
7.     return origin + randomDirection * randomDistance;
8. }
9.  

It might be a bit too puristic for the goal here, but this returns a random point in an annulus with uniform distribution in one pass. The other options above simply don't.

 

You are a genius! After 1 hour this is the only solution that works correctly