I tried to make a game similar to a game called pick a lock game. However, I was struggle with spawning randomly a point around the border of circle but not in the circle. For more details, I want to spawn a gameObject at the border of a circle only but not every where in the circle. Please help me :< Thank you very much and have a good day! p/s: I just a beginner to unity engine and syntax. It'll be great if you give me some advice of how to self-learning unity. Thank you so much

CodePudding user response：

First of all, I really like Dot Product. I took a random place around the reference object (which is in this case transform.position). Then take direction between randomPos and reference object. Calculated Dot Product with Vector3.Dot(). Take the angle between randomPos and our transform.forward with the result of Dot Product. Calculate the x and z values with Cos and Sin functions. Dot Product Angle always gives 0 to 180 values so I gave randomness to z-axis with multiply dotProductAngle with (Random.value > 0.5f ? 1f : -1f). If you don't do this, the given random position will be always in front of the player.

3D Space

private void SpawnSphereOnEdgeRandomly3D()
        {
            float radius = 3f;
            Vector3 randomPos = Random.insideUnitSphere * radius;
            randomPos  = transform.position;
            randomPos.y = 0f;
            
            Vector3 direction = randomPos - transform.position;
            direction.Normalize();
            
            float dotProduct = Vector3.Dot(transform.forward, direction);
            float dotProductAngle = Mathf.Acos(dotProduct / transform.forward.magnitude * direction.magnitude);
            
            randomPos.x = Mathf.Cos(dotProductAngle) * radius   transform.position.x;
            randomPos.z = Mathf.Sin(dotProductAngle * (Random.value > 0.5f ? 1f : -1f)) * radius   transform.position.z;
            
            GameObject go = Instantiate(_spherePrefab, randomPos, Quaternion.identity);
            go.transform.position = randomPos;
        }

Example in 3D

2D Space

private void SpawnSphereOnEdgeRandomly2D()
{
    float radius = 3f;
    Vector3 randomPos = Random.insideUnitSphere * radius;
    randomPos  = transform.position;
    randomPos.y = 0f;
    
    Vector3 direction = randomPos - transform.position;
    direction.Normalize();
    
    float dotProduct = Vector3.Dot(transform.forward, direction);
    float dotProductAngle = Mathf.Acos(dotProduct / transform.forward.magnitude * direction.magnitude);
    
    randomPos.x = Mathf.Cos(dotProductAngle) * radius   transform.position.x;
    randomPos.y = Mathf.Sin(dotProductAngle * (Random.value > 0.5f ? 1f : -1f)) * radius   transform.position.y;
    randomPos.z = transform.position.z;
    
    GameObject go = Instantiate(_spherePrefab, randomPos, Quaternion.identity);
    go.transform.position = randomPos;
}

Example in 2D

I hope this helps you out