Unity Ray Marching with Sphere Tracing

RayMarchingSpheres.cs

// unity ray marching with sphere marching : https://unitycoder.com/blog/2019/05/06/ray-marching/
// based on Ray Marching tutorial from The Coding Train https://www.youtube.com/watch?v=-6iIc6-Y-kk

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class RayMarchingSpheres : MonoBehaviour
{
    // place spheres under this root gameobject
    public Transform sphereRoot;

    // how many raymarch steps
    public int rayMarchSteps = 10;

    // list of step spheres
    List<Sphere> rayPoints = new List<Sphere>();

    bool isSearching = true;

    void Update()
    {
        if (isSearching == false) return;
        isSearching = false;

        // build ray to mouse (2D)
        var origin = Vector3.zero;
        var mousePos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
        mousePos.z = 0;
        var dir = (mousePos - origin).normalized;
        var ray = new Ray(origin, dir);


        // start from origin
        var currentPos = ray.origin;

        rayPoints.Clear();
        float totalDistance = 0;
        // raymarching steps
        for (int i = 0; i < rayMarchSteps; i++)
        {
            float smallestDistance = Mathf.Infinity;

            // find closest distance from currentPos sphere from spheres
            foreach (Transform t in sphereRoot)
            {
                var sphere = new Sphere();
                sphere.pos = t.position;
                // NOTE giving fixed radius, using default unity sphere
                sphere.r = 1f / 2f;
                var d = SignedDistance(currentPos, sphere);
                if (d < smallestDistance) smallestDistance = d;
            }

            // hit close enough, then stop marching
            if (smallestDistance < 0.001f)
            {
                isSearching = false;
                break;
            }

            // keep track of total distance
            totalDistance += smallestDistance;

            // record existing circle
            var s = new Sphere();
            s.pos = currentPos;
            s.r = 1f / 2f;
            s.minDist = smallestDistance;
            rayPoints.Add(s);

            // get next point position
            var newPos = ray.GetPoint(totalDistance);
            Debug.DrawLine(currentPos, newPos, rayPoints.Count % 2 == 0 ? Color.cyan : Color.yellow);
            currentPos = newPos;

            // draw cross at new currentpost 
            var c = rayPoints.Count % 2 == 0 ? Color.red : Color.blue;
            Debug.DrawRay(currentPos, Vector3.up * 0.1f, c);
            Debug.DrawRay(currentPos, -Vector3.up * 0.1f, c);
            Debug.DrawRay(currentPos, Vector3.right * 0.1f, c);
            Debug.DrawRay(currentPos, -Vector3.right * 0.1f, c);
        }
    }

    // draw spheres
    public void OnDrawGizmos()
    {
        Gizmos.color = Color.green;
        for (int i = 0, length = rayPoints.Count; i < length; i++)
        {
            Gizmos.DrawWireSphere(rayPoints[i].pos, rayPoints[i].minDist);
        }
        isSearching = true;
    }

    // signed distance to sphere
    float SignedDistance(Vector3 startPos, Sphere sphere)
    {
        var d = Vector3.Distance(startPos, sphere.pos);
        return d - sphere.r;
    }
}

Sphere.cs

using UnityEngine;

public struct Sphere
{
    public Vector3 pos;
    public float r;
    public float minDist;
}