CustomTrailRenderer.cs

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

public class CustomTrailRenderer : MonoBehaviour {

  IEnumerator Start() {
    var target = (Transform)null;
    while (target == null) {
      target = transform.parent;
      yield return null;
    }
    var target_g = target.gameObject;
    transform.parent = null;
    transform.position = new Vector3(0f, 0f, transform.position.z);
    transform.rotation = Quaternion.identity;
    transform.localScale = Vector3.one;
    var size = 0.6f;
    var color = (Color)(new Color32(0, 88, 231, 255));
    var max = 40 * 2;
    var step = 1.0f;
    var mesh = GetComponent<MeshFilter>().mesh;
    var last = target.position;
    var dt = 0f;
    List<Vector3> vertices = new List<Vector3>();
    List<Vector2> uv = new List<Vector2>();
    List<float> alphas = new List<float>();
    List<Color> colors = new List<Color>();
    List<int> triangles = new List<int>();
    while (true) {
      if (target_g == null) {
        Destroy(gameObject);
        yield break;
      }
      var modified = false;
      var current = target.position;
      var speed = dt > 0f ? Vector2.Distance(last, current) / dt : 0f;
      var alpha = Mathf.Min(1f, 0.05f + 0.95f * (speed / 100f));
      while (Vector2.Distance(last, current) >= step) {
        var p = Vector3.Lerp(last, current, step / Vector2.Distance(last, current));
        var r = target.eulerAngles.z * Mathf.Deg2Rad;
        var v = new Vector3(Mathf.Cos(r), Mathf.Sin(r), 0f) * size;
        vertices.Add(p + v);
        vertices.Add(p - v);
        alphas.Add(alpha);
        if (vertices.Count > max) {
          vertices.RemoveRange(0, 2);
          alphas.RemoveAt(0);
        } else {
          uv.Add(Vector2.zero);
          uv.Add(Vector2.zero);
        }
        last = p;
        dt = 0f;
        modified = true;
      }
      if (modified) {
        if (vertices.Count > 2) {
          {
            colors.Clear();
            var num = alphas.Count;
            var n = (float)num;
            var m = (float)max / 2;
            for (var i = 0; i < num; ++i) {
              var a = alphas[i];
              var c = color;
              c.a = a * (1f - (n - i) / m);
              colors.Add(c);
              colors.Add(c);
            }
          }
          {
            triangles.Clear();
            var num = (vertices.Count - 2) / 2;
            for (var i = 0; i < num; ++i) {
              triangles.Add(i * 2);
              triangles.Add(i * 2 + 1);
              triangles.Add(i * 2 + 3);
              triangles.Add(i * 2);
              triangles.Add(i * 2 + 3);
              triangles.Add(i * 2 + 2);
            }
          }
          mesh.vertices = vertices.ToArray();
          mesh.uv = uv.ToArray();
          mesh.colors = colors.ToArray();
          mesh.triangles = triangles.ToArray();
          mesh.RecalculateNormals();
          mesh.RecalculateBounds();
        }
      }
      yield return null;
      dt += Time.deltaTime;
    }
  }

}