The non-trivial problem with image processing.

I need to create an application on the similarity of this:

This mobile app for kids. There is an image that you want to draw a finger across the control points. During drawing validation occurs on the intermediate points. If the user deviates from the route, it is considered an error. Here, a small amount of images and simple drawings. I think that point is set manually, but I will be more complex images and much more. I would like to automate this process and do not put points manually. All images in PNG format. With GetPixel() I can get all the non-transparent pixels ie the same black outline of image. But the point is too much, because I take each pixel. How to reduce the number of points(pixels), so that the line width is equal to 1? Please help me find an algorithm to solve this problem? I would be grateful for any help. Its my code:

   public class PixelsGetter : MonoBehaviour {
         public List<Vector2> points = new List<Vector2>();
         public Transform container;
         public GameObject sphereObj;
         void Start()
         {
             sphereObj = Resources.Load<GameObject>("Sphere");
             Renderer rend = GetComponent<Renderer>();
             Texture2D texture = rend.material.mainTexture as Texture2D;
     
             for (int i = 0; i < texture.width; ++i)
             {
                 for (int j = 0; j < texture.height; ++j)
                 {
                    Color color = texture.GetPixel(i, j);
                     if (color.a!=0)
                     {
                         points.Add(new Vector2(i,j));
                     }
                 }
             }
         var obj = Resources.Load<GameObject>("Sphere");
             foreach (var item in points)
             {
                 CreateSphere(item);
             }
         }
        void CreateSphere(Vector2 point)
         {
             var newObj = Instantiate<GameObject>(sphereObj);
             newObj.transform.SetParent(container);
             newObj.transform.localPosition = new Vector3(point.x, point.y, 0);
         }
     }

Here is the result (left original image, to the right - the result created by the 29903 spheres):