How to find boundary point using bfs algorithm - algorithm

Question

I think of the 2D array as a coordinate and try to find a coordinate value with a value of 1.
So far, it's a very easy BFS problem, but what I want to do is look at the following picture.
While I'm looking for 1 or after I've found it all, I would like to know the coordinate values surrounding the boundary in the order of the arrow or the other direction.
What options do I need to add to get those information?
Below is the BFS code that I use now. I can get coordinate values from the BFS function as shown in the second picture.
class Node
{
public int x;
public int y;
public Node(int x, int y)
{
this.x = x;
this.y = y;
}
};
private int[] dx = new int[8] { -1, 0, 1, 0, 1, -1, -1, 1 };
private int[] dy = new int[8] { 0, -1, 0, 1, 1, -1, 1, -1 };
private Queue<Node> q = new Queue<Node>();
bool[,] visit = new bool[15, 15];
int[,] coordinates = new int[15, 15] { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0 },
{ 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0 },
{ 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0 },
{ 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0 },
{ 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }};
void BFS(int[,] pixel, int x, int y)
{
q.Enqueue(new Node(x, y));
visit[x, y] = true;
while (q.Count != 0)
{
Node cur = q.Dequeue();
for (int i = 0; i < 8; i++)
{
int r = cur.x + dx[i];
int c = cur.y + dy[i];
if (r >= 0 && c >= 0 && r < 15 && c < 15)
{
if (!visit[r, c] && pixel[r, c] == 1)
{
q.Enqueue(new Node(r, c));
visit[r, c] = true;
}
}
}
}
}
void main()
{
for (int y = 0; y < 15; y++)
{
for (int x = 0; x < 15; x++)
{
if (!visit[x, y] && coordinates[x, y] == 1)
{
BFS(coordinates, x, y);
}
}
}
}

Answer 1

we do not need BFS for finding boundary '1' values. We can simply loop over 2D grid and then for each '1', we can just check whether all of it's 4 adjacent (i.e up, down, left, right) values are '1' or not. If at least one of them is not '1', then it is a boundary point. Thanks!

Answer 2

find a coordinate value with a value of 1
Start by pre-processing the matrix
- Search all 1 values (this can also be done recursively)
- If 1 value does not have a 0 neighbor, it means it it not on edge - change it to 0.
After the pre-processing you are left with only the edge 1 values, and all others are 0.
I would like to know the coordinate values surrounding the boundary in
the order of the arrow or the other direction
To find out if the edge forms a closed loop, and get the nodes in the right order
apply BFS to the pre-processed matrix.
Seek a path from a node of your choice, back to the same node(a loop).