边界跟踪

public void GetBoundary(byte[,] image)
{
    for (int j = 0; j < image.GetLength(1); j++)
        for (int i = 0; i < image.GetLength(0); i++)
            if (image[i, j] == 255)               // Found first white pixel
                SquareTrace(new Point(i, j));
}

public void SquareTrace(Point start)
{
    HashSet<Point> boundaryPoints = new HashSet<Point>();  // Use a HashSet to prevent double occurrences
    // We found at least one pixel
    boundaryPoints.Add(start);

    // The first pixel you encounter is a white one by definition, so we go left. 
    // Our initial direction was going from left to right, hence (1, 0)
    Point nextStep = GoLeft(new Point(1, 0));               
    Point next = start + nextStep;
    while (next != start)
    {
        // We found a black cell, so we go right and don't add this cell to our HashSet
        if (image[next.x, next.y] == 0)
        {
            next = next - nextStep
            nextStep = GoRight(nextStep);
            next = next + nextStep;
        }
        // Alternatively we found a white cell, we do add this to our HashSet
        else
        {
            boundaryPoints.Add(next);
            nextStep = GoLeft(nextStep);
            next = next + nextStep;
        }
    }
}

private Point GoLeft(Point p) => new Point(p.y, -p.x);
private Point GoRight(Point p) => new Point(-p.y, p.x);