Boundary tracing

Boundary tracing, also known as contour tracing, of a binary digital region can be thought of as a segmentation technique that identifies the boundary pixels of the digital region. Boundary tracing is an important first step in the analysis of that region.
Algorithms.
Algorithms used for boundary tracing:
Square tracing algorithm.
The square tracing algorithm is simple, yet effective. Its behavior is completely based on whether one is on a black, or a white cell (assuming white cells are part of the shape). First, scan from the upper left to right and row by row. Upon entering your first white cell, the core of the algorithm starts. It consists mainly of two rules:
Keep in mind that it matters how you entered the current cell, so that left and right can be defined.
public void GetBoundary(byte[,] image)
 for (int j = 0; j < image.GetLength(1); j++)
 for (int i = 0; int 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);