How to find all borders of objects described pixel by pixel?

Question

Algorithms

Hello!
There is a two-dimensional array with pixels of two colors.
White - unoccupied space
Black - obstacles
Obstacles are some kind of shapes - squares, circles, etc.
I need to find only the border pixels of all the shapes in a fairly fast time.
Prompt good algorithms?
While doing this:
I run through all the black pixels and check if it has at least one black neighbor. If there is, then this is the boundary pixel.

Answer 1

If this is a two-dimensional array, where each pixel has coordinates, then why not take the indexes of the first black and the last black from each row.
In C# it would look like this

var arr = new bool[100][];
var contour = new List<List<int>>();

for (var y = 0; y < 100; y++)
{
  var firstX = Array.IndexOf(arr[y], true);
  var lastX = Array.LastIndexOf(arr[y], true);

  if(firstX != -1)
     contour.Add(new List<int> { firstX, y });

  if (lastX != -1)
     contour.Add(new List<int> { lastX, y });
 }