How to find the number of blobs in a 2d matrix?

Question

How can I find the number of blobs in a 2d matrix? SIZE MxN

A blob is a block of continuous X pixels. where the matrix contains X and O

XOOOXO
OXOXOX
XXOOXO

I would like to use 8-neighbourship (see here). So I would expect 2 blobs to be found in above example.

Answer 1

The idea is simple: Mark each continuous blob and count how many blobs were marked.

Here is some pseudo-code (you did not specify a programming language) to get you started:

numBlobs = 0;
foreach(item in matrix)
{
    res = Visit(item);
    if(res > 0) 
    {
        numBlobs = numBlobs + 1;
    }
}
return numBlobs;

The Visit function/method looks like this:

Visit(item)
{
    marked = 0;
    if(item.IsX() && !IsItemMarked(neighbour))
    {
        MarkItemAsVisited(item);
        marked = 1;
        foreach(neighbour in GetNeighbours(item))
        {
            marked = marked + Visit(neighbour);
        }
    }
    return marked;
}

All you have to do is to implement the other fucntions/methods but they are pretty straightforward.