Question

I want to find groups of contiguous cells in a matrix.

So for example let’s consider a 2D matrix below.

In the given matrix there are 2 contiguous groups of cells with value 1:

Here is one way to find these groups:

1. Assign 1st cell with value 1 a different value: let’s say A. Then examine cells with value 1 which are adjacent to A and set the value in those cells as A. Search this way until no more contiguous cells are found.

2. In the next step increment A to B and start with a cell having value 1. Then follow the same steps as above.

This is kind of brute force and it won’t be efficient in 3D. Does anyone know of any algorithm that I could use with a little tweaking?

Or any easy solution to this problem?

Answer 1

Here is some psuedo code for a simple flood fill algorithm:

>>> def flood(i, j, matrix):
...     if 0 <= i < len(matrix) and 0 <= j < len(matrix):
...         if matrix[i][j] == 1:
...             matrix[i][j] = 0
...             for dx, dy in ((-1, 0), (1, 0), (0, -1), (0, 1)):
...                 flood(i + dx, j + dy, matrix)

>>> count = 0
>>> while True:
...     i, j = get_one(matrix)
...     if i and j: #found a one
...         count += 1
...         flood(i, j, matrix)