Pixel Connectivity

Morphological processing starts at the peaks in the marker image and spreads throughout the rest of the image based on the connectivity of the pixels. Connectivity defines which pixels are connected to other pixels. A set of pixels in a binary image that form a connected group is called an object or a connected component.

Determining which pixels create a connected component depends on how pixel connectivity is defined. For example, this binary image contains one foreground object or two, depending on the connectivity. If the foreground is 4-connected, the image is all one object — there is no distinction between a foreground object and the background. However, if the foreground is 8-connected, the pixels set to 1 connect to form a closed loop and the image has two separate objects: the pixels in the loop and the pixels outside the loop.

0     0     0     0     0     0     0     0
0     1     1     1     1     1     0     0
0     1     0     0     0     1     0     0
0     1     0     0     0     1     0     0
0     1     0     0     0     1     0     0
0     1     1     1     1     0     0     0
0     0     0     0     0     0     0     0
0     0     0     0     0     0     0     0

Defining Connectivity in an Image

The following table lists all the standard two- and three-dimensional connectivities supported by the toolbox.

Value	Meaning
Two-Dimensional Connectivities
4-connected	Pixels are connected if their edges touch. Two adjoining pixels are part of the same object if they are both on and are connected along the horizontal or vertical direction.
8-connected	Pixels are connected if their edges or corners touch. Two adjoining pixels are part of the same object if they are both on and are connected along the horizontal, vertical, or diagonal direction.
Three-Dimensional Connectivities
6-connected	Pixels are connected if their faces touch. Two adjoining pixels are part of the same object if they are both on and are connected in: One of these directions: in, out, left, right, up, and down
18-connected	Pixels are connected if their faces or edges touch. Two adjoining pixels are part of the same object if they are both on and are connected in One of these directions: in, out, left, right, up, and down A combination of two directions, such as right-down or in-up
26-connected	Pixels are connected if their faces, edges, or corners touch. Two adjoining pixels are part of the same object if they are both on and are connected in One of these directions: in, out, left, right, up, and down A combination of two directions, such as right-down or in-up A combination of three directions, such as in-right-up or in-left-down

Choosing a Connectivity

The type of neighborhood you choose affects the number of objects found in an image and the boundaries of those objects. For this reason, the results of many morphology operations often differ depending upon the type of connectivity you specify.

For example, if you specify a 4-connected neighborhood, this binary image contains two objects; if you specify an 8-connected neighborhood, the image has one object.

0     0     0     0     0     0
0     1     1     0     0     0
0     1     1     0     0     0
0     0     0     1     1     0
0     0     0     1     1     0

Specifying Custom Connectivities

You can also define custom neighborhoods by specifying a 3-by-3-by-...-by-3 array of 0s and 1s. The 1-valued elements define the connectivity of the neighborhood relative to the center element.

For example, this array defines a “North/South” connectivity which can be used to break up an image into independent columns.

CONN = [ 0 1 0; 0 1 0; 0 1 0 ]
CONN =
     0     1     0
     0     1     0
     0     1     0

Note

Connectivity arrays must be symmetric about their center element. Also, you can use a 2-D connectivity array with a 3-D image; the connectivity affects each "page" in the 3-D image.