Opening
An opening is an erosion followed by a dilation, using the same structuring element. An opening can be used to remove some small objects, but without shrinking the image. The opening operator is a filter (it's an increasing and idempotent operator).
An important property of the opening (as well as of the closing) is that it is an idempotent operator. This means a second opening on your result image (with the same structuring element) gives the same result. So don't keep trying...
An example:
The first image is the original Lena image.
The second image is the result of a opening with a square structuring element
with side five.
The third image is the result of a opening with a square structuring element
with side eleven.
Notice the square artifacts due to the used structuring element. After opening
the number of light gray values has increased.
Lena hasn't become thicker or thinner.
To view the full-size figures before and after opening with a square structuring element with size three, five, seven, nine and eleven, or after opening with a disk-like structuring element with radius one, two, three, four or five, click on the respective links.
[ Mathematical Morphology ] -- [ Basics ] -- [ Erosion ]-- [ Dilation ] -- [ Closing ]