Closing is another operation from mathematical morphology that is derived from the fundamental operations of erosion and dilation. Like those operators, it is usually applied to black and white images, but can be applicable to grayscale images as well. Closing is similar in some ways to dilation in that it tends to enlarge the boundaries of foreground (bright) regions in an image (and shrink background color holes in such regions), but it is less destructive of the original boundary shape. As with other morphological operators, the exact operation is determined by a structuring element. The effect of the operator is to preserve background regions that have a similar shape to this structuring element, or that can completely contain the structuring element, while eliminating all other regions of background pixels.

For more information, see the IG_IP_close function.