Reflection of Structuring Elements

It is important to keep in mind that the structuring element you specify is applied in reflected form in morphological operations. The Grey‑scale Morphology tool automatically performs a reflection of the structuring element about the x- and y-axes. In most cases structuring elements are symmetric (such as the one shown in 3x3 grey‑scale morphological structuring element ) and are unaffected by reflection about the x- and y-axes. However, if the structuring element is asymmetric, you must take its reflection into consideration.

The reflections of variously shaped structuring elements are shown in the figure below.

Reflection of structuring elements

The offset values (profile) as well as the support of the structuring element are reflected. Also, the origin is reflected.

Note: Cognex nomenclature refers to the specified origin, not the reflected origin. As illustrated in the figure above, a 1x3 structuring element whose origin is (0,0) appears to have its origin at (2,2) when reflected, and a 3x3 structuring element whose origin is (2,0) appears to have its origin at (0,2) when reflected.

Asymmetric structuring elements can be used to detect asymmetric (directional) features. For example, you must specify an upright arrow to obtain the structuring element required to detect an upside down arrow.