The morphological gradient highlights sharp transitions in the input image. It depends less on edge directionality than the Sobel operator and is useful for locating faint but large scale structures. The morphological gradient is defined by,
It is thus the difference between a dilated image and an eroded image. Dilation removes small scale dark features and erosion removes small scale bright features. Dilation brightens the image and erosion darkens it. When the difference is obtained, all small scale features are gone and the contrast between the large scale features improved.