Ring dilation and erosion techniques for digital image processing

Abstract

Dilation and erosion operations are performed on imaged objects that are represented using a number of object pixels; the object pixels are a subset of a larger number of image pixels. A ring-shaped structuring element is specified by a radius and an origin. In the ring dilation operation, the boundary of the structuring element includes two concentric circles, one having a radius that is one pixel greater than the other so that the boundary is two-pixels wide. The origin of a copy of the structuring element is then overlaid onto each object pixel such that the boundaries of the copies intersect to form a dilated image feature. The image feature includes a feature boundary defined by the outermost portions of the boundaries of the overlapping copies of the structuring element. This feature boundary is identified and described using conventional chain-coding techniques. The dilation is then completed by filling an area defined within the feature boundary. To erode objects, a second ring-shaped structuring element is specified by a second origin and by a second radius. The origin of a copy of the second structuring element is overlaid onto each pixel within the object. A pixel within the object is selected only if the boundary of the copy of the second structuring element lies completely within the object when the origin of the copy is overlaid onto that pixel. The selected pixels represent an eroded version of the object.