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Rolling Ball Filter
The top hat filter is good for locating any features of a known size by adjusting the radius of the crown. Objects too large to fit into the crown of the hat are selectively removed. Reversing the logic to use the darkest values in both regions enables the same procedure to isolate dust or other dark features. By replacing the interior value by the mean of the surroundings, the dust can be selectively removed. In this application the method is called a rolling ball filter. This interactive tutorial illustrates the application of a rolling ball filter to remove details from an image.
The tutorial initializes with a randomly selected specimen appearing in the Specimen Image window. The Choose A Specimen pull-down menu provides a selection of specimen images, in addition to the initial randomly chosen one. Adjacent to the Specimen Image window is the Filtered Image window showing the result of applying the rolling ball filter to the specimen. The Filtering Mode buttons select whether to remove pixels that are Bright or Dark compared to their surroundings. The Neighborhood Radius slider adjusts the size of the interior region used in the top hat comparison (the outer or annular region is one or two pixels wide). The Threshold slider controls the difference between the brightest or darkest pixel value in the interior and surrounding regions that must be exceeded for the pixel to be replaced by the mean of the pixels in the surrounding region.
Contributing Authors
John C. Russ - Materials Science and Engineering Dept., North Carolina State University, Raleigh, North Carolina, 27695.
Matthew Parry-Hill, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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