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Rank Leveled Background

One method used to generate a background image is to remove the features from the image, leaving just the background. Of course, you can’t simply “remove” something from an image, you have to first identify the pixels that correspond to the features, and then decide what to replace those values with. One approach that has been used occasionally is to apply a Gaussian blur to the image with a large standard deviation. That is actually a poor technique for several reasons - large Gaussian filters are inefficient to apply, they mix the pixel values from the features into those of the background, rather than removing them, and the background produced is forced to vary gradually and can’t handle abrupt changes.

The preferred method uses a neighborhood ranking procedure. But rather than selecting the median value, either the brightest or darkest pixel in the neighborhood is kept. If the features are bright on a dark background, the procedure is to first replace each pixel with its darkest neighbor, and then in a second pass through the image to replace each pixel with its brightest neighbor. The individual operations are called erosion and dilation, and the combination is called an opening. If the features are dark and the background bright the order of the operations is reversed, a dilation followed by an erosion, and the combination is called a closing. Either a neighborhood with radius at least as large as half the width of the features must be used, or a smaller neighborhood used repeatedly until the features are removed. This interactive tutorial illustrates the process of generating and subtracting a background image produced by applying a rank filter (also called a morphological filter) to the original.

The tutorial initializes with a sample of rice grains imaged in the microscope appearing in the Specimen Image window. Adjacent to the Specimen Image window is the Filtered Image window showing the result of the filtering operation. Selecting the Background button shows the result produced by the ranking operation, and selecting the Leveled Result button shows the result of leveling the image contrast by removing that background. The Neighborhood Size slider controls the size of the neighborhood used for the ranking operation, while the Iterations slider controls the number of times that the erosion and dilation operations are performed.

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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