gradient filter menu

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Gradient Filtering is a method for enhancing the edges in an image. It produces an image that has dark areas where the original image has smooth areas, and light areas where the original image has areas of rapidly varying intensity (particularly near edges).

The Gradient filter works as follows:

  1. For each pixel in the image the system defines a neighborhood of that pixel. A 3 by 3 neighborhood, for example, would include the pixel and its eight immediate neighbors.
  2. The system calculates the average intensity for all the pixels in the neighborhood.
  3. The system adds up the absolute values of the differences between each neighborhood pixel's intensity and the average value.
  4. The pixel's intensity is replaced by this accumulated value.

The procedure for running the gradient filter is:

  1. You decide how big a neighborhood you want to use. A 3 by 3 neighborhood is typical. Indicate the dimensions of the neighborhood on the menu. The first number is the width of the neighborhood in pixels, and the second is the height. The larger the neighborhood, the longer it takes.
  2. You decide whether you want the system to stretch the image for you when it gets done. If you set the Auto-stretch field to Yes, the system will stretch the filtered image so as to clip off 0.05% of the pixels on the dark end and 0.05% on the white end. If you set it to No, the system will display a histogram of the filtered image and let you indicate the clipping levels yourself.
  3. You click on the FILTER menu field. This starts the filtering procedure.
  4. To see the result image, click on the DISPLAY menu field.

You can restrict the filter to work on only a portion of the image by specifying an Area of Interest (AOI). The AOI can be either rectangular or polygonal. To specify a rectangular AOI, first display the image, then click on USE RECTANGULAR AREA. The system will prompt you to indicate the rectangular area on the image. Do so with two clicks. When you then apply the filter, only the pixels in the rectangular area will be affected. Similarly, you can make an irregular AOI by first clicking on USE POLYGONAL AREA and then outlining the AOI with a series of clicks on mouse button 1. Terminate the drawing of the AOI by clicking with button 2. If you have restricted processing to an AOI and you want to make it apply to the entire image again, click on USE ENTIRE IMAGE.