b w local equalization menu

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Local Equalization is a method for enhancing the contrast of an image everywhere in the image. Most contrast enhancement schemes work globally, that is, they perform the same enhancement everywhere in the image. Local equalization enhances the contrast locally, which means that dark areas in the image get just as much enhancement as light areas. Local equalization is time consuming, but can be useful when all else fails. It 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 treats the neighborhood as a little image and calculates a contrast enhancement that would equalize the intensity histogram for that image. It then internally enhances the little image and adds the resulting pixel values into an accumulated image.
  3. When all the neighborhoods have been accumulated, each pixel in the image is replaced by the corresponding pixel intensity value in the accumulated image, divided by the number of times that pixel was accumulated into. Thus intensity of each pixel in the result image is the average of what that pixel would look like if each neighborhood that contained it were histogram-equalized.

The procedure for running local equalization is:

  1. You decide how big a neighborhood you want to use. A 16 by 16 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 click on the FILTER menu field. This starts the filtering procedure. It takes a long time.
  3. To see the result image, click on the DISPLAY menu field.

You can restrict the processing 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 initiate the processing, 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.