background subtraction menu

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Background Subtraction means removing unwanted background information from an image. Sometimes, for example, the illumination in an image is so uneven that it is not possible to get a reasonable segmentation. Background subtraction lets you modify the image to compensate for the unevenness in the illumination. The procedure works like this:

  1. You display the image by clicking on the DISPLAY IMAGE menu field.
  2. You select a window size by entering a value in the Averaging window size menu field. This value is the width of the averaging window in pixels. The system estimates the background intensity at a point in the image by calculating the median intensity value for all points in a neighborhood of that point. This neighborhood is a rectangular area, centered on the point, that is pixels wide and high. A size value in the 4-12 range is typical.
  3. You click on the SELECT BACKGROUND POINTS menu field. You should now indicate points in the displayed image that are representative of the background intensity level, that is, points that are relatively flat or uniform in intensity. The idea is that the image will be modified so that all the points you specify will have exactly the same intensity when the background is subtracted out. Your background points should be spread out as uniformly as possible across the image. If you make a mistake, you can click on the REMOVE BACKGROUND POINTS menu field and selectively remove points.
  4. When you have selected all your background points, click on the SUBTRACT BACKGROUND menu field. The system will calculate the background image, and subtract it from your image. You can see the result by clicking on the DISPLAY IMAGE menu field again. You can control the clipping of the background image as a B&W image file by entering an image name in the Image file menu field, and clicking on SAVE BACKGROUND IMAGE. By examining this image, you can see where you may need to add more background points.
  5. You can save and recall your background point layouts. To save a layout, enter a name in the Layout file menu field, and click on SAVE POINT LAYOUT. To recall one, select a name and click on RECALL POINT LAYOUT. You can add points to, or remove points from, recalled point layouts, and then save them again as many times as you want.