


       WELCOME TO THE FOURIER FILTERING DEMO

   This example demonstrates some of the filtering 
   capabilities of IDL using Fourier transformations. 

   The original image is shown at the left of the
   display. The image at the right is displayed as 
   selected by the droplist located below the image. 
   The original data is processed according to the 
   filter options shown to the left of the images.


   MENU OPTIONS
   ------------

   File Menu:
      Open...
      You can choose from among 16 different sets of 
      data from disciplines including astronomy,
      geology, and medicine.
      
      Quit
      Quit the Fourier Filtering Demo and return to 
      the IDL Demo main screen.

   
   Edit Menu:
      Select "Color Palette" to launch the XLOADCT 
      application. You can select different color 
      tables to help clarify the various images.


   Help Menu:
      Select "About Fourier filtering" for 
      information about the Fourier Filtering
      demo.


   FILTER OPTIONS
   --------------

   <<FUNCTION>> droplist
      Select between 3 filter functions:
      Butterworth, Exponential, and Ideal.

   <<ORDER>> droplist
       Select a filter order between 1 and 6.

   <<TYPE>> droplist
       Select the filter type: Low pass, High 
       pass, Band pass, or Band reject.


   <<FREQUENCY CUTOFF>> slider
       Select the frequency cutoff of the filter.

   <<BANDWIDTH>> slider
       Select the frequency bandwidth of the filter.
       This option is only enabled when the filter 
       type is set to Band pass or Band reject.

    PLOT VIEW OPTIONS:

       The droplist located under the display to the 
       right consists of the following options :

       o  Original signal
       o  Log power spectrum
       o  Filter plot
       o  Filter intensity image
       o  Filter cross section
       o  Shaded surface of filter
       o  Log filtered power spectrum
       o  Filtered signal


       Filter intensity image, Filter cross section, 
       and Shaded surface of a filter are enabled 
       only when viewing an image data set.





