June 10, 2004

Contents

Flatten Illumination using

FFT Low Cut Filtering

(Medium and High Pass Filtering)

Flattening background illumination can also be done in the spatial domain. (See Flatten Illumination exercise).  This exercise illustrates how to do it in the Fourier Domain.

• Open Sample Images / Betsy shipwreck.tiff with the File -> Open menu.
• Since the FFT requires square images where the sides are a power of two, make a 512x512 image with the File -> New menu.
• Click on the paint tool . Select a medium gray level by clicking near the middle of the LUT window, then click in the new blank image window to make the image a uniform gray. This is to minimize the brightness step at the boundary of the Betsy image. 

  In ImageJ, double-click on the color tool , select a medium gray from the ramp on the left, of the palette, click on the blank image, then use Edit / Fill.  The 512x512 image will now be uniform gray.  Click on the image again to cancel the selection.

• Paste the Betsy image into the middle of the blank image:
  • Click on the Betsy Image
  • Edit -> Select All menu
  • Click on the blank image
  • Edit -> Paste menu.

(Image reduced x2 for printing).

For ImageJ, skip this section. Use Process / FFT / Bandpass Filter, as described below.

• Take the FFT with the Process -> FFT -> FFT menu.
• 'Cut' a hole right in the middle of the FFT (eliminating the lower frequency components of the image.
  • Click on the oval selection tool .
  • With the shift key down, mouse a circle about this big: , where this is shown actual size. Move the selection (the marching ants circle) to the middle of the FFT image by dragging with the mouse (as shown here).
  • If necessary, make the eraser tool white (as shown here) by clicking on it, and then clicking on the white bar in the LUT window:.
  • Cut out the selection with the Edit -> Cut Selection menu. (or by using the Delete key - Mac only) or by using the command-x (control-X) key.

The center of the FFT should look like this:

• Take the inverse transform (Process -> FFT -> Inverse FFT) to show the filtered image.

• Click on the rectangle (or oval) selection tool , and select a representative part of the image having the gray levels of interest:

• Use the Process -> Enhance Contrast menu to enhance the contrast using only the range of gray within the selection.

• Compare this image with the final image in the flatten exercise. It is very similar except for the border.

Flatten the 512x512 image in ImageJ using Process / FFT / Bandpass Filter,

(Result not shown).  Then use Image / Adjust / Brightness/Contrast / Auto.