Pre-Processing > Frequency Filter

Modifies the Fourier transform of the Current Image with either custom or preset filters. Has many applications, ranging from background removal to edge-finding to artifact reduction.

1. Custom

a. Selection Tools

  • Outline: Fill/erase a polygon area by clicking to create nodes. Click the last node to close the polygon.
  • Ellipse: Click, drag, and release to fill/erase an elliptical/circular selection
  • Box: Click, drag, and release to fill/erase a rectangular/square selection
  • Freehand: Click, drag, and release to fill/erase a freehand selection. Selection closes automatically.
  • Line: Click, drag, and release to fill/erase a straight line selection
  • Pen: Click, drag, and release to fill/erase a pen (freehand line) selection
  • Circle Click: Click a point to define a center of circular selection. Pixel diameter of circles is determine by Pen Size.

b. Process Selection

  • Erase: Erase the selection from the image’s FFT
  • Keep: Keep the selection in the image’s FFT (discards the rest if no other selection has been processed, adds the selected intensity back if one has been processed)
  • Undo Undo the last fill/erase operation

b. Mirror/Invert Selection

  • Mirror X-Axis: Mirror current filter over X-axis
  • Mirror Y-Axis: Mirror current filter over Y-axis
  • Mirror Origin: Mirror current filter over origin
  • Invert: Invert current filter

b. Pen Size

Controls the thickness (in pixels) of the Line and Pen selection tools, as well as the diameter (in pixels) of the circles created by the Circle Click tool

2. Cutoff

Use a cutoff filter. Places a hard-edged circle filter around the center of the FFT.

  • Low-Pass: Keep frequencies within the filter
  • High-Pass: Keep frequencies outside filter
  • Cutoff Frequency: Size of the filter circle in frequency space

3. Ideal

Use an ideal filter. Places a 2D Gaussian filter around the center of the FFT.

  • Low-Pass: Keep frequencies within the filter
  • High-Pass: Keep frequencies outside filter
  • Sigma: Proportional to the size of the filter circle in frequency space

4. Annular

Use an annular filter. Places a ring filter around the center of the FFT.

  • Inside-Pass: Keep frequencies within the filter
  • Outside-Pass: Keep frequencies outside the filter
  • Outer Frequency: Size of the outer ring diameter
  • Inner Frequency: Size of the inner ring diameter

Tips

  • The points on the FFT will always be perpendicular to the direction of the artifact you wish to remove.
  • Example: SEM curtaining is causing vertical artifacts in the image, remove horizontal points in FFT to remove curtaining. (See Tutorial Video)

Tutorial

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