Pre-Processing > FFT Filter

Modifies the FFT 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: Select a polygon area by clicking to create nodes. Click the last node to close the polygon.
  • Ellipse: Click, drag, and release to create an elliptical/circular selection
  • Box: Click, drag, and release to create a rectangular/square selection
  • Freehand: Click, drag, and release to create a freehand selection. Selection closes automatically.
  • Line: Click, drag, and release to create a straight line selection
  • Pen: Click, drag, and release to create a pen (freehand line) selection
  • Circle Click: Click one or multiple points to define centers of circular selection. Pixel diameter of circles is determine by Pen Size. Double-click one point to create one circle. Click multiple points and press “Enter”, or double-click the last, to create multiple circles.

b. Process Selection

  • Erase: Erase the current selection from the image’s FFT
  • Keep: Keep the current 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)

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

Tutorial

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