FFT Filter

Applies a frequency filter to the image. This function performs three steps. First, it finds the fast Fourier transform (FFT) of the source image, which is a complex image. Then the function filters (truncates or attenuates) the complex image. Finally, it computes the inverse FFT. Generally, the FFT Filter is used to compute backgrounds in order to correct light drifts. The following controls are available.

• Step Name—Name to give the step.
• FFT Filters

  Image Source	Opens the original input image.
  Truncate	Truncates the frequencies of a complex image.
  Attenuate	Attenuates the frequencies of a complex image.

• Mode—Determines which frequencies to truncate or attenuate.

  Low Pass	Removes high frequencies.
  High Pass	Removes low frequencies.

• Truncation Frequency %—Percentage of the frequencies retained within a Fourier-transformed image. This percentage is expressed with respect to the length of the diagonal of the FFT image and the Mode value.
• Display FFT—Shows the complex image after filtering.