6.5. NL Filter


This filter is found in Image>Filters/Enhance/NL Filter. NL means "Non Linear". It originates from the "pnmnlfilt" program: more details at www.rt.com.

It joins smoothing, despeckle and sharpen enhancement functions. It works on the whole image, not on selection.



When Do preview is checked, parameter setting results are interactively displayed in preview.

Parameter settings

  • Alpha: Meaning of this value depends on the selected option.
  • Radius: Controls the strength of the filter (0.33-1,00).


  • Alpha Trimmed Mean: When Alpha is 0.00, this option works as a smoothing filter with radius determining strength. When Alpha is 0.5, this option works as a despeckling filter, good for single pixel noises without altering the rest of the image and radius allows fine tuning of filtering.
  • Optimal estimation: This option is good for filtering out dithering noises. Low alpha values give the most subtle filtering effect. High alpha values tend to smooth all parts of the image.
  • Edge enhancement: Alpha controls the amount of edge enhancement. Useful alpha values are between 0.5 and 0.9.