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Menu:   Processing > Filters… > Smooth > Sigma  
Script:   sigma  

Use this function to apply a non-linear smoothing to an image. Those pixels in the neighborhood which differ from the target pixel by less than Sigma are averaged, and the resulting value replaces the original one. If all the differences are greater than Sigma, the target pixel remains unchanged. As a result, the noise gets reduced while the fine structure preserved. If you choose this command, the following options will become available:

Displays the input image frame number. If you want to process another image, type or select the corresponding value.  

Displays the number of the frame in which the output image will be created. Depending on the Preferences. ImageWarp will set it either to the first available value or to the Input frame number. Type or select another value if you want the output image to be created in a different frame.  

Select this check box to watch the Preview of the output image.  

Lets you choose the size of the square neighborhood for filtering. This value must not exceed 2*Overscan+1. See Preferences for more details.  

Lets you adjust the filter's threshold. Only those pixels which differ from the central pixel by less than this value are taken for averaging. For better results Sigma should be close to the standard deviation of the noise distribution. You can determine it by acquiring a blank image under the same noise condition and using the Histogram tool on it.