Table of Contents

**What do the Noise filters do?**

Use the commands in this submenu to add noise to images or remove it. For more advanced capabilities, check out Erik Meijering's RandomJ package (Binomial, Exponential, Gamma, Gaussian, Poisson and Uniform) at www.imagescience.org/meijering/software/randomj/.

Adds random noise to the image or selection. The noise is Gaussian (normally) distributed with a mean of zero and standard deviation of 25.

Adds Gaussian noise with a mean of zero and standard deviation of 75.

Adds salt and pepper noise to the image or selection by randomly replacing 2.5% of the pixels with black pixels and 2.5% with white pixels. Note: this command only works with 8-bit images.

This is a median filter. It replaces each pixel with the median value in its 3 x 3 neighborhood. This is a time consuming operation because, for each pixel in the selection, the nine pixels in the 3×3 neighborhood must be sorted and the center pixel replaced with the median value (the fifth). Median filters a good at removing salt and pepper noise.

This is a selective median filter that replaces a pixel by the median of the pixels in the surrounding if it deviates from the median by more than a certain value (the **threshold).** It is useful for correcting, e.g., hot pixels or dead pixels of a CCD image.

**Radius** determines the area used for calculating the median (uncalibrated, i.e., in pixels). See *Process>Filters>Show Circular Masks* to see how radius translates into an area. **Threshold** determines by how much the pixel must deviate from the median to get replaced, in raw (uncalibrated) units. **Which Outliers** determines whether pixels **brighter** or **darker** than the surrounding (the median) should be replaced.