This filter widens and enhances dark areas of the active layer or selection.

For every image pixel, it brings the pixel Value (luminosity) into line with the lowest Value (the darkest) of the 8 neighboring pixels (3×3 matrix). So, a dark pixel is added around dark areas. An isolated pixel on a brighter background will be changed to a big “pixel”, composed of 9 pixels, and that will create some noise in the image.

In this image, the studied pixel has a red border and the studied 3×3 matrix has a green border. Thus when the pixel marked with “I” is inside the green border, the studied pixel turns to black.

A larger dark area will expand by one pixel in all directions:

The filter was applied 3 times.

On more complex images, dark areas are widened and enhanced the same, and somewhat pixelated. Here, the filter was applied 3 times:

Of course, if background is darker than foreground, it will cover the whole image.

This filter is found in the main menu under Filters → Generic → Erode.