Vysvětlení (bohužel v angličtině)

Dithering

Dithering is a technique used in computer graphics to create the illusion of more colors when displaying an image which has a low color depth. In a dithered image, the missing colors are reproduced by a certain arrangement of pixels in the available colors. The human eye perceives this as a mixture of the individual colors.

The Gradient tool uses dithering. You may also choose to use dithering when you convert an image to Indexed format. If you are working on an image with indexed colors, some tools (such as the pattern fill tool) may also use dithering, if the correct color is not available in the colormap.

The Newsprint filter uses dithering as well. You can use the NL Filter (Non Linear filter) to remove unwanted dithering noise from your image.

Also note that although GIMP itself uses 24-bit colors, your system may not actually be able to display that many colors. If it doesn't, then the software in between GIMP and your system may also dither colors while displaying them.

See also the glossary entry on Floyd-Steinberg dithering, which is used in GIMP.