Dithering is similar to halftoning in newspapers. Only a single color (black) is used in contrast to the paper (white), but due to using patterns the appearance of many shades of gray can be represented. In a similar way, OpenGL can dither the fragment from a high precision color to a lower precision color. I.e. dithering is used to find one or more representable colors to ensure the image shown on the screen is a best-match between the capability of the monitor and the computed image.
This is always needed when working with 8 Bit color-index mode, where only 256 unique colors can be represented, but the image to be drawn is actually calculated with higher precision. Dithering also applies when a RGBA32f color is converted to display on the screen, which is usually RGBA8. In RGBA mode, dithering is performed separately for Red, Green, Blue and Alpha.
Dithering is the only state that is enabled by default, the programmer has no control over how the image is manipulated (the graphics hardware decides which algorithm is used) besides enabling or disabling dithering with EnableCap.Dither.
GL.Enable( EnableCap.Dither ); // default GL.Disable( EnableCap.Dither );
The state of dithering can be queried through
Result = GL.IsEnabled( EnableCap.Dither );