If you simply google for ‘sky banding’ you’ll find a lot of photographs that exhibit ‘color banding’ over the sky gradient. It’s not an inherent problem of rombo.skylight nor necessarily a problem related to the sky but just something that can be seen on any smooth gradient. If there ain’t enough bits to fully represent the gradient, – banding may come out. However it may even be there for uncompressed raw floating point 32 bits images.
Generally the fix is to add some noise to the gradient. When in computer graphics noise is applied to color gradients to simulate colors not present in the palette by diffusing existing colors, this process is called dithering. The curious thing is that the term ‘dithering’ comes from WWII militar engineers that noticed that trajectory calculations were actually more precise when done directly on an airplane and less precise when done on the ground and just because the airplane was trembling while flying.
Dithering is used also in astrophotography where the telescope is slighty shifted between exposures to allows hot pixels, cosmic ray artifacts, and fixed pattern noise and even satellite or airplane trails to be removed during the stacking process. This is exactly what we do in rombo.skylight. We randomly shift environment rays to introduce some sort of pattern to avoid color banding. However we call this process ‘jittering’ as mental ray has already color dithering available.
You can find it under the Advanced panel in Environment Dome shader, at the very end.
This is a result from a jitter sky dome. Results may be a tad different from the non jittered sky because of the shifting in ray direction.