Why Smooth Skies Come Out Striped
Banding is what happens when a gradient runs out of numbers before it runs out of screen.
A clear sky at dusk changes colour so slowly that a large screen may need a thousand steps to draw it. An ordinary eight-bit file has 256 values per channel, and when a gradient is stretched across two thousand pixels each of those values has to cover a strip several pixels wide. The strips are the bands.
Compression makes it worse by flattening near-identical pixels into the same block, which is exactly the wrong economy for a photograph that consists of nothing but small differences. Some of the fix is upstream: a ten-bit source, a gentle grain, or the dither that good export tools add on purpose to break the edges apart. Some of it is downstream, since a monitor doing its own rough conversion can invent bands that were never in the file.
It shows up most on the quietest pictures, which is a small irony for anyone who chose a plain sunset to keep the desktop calm. Look for it in the upper third, where a wide expanse of one colour has the most room to step. A slight texture in the sky is not a flaw in the photograph so much as a wallpaper's way of staying smooth.
