Sharpening

The effect lightens one side of an edge and darkens the other, which the eye reads as detail. A small amount recovers softness introduced by lenses, by downscaling, or by heavy compression.

Overdoing it is the classic upload artefact. Too much produces bright halos around outlines, crunchy skin texture and shimmering on fine patterns, and compression makes all of it worse, since the encoder now has extra high frequency detail to spend bits on and less left for everything else. Sharpen at the end of the chain, judge it at full size on a normal screen, and back off from whatever looked right at 200 per cent zoom.