Resolution: How Big a Wallpaper Needs to Be
A screen is a fixed grid of pixels, and a wallpaper only has to cover it — the rest is margin, not merit.
A display is a countable number of pixels, and a wallpaper's only hard requirement is to have at least that many. A 1080p panel asks for 1920 by 1080; a 4K panel asks for 3840 by 2160. Give it less and the system stretches what it has, which reads as softness — and softness shows up first in fine texture, in moss, in sand, in the grain of rusted metal, long before it shows in an open sky.
More pixels than the screen can display is not a gain, only weight. The extra gets downsampled away, and what stays behind is a larger file sitting in memory. A little margin still helps, because aspect ratios rarely line up: 16:9, 16:10 and 21:9 all crop one another, and the crop always eats the long edge. Match the shape first, then argue about the numbers.
Where sharpness actually fails is narrower than it sounds. Long straight edges reveal the stair-step, smooth gradients reveal banding, and heavy compression reveals both — a badly saved 4K file looks worse than a carefully saved 1080p one. A wallpaper is seen between windows, a few seconds at a time. It needs to be clean enough to go unnoticed, and then it should get out of the way.
