The stripes are in the screen, not in the picture. Set a deep purple-to-navy gradient as a phone wallpaper in a dim room and, in the darkest third, you may find concentric rings or long stair-steps where the color should slide smoothly. Zoom into the source image on a better display and the ramp is clean. What you are seeing is called banding, and it is the seam of digital color arithmetic: displays of the common 8-bit class can set each channel to only 256 values, and where a gradient is dark enough, or stretched enough, neighboring steps drift apart far enough for your visual system to tell them apart. The staircase was always there; the wallpaper just gave it room to be seen. It is the desktop-sized version of the lesson inside why color sliders feel uneven—the smooth ramps you think you're setting are always secretly staircases.
A ramp that is secretly a staircase
Every pixel on a standard display is told, per channel, a number from 0 to 255. A "smooth" gradient file is therefore not a continuous ramp but 256 tiny ledges—some subset of which your screen must travel between. When the gradient spans the full tonal range, adjacent ledges differ by amounts no human eye can separate, and the staircase fuses into a ramp. Trouble starts when a gradient occupies a compressed slice of the range, especially near black: divide a dark fade across fewer and fewer usable steps and each jump between neighboring steps grows, in both physical luminance and in visual difference, until the edges between ledges become visible as contour lines. Perception sets the threshold—roughly, we notice luminance differences of about one percent in good conditions—which is the same discrimination limit explored in how close two colors can be and still look identical. Banding is simply that limit being crossed, not once, but in a repeating pattern your eye treats as structure.
Two accomplices usually widen the steps. Image compression loves flat gradients because they compress so efficiently—until it quantizes them into exactly the terraces you're trying to avoid, a habit inherited from video codecs squeezing dark night skies. And display gamma maps few stored values into the dark range your eye scrutinizes most, which is why banding gathers toward the shadowed end of a gradient rather than the bright end. Run the arithmetic on a typical wallpaper and the trouble becomes concrete: a gradient that spans only the bottom fifth of the tonal range has perhaps fifty usable code values to cross it, so each ledge drops several percent in luminance—comfortably past what a dark-adapted eye can separate. Widen the gradient across the full range and the same fifty steps spread over ten times the territory, shrinking each one below notice. The picture didn't get smoother; the staircase got denser underfoot.
Noise as an honest fix
The counterintuitive cure was discovered in the early 1960s, in the least glamorous corner of imaging: sending pictures from early spacecraft with as few bits as possible. Engineer Lawrence Roberts showed in 1962 that adding a small amount of pseudo-random noise before quantization—and subtracting the same noise at the receiver—lets a coarse staircase average out into a smooth, grainy ramp. Grain replaces contour. That trick, dithering, is why a slightly noisy photograph of a sunset looks better than the "clean" version that bands: the noise is doing work, breaking big visible steps into tiny invisible ones. The imaging literature later carried dithering from grayscale into color displays, and every modern GPU and many panels apply it automatically. When your phone renders a gradient smoother than the raw numbers say it should, grain is the mechanism.
What actually helps tonight
You cannot upgrade your panel's bit depth by shopping for wallpapers, but you can stop feeding the problem. Prefer gradients distributed across the mid-tones rather than plunging to near-black; the dark end is where steps separate. Favor files with a little inherent grain or add faint noise in any editor—two percent is plenty—to shake the terraces apart. If the device supports it, enable higher bit-depth color output, since 10-bit paths, the standard route for HDR content specified in ITU's BT.2100 recommendations, multiply the number of available steps per channel and shrink each step below the visibility threshold. To tell whether the file or the screen is at fault, view the same wallpaper on a better display or in a screenshot: if the stripes are baked into the file, no screen will save them, and if the file is clean, the display pipeline is quantizing. Either way, treat visible banding as diagnostic rather than offensive: on noisy night photos, grain and contour trade places in exactly this way. If you want a feel for how coarse the steps your eye tolerates really are, try rebuilding a target color in discrete jumps with Blind Sliders, where every adjustment lands blind in steps you can't smooth out.
Both strips use the same eight levels; the right one adds a noise pattern that lets the eye average the edges away, as dithering does in real displays.