Dithering

If colour banding is the disease, dithering is the cure, and the trade it makes is a good one: spend a little spatial detail to buy back colour detail. Where a ramp needs a value the display cannot produce, scatter pixels of the two nearest values it can, in the right proportion. From a normal viewing distance the eye integrates the pair and sees the colour that was never there. Nothing is smoothed and nothing is blurred, which is why dithering removes stripes while blur only softens their edges.

Three families do the scattering, and the names are worth knowing because they look different. Ordered dithering compares each pixel against a fixed threshold matrix, usually a Bayer matrix, which is cheap, deterministic, and leaves a faint crosshatch that becomes the look in retro work. Error diffusion, of which Floyd-Steinberg is the famous one, rounds each pixel and pushes the leftover error onto its neighbours, giving an organic scatter with no repeating pattern and no way to compute one pixel independently of the last. And random or blue noise simply perturbs each pixel a fraction of a level, which is what a noise layer over a CSS gradient is doing.

On the web the third one is the practical one. A grainy gradient is a dither: an feTurbulence field inlined as a data URI, laid over the wash at low opacity with a blend mode, and it costs nothing at runtime. Illustrator and Photoshop have shipped a dither checkbox on gradients for years, video encoders add grain for the same reason, and game engines dither in the shader at the very end of the frame. The strength is the craft: enough noise to break the step, not so much that a flat surface reads as textured. If the grain is visible as grain, it has stopped being a dither and become a decision.

Coarse dithering is also an aesthetic in its own right. The 1-bit Macintosh drew every grey it ever showed by dithering black and white, pixel art uses it for shading inside a tiny palette, and the deliberate roughness of a risograph print depends on it. Reaching for that on purpose is a different job from hiding a band, and the tell is the scale: a dither meant to disappear is one pixel across, and a dither meant to be seen is not.