Posterization is the close cousin of banding: when there are too few tonal levels to represent a smooth gradient, the continuous range flattens into a small set of discrete bands of constant tone, the way a poster printed with a reduced palette renders a photograph in a handful of flat colors. It shares banding's two roots - too low a bit depth (few code values per channel) and coarse quantization that rounds away the faint variation carrying the gradient - and lives in the same place, the smooth regions like skies, fades, and flat color fills, where the eye locks onto the abrupt steps between levels (amplified by the Mach-band effect). Like banding, posterization is a tonal-quantization artifact the popular full-reference metrics are nearly blind to: the per-pixel error of a flattened level is tiny, so PSNR, SSIM, and legacy VMAF under-report it, and a no-reference vision-based detector such as CAMBI is the right tool. The fixes are the same - higher bit depth, dithering, or a debanding filter.
