Banding, also called false contouring, is the staircase of flat stripes that appears where a picture should hold a smooth gradient - a clear sky, a fade to black, a studio wall. It has two compounding causes: too few brightness levels to draw the gradient (low bit depth) and a codec quantizing away the faint variation that carried it. It lives only in smooth, low-texture regions, which is also where the eye is freest to lock onto a straight low-contrast edge - human vision amplifies that step through the Mach-band effect, so a one-code-value contour looks glaring. This makes banding the worst case for the popular metrics: the per-pixel error is tiny, so PSNR can read about 48 dB and VMAF about 95 over plainly visible bands. Netflix states PSNR, SSIM, and VMAF are not designed to detect it, which is why CAMBI exists. Fix it with higher bit depth, dithering, or a debanding filter.
