Ringing is the faint halo of repeating ripples a codec leaves beside a sharp, high-contrast edge - a ghost of the edge echoing into the flat area next to it, like a struck bell that keeps sounding. Around black text on white it shows as a grey haze hugging each letter. The cause is the Gibbs phenomenon: an edge carries its energy in high-frequency DCT coefficients, quantization rounds those away, and a step rebuilt from too few frequencies overshoots (about 9% of the jump height) and oscillates. Ringing is a spatial artifact, visible in a single paused frame; its temporal form, mosquito noise, shimmers around moving edges. Unlike banding, ringing is genuine pixel error, so PSNR, SSIM, and VMAF do react to it - but it hides because it is localized in the thin band beside edges (a few percent of the frame), so whole-frame mean-pooling dilutes it. Codecs fight it with deblocking, HEVC's SAO, and AV1's direction-aware CDEF.
