Judder is uneven, jerky motion caused by frames being displayed for unequal lengths of time - a smooth pan develops a recurring stutter-step that repeats several times a second. The classic cause is a frame-rate conversion that does not divide evenly: showing 24-frames-per-second film on a 60 Hz display forces 3:2 pulldown, where each frame should occupy 2.5 refreshes but cannot, so the system holds one frame for three refreshes (50 ms) and the next for two (33 ms). That 50-versus-33 unevenness is the judder. The content moves at constant speed; only the display timing is uneven, and the eye reads the mismatch as jerkiness. It is distinct from stutter, which is irregular and comes from dropped or late frames rather than a steady cadence. The catch for measurement is that judder is a timing fault, not a pixel fault: every frame is pixel-perfect, so PSNR, SSIM, and even VMAF (whose Motion2 feature only measures how much content moves) report excellent quality on a clip that visibly judders.
