Freezing - more precisely a stall or rebuffering - is the event where playback stops because the player's buffer ran dry, and the last decoded frame sits frozen on screen (often under a spinner) until enough data arrives to resume. Over reliable transport like the TCP or QUIC under HLS and DASH, a network problem does not corrupt pixels; lost packets are retransmitted, which takes time, so the buffer drains and you get a freeze. It is the artifact that defeats a picture metric most completely, because during a freeze every frame on screen is a correct, fully decoded frame - simply the same correct frame held too long, so a full-reference PSNR, SSIM, or VMAF finds nothing wrong. The fault is in time, and a per-frame score has no time axis. Freezing also carries the hardest business numbers: a rebuffering ratio equal to 1% of duration costs about 5% of watch time, and startup past 2 seconds raises abandonment by roughly 5.8% per added second. Measure it with player-side QoE metrics (CTA-2066) and session models (ITU-T P.1203/P.1204), not picture scores.
