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Bilibili’s web upload page now uses the browser‑native WebCodecs API to decode video frames client‑side, replacing slower Canvas and Wasm‑FFmpeg pipelines, achieving 2.5–5× faster cover‑frame extraction with lower memory use, while supporting MP4/WebM and improving user experience.
By preloading the MP4 header and initial frames and routing playback through a local proxy that caches range‑requested segments in an LRU disk store, the system moves the moov box to the file start (or fetches it separately), cutting short‑video start‑up latency to roughly 800 ms and delivering near‑instant playback.
The article outlines the MP4‑based M4A container’s hierarchical box format, details the Sample Table (stbl) and its essential sub‑boxes (stts, stsc, stco/co64, stsz) that map playback time to file offsets, explains time‑scale conversion, optional boxes, and provides pseudo‑code and parsing tips for precise random‑access seeking.