Transcoding
Transcoding (Format Re-encoding)
The process of converting a video or audio file from one codec or format to another by fully decoding the original and re-encoding it, typically to change the codec, bitrate, resolution, or container format.
技術的詳細
Transcoding is computationally intensive because it involves complete decode → process → re-encode. Each re-encoding with a lossy codec introduces generation loss (accumulated quality degradation). Transmuxing (container change without re-encoding, e.g., MKV to MP4 when both use H.264) avoids quality loss. Hardware acceleration (NVENC, QuickSync, VideoToolbox) significantly speeds up encoding. FFmpeg is the standard tool, and FFmpeg.wasm brings it to the browser via WebAssembly. Key transcoding parameters include target codec, bitrate (or CRF for quality-based encoding), resolution, frame rate, and audio settings.
例
``` Video codec comparison (1080p, 1 minute): H.264 (AVC): ~15 MB @ 2 Mbps — universal support H.265 (HEVC): ~10 MB @ 1.5 Mbps — 40% smaller, patent costs VP9: ~10 MB @ 1.5 Mbps — royalty-free (Google) AV1: ~8 MB @ 1 Mbps — royalty-free, best quality Encoding speed: H.264 > VP9 > H.265 >> AV1 (10x slower) ```