The Power of WebAssembly: In-Browser Video Compression
A deep dive into how we run FFmpeg — a 20-year-old C library — entirely inside your web browser using WebAssembly.
What Is WebAssembly?
WebAssembly (WASM) is a binary instruction format that runs in modern browsers at near-native speed. It lets developers compile C, C++, and Rust code and run it in the browser — unlocking capabilities previously reserved for desktop apps.
FFmpeg in the Browser
FFmpeg is the world's most powerful open-source multimedia framework. It handles encoding, decoding, and transcoding for virtually every video and audio format. By compiling FFmpeg to WebAssembly via Emscripten, we can run the full FFmpeg pipeline inside a browser tab.
How We Use It
When you compress a video on SimpleCompressor:
1. You select a file — the raw bytes stay in your browser's memory 2. FFmpeg.wasm loads lazily — only fetched when needed (~30 MB WASM binary) 3. FFmpeg runs in a Web Worker — so your UI stays responsive 4. H.264 encoding at your chosen CRF — CRF 20 is high quality, CRF 32 is smallest 5. The compressed file is returned as a Blob — never touches a server
Privacy Guarantee
Since everything runs locally, your video files are never uploaded. This is critical for sensitive content like internal meetings, medical footage, or legal recordings.
Performance
Modern CPUs can compress a 5-minute 1080p video in roughly 2-3 minutes using CRF 26 (medium quality). For smaller files or lower resolutions, it's much faster.