AV1 Video Encoding in Browser with rav1e and Wasm

Yes, rav1e (the library form of librav1e) can be compiled into WebAssembly (Wasm) to encode AV1 videos directly inside a web browser. Because rav1e is written in Rust, it leverages Rust’s premier support for WebAssembly compilation. This allows developers to run high-quality AV1 video encoding client-side without relying on external server-side infrastructure.

How to Compile and Use rav1e in Wasm

To run rav1e in a browser, you must compile the Rust source code into a WebAssembly module. This is typically achieved using wasm-pack, a tool that compiles Rust to Wasm and generates the necessary JavaScript glue code.

1. Set up the Rust library: Create a Rust wrapper around the rav1e crate that exposes the encoding functions you need.
2. Compile to Wasm: Run wasm-pack build --target web to compile the crate.
3. Handle Video Inputs: In the browser, you can extract raw video frames using the WebCodecs API (such as VideoDecoder or VideoTrackProcessor) or read pixel data from an HTML5 Canvas.
4. Pass Data to Wasm: Send the raw YUV or RGB frame data to the WebAssembly module, where rav1e processes the frames and outputs the compressed AV1 bitstream back to JavaScript.

Performance Considerations

While functional, encoding AV1 video in a browser is highly CPU-intensive. To make browser-based encoding practical, you must optimize the environment:

• Enable Wasm SIMD: WebAssembly Single Instruction, Multiple Data (SIMD) is crucial for video encoding. Compiling rav1e with SIMD enabled provides a significant speedup for vector operations.
• Enable Multithreading: Video encoding benefits greatly from parallel processing. By using Web Workers and enabling SharedArrayBuffer in the browser, rav1e can utilize multiple CPU cores.
• Adjust Speed Presets: rav1e offers speed levels from 0 (slowest, highest quality) to 10 (fastest). For browser environments, you should use fast presets (such as 8 to 10) to ensure the encoding process finishes in a reasonable timeframe.

Current Limitations

• Processing Speed: Even with SIMD and multithreading, browser-based encoding is significantly slower than native execution. Real-time encoding of high-resolution video (like 1080p or 4K) is currently not feasible in a browser.
• Memory Constraints: Standard WebAssembly is limited to a 4GB linear memory space. Encoding long or high-resolution videos can exceed this limit if frames are buffered in memory.
• Battery Drain: The high CPU usage required for AV1 encoding will quickly drain the battery on mobile devices.

Practical Use Cases

Despite the performance limitations, using rav1e in the browser is ideal for specific scenarios:

• Short Clip Generation: Creating short GIFs, social media clips, or memes directly on the client side.
• Privacy-Focused Apps: Encoding video locally so user data never leaves their device.
• Cost Reduction: Offloading encoding tasks from expensive server-side cloud infrastructure to the user’s local machine.