How this MP4 converter works
An MP4 converter takes a video saved in another container or codec and writes it again as an MP4 that plays on almost every phone, browser and editing app. When the video is already H.264, VimCut copies that stream into the MP4. Otherwise it re-encodes the first video track to H.264 and the first audio track to AAC.
The output uses the yuv420p pixel format, which keeps it playable on every device, and the MP4 is laid out so it can start playing before it has fully loaded. Only the first video and audio stream are kept, because some sources carry extra data or subtitle streams that MP4 cannot hold. When your browser supports it, the work uses the built-in hardware video encoder. Otherwise it falls back to WebAssembly (ffmpeg.wasm). A progress bar shows how far the conversion has got.