How WebP to PNG Conversion Works in Your Browser
WebP is a RIFF-derived container that stores image data in one of two chunk types: VP8 for lossy frames or VP8L for lossless frames with an optional alpha channel. This page never parses those chunks itself — instead it hands the file to the browser's native WebP decoder. The file becomes a Blob URL, an HTMLImageElement is pointed at it, and the engine decodes the compressed bitstream into an in-memory bitmap, exactly as it would for any image loaded on a website.
That bitmap is then drawn into a Canvas 2D context whose backing store matches the image's natural dimensions. Crucially, nothing is composited underneath the drawing: unlike a WebP-to-JPG conversion, no opaque backdrop is painted first, so fully and partially transparent pixels survive the round trip. When canvas.toBlob() is called with 'image/png', the browser runs its native PNG encoder, which filters scanlines and applies DEFLATE compression to produce a lossless output stream.
Because the heavy lifting happens inside the browser engine, there is no WebAssembly module to download, no worker script, and no network dependency. On engines that support it, createImageBitmap() or an OffscreenCanvas could push the work off the main thread for very large images — but the decode itself is so fast that even a 4000×3000 pixel photo typically converts in well under a second. The resulting PNG preserves the full RGBA pipeline from decode to download, which makes it a safe interchange format for editors, print shops, and archival storage that do not yet read WebP natively.