JPEG to JXL Converter: Lossless Re-Encode in Browser

JPEG XL came out of the same ISO working group that standardized JPEG, and it was built partly as an exit ramp from a format that has been the web's default photo codec since the early 1990s. This converter takes a finished .jpg and writes a .jxl, encoding the picture losslessly so the output decodes to the same pixels you started with. The mechanics are specific to how the browser handles each format. Your JPEG is decoded to a pixel grid by the browser's native decoder, drawn to an off-screen canvas with image smoothing disabled, then read back as raw RGB and handed to the @jsquash/jxl WebAssembly encoder with lossless mode on. Because it works from the decoded RGB buffer rather than the original DCT coefficients, this is a pixel-faithful re-encode, not JXL's bit-exact JPEG recompression mode. The pixels are preserved; the original JPEG container, its EXIF tags, and its color profile are not carried over. If a file trips up the lossless path, the encoder retries at quality 0.99 with high effort rather than failing outright. One honest caveat: lossless coding of photographic RGB is expensive, so a .jxl produced this way is often similar in size to the source JPEG and can be larger, especially for noisy or heavily detailed photos. You gain a modern codec and guaranteed pixel fidelity, not a guaranteed smaller file. Everything runs in your tab via WebAssembly, so the image never leaves your machine, which is what you want when the JPEGs are IDs, receipts, or anything you would not drop into a random upload form.