ShellNewHandler VS JPEG XL Convert

JPEG XL Convert's answer:

Two groups, mostly.

The first is anyone who has been handed a file their computer refuses to open: a .jxl someone exported, or an iPhone .heic photo that Windows shows as a blank icon. These people are not technical, they do not want to install anything, and they are not going to use a command line. They just want the picture back in a format that works everywhere.

The second is photographers, designers and web developers who are deliberately moving to JPEG XL for the file-size savings, and who need a quick way to convert or inspect a file without opening a paid desktop app or building libjxl themselves.

There is a third group worth mentioning, because it is the reason the tool works the way it does: people who cannot upload the images at all. Because every conversion happens locally in the browser and nothing is ever sent to a server, it gets used with medical images, legal documents, unreleased product photos and client work under NDA, where handing the file to a third-party converter is simply not an option.

JPEG XL Convert's answer:

JPEG XL is a genuinely better image format. Files are substantially smaller at the same quality, it can recompress an existing JPEG losslessly, and it handles wide colour and high bit depth. The problem is that almost nothing opens a .jxl file. Chrome pulled its support, Windows shows a blank thumbnail, and the standard advice is to build libjxl and use the command line. That is a reasonable answer for a developer and a dead end for everybody else.

The other half of the problem is what the existing web converters do with your files. They upload them. Once your photo is on someone else's server it is their copy, on their disk, under their privacy policy and their retention period, and you have no way to check what happens to it.

The tool was built to solve both at once: compile libjxl to WebAssembly so the browser itself can decode and encode JPEG XL, and then never send the file anywhere. You drop a file on the page and the conversion runs on your own machine. That is not a promise in a privacy policy, it is something you can verify: open the browser's Network tab while you convert and there is no upload.

HEIC came next, for the simple reason that the most common version of "my computer will not open this photo" is an iPhone picture sitting on a Windows PC. The written guides followed after that, because a lot of the people arriving did not just need the conversion, they needed someone to explain what the file actually was.

JPEG XL Convert's answer:

The important part is that the actual image codecs are the reference implementations, compiled to WebAssembly and running inside the browser tab rather than on a server.

• libjxl compiled to WebAssembly, including cjxl for encoding, which is the same encoder the JPEG XL project ships for the desktop. The build detects WASM SIMD and threads at runtime and uses them where the browser allows it.

• libheif compiled to WebAssembly for reading Apple HEIC and HEIF photos.

• Plain JavaScript with no framework, plus OffscreenCanvas and createImageBitmap for the raster steps, and a worker for the heavier HEIC decoding so a large photo does not freeze the page.

• Static HTML and CSS, hand-written and generated by small Node scripts at build time. There is no framework, no bundler runtime and no client-side router.

• Hosted on Cloudflare Pages. There is no backend, no database and no upload endpoint, which is what makes the privacy claim structural rather than a policy: there is no server that could keep your files even if it wanted to.

JPEG XL Convert's answer:

Three reasons, and one honest reason not to.

Your files stay with you. The competing online converters upload your images to their servers to do the work. If the photos are client work, medical scans, documents or anything you would not email to a stranger, that is a real problem and no privacy policy fixes it. Here the conversion runs in your browser, so the question never comes up.

No limits and nothing to install. No sign-up, no daily quota, no file-size cap, no watermark, no paid tier holding the useful part hostage. Server-based converters have to meter you because bandwidth costs them money; this one costs nothing to run, so it does not.

It actually opens the files. A lot of tools convert to JPEG XL but cannot show you a .jxl you already have, and Windows still shows those files as blank icons. There is a viewer here, and the same applies to iPhone HEIC photos. There are also written guides explaining what these formats are and why your computer refuses them, which is usually the real question behind the search.

When not to use it: if you need to convert video, audio, spreadsheets or documents, or batch-process thousands of files on a schedule, a general-purpose service like CloudConvert or a desktop tool is the better answer. This one does images, and it does them without ever taking a copy.

JPEG XL Convert's answer:

Everything happens on your own machine. The converter is libjxl and libheif compiled to WebAssembly, so your browser does the decoding and encoding itself and the file is never uploaded to a server.

That is the part you do not have to take on trust. Open the browser's Network tab, convert an image, and you will see there is no request carrying your file. Most "free online converters" send your photos to their servers and then ask you to believe a privacy policy about what happens next. Here there is no server to send them to: the site is static, with no backend, no database and no upload endpoint.

The practical consequences are the ones people actually notice. Nothing to install and no sign-up. No file-size cap and no daily limit, because there is no bandwidth being paid for. No watermark. It works offline once the page has loaded, and it works on a phone. And it opens .jxl and .heic files that Windows and most browsers still refuse to display at all.