Convert SVG to JXL

SVG is read by every browser shipped this century, by every design tool, by most office software, by anything that renders HTML in an email, and by a text editor if you are desperate. JPEG XL is read by libjxl and by the software that has linked it. That is the trade this conversion makes, and it is the largest thing on the page.

It was measured rather than assumed. A test drawing converted here produced a valid 20,368-byte JPEG XL, and handing that file straight back to the same browser’s decoder failed with "The source image could not be decoded". Chrome shipped support behind a flag and removed it in 2023; Safari is the major browser that reads one today.

The same 140-byte drawing, rendered 1,024 pixels wide and converted to every raster target this site writes: 9,730 bytes as AVIF, 20,368 as JPEG XL, and 4,195,304 as an uncompressed TIFF. JPEG XL, the newest format in the list, produced twice the bytes of AVIF.

That ordering is specific to small flat artwork and reverses on photographs, where JPEG XL is excellent. Quoting the photographic result as a reason to convert drawings is the mistake this section exists to prevent — and every number in the list is enormous next to the 140 bytes the drawing already occupied.

The format was built around two things that matter enormously in photographic archives. It can re-encode an existing JPEG losslessly, cutting roughly a fifth off a photographic library without touching a pixel, and it handles very high bit depths and very large dimensions for scanned and sensor-derived material.

A drawing offers it neither. There is no JPEG to transcode, no sensor noise to model, no gradient beyond what the artwork happens to contain and no bit depth beyond eight. What remains is a general-purpose encoder applied to flat fills and hard edges, which is where every modern codec compares worst against a vector or a PNG.

JPEG XL has two legal file forms. The bare codestream begins with the two bytes FF 0A. The container form wraps the same codestream in an ISOBMFF box structure beginning 00 00 00 0C 4A 58 4C 20, which is what lets a file carry Exif, XMP and an ICC profile alongside the image.

The files written here are the bare form, confirmed by reading the first bytes of the output. libjxl reads both, and a strict ingest configured to expect boxes will reject one that has none. If a pipeline refuses the file, check the header before assuming the encoder produced something invalid.

An SVG has no pixel size, so the conversion picks one: 1,024 pixels wide by default with the height following the drawing’s proportions, and the scale setting covers 0.1 to 10 for anything from 102 to 10,240 pixels.

That choice is permanent. The whole reason to keep a drawing as a vector is that the size question stays open — the same file serves an icon, a page banner and a print sheet. Rendering it answers the question once, and no later step can reopen it: enlarging the JXL interpolates pixels that were never drawn, and tracing it back into paths produces a different, worse file.

The alpha channel is supported by the format and preserved through the conversion, so a cut-out drawing does not acquire a background — unlike the GIF path on this site, which turns the same transparent pixels opaque black.

What is not offered is a lossless switch. The quality control starts at 82 and the pair is classified as lossy, which honestly describes what the encoder is asked to do. If the reason for converting is archival fidelity, that is an argument against this path rather than for it: the SVG is already exact and PNG is lossless without a setting to get wrong.

GIMP and ImageMagick read JPEG XL today, both are the kind of software that persists, and libjxl is open source with the specification published as ISO/IEC 18181. The decoder is not going to vanish the way a proprietary one might.

Against that, the format has been standardised since 2021 and its browser support has moved backwards rather than forwards in that time. Archival formats are chosen for confidence that something will read them, and on that measure PNG, TIFF and — for a drawing — SVG itself are all ahead by a wide margin.

This is worth stating plainly because it cuts against the instinct that newer means better preserved. An SVG is XML: plain text, an open W3C specification, readable by a human with a text editor if every renderer on earth disappeared, and diffable in version control so its history is legible. Very few formats in any category are in that position.

A rendered copy is a derived artefact, useful for showing the drawing to something that cannot read one. Keep the SVGs as the record and treat the JXLs as previews. An archive that replaced its vectors with a lossy raster in a format two browsers dropped would be a genuinely bad outcome, and it happens by converting a folder without thinking about which file is the master.

Drop the whole set and each drawing is rendered at the same scale and encoded on its own, with the results returned as a ZIP. The JPEG XL encoder is one of the two large WebAssembly modules on this site, so the first file pays for the download and the rest follow quickly.

Check a sample in whatever will consume them before converting the rest. Since the browser may not display a JXL at all, the usual habit of opening one to confirm it looks right does not work — and a batch that quietly produced files nothing in the pipeline accepts is an expensive thing to find out late.