Convert JXL to ICO

JPEG XL’s recorded uses are archival and photography, and the people who have adopted it have done so at the front of a pipeline: it is where the original lives, because it holds up to 32 bits per channel, a full alpha channel and wide-gamut colour, and because it can absorb an existing JPEG losslessly.

What that leaves is a gap at the far end. Everything downstream — a site’s favicon, a Windows shortcut, an installer’s icon — wants formats that were specified decades before the master format existed, and ICO is the oldest of them, published by Microsoft in 1985. This conversion is that last step, and it is the one nothing else in a modern toolchain reliably covers.

JPEG XL support in browsers is recorded here as partial rather than universal, and that is not an academic detail — it is frequently why somebody is looking for a converter at all. The file will not preview in a tab, will not open from a file manager, and produces a blank or a download prompt everywhere it is tried.

The decoder used on this page is a WebAssembly module the site fetches and runs itself, so the browser’s own opinion about JPEG XL never comes into it. A file that shows as nothing in the browser converts here perfectly well, which also means you can use this to find out whether a .jxl you were sent contains what you expected.

Whatever the source measures, it is first scaled so that its longest edge is 256 pixels. That is the format’s ceiling rather than a policy: each dimension in an ICO directory entry is stored in a single byte, and 256 is recorded as zero because the number does not fit in the field. The sizes that actually go into the file come off that image — 48, 32 and 16 with the default set, each squared, centred and written as its own PNG.

For a master image that is a very large reduction, and it changes where the effort belongs. Nothing above 256 pixels contributes to the result, so a 6,000-pixel original is no better a starting point than a 600-pixel one, and with the default set nothing above 48 contributes either. What decides whether the icon is any good is the crop and the shapes in it, both of which have to be right before the scale happens.

JPEG XL stores transparency as a proper channel with every intermediate value available, and the PNG written inside the ICO stores the same. So a logo with soft antialiased edges, a shadow that fades out, or a mark drawn with a feathered stroke all survive into the icon with those edges intact.

That is the practical difference between this source and an older one. Formats with a single transparent colour give an icon a hard, jagged outline that looks wrong against both light and dark browser themes, and no setting fixes it. Starting from a master with real alpha means the icon composites correctly over whatever is behind it, which is most of what makes an icon look professionally made.

A master is usually the full composition — the whole mark, or a photograph with room around the subject. An icon is looked at in a 16 or 32 pixel box, and at that size a full composition is a smudge no matter how good the source was.

Crop first, hard, and crop square. Take the symbol, the monogram or a single distinctive element, centre it, and leave a small margin so it does not touch the edges. If the mark is a wordmark, use the initial rather than the words. A non-square master is not refused — it is centred on the square with transparency around it, so nothing is stretched — but the mark is then drawn across a fraction of an icon that is already only sixteen pixels wide. This is a design decision the conversion cannot make, and it is the difference between an icon and a coloured square.

JPEG XL can hold 32 bits per channel and wide-gamut colour. The image inside an ICO is a PNG with eight bits per channel and, in practice, no colour management applied by the browsers that draw it.

That reduction is normally invisible and occasionally is not. A mark defined in a wide gamut — a saturated brand orange or green that sits outside sRGB — will shift slightly when it is written into an icon, and it will shift differently again depending on how the operating system draws the tab. If the brand colour is contractual, check the icon against the specification rather than assuming it came through, and expect to nudge it rather than to match it exactly.

ICO was designed to hold several images — 16 pixels for a list, 32 for a desktop, 256 for a preview — so that the system picks the right one rather than scaling. The "Icon sizes" control is where you say which. The default writes the classic favicon three; "All common sizes" writes six, which is the set a Windows application resource expects and the one to choose if the icon is going into a build rather than onto a web server.

What the control cannot do is redraw. Every entry is your master reduced to that size in a filtered pass, so the 16 is a small version of the same drawing rather than a simplified one with its strokes snapped to the pixel grid. For a favicon that is the whole job. For a desktop application it is most of it: the remaining step, if the mark has fine detail, is to draw a simplified 16 by hand and swap it into the file in an icon editor, and the file this page produces is a sound input to that.

The favicon use is the one most people meet, but ICO predates the web by several years and it is still what Windows wants for a shortcut, an executable’s resource, an installer and a file-type association. A cross-platform application build will ask for an ICO, an ICNS and a set of PNGs, and this covers the first.

That is worth knowing because the requirements differ. A favicon wants the small sizes; an application icon is specified as a set running up to 256 and often with hand-tuned small versions. Both come out of this page, from different settings — but if the destination is a build script rather than a web server, read what it expects before assuming the default set is the whole job.

Both halves of this conversion run locally: the JPEG XL decoder is a WebAssembly module in the page, and the icon container is written in JavaScript beside it. No request carries the file, there is no account, and nothing is retained.

For this pair specifically that is more than a principle. A master file is the version with the layers flattened but nothing else given away — the full resolution, the untouched colour, sometimes the metadata — and it is the last thing worth handing to a free converter in exchange for a 4 KB icon.