JXL to ICO Converter

Encoding: the JXL uses lossy VarDCT or lossless modular coding; existing JPEGs can be repacked without loss; ICO uses each size stored as a headerless BMP or as a complete PNG, and the download is a multi-size ICO with PNG-compressed icons (16, 32, 48 and 256 px by default).

Bit depth: 8 to 16 bits per channel, or floating point in the JXL; ICO allows 1 to 32 bits per pixel, set per size, and the ICO written here has 32 bits per pixel. High-bit-depth JPEG XL files are reduced to 8 bits per channel.

Colour: sRGB, Display P3, linear and other spaces described in the codestream in the JXL; ICO supports RGB or a palette, per image, written here as 8-bit sRGB.

Transparency: the JXL stores it as an extra channel in the codestream; the ICO keeps it as 8-bit alpha in 32-bit and PNG entries, or a 1-bit mask in old icons.

Animation: frames inside a single codestream; the ICO holds one image, so only the first frame is converted.

Metadata: the JXL can carry EXIF and XMP boxes in the container form, often stripped by encoders; ICO can carry only a directory of the sizes inside, with no EXIF, but nothing is copied across.

JPEG XL is a next-generation format designed to replace JPEG. It compresses better than JPEG and WebP, supports lossless, HDR and progressive decoding, and can losslessly repack existing JPEGs.

A JPEG XL file is either a bare codestream starting with FF 0A or an ISO base media container whose 'jxlc' box holds the codestream. A recompressed JPEG keeps a 'jbrd' box that lets the original JPEG bytes be rebuilt exactly.

JXL files usually come from image editors, archival pipelines and lossless JPEG recompression.

ICO is a container that bundles several sizes of the same icon (16, 32, 48, 256 px) into one file. It is still the most compatible format for website favicons and Windows app icons.

An ICO starts with a 6-byte ICONDIR (type 1 for icons) and a 16-byte entry per image giving width, height, colour count and offset. Each image is either a BMP without its file header plus an AND mask, or a complete PNG.

ICO is the better choice for favicons and Windows icons, while JXL suits archiving and future-proof high quality storage.

ICO strengths: Multiple sizes in one file; Required by Windows and supported by every browser as a favicon. Drawbacks to weigh: Max 256×256 per image; Not for photos.