BMP to JXL Converter

Encoding: the BMP uses usually none; RLE for some 4 and 8-bit files; JXL uses lossy VarDCT or lossless modular coding; existing JPEGs can be repacked without loss, and the download is a JPEG XL encoded with libjxl compiled to WebAssembly, lossy at the quality you choose or lossless.

Bit depth: 1, 4, 8, 16, 24 or 32 bits per pixel in the BMP; JXL allows 8 to 16 bits per channel, or floating point, and the JXL written here has 8 bits per channel.

Colour: RGB or an indexed palette in the BMP; JXL supports sRGB, Display P3, linear and other spaces described in the codestream, written here as 8-bit sRGB.

Transparency: the BMP stores it as an optional fourth byte in 32-bit files, which many programs ignore; the JXL keeps it as an extra channel in the codestream.

Metadata: the BMP can carry nothing beyond the header's resolution fields (and a colour profile in rare V5 headers); JXL can carry EXIF and XMP boxes in the container form, often stripped by encoders, but nothing is copied across.

BMP stores raw, uncompressed pixels. Files are huge but trivially simple, which is why some legacy Windows software, embedded displays and printers still ask for it.

A BMP is a 14-byte file header ('BM' and the offset to the pixels), a DIB header (40 bytes, or 108 and 124 bytes in the V4 and V5 versions that add colour masks and alpha), an optional palette, then pixel rows padded to 4 bytes and normally stored bottom-up.

BMP files usually come from Windows Paint, legacy Windows programs, embedded displays and some scanners.

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 is the better choice for archiving and future-proof high quality storage, while BMP suits legacy software and embedded systems that require it.

JXL strengths: Excellent lossy and lossless compression; Progressive decoding and HDR; Huge image sizes. Drawbacks to weigh: Browser support is limited (Safari only by default); Few apps can open it yet.