koni_jxl 0.1.0

Pure Dart JPEG XL (JXL) codec — decode .jxl files to pixels and encode lossless .jxl, with zero native dependencies on every Dart/Flutter platform.

koni_jxl

A pure-Dart JPEG XL (.jxl) codec — decode and losslessly encode, with zero native dependencies and zero runtime package dependencies. Works on every platform Dart runs on: Android, iOS, macOS, Windows, Linux, and (with reduced lossy speed) the web.

Correctness is verified bit-exact against libjxl's djxl on a generated corpus, the official conformance suite, and real-world JPEG-transcoded manga chapters.

import 'package:koni_jxl/koni_jxl.dart';

// Cheap header-only inspection (reads a few hundred bytes):
final info = JxlInfo.parse(bytes);
print('${info.width}x${info.height}, ${info.bitsPerSample}-bit');

// Full decode to interleaved RGBA (sRGB, EXIF-oriented):
final image = JxlDecoder.decode(bytes);
final rgba = image.toRgba8();

// All frames of an animation:
final anim = JxlDecoder.decodeAnimation(bytes);

// Lossless encode from raw RGBA:
final jxl = JxlEncoder.encodeLossless(rgba,
    width: image.width, height: image.height, hasAlpha: true);

For Flutter widgets (JxlImageProvider, animation and progressive playback, background-isolate encode), use koni_jxl_flutter.

Features

Decoding

• Modular (lossless) still images — bit-exact vs libjxl
• VarDCT (lossy) still images — within ~1 RMSE of libjxl
• Animation (all frames, durations, loop count)
• Splines, progressive DC (LF) frames, multi-pass AC
• Streaming decode with a 1:8 DC preview (JxlStreamingDecoder) for blurry-then-sharp display
• Grayscale/RGB, palette, alpha, 8/16-bit, EXIF orientation
• Embedded ICC profiles; header-only JxlInfo.parse

Encoding (lossless)

• JxlEncoder.encodeLossless / encodeLossless16 from raw pixels
• JxlEncoder.encodeImage for JXL→JXL transcodes
• Per-image LZ77 / palette / YCoCg RCT selection; every output is verified bit-exact through this decoder and djxl

Robustness — all decode surfaces throw only JxlException on malformed input (mutation-fuzz verified); JxlLimits caps header-driven allocations.

Not yet supported (decoding throws JxlUnsupportedException with a stable feature id): spot-color rendering, JPEG bitstream reconstruction, and float (HDR) sample formats. Encoding is lossless-only.

Performance

Apple Silicon, AOT, single-threaded: a 1536×2200 lossless page decodes in ~60–410 ms; typical lossy pages in ~0.3–0.5 s, using Float32x4 SIMD across the lossy pipeline (native on AOT targets; emulated on the web).

Command-line tools

dart run koni_jxl:jxl_info image.jxl         # header info
dart run koni_jxl:jxl_dec  image.jxl out.ppm # decode to PNM/PAM
dart run koni_jxl:jxl_enc  in.ppm  out.jxl   # lossless encode

License

MIT. Ported from jxlatte (MIT); see NOTICE in the repository.