ImcodecNative

Optional AVIF and HEIF/HEIC codecs, plus libjxl and libwebp codec replacements, for Imcodec. Imcodec itself remains pure Dart and never depends on this package.

The same engines run on Android, iOS, macOS, Windows and Linux through Dart code assets, and in browsers through a bundled WebAssembly module. There are no platform codec dependencies, platform channels, runtime CDN downloads or browser-specific image decoders.

Usage

Add imcodec_native to your application's dependencies, then initialize it once before using the generic Imcodec helpers:

import 'package:imcodec_native/imcodec_native.dart' as img;

await img.ImcodecNative.initialize();

final img.ImageFormat? format = img.ImageFormat.sniff(encodedBytes);
final img.Image image = img.decodeImage(encodedBytes);
final avif = img.encodeImage(image, format: img.NativeImageFormat.avif);
final heic = img.encodeHeif(
  image,
  options: const img.HeifEncodeOptions(quality: 85),
);
final jxl = img.encodeJpegXl(image); // libjxl, lossless
final webp = img.encodeWebP(image); // libwebp, lossless
final lossyWebP = img.encodeWebP(
  image,
  options: const img.WebPEncodeOptions(quality: 82),
);

The package re-exports Imcodec. NativeImageFormat extends ImageFormat and implements InspectableFormat; neither AVIF nor HEIF is declared in Imcodec. Their signatures and metadata inspectors are registered through ImageFormatRegistry, while their codec implementations are registered separately through ImageCodecRegistry. Adding an import alone has no side effects. ImageFormatRegistry.formats is the complete ordered collection of currently supported core and add-on formats.

Select only the registrations you want:

await img.ImcodecNative.initialize(jpegXl: false, webP: false);

These flags select registrations, not compiled components. Calls are additive. ImcodecNative.unregister() removes this package's current registrations and restores the original generic JPEG XL/WebP codecs. Registration replaces a complete generic codec entry, so registering JPEG XL or WebP replaces both its Dart encoder and decoder. Direct instances such as const JpegXlCodec() and WebPCodec() retain their Dart implementation. Dedicated AvifCodec, HeifCodec, NativeJpegXlCodec and NativeWebPCodec work without global registration.

NativeImageFormat.heic, HeicCodec, HeicEncoder, HeicDecoder and the encodeHeic/decodeHeic helpers are aliases of their canonical HEIF spelling.

On native platforms, synchronous ImcodecNative.register() is sufficient. Call it inside each application-created isolate that uses generic helpers; the registry is isolate-local. On browsers, await initialize() before any native converter or helper, including asynchronous decoder APIs.

Codecs and background work

The codec/encoder/decoder APIs follow Imcodec's dart:convert contracts:

const codec = img.AvifCodec();
final encoded = codec.encoder.convert(
  image,
  encodeOptions: const img.AvifEncodeOptions(quality: 80, speed: 6),
);
final decoded = codec.decoder.convert(encoded);

final lossless = img.encodeAvif(
  image,
  options: const img.AvifEncodeOptions(lossless: true, speed: 9),
);
final background = await img.encodeAvifWith(
  img.onBoundedIsolates,
  image,
  options: const img.AvifEncodeOptions(quality: 80),
);
final pixels = await img.decodeHeifAsync(heicBytes);
final nativeWebP = await img.encodeWebPWith(
  img.onBoundedIsolates,
  image,
);

Codec and encoder instances are immutable resources. Compression and allocation settings live in typed per-operation options, matching Imcodec 0.4's API.

Decoder *Async helpers use a native isolate or a module Web Worker. Inputs are copied before scheduling and returned buffers own their memory. Browsers reuse one Worker and one Wasm instance, correlate concurrent requests, and replace the Worker after a transport failure. Codec errors affect only their own request.

Encoding follows Imcodec's encodeWith convention. The AVIF, HEIF, JPEG XL and WebP *With helpers accept a ParallelRunner and submit one complete native operation to it without requiring codec registration in its worker isolate. Output matches synchronous encoding byte for byte. onBoundedIsolates moves the operation off the calling isolate on native platforms; runSequentially keeps it inline, including in browsers.

Native calls sharing an engine are serialized to protect its global tables; HEIF/AVIF, JPEG XL and WebP use separate locks. Browser Worker decode requests are serialized inside the background Worker.

Format behavior

Format	Decoding	Encoding
AVIF	libheif + libaom; primary still image, alpha, 8/10/12-bit samples	libaom; lossy 4:2:0 or lossless RGB 4:4:4; lossless alpha
HEIF/HEIC	libheif + libde265; primary HEVC still image, alpha, high depth	Kvazaar; lossy 8-bit 4:2:0 colour, lossless alpha
JPEG XL	libjxl; first visible frame, orientation, straight alpha, uint8/uint16/float32 samples and output ICC	libjxl; lossless RGBA, including RGB beneath transparent pixels
WebP	libwebp; still image or first composited animation frame, straight RGBA8	libwebp; lossless or lossy RGB, lossless alpha

HEIF is a container: HEVC and AV1 payloads are bundled, not arbitrary optional HEIF compression methods. Sequence-only brands and MP4 videos are not sniffed as still images. Collections return the primary image; there is no animation, multi-image, depth-map or gain-map API. Crop, rotation and mirroring are applied by libheif. HEIC colour remains lossy even at quality 100.

decodeAvifData, decodeHeifData and registered decodeImageData preserve high-depth input. HEIF-family and integer JPEG XL samples use little-endian, full-range uint16 RGBA; floating-point JPEG XL uses little-endian float32. Ten- and twelve-bit integer samples are normalized to 0–65535. Ordinary decodeImage returns RGBA8. Samples retain the decoder output transfer function; there is no display tone mapping. libjxl supplies the ICC profile describing its output pixels. NCLX-only HEIF colour descriptions are not exposed by Imcodec's current metadata model. Encoding accepts RGBA8 only and does not copy source metadata.

inspectImage retains bounded ICC, EXIF and XMP packets. EXIF is the opaque HEIF metadata payload, including its four-byte TIFF-offset prefix. Decoding attaches the ICC payload to DecodedImage. Pixel, decoded-byte, ICC, descriptive-metadata and encoded-output limits are checked independently; codec working memory is additional to the returned pixel buffer. Lower the defaults for untrusted images or constrained devices.

Platform builds

Dart 3.13 or newer and a matching Flutter SDK are required. Native builds use the build hook automatically, including when running tests. The first build downloads the pinned sources over HTTPS, verifies every SHA-256 digest and compiles them; subsequent builds reuse a shared source cache and a target-specific compilation cache. No Emscripten installation is needed by applications.

To prepare a package checkout or Pub installation for an offline native build, download the same verified archives in advance:

dart run imcodec_native:prepare_library

The command stores them beside the installed package. Normal application builds need no manual command, but their first native build requires network access if this cache has not already been prepared.

Platform	Build requirements
Android	CMake 3.25+, Android NDK selected by Flutter; ARMv7, ARM64, x86 and x64 toolchains
iOS	macOS, Xcode and CMake 3.25+; ARM64 device, ARM64/x64 simulator
macOS	Xcode command-line tools and CMake 3.25+; ARM64/x64
Windows	Visual Studio C++ toolchain and CMake 3.25+; x64/ARM64
Linux	CMake 3.25+, C/C++20 toolchain and Make or Ninja
Web	Bundled .mjs, .wasm, loader and worker assets; no native compiler

Android uses a statically linked C++ runtime and 16 KB-compatible ELF segment alignment. Release libraries are stripped on Android. Native outputs expose only the small bridge ABI and bundle all codec engines into one dynamic library. IMCODEC_NATIVE_CMAKE can select the CMake executable. Cross-compiling Linux requires an appropriate compiler toolchain; a host binary is not a substitute.

Flutter bundles Web assets automatically. For a custom asset server, pass assetBaseUrl to initialize(). Keep all four files together, serve .wasm as application/wasm, and allow the module/worker URLs in your CSP. WebAssembly compilation must also be permitted (typically script-src 'self' 'wasm-unsafe-eval' and worker-src 'self'). The worker script must be same-origin. No SharedArrayBuffer, COOP or COEP headers are needed. If the host nevertheless enables cross-origin isolation, add blob: to worker-src; the loader uses a tiny blob bootstrap so the Worker inherits the document's COEP policy even when the asset response omits that header.

See docs/BUILDING.md for rebuilding, testing and replacing the engines.

Licensing

The Dart integration and bridge are MIT licensed. Codec components have their own licenses, including LGPL-3.0-or-later for libheif and libde265. Published Pub packages omit upstream source archives; repository tags retain the exact LGPL sources and complete reconstruction material. See docs/SOURCE_OFFER.md and docs/THIRD_PARTY_NOTICES.md. Application distributors must retain applicable notices and satisfy the relevant source/relinking requirements. Codec software licenses do not grant all possible HEVC patent rights; assess your distribution's requirements.

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