imcodec 0.1.3

Focused BMP, JPEG, PNG, QOI, TGA, TIFF, and WebP codecs for RGBA images in Flutter.

Imcodec

Imcodec is a focused Flutter image codec for BMP, JPEG, PNG, QOI, TGA, TIFF, and WebP. It keeps a small straight-alpha RGBA image model and exposes synchronous Dart encoders, which makes expensive encoding suitable for Isolate.run.

Decoding is asynchronous at the public boundary. BMP, QOI, TGA, and TIFF use pure-Dart decoders; JPEG, PNG, and WebP use Flutter's platform codecs. Animated PNG and WebP inputs currently return their first frame. Flutter's premultiplied render pipeline may round RGB values on translucent pixels and discards hidden RGB values where alpha is zero.

Usage

import 'dart:isolate';
import 'dart:typed_data';

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

Future<Uint8List> exportWebP(
  Uint8List straightRgba,
  int width,
  int height,
) => Isolate.run(() {
  final img.Image image = img.Image.fromBytes(
    width: width,
    height: height,
    bytes: straightRgba.buffer,
    bytesOffset: straightRgba.offsetInBytes,
    numChannels: 4,
    order: img.ChannelOrder.rgba,
  );
  return img.encodeWebP(image);
});

The encoder entry points mirror the subset used by almost any image editor:

final Uint8List png = img.encodePng(image);
final Uint8List jpeg = img.encodeJpg(image, quality: 90);
final Uint8List webp = img.encodeWebP(image); // lossless VP8L
final Uint8List bmp = img.encodeBmp(image);
final Uint8List tga = img.encodeTga(image); // RLE by default
final Uint8List qoi = img.encodeQoi(image);
final Uint8List tiff = img.encodeTiff(image); // PackBits by default

Decode supported data with format detection or a format-specific function:

final img.Image decoded = await img.decodeImage(encodedBytes);
final img.Image png = await img.decodePng(pngBytes);

decodeImage defaults to a 100-million-pixel allocation limit. Supply a lower maxPixels value when input comes from an untrusted source.

Format behavior

• BMP is encoded as a V4 32-bit bitmap with explicit RGBA bitfields. The decoder supports uncompressed palette, 16-bit, 24-bit, 32-bit, and bitfield images; BMP RLE compression is not currently supported.
• PNG is encoded as non-interlaced 8-bit RGBA with adaptive row filters.
• JPEG is baseline JPEG with selectable 4:4:4 or 4:2:0 chroma sampling. Transparency is composited against white.
• QOI is encoded and decoded losslessly according to the Quite OK Image specification.
• TGA supports color-mapped, true-color, and grayscale input, with raw or RLE pixel data. Output is 32-bit true-color and uses RLE by default.
• TIFF import supports little- and big-endian baseline files, eight-bit RGB, RGBA, grayscale, and palette pixels, strips, all eight orientations, horizontal prediction, and uncompressed, PackBits, or LZW data. Planar, tiled, JPEG-compressed, and samples wider than eight bits are not currently supported. Output is little-endian, chunky, eight-bit RGBA using PackBits by default; pass TiffCompression.none for uncompressed output.
• WebP is lossless VP8L and preserves alpha. Quality is intentionally not an option until a lossy encoder is added.

The JPEG and WebP encoder implementations contain code derived from the MIT licensed Dart image package.