act_thumbhash

A pure Dart implementation of the ThumbHash image placeholder algorithm by Evan Wallace.

ThumbHash encodes an image into a compact binary hash (~25–35 bytes) using a Discrete Cosine Transform, which can then be decoded into a small lossy placeholder image. This is useful for displaying image previews while the full-resolution version loads.

For Flutter-specific widgets and image providers, see act_thumbhash_flutter.

Installation

dependencies:
  act_thumbhash: ^1.0.0-dev.2

Usage

Encoding

import 'dart:convert';
import 'dart:typed_data';
import 'package:act_thumbhash/act_thumbhash.dart';

// Your RGBA image data (4 bytes per pixel)
final Uint8List rgba = getImageRgbaBytes();
final int width = 100;
final int height = 75;

// Async (recommended — runs in a separate isolate)
final hash = await ThumbHash.encodeAsync(width, height, rgba);

// Sync
final hash = ThumbHash.encodeSync(width, height, rgba);

// Encode as base64 for storage or transport
final base64Hash = base64.encode(hash);

Images larger than 128px on their longest side are automatically downscaled before encoding using nearest-neighbor interpolation.

Decoding

import 'package:act_thumbhash/act_thumbhash.dart';

final Uint8List hash = getStoredHash();

// Async (recommended)
final result = await ThumbHash.decodeAsync(hash);

// Sync
final result = ThumbHash.decodeSync(hash);

print('Size: ${result.width}x${result.height}');
final Uint8List rgba = result.rgba; // RGBA pixel data

Custom output size

The baseSize parameter controls the size of the decoded placeholder (default: 32):

// Larger placeholder for hero images
final result = await ThumbHash.decodeAsync(hash, baseSize: 64);

// Smaller for list thumbnails
final result = await ThumbHash.decodeAsync(hash, baseSize: 24);

API

License

MIT — see LICENSE.