PyGFWX

A Python implementation of the GFWX (Good, Fast Wavelet Codec) for educational purposes.

Test image: "Orange tabby cat sitting on fallen leaves" by Hisashi (derivative work: Caspian blue), CC BY-SA 2.0

Overview

PyGFWX is an educational implementation of the GFWX wavelet codec, created to learn and understand wavelet-based image compression. The implementation prioritizes readability over performance - it's written in pure Python with NumPy for data structures, but no optimized libraries for the core algorithms.

Note: This is a learning tool, not a production codec. For high-performance encoding, use the reference C++ SDK. PyGFWX is useful for understanding how wavelet compression works, experimenting with the algorithm, and educational purposes.

Compression Demo

Using the cat image above (768×1024 RGB), here are compression results at different quality levels:

Original uncompressed size: 2.25 MB (768×1024×3 bytes)

Features

• Wavelet compression using lifting scheme (5/3 and 9/7 wavelets)
• Golomb-Rice entropy coding with context-adaptive modeling
• Progressive decoding with downsampling support
• Multiple color modes: Mono, RGB, RGBA, Bayer patterns
• Quality range: Lossy (1) to lossless (1024)
• Multi-layer support: Stereo images, depth maps, animation frames
• Custom color transforms: Programmable UYV, A710, and custom transforms
• Metadata support: JSON, key-value, or binary metadata blocks

Why Pure Python?

This project implements the full codec in readable Python to make it easy to:

• Step through the compression pipeline with a debugger
• Understand each stage: wavelets, quantization, entropy coding
• Experiment with modifications to the algorithm
• Learn wavelet compression without deciphering optimized C++

The code is validated against the reference SDK to ensure correctness, but runs significantly slower (expect 100x+ slower than the C++ implementation).

Installation

This project uses uv for dependency management.

# Clone the repository
git clone https://github.com/chiplukes/pygfwx.git
cd pygfwx

# Install dependencies
uv sync

# Install with development dependencies
uv sync --all-extras

Quick Start

from pygfwx import encode, decode
import numpy as np

# Create or load an image (numpy array)
image = np.random.randint(0, 256, (256, 256), dtype=np.uint8)

# Encode (compress)
compressed = encode(image, quality=512)

# Decode (decompress)
decoded = decode(compressed)

# Verify
assert np.array_equal(image, decoded)  # True for lossless (quality=1024)

Project Structure

pygfwx/
├── src/pygfwx/          # Main Python package
│   ├── core/            # Core codec implementation
│   │   ├── bitstream.py     # Bit-level I/O
│   │   ├── header.py        # GFWX header parsing
│   │   ├── lifting.py       # Wavelet transforms
│   │   ├── golomb_rice.py   # Entropy coding
│   │   ├── context.py       # Context modeling
│   │   ├── encoder.py       # Coefficient encoding
│   │   ├── decoder.py       # Coefficient decoding
│   │   ├── transforms.py    # Color transforms
│   │   ├── bayer.py         # Bayer/RAW support
│   │   ├── multi_layer.py   # Multi-layer images
│   │   └── metadata.py      # Metadata utilities
│   ├── streaming/       # Progressive/streaming support
│   └── utils/           # Helper functions
├── cross_codec/         # SDK wrapper and validation tools
├── tests/               # Test suite (385+ tests)
├── examples/            # Example scripts
├── notes/               # Documentation
└── gfwx-sdk/            # Reference C++ SDK

Documentation

• Compression Overview - How GFWX compression works
• Bitstream Format - GFWX file format specification
• Lifting Scheme - Wavelet transform details
• Golomb-Rice Coding - Entropy coding explanation
• Project Plan - Development roadmap

Running Tests

# Run all tests
uv run pytest

# Run with coverage
uv run pytest --cov=pygfwx

# Run specific test file
uv run pytest tests/test_bitstream.py

# Run SDK comparison tests (requires built SDK)
uv run pytest -m sdk

Development

Code Quality

# Check linting
uv run ruff check src tests

# Format code
uv run ruff format src tests

Building the SDK

The reference SDK is needed for validation tests:

cd gfwx-sdk/build
cmake ..
cmake --build . --config Release

Quality Levels

How It Works

GFWX uses a wavelet-based compression pipeline:

1. Color Transform - Convert RGB to decorrelated color space (UYV)
2. Wavelet Transform - Apply lifting-based wavelet decomposition
3. Quantization - Scale coefficients based on quality setting
4. Entropy Coding - Golomb-Rice coding with context modeling

┌─────────┐    ┌─────────┐    ┌────────┐    ┌─────────┐
│  Image  │ -> │  Color  │ -> │Wavelet │ -> │Quantize │ -> ┌──────────┐
│  RGB    │    │Transform│    │ Lift   │    │ Scale   │    │ Entropy  │
└─────────┘    └─────────┘    └────────┘    └─────────┘    │  Code    │
                                                           └────┬─────┘
                                                                │
                                                                v
                                                         ┌──────────┐
                                                         │Compressed│
                                                         │  Data    │
                                                         └──────────┘

References

• GFWX Website
• GFWX Whitepaper
• Reference SDK

License

BSD 3-Clause License (same as GFWX)

Acknowledgments

• Graham Fyffe for creating GFWX
• USC Institute for Creative Technologies
• GFWX is "thoroughly tested using several pictures of cats" 🐱

This is an educational project focused on understanding wavelet-based image compression.