jpggen

Generate surprisingly structured images by sampling random (but biased) JPEG DCT coefficients and letting the decoder reconstruct them. Supports color (YCbCr 4:4:4) and grayscale, with “waves” patterns for coherent global structure or fully random per-block coefficients.

Introduction

Compression and prediction are symmetric. A perfect compressor is a perfect predictor, and vice versa. Therefore, compression algorithms can be used as generative algorithms by sampling random data from simple distributions and decompressing, conjuring structure from noise. Standard compression algorithms, such as the one used by JPEG, can be used as generative “AI.” The structure of the generated images reveals how JPEG models the distribution of images. JPEG, being hand-designed, has a less interesting model of the image world than something such as image diffusion models, but interesting patterns emerge.

Example

The image below was generated with the default settings:

Command used:

cargo run --release -- --output example.jpg

Build and install

• Prerequisites: Rust (stable)
• Build:

  cargo build --release

• Run:

  cargo run --release -- -o out.jpg

Usage

Usage:

jpggen [OPTIONS] --output <PATH>

Options:

• Output and size

  • -o, --output Output JPEG path (required)
  • -w, --width Image width in pixels (default: 512)
  • -H, --height Image height in pixels (default: 512)

• Determinism

  • --seed Seed for reproducibility (optional)

• Pattern of coefficients

  • --pattern <waves|random> Coefficient pattern (default: waves)
    • waves: coherent global sine modulation across blocks
    • random: independent random ACs per block
  • --num-waves Waves mixed per active AC index (default: 3; waves only)

• Frequency selection

  • --nonzero Number of active AC positions - waves: how many AC indices are active globally - random: approximate non-zeros per block (default: 4)
  • --freq <low|all> Restrict which AC positions are used (default: low)
  • --low-span <1..63> If --freq low, use zig-zag indices 1..=low-span (default: 12)

• Amplitude and JPEG quality

  • --max-ac-bits <1..10> Max AC category (amplitude range); higher = stronger (default: 6)
  • --quality <1..100> JPEG “quality” scaling (default: 50)

• Components and DC behavior

  • --grayscale Force grayscale (single-component JPEG)
  • --dc-random-walk Add a tiny DC random walk (slow gradients)

Examples:

• Default color, waves:

  cargo run --release -- -o waves.jpg -w 1024 -H 768 --seed 42 \
    --nonzero 6 --freq low --low-span 12 --quality 45 --num-waves 4

• Trippier color (stronger amplitudes):

  cargo run --release -- -o waves_strong.jpg --seed 7 \
    --nonzero 8 --max-ac-bits 8 --quality 40 --num-waves 5

• Grayscale, random per-block ACs:

  cargo run --release -- -o random_gray.jpg --grayscale \
    --pattern random --nonzero 6 --freq all --max-ac-bits 6 --quality 50

How it works

• Writes a valid baseline JFIF JPEG with standard Huffman tables and scaled quantization tables.
• Instead of starting from pixels, directly samples quantized DCT coefficients in zig-zag order.
• DC is optionally a tiny random walk; ACs are either:
  • random: independent selections per block
  • waves: per-index sine modulation over the block grid for global coherence
• The decoder’s inverse DCT “turns” those frequency-domain samples into images that reflect JPEG’s hand-designed basis and coding assumptions.

License

MIT License - see LICENSE for details.