JPEG Pure-JS Implementation Notes

Current Status

The JPEG format handler implements a baseline DCT (Discrete Cosine Transform) encoder and decoder that works correctly for the most common JPEG images:

Decoder (Baseline DCT)

  • ✅ Baseline DCT JPEG (SOF0) - fully implemented
  • ✅ Huffman decoding (DC and AC coefficients) - fully implemented
  • ✅ Quantization table parsing - fully implemented
  • ✅ IDCT (Inverse Discrete Cosine Transform) - fully implemented
  • ✅ YCbCr to RGB color conversion - fully implemented
  • ✅ Grayscale JPEG support - fully implemented
  • ✅ JFIF (APP0) metadata parsing - DPI support
  • ✅ EXIF (APP1) metadata parsing - author, description, copyright, creation date
  • ✅ Restart markers (DRI) - fully implemented
  • ❌ Progressive JPEG (SOF2) - not yet implemented
  • ❌ Interlaced/progressive scan support - not yet implemented
  • ❌ Arithmetic coding - not yet implemented
  • ❌ Lossless JPEG - not yet implemented
  • ❌ Extended sequential DCT - not yet implemented
  • ✅ Chroma subsampling - 4:4:4, 4:2:2, and 4:2:0 fully tested and supported

Encoder (Baseline DCT)

title: "JPEG" parent: "Implementation Notes" nav_order: 1

JPEG Pure-JS Implementation Notes

  • Decoder: baseline DCT (SOF0) implemented. Supports Huffman (DC/AC), DQT, IDCT, YCbCr→RGB, grayscale, and chroma subsampling (4:4:4, 4:2:2, 4:2:0). Progressive, arithmetic, and lossless JPEGs are not supported.
  • Encoder: baseline DCT implemented with adjustable quality, standard Huffman tables, and JFIF/EXIF marker support. Defaults to 4:4:4 (no subsampling). Progressive encoding and optimized Huffman tables are not implemented.
  • Metadata: APP0 (JFIF) and APP1 (EXIF) are parsed and written; orientation is preserved in metadata but not auto-applied during decode.
  • Limitations: no progressive decode/encode, no arithmetic coding, limited CMYK/16-bit support.
  • Key files: src/formats/jpeg.ts, src/utils/jpeg_decoder.ts, src/utils/jpeg_encoder.ts.
  • Tests: test/formats/jpeg.test.ts, test/purejs_roundtrip.test.ts, test/formats/jpeg_subsampling.test.ts.
  • Notes: prioritize correctness and compatibility across runtimes. Future work: progressive support, chroma-subsampled encoding, and optimized Huffman tables.