Home

PJA

PJA (packJPG Algorithm) recompresses JPEG, JPEG-LS and JPEG 2000 files without loss: a photo shrinks by about a quarter, a JPEG 2000 codestream by about 8% (HTJ2K 11%, scanned 1-bit pages 70%), and pja x gives back the original byte for byte. It is written in FreePascal, with the coefficient model's core in C and some x86-64 assembly, and no libraries; it runs on Linux, macOS and Windows (XP and later). PJA is the successor of packJPG: a new codec and format of its own, not compatible with .pjg.

pja a photo.jpg          # -> photo.pja, about 27% smaller
pja x photo.pja          # -> photo.jpg, the same file byte for byte

Nothing is written under an output's name unless it verified: a decompresses its own result and compares it with the input first, and x checks the size and the BLAKE3 hash the .pja carries.

The family

PJA belongs to a family of recompressors that share that rule.

Sibling	Formats	Successor of
PMPA	MPEG audio (MP1, MP2, MP3)	packMP3, packMP2
PRA	raster images (PNM, BMP, TIFF, PSD, HDR, ...)	packPNM
PPA	the PNG family (PNG, APNG, MNG, JNG)	packPNG
PAIA	animated images (GIF, WebP, FLI/FLC, ANI, ...)	(new)
PJA	JPEG, JPEG-LS and JPEG 2000	packJPG

Pages

• Installing: the release binaries for each platform, and checking them
• Using pja: commands, switches, examples
• What PJA handles: which JPEG, JPEG-LS and JPEG 2000 files, and what goes as bytes
• Benchmarks: size and speed against packJPG, JPEG XL, Lepton, brunsli, general-purpose compressors and archivers
• Safety and robustness: damage, fuzzing, full disks, links and kills, measured against the others
• Reporting bugs: pja report, and what it does and does not contain
• The zpaq-std plugin: PJA inside zpaq archives
• Library and 7-Zip: libpja, and PJA as a 7-Zip codec
• Platforms: what is built and tested where
• Building: from source, and running the tests
• Source layout and FreePascal pitfalls: for contributors
• How it works: a short tour, with links to the full notes
• Releases: what each version changed
• FAQ

The repository's own documents go deeper: CLI.md (the command), CODEC.md (the codec, everything measured and tried), FORMAT.md (the .pja file) and POLICY.md.

The formats are stable since 0.9.0: what is fixed, and what may still come, is in doc/STABILITY.md.