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packJPG v4.0 — cross-component lazy prediction

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@YadeWira YadeWira released this 21 Apr 19:24
· 26 commits to master since this release
v4.0
c24db69

What's new in v4.0

packJPG v4.0 introduces a format change (version byte 0x28 / 40) that adds cross-component adaptation to the PJG coder: when encoding the chroma components (Cb/Cr) of 4:4:4 JPEGs, the bit-length of the co-located luma (Y) coefficient is folded into the arithmetic model context. This tightens the prediction in exactly the frames where chroma is most correlated with luma (portraits, text, sharp diagonals) and delivers a measurable ratio win on photographic corpora.

A -legacy flag is provided to emit v3.1d-compatible PJGs for interoperability. The decoder auto-detects the version byte, so all existing v3.1d archives decompress without any user action.

Highlights

  • Better compression: −0.532 % on a 151-JPEG mixed corpus; up to −5 % on 4:4:4 photographic images. Subsampled (4:2:0 / 4:2:2) files are byte-identical to v3.1d by design
  • Parallel encode/decode (-sfth): 3-worker pipeline (Y/Cb/Cr), unchanged encode speed on 4:2:0, faster on corpora with many files (-th4)
  • Backward-compatible decoder: v4.0 reads both v4.0 and v3.1d PJG files. v3.1d cannot read v4.0 files — use -legacy during the transition
  • -legacy flag: emit v3.1d-format output from v4.0 encoder (byte-identical to v3.1d)
  • Fuzzer harness: source/test/pjg_decode_fuzzer.cpp — libFuzzer + ASan + UBSan; 15-min campaign, 0 crashes
  • LTO build (-flto=thin on clang): ~8 % encode speedup at no ratio cost
  • Windows XP: v4.0 is the last feature release for XP. Future XP releases are bugfix-only (v4.0a, v4.0b, …)

Validation

Check Result
Round-trip (202 JPGs) 283 / 283 PASS, 0 fail
-legacy round-trip 151 / 151 PASS
MT stress (-th8, 50 iters × 151 files) 393 / 393 OK
ThreadSanitizer (4 threads × 10 iters) 0 data races
libFuzzer + ASan + UBSan 0 crashes, 0 sanitizer reports

Incompatibility notice

PJG files produced by v4.0 cannot be decoded by packJPG v3.1d or earlier. Use -legacy if downstream consumers have not been upgraded yet. This is the first intentional format break since v2.0 (2007).

Downloads

File Platform
packJPG_linux_x64 Linux x86-64 (generic)
packJPG_linux_x64_native Linux x86-64 (optimized for build host — do not distribute)
packJPG_win_x64.exe Windows x86-64 (Vista+)
packJPG_win_x86.exe Windows x86 32-bit (Vista+)
packJPG_win_xp.exe Windows XP 32-bit
Assets 7
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