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@YadeWira YadeWira released this 21 Apr 19:59
· 24 commits to master since this release
v4.0a
33ebada

v4.0a (2026-04-21) — bugfix

  • fix: decode_jpeg() leaked a BitReader (32 bytes) when jpg_parse_jfif()
    returned an error mid-scan loop. Found by libFuzzer + LeakSanitizer on a
    malformed JPEG; added delete huffr before the early return.

v4.0 (2026-04-21) — format break: cross-component lazy prediction

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 and decode PJG v3.1d (0x1F / 31)
files for interoperability. The decoder detects the version byte and
dispatches accordingly, so all existing v3.1d archives decompress to
byte-identical JPEGs without user intervention.

  • new: cross-component lazy prediction for 4:4:4 chroma DC and AC passes
    in pjg_encode_dc, pjg_encode_ac_high, pjg_encode_ac_low and their
    decoder counterparts. Y coefficient bit-length (BITLEN1024P, clamped to
    7) is combined with the existing neighbourhood bit-length into a compound
    context ctx_shift = (ctx_len << 3) | y_clen. The mod_len model is
    widened from max(11, segm_cnt) to 128 (AC) or (max_len+1) << 3 (DC).
    Gated on cmp != 0 && cmpc >= 2 && cmpnfo[cmp].bc == cmpnfo[0].bc so
    subsampled (4:2:0 / 4:2:2) files fall back to the v3.1d path byte-for-byte
  • new: -legacy flag — emit v3.1d-format PJGs; output is byte-identical to
    packJPG v3.1d on the same input. Thread-local so concurrent MT batches
    can mix v4.0 and v3.1d encoding safely
  • new: dual-version decoder — v3.1d PJGs are detected by their version byte
    and decoded via the v3.1d path (no cross-component context). v4.0 PJGs
    require packJPG v4.0+ to decompress
  • measurement (153-JPEG mixed corpus, 151 round-trippable):
    • v4.0 total PJG: 60,066,170 B vs v3.1d 60,387,566 B → −0.532 %
    • -legacy output: 60,387,566 B, byte-identical to v3.1d
    • top per-file wins: 4:4:4 photographic JPEGs (e.g. 827C1CF27.jpg −5.01 %);
      subsampled JPEGs neutral by design (gate bypasses cross-comp)
  • -sfth path continues to emit v3.1d-format PJGs: the parallel encoder
    processes components concurrently, so Y is not available as context when
    Cb/Cr are being encoded. pjg_use_crosscomp_now is kept false for sfth;
    sfth output remains byte-identical to v3.1d
  • validation:
    • 151 / 151 round-trip byte-exact on corpus
    • -legacy round-trip: 151 / 151 byte-exact
    • MT stress (-th8, 50 iters on 151 files): 393 / 393 OK, 0 mismatches
    • ThreadSanitizer (4 threads × 10 iters): 39 / 39 OK, 0 data races
    • lib_roundtrip_test: 151 OK, ratio 76.59 %
    • libFuzzer + ASan + UBSan 15 min: 0 crashes, 0 sanitizer reports,
      coverage grew 2596→3012 edges, corpus 20→23
  • new: source/test/pjg_decode_fuzzer.cpp + build_fuzzer.sh — libFuzzer
    harness over pjglib_convert_stream2mem covering both directions of the
    codec (auto-detects JPG vs PJG by leading bytes)
  • incompatibility notice: PJG files produced by v4.0 cannot be decoded
    by packJPG v3.1d or earlier.     Use -legacy during the transition if
    downstream consumers have not been upgraded. This is the first intentional
    format break since v2.0 (2007)
  • build: Linux builds now use -flto=thin (clang) for ~8 % encode speedup
    via cross-TU inlining; ratio and output are bit-identical
  • build: check_value_range rewritten with std::minmax_element for
    cleaner SIMD-friendly range validation
  • test corpus: test-files/ directory added (5 representative JPEGs:
    baseline 4:2:0, 4:4:4, grayscale, photographic, solid color) for CI
    smoke tests
  • Windows XP support policy: v4.0 is the last feature release for
    Windows XP. Future XP releases will be bugfix-only (versioned v4.0a,
    v4.0b, …). New features introduced in v4.1 and later will not be
    backported to the XP build
  • maintainer: Yade Bravo (https://github.com/YadeWira/packJPG)
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