feat(png)!: migrate PNG encode to gamut-png - #47
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PNG encoding moves from zune-png to gamut_png::PngEncoder. The 8-bit path keeps the existing 16->8 high-byte conversion into ImageRef<Rgb8>; the 16-bit path hands the RgbImage samples to ImageRef<Rgb16> directly, dropping the manual big-endian packing (gamut serialises 16-bit PNG itself). Decode stays on zune-png until the gamut decoder migration (#32) — encode-only swap, no dual encode backends. Metadata now embeds through the encoder itself — with_exif (bare TIFF from the gamut-exif builder), with_icc_profile, with_xmp — replacing the img-parts PNG chunk muxing on this path; the PNG path no longer pulls container-embed or zune-runtime. A new metadata round-trip test asserts eXIf/iCCP/iTXt chunk presence plus exact 16-bit pixel round-trip through the zune decoder. Config surface: ZunePngEncodeConfig -> PngEncodeConfig, exposing exactly gamut's encoder knobs (compression level Store/Fast/Default/Best, filter strategy None/Fixed/MinSumAbs/BruteForce, auto_reduce) — no invented options. EncodeOptions::PngZune -> PngGamut; encode codec id png/zune -> png/gamut. Feature graph: tier-4 collapses, png-encode = dep:gamut-png; png-encode-zune deleted; zune-png remains only behind png-decode-zune. Baseline vs zune (1024x1024 synthetic 16-bit gradient, release, avg of 5, default options; ref gamut#224): output size 6,302,493 -> 26,245 bytes (-99.6%; zune 0.5 emitted essentially uncompressed output), encode time 20.5ms -> 96.3ms (16-bit) and 2.6ms -> 47.3ms (8-bit) — the size win costs real filtering+DEFLATE time; Store/Fast levels are exposed for speed-first callers. Gradient content exaggerates the size ratio. BREAKING CHANGE: ZunePngEncodeConfig/EncodeOptions::PngZune renamed; PNG encode output bytes change (properly compressed).
This was referenced Jul 18, 2026
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Closes #23.
Stacked on #46; merge order #42 → #44 → #45 → #46 → this.
What is Delivered
gamut_png::PngEncoder(decode stays zune-png — that is Migrate PNG decode to gamut-png #32). 8-bit keeps the existing 16→8 conversion intoImageRef<Rgb8>; 16-bit feeds samples straight toImageRef<Rgb16>, deleting the manual big-endian packing. No dual encode backends:png-encode-zuneis gone,png-encode = dep:gamut-png, zune-png remains only behindpng-decode-zune(grep-verified).with_exif(bare TIFF from the Migrate metadata stack to gamut-metadata #19 gamut-exif builder),with_icc_profile,with_xmp— replacing img-parts PNG chunk muxing on this path. The PNG path dropscontainer-embed/zune-runtimeentirely.PngEncodeConfigreplacesZunePngEncodeConfig, exposing exactly gamut's knobs (compression Store/Fast/Default/Best; filter None/Fixed/MinSumAbs/BruteForce;auto_reduce) — nothing invented, per the intentional-API rule.EncodeOptions::PngZune→PngGamut; codec idpng/zune→png/gamut.Bench vs the zune baseline (issue requirement; ref gamut#224)
1024×1024 synthetic 16-bit gradient, release, avg of 5, defaults:
zune 0.5 was emitting essentially uncompressed output; gamut's filtering + DEFLATE costs real time for a massive size win. Gradient content exaggerates the ratio;
Store/Fastare exposed for speed-first callers.Verification Done
Run independently after implementation:
cargo test --workspace384/0;--features full533/0 (fixtures ran);--features serde366/0; clippy workspace + full-features-D warningsboth 0 issues;--no-default-features --features pngstandalone builds clean; feature-graph grep confirms zune-png is decode-only. lefthook green.