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Fix chroma artifacts (issue #44 root cause): restrict green-diff decorrelation to lossless mode only #47

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Merged
merkalev merged 1 commit into from
Aug 25, 2026
Merged

Fix chroma artifacts (issue #44 root cause): restrict green-diff decorrelation to lossless mode only #47

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15 changes: 6 additions & 9 deletions src/v2_core.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -626,18 +626,15 @@ Status encode_image(const ImageView& image, const EncodeOptions& options,
options.tile_size < 16 || options.tile_size > 256 || image.width > 65535 || image.height > 65535 ||
options.metadata.size() > 16u * 1024u * 1024u)
throw std::invalid_argument("invalid encode options");
// Known-flaw A2: channels were entropy-coded independently. Apply the
// reversible mod-256 green differencing (G kept, R-G / B-G residual
// planes) so chroma planes become near-flat and compress far better.
// Pixel-wise and exactly invertible, so tiling/ROI/threading are
// unaffected. Signaled by container flags bit 1.
// Channel decorrelation (known-flaw A2): reversible mod-256 green
// differencing, signaled by container flags bit 1 and mirrored by the
// Python reference decoder.
// Known-flaw A2: reversible mod-256 green differencing for 8-bit
// RGB/RGBA. Lossless ONLY: quantization errors in the residual
// planes are amplified by the mod-256 undo in dark areas (issue
// #44), producing chroma artifacts. Lossy tiles code raw RGB.
constexpr bool kDecorrelateEnabled = true;
std::vector<uint8_t> color_work;
const bool color_decorrelated =
kDecorrelateEnabled && (image.channels == 3 || image.channels == 4)
kDecorrelateEnabled && options.lossless
&& (image.channels == 3 || image.channels == 4)
&& image.bytes_per_sample == 1;
if (color_decorrelated) {
color_work.assign(image.data,
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