Global Tint direction

Negative (left) Tint adds green; positive (right) adds magenta, as the slider's track shows and as in a professional photo organizer and the DNG specification (issue #188). It is a global white-balance change, not a Nikon-specific decoder fix.

Reproducible evidence

The existing UI already declares Track::Tint as green → magenta (crates/develop/src/controls.rs) and paints a green left endpoint and purple right endpoint (crates/ui-egui/src/widgets.rs). Local mask tint also implements the correction direction: positive tint reduces green (crates/pipeline/src/finish.rs). The old global conversion contradicts both.

The synthetic tint_negative_is_green_and_positive_is_magenta regression renders an 18% neutral patch. Before the fix, setting global Tint to -50 produces sRGB [129, 113, 128]: red and blue exceed green, so the result is magenta. The test asserts the output direction for rendered sources, relative-WB RAW, and calibrated RAW with a nonzero As Shot reference. The complementary auto/picker test checks that correcting a green patch returns positive Tint and neutralises the patch.

Run the evidence without private samples:

cargo test -p imago-pipeline tint_ -- --nocapture
cargo test -p imago-gpu --test it equivalence::tint_directions -- --nocapture
cargo test -p imago-color cct

Why the sign changes

temp_tint_to_xy describes the source illuminant removed by Bradford adaptation. To add magenta to the output, the source white must be greener (positive Duv), not more magenta. Therefore the correction convention requires Duv = +tint / TINT_SCALE, and the inverse conversion must return tint = +Duv * TINT_SCALE.

The CPU and GPU consume the same wb_matrix_for result. RAW white-point inference, auto WB and the picker use the inverse conversion. Both directions change together; local mask tint and calibration shadow tint are unchanged. The render-cache version increases to invalidate stale output.

Existing edits

Before this change the sign was the other way round, so a saved non-zero custom Tint (a catalog edit, a preset, history) now renders with the opposite green/magenta shift; As Shot, Auto and the presets that compute their values are unaffected, and re-running Auto or the picker gives values in the new convention. Tint values imported from a professional photo organizer XMP (crs:Tint) were rendered the wrong way round before and now render as in a professional photo organizer. No saved values are rewritten.