Issue #212: Raw: keep vendor white balance when writing DNG

NEF (and other vendor raws without a colour matrix) carry their as-shot
white balance as vendor multipliers, not as a DNG AsShotNeutral. The DNG
writer only copied AsShotNeutral/AsShotWhiteXY, so Convert to DNG and
Copy as DNG produced files with no white balance at all, which develop
dark and green. Write the reciprocal of the vendor multipliers as
AsShotNeutral when the source has no DNG white of its own.

crates/raw/src/dngwrite.rs

@@ -177,6 +177,9 @@ pub fn write_dng(raw: &RawImage, opts: &DngWriteOptions) -> Result<Vec<u8>> {
         ifd.set(t::AS_SHOT_NEUTRAL, rat_vec(&v));
     } else if let Some(xy) = c.as_shot_white_xy {
         ifd.set(t::AS_SHOT_WHITE_XY, rat_vec(&[xy.x, xy.y]));
+    } else if let Some(m) = raw.wb_multipliers.filter(|m| m.iter().all(|v| *v > 0.0 && v.is_finite())) {
+        // vendor raws (NEF, ARW…) carry white balance as multipliers: the DNG neutral is their reciprocal
+        ifd.set(t::AS_SHOT_NEUTRAL, rat_vec(&m.map(|v| 1.0 / v as f64)));
     }
     ifd.set(t::BASELINE_EXPOSURE, Value::SRational(vec![srational(c.baseline_exposure)]));
     // the source's own profile look travels with its data (conversions, smart previews, merges)

crates/raw/tests/dng.rs

@@ -453,6 +453,22 @@ fn colour_transform_from_decoded_dng() {
     assert!(o.iter().all(|v| (v - 1.0).abs() < 1e-9));
 }
 
+/// Issue #212: a NEF converted to DNG came out dark and green. NEF white balance is vendor multipliers
+/// (no colour matrix, no `AsShotNeutral`); the DNG must carry them as its as-shot neutral.
+#[test]
+fn vendor_white_balance_survives_dng_conversion() {
+    let mut raw = synthetic(16, 16, Some(Cfa::bayer("RGGB").unwrap()), 1);
+    raw.color = ColorData::default();
+    raw.wb_multipliers = Some([2.25, 1.0, 1.7421875]);
+    let before = lightcraft_raw::color::camera_transform(&raw, lightcraft_raw::color::as_shot_white_xy(&raw));
+    let back = decode(&write_dng(&raw, &DngWriteOptions::default()).unwrap()).unwrap();
+    let after = lightcraft_raw::color::camera_transform(&back, lightcraft_raw::color::as_shot_white_xy(&back));
+    for c in 0..3 {
+        assert!((before.wb[c] - after.wb[c]).abs() < 1e-3, "{:?} vs {:?}", before.wb, after.wb);
+    }
+    assert!((after.wb[0] - 2.25).abs() < 1e-3 && (after.wb[2] - 1.7421875).abs() < 1e-3, "{:?}", after.wb);
+}
+
 #[test]
 fn rejects_unsupported_and_broken() {
     let (w, h) = (8, 8);