A linear DNG in and out: the writer, and a three-sample RawImage

dr-export gains write_linear_dng — LinearRaw, DNG 1.4, u16 samples at
the sensor's scale, the body's matrices with their illuminants, the
as-shot neutral, the EXIF block an export writes — streamed strip by
strip through a closure so the composite is never held (FR-MRG-11). The
tiff crate's directory is a map, so PhotometricInterpretation is written
over what new_image set, which is the trick the S15.1 spike thought it
had to hand-roll around. The test reads the file back through rawler.

dr-decode's RawImage carries samples_per_pixel (a linear DNG is 3), the
body's profile with its calibrations mapped back to EXIF illuminant
codes, and the cleaned make and model. The GPU uploads a three-sample
image as it is, normalised by black and white like a photosite, through
a full f16 conversion — subnormals kept, because a 14-bit LSB sits at
f16's smallest normal and rounding it to zero would crush exactly the
shadows the file was written to keep.

core/dr-decode/src/lib.rs

@@ -134,6 +134,22 @@ pub struct RawImage {
     pub base_curve: BaseCurve,
     /// The usable region of `data`, excluding masked and border photosites.
     pub crop: CropRect,
+    /// TRACES: FR-MRG-3
+    /// Samples per photosite in `data`: 1 for a colour-filter-array capture,
+    /// 3 for a *linear* DNG — demosaiced RGB, still camera-space, which is
+    /// what a merge writes. With 3, `cfa_pattern` means nothing, `data` is
+    /// `width × height × 3` interleaved, and the GPU uploads it as it is
+    /// rather than demosaicing.
+    pub samples_per_pixel: u8,
+    /// TRACES: FR-MRG-3
+    /// The body's colour profile as the file carried it, for a composite to
+    /// carry on: calibrations and the as-shot neutral. `None` for a body the
+    /// decoder has no matrix for.
+    pub profile: Option<profile::CameraProfile>,
+    /// The body, as rawler cleans the names: what `Make`/`Model` say and what
+    /// the base-curve database matches on.
+    pub make: String,
+    pub model: String,
 }
 
 /// TRACES: FR-RAW-3
@@ -559,6 +575,21 @@ fn decode_unguarded(bytes: &[u8]) -> Result<RawImage, DecodeError> {
         image.camera.clean_model.as_str(),
     );
 
+    // TRACES: FR-MRG-3
+    // A linear DNG — three samples per pixel, no colour filter array — is a
+    // composite this application wrote (or any other demosaiced DNG). It
+    // carries the same scale, matrices and neutral as a CFA file and goes
+    // through the same profile; only the demosaic is skipped.
+    let samples_per_pixel = match image.cpp {
+        1 => 1u8,
+        3 => 3,
+        other => {
+            return Err(DecodeError::Unsupported(format!(
+                "{other} samples per pixel; only CFA (1) and linear RGB (3) are handled"
+            )))
+        }
+    };
+
     let data = match image.data {
         rawler::RawImageData::Integer(v) => v,
         rawler::RawImageData::Float(v) => {
@@ -627,6 +658,10 @@ fn decode_unguarded(bytes: &[u8]) -> Result<RawImage, DecodeError> {
         wb_coeffs,
         color_matrix,
         base_curve,
+        samples_per_pixel,
+        profile,
+        make: image.camera.clean_make.clone(),
+        model: image.camera.clean_model.clone(),
     })
 }
 

core/dr-decode/src/profile.rs

@@ -392,6 +392,21 @@ impl CameraProfile {
     }
 
     /// The calibrations this profile was built from, coolest first.
+    /// TRACES: FR-MRG-3
+    /// The calibrations as a DNG carries them: `(CalibrationIlluminant,
+    /// ColorMatrix)` with the EXIF light-source code, for a composite to
+    /// write the profile of the body that took its sources.
+    ///
+    /// The code is recovered from the temperature, which is lossy only for
+    /// illuminants this profile never kept: `extract` drops calibrations
+    /// whose illuminant has no temperature, so every one here maps back.
+    pub fn dng_calibrations(&self) -> Vec<(u16, [[f32; 3]; 3])> {
+        self.calibrations
+            .iter()
+            .map(|c| (illuminant_code(c.temperature), c.xyz_to_cam))
+            .collect()
+    }
+
     pub fn calibrations(&self) -> &[Calibration] {
         &self.calibrations
     }
@@ -528,6 +543,35 @@ fn illuminant_temperature(illuminant: Illuminant) -> Option<f32> {
     })
 }
 
+/// The EXIF `LightSource` code for a calibration temperature — the inverse
+/// of [`illuminant_temperature`], on the temperatures it produces.
+fn illuminant_code(temperature: f32) -> u16 {
+    // Nearest of the table, so a temperature that came through a float
+    // round-trip still lands on its illuminant. Where two illuminants share
+    // a temperature (D55 and Daylight, D65 and Cloudy, D75 and Shade) the
+    // CIE standard one is written: it is what every profile database means.
+    const TABLE: &[(f32, u16)] = &[
+        (2856.0, 17), // A
+        (3200.0, 24), // ISO studio tungsten
+        (3500.0, 15), // white fluorescent
+        (4150.0, 14), // cool white fluorescent
+        (4230.0, 2),  // fluorescent
+        (4874.0, 18), // B
+        (5000.0, 13), // daylight white fluorescent
+        (5003.0, 23), // D50
+        (5503.0, 20), // D55
+        (6430.0, 12), // daylight fluorescent
+        (6504.0, 21), // D65
+        (6774.0, 19), // C
+        (7504.0, 22), // D75
+    ];
+    TABLE
+        .iter()
+        .min_by(|a, b| (a.0 - temperature).abs().total_cmp(&(b.0 - temperature).abs()))
+        .map(|(_, code)| *code)
+        .unwrap_or(255)
+}
+
 /// Compose a forward matrix into camera RGB → linear sRGB.
 ///
 /// `forward` takes white-balanced camera RGB to XYZ under D50, which is the