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(),
     })
 }